journal of experimental psychology · jerk, is to maintain by means of a constant reflex influence,...
TRANSCRIPT
Journal of
Experimental PsychologyCopyright <D 1974 by the American Psychological Association, Inc.
VOL. 103, No. 6 DECEMBER 1974
A STUDY OF THE KNEE JERK
BY
EDWIN B. TWITMYER
University of Pennsylvania
Knee-jerks to the sound of a bell were obtained from six 5s after 150-238trials during which the two patellar tendons were struck 0.5 sec. after the soundof the bell. These knee-jerks to the bell alone were not the result of S's volun-tary effort, and attempts to inhibit the kicks were wholly unsuccessful. Withincreased numbers of paired stimulations, the regularity of responding to thebell alone was greatly increased, and the pattern of the knee-jerks was exactlythe same as that of knee-jerks resulting from blows to the patellar tendon. Theknee-jerks to the bell can be explained only in terms of reflex action, whereinrepeated association of the functioning of the motor cells of the lumbar segmentof the spinal cord with excitation of centers of the medulla resulting from thesound of the bell resulted in the development of an unusual reflex arc.
I, The tendons of the different portions of„,, . . . . . .. . i , u
tne quadriceps unite at the lower portionThe knee jerk is immediately caused by of ̂ ̂ fa gQ ag tQ form & gi le &the sudden and vigorous Contraction of the tendon which ijlvegts the Ua th h_quadriceps muscle of the thigh. The quad- Qut itg who]e gurface ̂ underneathnceps is the great extensor muscle of the where a free articular gurface ig nted
leg, forming a large fleshy mass, wh.ch and continues downward to be inserted incovers the front and sides of the femur. ^ fa fa prominence of the tibia.Shemngton* has demonstrated by expen- From ̂ ,ower ̂ ^ of ̂ Ha tQ ̂mentation upon animals that two of its four insertion ^ ̂ tuberosit of the tibia it isdivisions the vastus mternus covering the known &s ̂ H ntum tellae. The
inner surface of the femur and the crureus, ^ therdore be properly regardedcovering the front of the femur, are chiefly &g & segamoid bo devd d in the tendoninvolved in the knee jerk mechanism. of ^ dric and the Hgamentum
These two portions are so intimately tdlae immediately below the lowerblended anatomically as to form a single fidge of the patella_one_half inch to an
muscle. So considered it is related by its inch__the tendon passes ove]- a siightiy
superficial surface with the psoas and ilia- hollowed space in the parts beneath,cus, the rectus, sartorius, pectineus, adduc- The nervous structures involved in thetors, and fascia lata; by its deep surface knee jerk mechanism have also been deter-with the femur, sub-crureus, and synovialmembrane of the knee joint. 1 Journal of Physiology, Vol. 13, p. 666.
1047
1048 EDWIN B. TWITMYER
Editor's note:This paper is E. B. Twitmyer's PhD thesis, Uni-
versity of Pennsylvania, 1902. The thesis wasprivately printed in a limited edition, so that inspite of its considerable historical importance, it hasbeen virtually inaccessible to interested psycholo-gists. It is being reprinted after consultation be-tween the Editor of the Journal and Dr. Arthur W.Melton, Chairman of the Publications and Com-munications Board, and with the permission of theUniversity of Pennsylvania. The Editor wishes tothank Professors Francis W. Irwin and Eliot Stellarof the University of Pennsylvania for their assis-tance in obtaining a copy of the thesis and thenecessary permission.
Twitmyer's thesis reports the first Americandiscovery of the conditioned reflex, but it should beborne in mind that by 1902 vigorous and purposeful
research on conditioned reflexes had already beencarried out in the laboratories of Pavlov andBekhterev. Section III of the thesis was reportedby Twitmyer at the Christmas 1904 meeting of theAmerican Psychological Association, and this reportwas abstracted (E. B. Twitmyer. Knee-jerks with-out stimulation of the patellar tendon. Psychologi-cal Bulletin, 1905, 2, 43-44). The thesis and theabstract show that Twitmyer realized that he hadmade an important discovery, "an association in areflex arc" such that "auditory excitation ... hasbeen substituted for the excitation of the peripheralsensory fibers terminating in the quadriceps muscleand tendon." Twitmyer's APA presentation, how-ever, did not even elicit significant discussion. Theimportance of his findings were totally unrecognizedat the time, in spite of the fact that, as Irwin com-mented, "Brief as it is, this abstract presents per-
mined with exactness. Sherrington2 haslocated the position of both the afferentand efferent conduction paths. He foundthe afferent path to lie in the posterior rootof the fifth lumbar of the Rhesus monkey,which corresponds to the fourth of man.This posterior root receives afferent fibersfrom the obturator and anterior cruralnerves, and from the external and internalpopliteal nerves. By the method of sever-ance and other modes of destruction of theconductivity of the nerves entering theroot, e. g., cooling to the freezing point, etc.,Sherrington found that of the fibers enter-ing the root from these sources, those onwhich the jerk depends are not from anyexcept the anterior crural nerve, and in theanterior crural nerve only those fibers whichissue from the vastus internus and crureusmuscles.
The peripheral terminations of the sen-sory conduction path are therefore foundin the muscles whose contraction causes themovement. Further it is probable thatsome of these fibers also terminate in thetendon of the muscle.3
By the same method the efferent conduc-tion path was found in the anterior rootsof the fifth and fourth lumbar nerves ofthe Rhesus (fourth and third of man) andwas traceable along the anterior cruralnerve into those of the muscular branchesof that trunk which supply the vastusinternus and crureus divisions of the quad-riceps, i. e., in exactly the same muscles in
which the afferent condition path has itsperipheral termination.
Pathological evidence, offered by casespresenting degeneration of posterior nerveroots and certain portions of the posteriorcolumns of the cord in which the knee jerkis observed to be entirely absent, makes itclear that the spinal center involved in theknee jerk mechanism is situated somewherebetween the second and fifth lumbar seg-ments inclusively. Mills4 agrees withEdinger, Starr and others in placing it inthe second or third lumbar segments orprobably both.
A blow on the patella tendon (a) stimu-lates the peripheral sensory fibers termi-nating freely in the superficial tissue, (b)stimulates the peripheral sensory nervefibers terminating in the tendon, (c) de-presses the tendon thereby drawing it tautand exerting through its attachments a
1 Journal of Physiology, Vol. 13, p. 666.3 As to the manner in which sensory nerve fibers
terminate in the tendons, Phillipp Stohr, in Lebrbuchder Histologie, 1901, says: "Die Sehnenspindeln sindmeist spindelformige Auftreibungen von Sehnen-biindeln, die von einer gut entwickelten bindegewe-bigen Hiille umgeben werden. Das eine Ende derSpindel geht in Sehnenbiindel liber, das andere setztsich in Muskelfasern fort Die an die Mitte her-antretenden Nervenfasern theilen sich widerholt,verlieren ihr Mark und gehen in ein reich entwicke-Ites Astwerk liber mit oft keulenformig angeschwol-lenen Enden.
4 Journal of Nervous and Mental Diseases, 1899,p. 142.
A STUDY OF THE KNEE JERK 1049
fectly clearly the essentials of the discovery, a sparksurely sufficient to fire the imagination of anyonewho was adequately prepared for it [F. W. Irwin.Edwin Burket Twitmyer: 1873-1943. AmericanJournal of Psychology, 1943, 56, 451-453]." Thethesis shows that Twitmyer was adequately pre-pared for it, even if American psychology in 1904was not. Twitmyer himself did not follow up hisfinding with further research on the conditionedreflex. Upon occasion in personal conversations herecalled his disappointment at the reception of hisdiscovery in 1904, but later psychologists could onlyspeculate in trying to account for his failure topursue such a promising and fundamental line ofresearch. (K. M. Dallenbach. Twitmyer and theconditioned response. American Journal of Psy-chology, 1959, 72, 633-638.) Twitmyer's thesis, hisabstract, Irwin's necrology, and Dallenbach's con-
jectures are worth reading today.The thesis is reprinted in very nearly its original
form with a few minor typographical errors cor-rected. The abstract has been rewritten, and con-sists of quotations and paraphrases from Twitmyer'sabstract of his APA report. Reproductions ofeight knee-jerk records have been deleted and someslight changes have been made in the footnoteswhich are here numbered serially. Section III re-ports the discovery of the conditioned reflex andthe further experimentation which confirmed itsnature.
Requests for reprints should be sent to David A.Grant, W. J. Brogden Psychology Building, 1202West Johnson, University of Wisconsin—Madison,Madison, Wisconsin 53706.
David A. Grant, Editor
longitudinal stress upon the fibers of thequadriceps, thus offering a stimulus to theperipheral sensory nerve fibers terminatingin the muscle itself, (d) by depressing thetendon serves as a mechanical stimulus tothe quadriceps.
With the given anatomical structurescomprising the knee jerk mechanism andthe given results of a blow on the ligamen-tum patellae, three explanations of thephysiological cause of the knee jerk arepossible.
1. The knee jerk is a reflex. Stimulationof the sensory fibers terminating in thesuperficial tissue by contact or by an elec-trical current does not produce the move-ment, consequently if the jerk is a reflex,the peripheral excitation has its origin inthe tendon or muscle (or both). In eithercase the course of the afferent impulse is thesame. "From the tendon of the patella,the excitation is carried to the spinal cordby crural sensory fibers, and enters by thedorsal root into a certain portion of thedorsal column, and thence passes to thedorsal horn; next it takes its course throughthe intermediate gray matter until itreaches cells of the ventral horn; and thencethe motor excitation goes through themotor roots and crural nerve to the an-terior muscles of the thigh.6" Waller6 ob-jects to this theory on the ground that thetime elapsing between the blow and theresponse is too short for the transmission
of an excitation over a reflex arc. By exactmeasurement he found the time betweenthe percussion of the tendon and the con-traction to be between .03 and .04 secondand between the direct percussion of themuscle and the contraction .03 second.Exner's reflex, the winking of the eyelid,has a latent time of .05 seconds. Asidefrom this one instance a knowledge ofnormal reflex times is still wanting.
2. The knee jerk is a muscular contrac-tion due to the direct mechanical stimula-tion of the muscle. On this theory thefunction of the nervous structures involved,i. e., an afferent and efferent conductionpath and a spinal segment, any interrup-tion of which results in a total loss of thejerk, is to maintain by means of a constantreflex influence, the tonicity of the muscle.With the tone present the muscle reactsdirectly to the mechanical stimulus. Op-posed to this theory is the fact that theknee jerk may be present when muscle toneappears to be wanting and may be absentin the case of men who apparently have anormal amount of tone.7 Further thetheory does not satisfactorily explain thefacts of reinforcement, e. g., the increase inthe extent of the kick when the hands areclinched just before the tendon is struck.
6 Mills, Journal of Nervous and, Mental Disease,1899, p. 142.
6 Journal of Physiology, Vol. 11, p. 384.7 See Diagram, p. 1055.
1050 EDWIN B. TWITMYER
Lombard8 made a study to determine thechanges in the tone and irritability of thequadriceps and found that neither in-creased under reinforcing conditions.
3. The knee jerk is first due to themechanical stimulation of the muscle andsecond to the reflex excitation, i. e., themuscle contracting in response to the me-chanical stimulus represents the beginningof the kick, while the reinforcement orcontinuance of the movement is the resultof the reflex impulse. Lombard9 reportsthe results of one case which showed anirregularity "which one might expect if,when the original contraction of the musclehad reached its highest point or when themuscle had even begun to relax, a secondimpulse had reached it, and caused it tocontract still further." Although this re-sult lends support to the theory, the non-observance of this irregularity in all kneejerk records is not evidence to the con-trary. The reflex influence may graduallybecome operative quite before the move-ment due to the mechanical stimulationhad reached its maximum height, in whichcase the irregularity would not appear inthe record. Improved methods of record-ing the excursion of the leg may revealdifferent rates of movement during differentdivisions of the excursion and therebyfurnish more conclusive evidence on thispoint. Some evidence for this theory ispresented in Part III of this study.
The following study comprises (1) anattempt to determine as nearly as possiblethe extent of the unaugmented or normalknee jerk for normal subjects and (2) aconsideration of the modifications it isobserved to undergo during an extendedperiod of experimentation. When the pa-tella tendons are struck at exactly the sameplace with blows of constant force and atregular intervals, no two of the resultingknee jerks are of the same extent. Thisvariation is usually referred to some acci-dental stimulus acting upon the subject,e. g., a loud or distracting sound or to anidea to which the subject directs his atten-tion, an emotional state of greater or lessintensity, any one of which conditions mayproduce a variation in the extent of the
jerk, or in a general way to a change in theactivity of the central nervous system.10
From the very nature of the case suchsources of variation are not wholly avoid-able. However secure the subject may bekept from accidental sensory stimuli, thestream of consciousness is never altogetherwithin control of the experimenter and theorganic processes of digestion, circulation,etc., are constantly producing some slightor more profound modification in the equi-librium of the nervous system.
The results of a study made by Noyes11
on the unaugmented knee jerk in sleep ina case of terminal dementia suggest a moredefinite explanation for the variations oc-curring in kicks following in close succes-sion when the conditions remain preciselythe same. Noyes obtained the knee jerkcurve and the Traube-Hering curve for thesame period and found a well-marked co-incidence between them. A comparisonshows that the Traube-Hering curve de-scends lowest in that part of the group ofknee jerks where the kicks are longest andat the place where the Traube-Hering curveis highest the knee jerks are much dimin-ished. A rise in the Traube-Hering curveindicates increased blood pressure in thearm, and a fall in the curve corresponds todiminished blood pressure.
On the theory that increased blood pres-sure in the extremities means lessenedblood pressure in the central nervous sys-tem there is relative anemia of the brainand cord when the Traube-Hering curveis at its height, and relative hyperanemiaof the brain and cord when the curve islowest. The diminished knee jerk wouldthen follow from the lessened functional
8 Journal of Physiology, Vol. 10, p. 122.• American Journal of Medical Science, Vol. 93,
p. 88.10 In a consideration of these variations Sommer
says: "Es mussen demnach in den betreffendenIndividuen Krafte wirksam sein, welche den Reflexhemmen oder verstarken. Diese Curvenreihen sindder einfachste Ausdruck eines variablen Momentesim Nervensystem, vermoge dessen bei gleichemReiz verschiedene Wirkungen zustande kommen."—Psychopatholog. Untersuchungs-Methoden, 1899, p.30.
11 American Journal of Psychology, Vol. IV, No. 3.
A STUDY OF THE KNEE JERK 1051
activity of the spinal cord at the height ofthe Traube-Hering wave, while an in-creased knee jerk from increased func-tional activity of the cord would follow atthe low phase of the peripheral Traube-Hering curve.
Although these results were obtainedfrom a demented subject there is no reasonfor believing that the same relation wouldnot maintain between the two curves fora normal subject provided all cerebral in-fluences could be successfully removed orinhibited. Provided Noyes' inferences asto the influences affecting the knee jerk inthe demented case are legitimate it is notevident why the same inferences do notapply to the normal subject. This factmay therefore be looked upon as one of thesources of the variation occurring in theextent of the jerks in a given group.
In 1887 Lombard12 made a study of thecharacter and extent of this so-callednormal knee jerk and in addition to varia-tions occurring in successive kicks, foundvariations corresponding to the time of dayat which the experiments were made. Thisstudy comprised a series of experimentsmade on one subject, during fourteen con-secutive days in which the jerk of the rightleg was examined seven times a day,twenty-five experiments being made ateach examination.13 The hours chosenwere as follows, viz., 8.15 immediatelyupon arising; -9.15 soon after breakfast;1.15 just before lunch; 2.15 just after lunch;6.15 must before dinner; 8.00 soon afterdinner, and 11 just before retiring. Theaverages of the results for the differentperiods were as follows: 25, 65, 43, 47, 30,40, 27 mm.
From these results Lombard concludesthat there is a diurnal variation of theknee jerk. "The variation correspondswith the gradual loss of vigor which thebody as a whole suffers from morning tillbedtime. This decline is an interruptedone and further corresponds to the tem-porary and partial recoveries which thebody undergoes as the result of the freshsupplies of nutriment and rest which itobtains at each meal."
Lombard further observed that the aver-
age knee jerk for any given day did notalways correspond with the average kick ofother days in the series. In a comparisonof the mean temperature and mean baro-metric records for the same period he foundthat there was a marked correspondencein three curves. In general as the tempera-ture rises the knee jerk becomes less and asthe temperature falls the knee jerk becomeslarger. On the other hand the knee jerkrises and falls with the barometric curve.
A few preliminary series of experimentsupon several subjects is sufficient to es-tablish the fact that the knee jerk is sub-ject to marked -individual differences.14 Thequestion as to what may be considered thenormal jerk of the normal subject thereforepresents itself. In order to study this prob-lem, an extended series of experiments wasperformed upon seventeen subjects, seniorsin college and graduate students, all ofwhom were in a healthy and normal con-dition. One subject, however, during theperiod of time over which the experimenta-tion extended, developed certain nervoussymptoms which will be noted later. Theexperiments, divided into four groups, weremade on four different days, a group eachday, usually with an interval of one week,but in some few cases, at an interval oftwo or three weeks. The experiments on agiven subject were made at the same houreach day and consequently always followedthe same amount of preceding college work.A group consisted of either forty or forty-
12 American Journal of Psychology, Vol. I, No. 1.13 For the assumed normal condition Lombard had
the subject lie upon his left side upon a comfortablecouch, the back and head being supported. Theright thigh rested in a splint of plaster of Paris,shaped so as to conform to the inner and posteriorsurface, and of such a height as to hold the kneeon a level with the hip joint. The right foot wassupported at the same height by a swing suspendedby a cord from the ceiling. In this position themuscles were passive, and the whole body was asfar as possible in a state of rest.
14 Sommer views the knee jerk phenomenon as themotor expression of a definite disposition of thenervous system and maintains "dass aus demNachweis solcher Schwankungen von motorischenReactionen ein Schluss auf eine gewisse generalleDisposition der Nervensubstanz eines Individuumsgemacht werden konnte."
1052 EDWIN B. TWITMYER
five experiments, which were divided intoseries containing five experiments each.The initial, middle and final series, i. e.,the first, fifth, and eighth or ninth, werealways made under the assumed normalconditions, the remaining series of thegroup involving various augmenting con-ditions, e. g., muscular contraction, mildelectrical shock to various portions of thebody, pain stimulus, etc.
In all of the experiments the force ofthe blows on the tendons and the intervalsbetween the blows were kept constant.The subjects were directed to allow thebody to relax and to avoid, as far as pos-sible, giving active attention either to theexperiment or to any ideas which mightcome into mind.
In order to eliminate as far as possibleall variations due to the augmenting effectof accidental stimuli, the experiments wereperformed in a room from which practicallyall noises from other parts of the buildingand streets were excluded. The room ispartitioned off from a larger room on thetop floor of a building by double walls, thespace between being filled with sawdust.The entrance is closed by heavy doubledoors; the only other means of communica-tion is a small opening in the wall directlybehind the chair upon which the subjectis seated, which permits the passage of twofine linen threads, attached to the heels ofthe subject in the experimenting room,to the recording apparatus in the roomadjoining.
The room is illuminated by a singleincandescent lamp so placed that it doesnot shine directly on the eyes of the sub-ject. The walls, ceiling and carpet areneutral gray in color. Ample ventilationis provided.
The subject is comfortably seated in aMorris chair, the legs of which had beenlengthened about 18 inches. The back ofthe chair is inclined at an angle of 45degrees. In this semi-reclining position thegreater portion of the weight of the bodyis supported upon the pelvis, thus reducingthe pressure of the flexor muscles of theunder part of the thigh to a minimum; theknees extended some distance beyond theedge of the seat; the lower legs are thus
free to swing back and forth without strik-ing the edge of the seat, the feet clearingthe floor. The head is supported in acomfortable position and the arms are al-lowed to rest extended on the broad flatarms of the chair. The cushion is slightlyhollowed out at the edge of the seat toconform to the contour of the legs, therebypreventing them from being moved out ofposition. In this position the muscles ofthe body are thoroughly relaxed and thedistracting effects of complicated harnessand apparatus are avoided.
The bell, the use of which is describedlater, was fastened to the wall just back ofthe subject in the median plane of thehead and body and on a level with thehead. Its ring consisted of a single tap ofmoderate intensity.
The apparatus employed to give the blowon the tendons consists of two hammersswinging as pendulums from a supportingframe. This frame is constructed of twotelescoping upright rods, secured in heavyiron bases, resting on the floor, and con-nected above by a horizontal brass bar.The frame is moveable and after adjust-ment can be secured in position by bracesclamped to the arms of the chair and theupper ends of the upright rods. The clampson the horizontal bar furnish the point ofsupport for the hammers. These clampscan be moved to any position along thebar. The hammer is made of lead, cylindri-cal in form, 6 cm. long and 2.5 cm. in di-ameter; the pole coming in contact withthe tendon is spherical in shape. It is sus-pended from the moveable clamp by abrass handle or arm made of two pieceswhich slide over each other and are fastenedtogether by a thumb screw to give anydesired length. When the hammer is ad-justed for the experiment, the pole barelytouches the ligamentum patellae. It isthen drawn back by the experimenteruntil it is caught by an electro-magnet, alsosupported from the moveable clamp on thehorizontal bar, and which can be furtheradjusted at any height within 90 degreesfrom the vertical position. When thuscaught by the magnet it is held until thecurrent is broken by the interrupting in-strument in the recording room. This ar-
A STUDY OF THE KNEE JERK 1053
rangement of the hammers provides forexact adjustment and makes it possible tokeep the force of the blow, once determinedupon, absolutely constant for all subjects.In the experiments herein reported, thelength of the arm of the hammer was keptat 23 cm., the arc through which thehammers swung at 50 degrees.
By means of an instrument (interrupter)in the recording room, the signal bell andthe hammers were operated automaticallyat regular intervals. This instrument con-sists of a brass cylinder, 19 cm. in diameter,which is rotated at a constant rate aboutits vertical axis by means of a weight andescapement mechanism. The circumfer-ence of the cylinder is surrounded with anumber of rows of small holes, placed atregular intervals. Brass pegs, 6 mm. longare inserted into these holes and project 5mm. beyond the surface of the cylinder.The time interval required determines inwhich holes of a row the pegs are to beplaced. During the rotation of the cylinderthese projecting pegs come into contactwith two contact pieces fastened on an up-right. For the present experiment theupper contact piece was so adjusted thatthe upper row of pegs in the cylinder inpassing closed momentarily a circuit therebyringing the signal bell; in the lower contactpiece the peg of the lower row broke acircuit momentarily, causing the electro-magnets to release the hammers. By theadjustment of these two contact pieces, thetime elapsing between the ringing of thebell and the fall of the hammers can bemade any desired amount. For the experi-ments herein reported the interval was .5second.
The recording apparatus consists essen-tially of two pens to each of which arefastened the strings attached respectivelyto the right and left heels of the subject inthe experimenting room. The pen carriageruns upon two steel wires drawn very tautand consists of two parts: an ink reservoirand a pen which is supplied by it with acontinuous flow of ink. The pen inscribesthe excursion of the foot upon an endlessroll of paper which passes over the tracingsurface beneath the pen. The carriage isdrawn back after the outward excursion by
means of a small weight fastened to therear end of the carriage by a string playingover a small pulley. As the total amountof the friction of the pen on the paper andthe carriage on the wires is extremely smalla weight of 17 grams is sufficient for thepurpose and offers practically no resistanceto the outward kick of the leg. The tworollers which receive the paper are con-nected by belts to a pulley which the personrecording turns by hand; by this means therate of movement of the two rolls at anyone time is the same and the two curvestherefore correspond. The recorder beginsto turn the pulley just before the tendonsare struck and continues turning until thelegs have come to rest. The first kick outof the legs and all the subsequent swingsare therefore recorded. (See Record No. 1.)When the pens are at rest, a movement ofthe paper will cause a horizontal line to beinscribed. This base line is a broken line;its rise and fall indicates that the leg afterthe kick does not always drop back intoexactly the same position. The extent ofany given initial kick as well as any sub-sequent swing is taken to be the distancefrom the extremity of the line to the baseline immediately preceding and in thetables given is expressed in millimeters.
In order to determine the individual dif-ferences in the normal jerk an average ofall the normal series in the four groups wasobtained. Since no two kicks are of thesame extent, even when taken under pre-cisely similar conditions, an average of adetermined number of trials must be takenas the index of the extent of the kick underthe given conditions. The mean variationis frequently considerable but the averagesobtained from several groups under similarconditions usually correspond very closely.Although the averages obtained in thismanner may not be looked upon as absoluteindexes of the extent of the kick, they,nevertheless, afford a satisfactory meansof comparison between results obtainedfrom different individuals and under variedconditions.
In Table I are given the averages forthe seventeen subjects for both the rightand left legs, each average representingthe results of 60 experiments. The sub-
1054 EDWIN B. TWITMYER
TABLE I.
Subject
Right Leg.
Left Leg . . .
A
0
0
B
16.4
8.9
C
20.7
20.8
D
24.6
28
E
35.2
22.3
F
52.8
69.9
G
68.8
64.3
H
71.9
73.3
I
73.3
37.9
J
81.4
76.6
K
92
95.7
L
92.6
104.4
M
98.2
98
N
103.7
109.4
O
122.9
136.9
P
135.5
144.6
Q
148.4
165.3
jects have been arranged in order withreference to the increase in the averagesfor the right leg.16 It will be noticed thatthe increase in the averages for the left legcorresponds very closely." This order iskept in the tables following. The averagenormal jerk for the subjects reported there-fore varies from 0 to 165 mm. Thesemarked individual differences are not dueto any of the sources of variation found byLombard.17 Although the subjects wereexperimented upon at three different periodsof the day, no correspondence is found toexist between the extent of the kicks forthe different subjects and the time of dayat which the experiments were made. Theexperiments were performed on SubjectsB, C, F, G, K and M between 9 and 11 A.M.,the period at which Lombard found thelargest kicks occurring with his subject.The averages for B and C are extremelysmall while no subjects in this number wereamong those giving very large kicks. Theexperiments were made on Subjects A, Eand N between 11 and 1 o'clock and onSubjects D, H, I, L, O, P and Q, most ofwhom were among those giving the largestkicks, between 3 and 5 P.M.
Further the experiments were all madeduring the months of February, March andthe early part of April, while the buildingwas being heated, the temperature of theroom being kept at about 70 degrees. Con-sequently these individual differences can-not be ascribed to this cause of variation.
Finally, the non-correspondence in theextent of the kicks obtained from subjectsexperimented upon at the same period ofthe same day is sufficient evidence that thebarometric conditions are not responsiblefor these differences.
From the results it appears impossibleto fix upon any given amount as repre-senting the extent of the normal knee jerkof the normal man, with a blow of givenforce. Nine of the seventeen subjects gave
averages falling within wide limits, 50 to110 mm., and in the absence of more defi-nite knowledge we may arbitrarily desig-nate an average falling within these limitsas normal. There is no reason, however,for looking upon averages falling withoutthese limits as indicative of an abnormalcondition. There may even be a totalabsence of the jerk in apparently soundsubjects (Subject A). Wide departuresfrom the limits, thus arbitrarily determined,therefore have no significance. They may,however, direct the study of the cause ofthese individual differences to the cases inwhich they are most marked.
A careful examination showed that theanatomical formation of the knees of thedifferent subjects varied slightly, the hollowbeneath the ligamentum patellae beingmore or less well marked. This of courseallows a greater or less displacement of thetendon when struck, and in consequencethe quadriceps is offered a mechanical stim-ulus of varying intensity. No correspon-
16 The averages given for Subject D have little orno value. In Group I this subject gave large kickswith regularity, the average for all the normalsbeing 92.1 and 103.3 for the right and left legs,respectively. In Group II the extent of the kickswas greatly diminished and in Groups III and IVno response was obtained under normal conditions.For the remaining subjects the averages fairly repre-sent the actual results.
16 During the period covered by the experimenta-tion Subject I, especially at the time when GroupsIII and IV were employed, was suffering from someslight sensory disturbances which were restrictedentirely to the left side of the body. These dis-turbances manifested themselves in fugitive sensa-tions of pressure along the left side of the lumbarand sacral regions of the cord and in numbness ofthe little finger, little toe and restricted areas on theabdomen. A medical examination revealed nothingfurther than that the subject was suffering fromnerve exhaustion due to overwork. This fact prob-ably accounts for the marked discrepancy betweenthe averages for the right and left legs.
17 American Journal oj Psychology, Vol. I, No. 1.
A STUDY OF THE KNEE JERK 1055
dence between this fact and the differencesin the extent of the kick could be observed.
On the assumption that the knee jerk isa muscular contraction, due to a mechanicalstimulus, differences in the extent of thekick for different subjects, when the in-tensity of the stimulus is kept constant,must be referred to a difference in thetonicity of the quadriceps, i. e., its capacityto respond with a contraction of givenenergy to a stimulus. This tone is depen-dent upon certain relations of the muscleto the central nervous system. It may beassumed that in the normal subject, thetone of the various portions of the muscula-ture of the body is relatively uniform. Weshould then expect to find a correspondencebetween the work done by any two or moregroups of muscles. A comparison betweenthe contraction of the quadriceps in theknee jerk and the contraction of the musclesof the hands and arms in the dynamometertest gives no such correspondence. Formany reasons the dynamometer test is un-satisfactory. The ability to make a highrecord involves mental factors quite asmuch as the mere tonic condition of themuscles. The test, in this instance, how-ever, was unfamiliar to the subjects experi-mented upon and the results may thereforebe considered more of an index of themuscular than of the mental conditionsinvolved.
In the curve the heavy line representsthe average normal jerk of the right leg forten subjects. The broken line representsthe average of five trials with the dyna-mometer with maximum energy.
The wide divergence of the two curvessuggests that the knee jerk is more depen-dent upon differences in the irritability orconductivity, or both, of the nervous struc-tures involved in the knee jerk mechanismthan upon differences in muscle tone. If
The dynamometer test was made with the elevensubjects indicated. The knee jerk curve (the heavyline) is constructed from Table I.
this inference is legitimate some evidenceis offered for the view that the normal jerkis more dependent upon the reflex influencesinitiated by the blow on the tendon thanupon the response of the quadriceps muscleto a mechanical stimulus.
A comparison of the results obtained inthe initial, middle and final series of a givengroup shows that there is usually a markeddecline in the extent of the normal kickduring the course of the group. Given inTable II are the averages that represent theresults obtained in the above-named seriesfor all four groups. The initial series wasalways taken after one or two preliminarytrials in the adjustment of the apparatus;the middle and final series were precededby series in which augmenting stimuli ofvarious kinds were employed. For threesubjects (H, K, O) the average of the finalseries is slightly greater than the initialseries. For the remaining subjects (exceptA, who gave no kicks at all) there is adecrease which is well marked in most in-stances. For ten subjects, the value repre-senting the middle series falls, where weshould expect to find it, between the valuesrepresenting the initial and final series.
TABLE II.
Subject
InltialSeriea.
Middle Series,
Final Series..
A
0
0
0
B
17.7
18
14
C
24.4
21.7
13.1
D
37
34
2.7
E
57.4
26.4
22
F
71.7
45.9
41
G
69.6
75.5
61.4
H
63.5
81.6
70.6
I
91.4
76.6
56.3
J
92
74
78.4
K
90.6
97.6
98.9
L
109.1
93.1
79.2
M
103.9
106.8
74.6
N
120.4
96.6
88.5
O
111.5
127
126
P
163.3
133.4
134.9
Q
152.2
H6.7
146.3
1056 EDWIN B. TWITMYER
TABLE III.
Subject
Group I . . .
Group IV..
A
0
0
B
14.4
25.1
C
12.1
36.4
D
92.1
0
E
16.2
18.7
F
26.8
51.4
G
80.2
60.9
H
70.8
67.9
I
72.1
96.7
J
99.5
93.1
K
112.5
113.6
L
105.5
103.2
M
79.5
111.7
N
102.5
104.9
O
83.7
180.8
P
121.4
173.9
Q
191.5
97.8
This decrease may be due either tofatigue of the muscles or nerves, or both,or to an accommodation of the subject tothe constantly recurring stimuli. Neitherthe nervous excitation nor the muscularcontractions seems to have been of suffi-cient intensity or duration to have producedan appreciable fatigue of the structures in-volved. On the other hand, the fact that aseries of experiments under slightly unusualconditions, when employed after the com-pletion of a group, always resulted in kickssomewhat increased in extent, offers someevidence that the accommodation of thesubject to the stimulus and the surroundingconditions is responsible for the decrease.It also indicates that the accommodationconsists rather in a mental quiescence thanin a purely physiological modification ofthe nervous system.
The increase in the extent of the jerkwhen definite mental states are employedas augmenting stimuli and the decrease inthe kick (frequently approximating zero)when consciousness is relatively emptiedof content led Witmer18 to look upon thekick as a function of its mental augmenta-tion. This view finds corroboration in thefact that the subjects whose results areherein reported gave testimony that mentalactivity seemed to wane during the experi-mentation and even frequently complainedof drowsiness toward the end of the group.The absence of the reflex influences depen-dent upon vigorous mental activity maytherefore account for the observed decreasein the final series.
Although the extent of the kick tends todecrease during any one group of experi-ments there is some evidence for believingthat the experimentation from week toweek tends to develop the reaction incertain subjects. Table III gives the aver-ages of the normal kicks in Groups I andIV. Subjects G and Q gave decreasedkicks in Group IV while Subject D failed
to give any normal kicks in Group IValthough reacting when augmenting stimuliwere employed. In Group IV no responsewhatever could be obtained, even with in-tense augmenting stimuli. Subjects B, C,F, I, M, 0 and P gave marked increases inGroup IV while the results from the re-maining subjects differ but little from theresults of Group I.
II.
In addition to the differences in theextent of the initial kick out of the legsfollowing blows of constant force on thetendons, the subsequent swings of the legsbefore coming to rest present certain varia-tions. Record I19 is the curve of one sub-ject (Subject M) for the right leg in anormal series of Group I. The tracingabove the baseline represents the actualexcursion of the leg outward from the posi-tion at which it is at rest.20 Experiments 1,4 and 5 yield records which correspond veryclosely both in the extent of the initial kickand the number and extent of the subse-quent swings. In Experiment 2 the heightof the initial kick is equal to the initialkick of Experiment 5, but the first subse-quent swing is considerably greater and anadditional subsequent swing appears. Ex-periment 3 also yielded three subsequentswings, but this may be due to the greaterextent of the initial kick. This record isfairly typical of the tracings obtained fromthe other subjects experimented upon.When first observed the subsequent swingsappear as merely the oscillations of the legto and fro as a pendulum but the evidence
18 A paper before the American Psychological As-sociation, 1895.
19 [Reference to omitted records (pp. 38-39 of thethesis).] .
20 On account of the limitations of the recordingapparatus used, the tracings below the horizontalline have no meaning.
A STUDY OF THE KNEE JERK 1057
furnished by the graphic record of the dif-ferences between the extent of the firstsecondary swings following initial kicks ofequal extent disproves this view. Thatthese subsequent swings are not merely themovements of the legs coming to rest as apendulum is further established by a com-parison which Sommer21 made between theknee jerk curve and the curve obtained byraising the leg of a corpse, immediatelyafter death, releasing it and taking therecord of the swinging leg in the samemanner. The curve in the latter case withthe given height consisted of five depres-sions below the base line and four eleva-tions,- the leg coming to rest as a pendulumin exactly the same place on each trial.Equal elevations of the legs were followedby the same number of subsequent swings.On the other hand, fewer oscillations takeplace after the first kick out in the normalknee jerk. The curve obtained by Sommer,when the initial kick was about the sameheight as the elevated leg of the corpse,consists of an elevation, a relatively lessdepression below the initial level, of asecond elevation with return to the initiallevel.
Record II is the result of a normal seriesfor the same subject taken in Group IV.Eighty-five or more experiments were there-fore made between the two records. Al-though the conditions were exactly similarin the two instances, it will be observedthat together with the increase in the extentof the initial kick [reference to omittedfigure] there is a marked difference in theappearance of the subsequent swings. Boththe number and extent of the swings isgreatly increased. In the last three experi-ments of the series the extent of the firstsecondary swing and in the third experi-ment the second secondary swing is greaterthan the initial kick.
Although the secondary swings in thenormal series of later experiments becomeactually greater than those in experimentsat the beginning of experimentation, as inthe case cited, for a limited number of sub-jects, there is a decided tendency for the ex-tent of the secondary swings gradually toincrease for all subjects as the experimenta-
tion proceeds. Some subjects, however,show more marked variation in the numberof subsequent swings. Record III (SubjectH), which is the record of one experiment ina normal series in Group III, even thoughthe initial kick is less than those of RecordII (Subject M), shows a larger number ofsubsequent swings. The record of this oneexperiment is typical of the records ob-tained in all the experiments in the normalseries of this group for Subject H. Therecords of the normal series in Group I(Record IV) show two and three less sub-sequent swings following initial kicks nearlyequal in extent. The increase in the num-ber of swings develops gradually through-out the intervening experimentation.
These same variations are much moreapparent when some augmenting stimulusis employed to reinforce the normal jerk.Records V and VI were taken when thesubject (Subject M) clinched both handsvigorously just before the blow on thetendons. Record V shows the result of aseries in Group I, Record VI the result ofa series in Group III for the same subject.The variations observed in the normalseries are again presented. The subsequentswings increase in number and extent withthe increase in the total number of preced-ing experiments. A much greater numberof subjects give secondary swings largerthan the initial kick when some augmentingstimulus is employed than in the normalseries; a marked increase is obtained in theremaining subjects.
A comparison of Records VII and VIII(Subject H) taken in augmented series inGroups III and I, respectively, shows notonly an increase in the extent of the subse-quent swings but a remarkable increase inthe number of swings. This record is fairlytypical of the records obtained for the otherexperiments in the series mentioned.
Sommer22 concludes from the marked dif-ferences between the excursion of the leg ofa corpse when elevated and allowed toswing until at rest and the curve of theknee jerk, that in the latter instance some
21 Psychopathologischen Methoden, p. 28.82 Psychopathologischen Methoden.
1058 EDWIN B. TWITMYER
energy is somewhere operative which in-hibits the previous mechanical pendulummovements and that this inhibition is con-ditioned by "Innervationszustande" mainlycerebral in origin, and not by the mechan-ism of the knee joint. Many records of theknee jerk, presenting variations in both theform and number of oscillations followingthe initial kick out of the legs, for the mostpart obtained from subjects presentingneuropathological conditions, substantiatesthe view that the character of these oscilla-tions as well as the initial kick are an ex-pression of a variable disposition of thenervous system. This is certainly true as ageneral statement, but on the other handthe results of Sherrington,23 which showthat at the knee joint excitation of theafferent fibers coming from one set ofantagonistic muscles induces reflex toniccontractions of the opposing set, despitethe fact that the opponent muscles are notinnervated from the same spinal segment,seem to offer a more definite and immediateexplanation of the phenomenon.
Severance of the great sciatic trunk inlower animals produces an increased brisk-ness of the knee jerk. This Sherrington24
found to depend solely upon the cutting ofthat portion of the trunk which is destinedfor the hamstring muscles. This severanceresults in a loss of the tone of the hamstringmuscles in consequence of which there is arelaxation of the flexor tension on the leg.This allows the leg to swing out withoutopposition. Certain experimental resultsobtained by Sherrington supplement thismechanical explanation with a more satis-factory physiological explanation. In thisstudy he was able to establish three impor-tant facts concerning the contraction of theextensor muscles produced by stimulatingthe motor nerve to the flexors.
1. If for the excitation of the motor rootto the flexors a series of induced currentsare employed, succeeding each other at arate slow enough to produce not perfecttetanisation, but tremulent contraction ofthe muscles, the contraction obtained inthe extensor muscles coincidently was,nevertheless, perfectly steady and tetanic,although not vigorous.
2. If the flexor muscles are severed fromconnection with the knee joint, so thattheir contraction cannot affect the joint,and if the knee jerk be elicited before,during and after stimulation of the motorroot to the flexor muscles, during the exci-tation, when those flexor muscles were con-tracting, the knee jerk, brisk previouslyand brisk later, disappeared or almostdisappeared.
3. If the sensory spinal roots belongingto the hamstring nerve are severed, thestimulation of a motor root to the ham-string muscles is no longer accompaniedby contraction of the extensor muscles ofthe knee, even when strong stimulation isemployed.
From these facts Sherrington concludesthat the degree of tension in one muscle ofan antagonistic couple intimately affectsthe degree of tonicity in its opponent notonly mechanically but also reflexly, throughafferent and efferent channels and thespinal cord.
It can be maintained, therefore, that ablow on the patella tendon primarily initi-ates a nervous excitation which is reflectedin the second or third lumbar segment intothe efferent tracts to the quadriceps muscle;as a secondary result an excitation passesdown over the intervening segments of thecord and is reflected into the division of thegreat sciatic nerve distributed to the ham-string muscles. This nerve impulse mo-mentarily increases the tone of the ham-string muscles, possibly produces mild con-tractions, consequently not only the initialkick but also the subsequent swings of theleg as well are opposed. In the corpsethese reflex influences are totally absentand the leg is free to swing as a mechanicalpendulum.
Upon this theory, how can the increasein the number and extent of the subsequentswings following the initial kick, observedto take place as the total number of experi-ments is increased be explained? Let usassume that the repeated transmission ofreflex excitation along a reflex arc, including
23 Proceedings of the Royal Society, Vol. 52, p. 563.24 Loc. cit.
A STUDY OF THE KNEE JERK 1059
the spinal segment, an afferent and efferentconduction path or from a given segmentin the cord to a segment somewhat removed,develops a pathway offering increasinglyless resistance. Evidence bearing on thispoint is presented in Part I of this mono-graph. A blow of constant force on thetendons will then result in a more and moreintense stimulation of both the quadricepsand its antagonist. This increase will berelatively constant, consequently the extentof the initial kick is still checked by thesimultaneous stimulation of the hamstringmuscles. The actual extent of the kick,however, tends to increase somewhat withthe continuation of experimentation (seep, 1056). With the increased intensity of thereflex influence, the flexors are thrown intomore violent contraction, which contrac-tion now becomes an adequate stimulus tothe sensory fibers terminating in the flexorsand as a result there is a vigorous excitationto the cord and ultimately to the quadri-ceps. This excitation therefore contributesits quota to the second outward excursionof the leg and frequently is sufficientlyvigorous to produce a kick larger than theinitial kick. This view is fully corroboratedby Sherrington's25 experiment in which hesucceeded in greatly augmenting the extentof the knee jerk by merely compressing thehamstring muscle when it is completelydissected away from its attachments, whilethe nerves were still intact.
III.
During the adjustment of the apparatusfor an earlier group of experiments withone subject (Subject A)26 a decided kick ofboth legs was observed to follow a tap of thesignal bell occurring without the usual blowof the hammers on the tendons. It was atfirst believed that the subject had merelyvoluntarily kicked out the legs, but uponbeing questioned, he stated that althoughquite conscious of the movement as it wastaking place, it had not been caused by avolitional effort, and further, that the sub-jective feeling accompanying the movementwas similar to the feeling of the movementfollowing the blow on the tendons with theexception that he was quite conscious that
TABLE IV.
Series III
Series VII . .
Series VIII
Exp; 3o
Exp. 245
Exp. 3
Right
1302S5
244145183
105
Left
193195
291204240
168
the tendons had not been struck. Twoalternatives presented themselves. Either(1) the subject was in error in his intro-spective observation and had voluntarilymoved his legs, or (2) the true knee jerk(or a movement resembling it in appear-ance) had been produced by a stimulusother than the usual one.
In a group of experiments, consisting ofeight series, designed primarily to studythe effects of motor augmentation, threeseries of five experiments each were plannedto investigate the phenomenon. In SeriesIII the subject was directed to clinch bothhands at the sound of the bell and twice inthe series, the third and fifth experiments,the hammers were dropped as usual butwere caught before they had struck thetendons. Again in Series VII the subjectwas directed to clinch both hands and thehammers were caught in the second, fourth,and fifth experiments. In Series VIII, anormal series, the bell sounding as before,the hammers were caught in the thirdexperiment.
These experiments were performed oneweek subsequent to the first observation ofthe phenomenon. The subject was,in abso-lute ignorance as to the nature of the ex-periments in the group and had no way ofknowing when the hammers would not beallowed to strike the tendons. Table IVgives in millimeters the results of theexperiments when the hammers were notdropped. It will be observed that in each
25 Proceedings of the Royal Society, Vol. 52, p. 563.26 Subjects A, B, etc., do not correspond with the
subjects thus indicated in Parts I and II. Theexperiments reported here were made the yearprevious; the report includes the results for allsubjects upon whom the series were performed.
1060 EDWIN B. TWITMYER
TABLE V.
Series
VII
VIII
Experiment
3
5
2
4
5
3
A
Right
130
255
244
145
183
105
Left
193
195
291
204
240
168
I
Right
40
61
Left
45
70
10
c
Right Left
E
Right
98
>
Left
60
5
E
Right
,
Left
of the six trials in which the tendons werenot struck the subject responded with &decided kick. To all appearances the move-ment had the explosive, jerky character ofthe kick following the actual blow on thetendons. The subject again testified thatthe movement had not been producedvoluntarily.
The same series of experiments was thenperformed with five other subjects whohad not been made acquainted with thenature of the series and the results ob-tained from Subject A. Table V gives theresults of these trials for the six subjects.
Although results were obtained from twoadditional subjects under the conditionsdescribed, the results obtained from SubjectA were not considered to have been whollyconfirmed. Subjects C, E and F gave noresponse when the tendons were not struck.Subjects B and D gave some kicks but notwith the regularity of Subject A. Theyagreed, however, with Subject A in sayingthat the kicks which occurred were whollyinvoluntary.
In order more fully to confirm the resultsobtained from Subject A, two additional
TABLE VI.
Series I
Right
.140101160
32136
Left ,
180133220
50171
Series II
Right
1971282511
106
Left
2601484125
100
series were employed in which the bellsounded as usual and the hammers werecaught each time before striking the ten-dons. In each series the bell struck everyfifteen seconds with an interval of two min-utes between the two series. For the firstseries the subject was given no directionswhatever; for the second series he wasdirected to clinch both hands in all experi-ments except the fourth. In the latterseries the subject, without the knowledgeof the experimenter at the time, made anattempt to inhibit the kicks. [See Table VIfor the results obtained.] With the sub-ject still remaining seated in position duringthe following five minutes there was nofurther movement of the legs after thetapping of the bell had ceased.
One week later, no experiments havingbeen performed on the subjects in the mean-time, a new group was employed, fourseries of which contained experiments inwhich the hammers were not permitted tostrike the tendons. In Series III the sub-ject was as before directed to clinch bothhands at the sound of the bell, tendons notstruck in Experiments 3 and 5; Series IV,the subjects said "ah" when the bell struck,tendons not struck in Experiments 1, 3 and4; Series VI, Experiment 4, slight pin prickon the left thigh; Experiment 5, same onright thigh, tendons not struck in eithercase; Series VIII the subject was directed tothink vividly of clinching both hands with-out making any actual contractions; ten-dons not struck in Experiments 1 and 4.Table VII gives the results for the sixsubjects.
A STUDY OF THE KNEE JERK
TABLE VII.
1061
Series
II
IV. . .
VI
VIIT
Experiment
3
5
1
3
4
4
5
1
4
j
Right
108
. 84
130
143
87
89
171
125
174
i
Left
17S
104
151
110
121
302
169
190
360
1
Right
20
127
264
232
j
Left
11
jino
..'ISL'330
178
(
Right
'il"*"1
50
Left
•* i*r ̂ '* )
44
i
Right
63
35
26
.•*!'22
)
Left
57
35
32
10
0
i
Right
;
Left
J — . .
I
Right
20
1 .1. s •'•-^—A — I
— — .
41
~
Left
26
65
Comparing the results of this group forall the subjects with the previous one itwill be observed that more kicks were ob-tained in proportion to the number of ex-periments made. Kicks were recorded forSubjects C and F, both of whom had givenno result in the first group. Subject Aconsistently kicked each time and SubjectE alone failed to give any response.
The week following a final group, con-sisting of eight series, was employed, inwhich five of the series contained experi-ments without the blows on the tendons.Series II, the subject was directed to clinchboth hands at the tap of the bell, the ten-dons not struck in Experiments 3 and 5.Series IV the subject received a very mildelectric shock from electrodes held in thehands. An induced current, resulting froman instantaneous make and break of theprimary current of an induction coil wasused to produce the shock which was notsufficiently strong to cause apparent con-tractions of the muscles of the hands andforearms. The current was closed by theexperimenter reacting to the sound of thebell with a telegraph key; with probablylarge variations the shock occurred about.125 second after the stroke of the bell.The hammers did not strike the tendons inExperiments 2 and 5. In Series VI thesubject clinched both hands at the soundof the bell and the shock, as in Series IV,was employed in addition. The hammers
did not strike the tendons in Experiments1, 3, 4 and 5. Series VII was a normalseries, the hammers being caught in thesecond experiment. Series VIII, same con-ditions as Series VI, the tendons not struckin any of the five experiments.
Table VIII gives the results for the sixsubjects.
In the fourteen experiments of this finalgroup in which the tendons were not struck,Subjects A and F gave decided kicks everytime, with the single exception of SubjectF with the left leg in Experiment 1, SeriesVI. Subjects B and C failed to respondin only two experiments, while Subjects Dand E gave three and four kicks respec-tively. A much greater proportion of kickswas therefore obtained in this group thanin either of the two preceding groups.
The results of these experiments warrantthe opinion that the occurrence of the kickwithout the blow on the tendons cannot beexplained as a mere accidental movementon the part of the subjects. On the con-trary the phenomenon occurs with suffi-cient frequency and regularity to demandan inquiry as to its nature.
Excepting in Series VI of the secondgroup and Series IV, VI and VIII of thethird group, the tap of the bell was clearlythe immediate stimulus to the movement.When the bell ceased striking no furtherkicks occurred without the blows on thetendons. The clinching of the hands,
1062 EDWIN B. TWITMYER
TABLE VIII.
Series
IV
VI
VII
V I I I . . . .
Experiment
3
5
2
5
1
3
4
5
2
1
2
3
4
5
l
Right
142
90
228
107
140
160
156
14S
114
95
185
135
110
185
i
Left
192
128
360
130
160
215
184
152
130
108
201
156
242
216
I
Right
128
255
185
220
328
240
280
263
275
247
265
255
3
Left
162
305
206
245
360
295
310
295
327
276
360
234
(
Right
184
142
97
23
60
58
154
90
55
52
30
62
Left
110
112
80
15
49
27
155
64
35
15
15
30
I
Right
12
45
22
>
Left
9
31
10
I
Right
12
52
16
28
:
Left
25
8
12
12
I
Right
33
18
51
2
18
55
72
53
15
28
25
75
35
46
-
Left
36
5
65
20
30
80
30
2
10
30
55
45
18
which was employed to produce an augmen-tation of the normal jerk, even when per-formed with maximum energy on the partof the subject, elicited no movements ofthe legs when the bell was not striking.In Series VI of the second group the painfulprick on the thigh and in Series IV, VI andVIII of the third group, the electric shockwas employed as the augmenting stimulus.Subsequent attempts to produce the kickswith these stimuli when the bell was notstriking were sometimes successful and itmay be questioned whether these stimulidid not immediately cause the movementin the experiments indicated rather thanthe stroke of the bell. Assuming this to bethe case there is still no reason for believingthat the movement differed in characterfrom the movements following the strokeof the bell alone. The painful prick on thethigh was not sufficiently intense to causethe subjects consciously to react in aneffort to remove, or get away from, thestimulus, and as a reflex movement in re-sponse to a pain stimulus, the reaction
would naturally have manifested itself inwithdrawal of the thigh from the stimulus,or perhaps a movement of the entire bodyrather than in a simple kick out of thelegs. The induced current employed wasnot strong enough to be painful and, ex-cepting a few instances, produced no per-ceptible contractions of the muscles of thehands and arms, much less of the body andlower extremities. The movement in thefirst instance cannot, therefore, be viewedeither as a voluntary effort or a generalreflex movement in response to a painfulstimulus nor in the second instance as amovement due to the diffusion of the cur-rent through the body to the muscles ofthe legs. We are therefore justified in look-ing upon the movements following thesestimuli as identical in character to themovements obtained with the bell alone orwith the bell and some voluntary muscularcontraction as a source of augmentation.
Can these movements, then, be lookedupon as the true knee jerk phenomenonin which the appropriate stimulus, i. e., a
A STUDY OF THE KNEE JERK 1063
blow on the tendons has been replaced bya new and unusual stimulus?
These movements are not the result ofvoluntary effort. Each subject gave un-qualified testimony on this point. Thistestimony is further corroborated by thefact that the subjects were kept in absoluteignorance as to the nature of the experi-ments about to be performed and conse-quently did not know when to expect theblow and when not to expect it. Further,the position in which the subjects wereplaced made it impossible for them to ob-serve when the hammers were about to becaught before striking the tendons. Thekinaesthetic sensations resulting from thesemovements and the more general subjec-tive feeling could not be distinguished fromthose aroused when the movements fol-lowed the actual blows on the tendons.The subjects were, however, always awareof the fact when the tendons had not beenstruck. Considerable confidence may beplaced in this testimony, for all the sub-jects had been students of psychology fortwo years and were consequently familiarwith the method of introspective examina-tion. Therefore, as far as the data of intro-spection is concerned, it can be definitelystated that there is no difference betweenthese movements and the true knee jerk.
An effort on the part of Subject A toinhibit the kicks in the supplementaryseries following Group II and in all of theexperiments of Group III was wholly un-successful. (The attempt to inhibit themovement was the subject's own initiativeand was not reported to the experimenteruntil the experimentation had been dis-continued.) Later experiments on anothersubject completely confirmed this result.This corresponds exactly with the well-known fact that the knee jerk cannot bevoluntarily inhibited without actual con-traction of the flexor muscles of the thigh.
To the observer these movements re-sembled the knee jerk in every detail. Thecharacteristic jerky or explosive appearancewas very evident. When the tracings onthe record were compared no differencecould be discovered. If differences reallyexist a much more rapid recording device
will be required to detect them. In propor-tion to the extent of the kick, the numberand appearance of the subsequent swingsof the leg correspond to the number andappearance of the swings following theknee jerk.
Except in a few cases the average extentof the kicks without the blow on the ten-dons was somewhat less than that of thekicks following the blow. Tables IX andX make a comparison of the extent ofthe kicks in the two cases. The series inwhich Subjects A, B, C and F kicked everytime, without a blow on the tendons, wereselected for the tabulation.
In a study of the tables it is observedthat the relation between the extent of thekicks of the right and left legs correspondsalmost exactly with the results obtainedwhen the tendons are struck. Subjects Aand B give a uniformly greater kick withthe left leg than with the right when thetendons are struck both in normal andaugmented series. Subject C gives largerkicks with the right leg. This relationmaintains in the kicks obtained withoutthe blows on the tendons. The results forSubject F are not as consistent as theresults for the other subjects; this was,however, not unexpected, for in a longseries of normals and augmented kicks, thesubject frequently failed to give any re-sponse to a blow on the tendons, oftenkicked with one leg only and when resultswere obtained for both legs, neither wasconsistently larger than the other.
Further, the relation between the extentof the initial kick out of the legs and thefirst secondary swing remains constant foreach subject whether the movement followsthe blow on the tendons or whether itfollows the sound of the bell alone. Sub-jects A and B give secondary kicks some-what larger than the primary kicks andSubjects C and F vice versa.
The movement of the legs following .thetap of the bell, without the blows on thetendons, has the characteristics of a simple,immediate reaction to a stimulus. Uponthe unanimous testimony of the subjects,it was not produced voluntarily, i. e., therewas no idea of the movement in conscious-
1064 EDWIN B. TWITMYER
TABLE IX.
Subject A
Group I, Series iii and vii (subjectclinch both hands) .... Av. 5
Group II, Series ii (subject clinchboth hands) Av. 2
Group II, Series iv (subjectsaid "ah") ... . Av. 2
Group II, Series viii (thinkingof clinching hands) Av. 2
Supplementary (normal) Av. 5
Group I I I , Series vi (clinch handsand electric shock) Av. 4
Group I I I , Series viii (electric
Tendons Not Struck
Primary
Right
211.4
96
136.5
150
113.8
150
142
Left
224.6
139.5
130.5
275
150.8
187.5
184.6
Secondary
Right
225.8
65
161.5
231
111
206.5
197.6
Left
248
91.5
193.5
281
143.4
285.7
267.2
Tendons Struck
Primary
Right
228.5
164.5
99
173.5
118.2
165
154.3
Left
282.7
255
134.5
296.5
123.4
177
253
Secondary
Right
277
224
180
311.5
201.8
230
313
Left
302
243
273
303.5
274.4
315
356
ness, antecedent to the movement itself.It may, therefore, be held, tentatively atleast, that the movement is a reflex action.The afferent excitation must thereforereach the cord at the level of the medulla(or in the case of the pain stimulus of thecervical cord) and then passes down to thesecond or third lumbar segment in which
the cell bodies of the efferent conductionpath are located. Here then we have anew and unusual reflex arc. With a stimu-lus of moderate intensity, a reflex move-ment is always restricted to the group ofmuscles most intimately connected withthe sensitive part. The cell bodies of theefferent conduction path are situated in
TABLE X.
Subject B
Group III , Series vi (clinch bothhands and electric shock) Av. 4
Group III, Series viii (electricshock, both hands) Av. 5
Subject C
Group I I I , Series vi . . Av 4
Group I I I , Series viii . . Av. 5
Subject F
Group I I I , Series vi Av. 4
Group III , Series viii. . . . Av. 5
Tendons Not Struck
Primary
Right
267
261
64.2
57.8
52
41.9
Left
302.5
298.4
42.7
31.8
35
31.6
Secondary
Right
227
204
19.7
16.4
21.5
12.4
Left
286.5
250.4
7.5
9
14.5
11.6
Tendons Struck
Primary
Right
287
235.6
142
173.3
55
91.7
Left
312
243
85
173
130
125
Secondary
Right
357
281.7
25
80.3
12
40
Left
360
289
15
81.3
15
43
A STUDY OF THE KNEE JERK 106S
the same or immediately adjacent segmentswith the terminating fibers of the afferentpaths. If the intensity of the stimulus isincreased, the reflex movement may be-come more widely diffused but is still con-fined for some time to muscles in the neigh-borhood of the organs stimulated. Increasethe intensity of the stimulus still furtherand the response grows more and moregeneral. In the case under consideration,however, the movements cannot be viewedas a general reflex response to an intensivestimulus. In no instance was the sound ofthe bell or the electrical shock sufficientlyintense to produce diffused movementseven of neighboring parts; on the contrary,the only movement of the body was con-fined to a kick out of the legs which alwaysdisplayed a definite character.
All the experiments which had been per-formed in the investigation of other ques-tions, previous to each of the three groupsreported in this section of the monograph,may be considered as preliminary or pre-paratory. In these experiments the soundof the bell immediately preceded the blowon the tendons. One hundred and twenty-five such experiments had been performedon Subject A before the kick without theblow on the tendons was observed. AfterISO trials the phenomenon was obtainedfrom Subjects B and D, after 185 from E,after 220 from C and after 230 from F.These experiments were not in a continu-ous series but were performed at weeklyintervals on five different days. Later twosubjects, not reported in this study, werefound who gave these kicks after thirtytrials with the bell immediately precedingthe blow. After thirty-five additionaltrials one of the subjects kicked regularlywithout the blows on the tendons. Theoccurrence of the phenomenon, therefore,depends upon the preliminary simultane-ous occurrence of the sound of the bellwith the kick produced in the usual way,i. e., a blow on the tendon. After a certainnumber of such trials, the number varyingfor different subjects, the association of thesound of the bell and the kick becomes sofixed that the bell itself is capable of serv-ing as a stimulus to the movement. Physi-
ologically the repeated association of thefunctioning of the motor cells in the lumbarsegment of the cord, upon which the kickimmediately depends, with the excitationof centers in the nuclei of the medulla con-nected with the auditory conduction path,has resulted in developing a fixed relation-ship between them. The impulse enteringthe latter therefore finds an accustomedchannel to the former. "Considerations ofembryology and comparative anatomy" inthe opinion of Dercum, "point to the con-clusion that the nervous system, thoughinextricably complex and composed of analmost infinite number of parts, acts as awhole, and that its parts are so closely re-lated and interdependent that no one partcan move unless every other part, no matterhow slightly or how profoundly, movesalso." According to this view every in-coming impulse, of whatever origin, besideseffecting a change in the appropriate cen-tral cells, diffuses itself over the entire cen-tral system. The excitation aroused bythe tap of the bell, therefore, always in-fluences, to a greater or less degree, themotor centers of the cord. The results ofthe experiments herein reported, however,would seem to indicate that it is only aftera habit of interaction between the two in-volved centers has been developed by repe-tition, i. e., when the connecting pathwayof discharge has become well worn, thatthe sound of the bell alone is an adequatestimulus to the movement.
A comparison of Tables V, VII and VIIIshows that the number of kicks obtainedwithout the blows on the tendons for eachsubject gradually increases during the threeseries. In the first group containing experi-ments definitely planned to study the phe-nomenon, Subject A did not fail to respondin any trial. An equal number of pre-liminary experiments has been performedon the remaining five subjects, but onlytwo gave kicks and these with no regularity.For all the subjects, in 36 possible times,11 kicks, or 30 percent, were obtained. InGroup II, after the additional series ofGroup I had been performed under pre-cisely similar conditions, the six subjectsgave 21 kicks out of a possible 54, or 40
1066 EDWIN B. TWITMYER
percent. In Group III, for which all thepreceding experiments can be considered aspreliminary, the six subjects gave kicks in87 percent of the times in which the tendonswere not struck. Further, two of the sub-jects reacted each trial and two in all trialsbut two. Hence, not only the frequencybut also the regularity of the occurrenceincreases with the total number of experi-ments. It may be possible that the em-ployment: of the more exciting electricstimulus in the last group partly accountsfor the marked increase. It does not whollyexplain it however. When this stimuluswas tried on a subject upon whom noprevious experiments had been performedno responses were obtained without theblows on the tendons.
If the knee jerk following a blow on thetendons is a true reflex, it differs from themovements of the legs following the tap ofthe bell merely in that in the latter an audi-tory excitation (in some instances possiblyan electrical shock) has been substitutedfor the excitation of the peripheral sensoryfibers terminating in the quadriceps muscleand tendon. There is consequently an as-sociation in a reflex arc of centers in themedulla or cervical cord with the motorcells of the third lumbar segment insteadof two centers represented in the same level,i. e., the third lumber segment.
On the other hand if the knee jerk is nota true reflex but merely the result of animmediate response to a mechanical stimu-lation of the muscle, and the movement ofthe legs without the blow is a true reflex,a comparison of the latent times of thetwo movements should show a marked dif-ference. In the mere observation of thetwo phenomena no marked difference seemsto exist. The matter can be settled onlyby an exact measurement. This problemwill be undertaken by the writer in the nearfuture.
The results of the experiments herein re-ported seem to give considerable support tothe theory held by Lombard, DeWattville,Mills and others that the knee jerk is duefirst to direct stimulation of the muscle
and secondly to reflex influence. The re-sults show that the extent of the kickfollowing a blow on the tendons, with veryfew exceptions, is greater than the kickfollowing the sound of the bell alone, allother conditions being kept constant. Ifin the knee jerk, the movement is initiatedby the contraction of the muscle due to themechanical stimulation and then reinforcedor continued by a reflex excitation comingfrom the spinal center, the difference be-tween the extent of the kicks following thestroke of the bell or the electrical shockand those following the blows on the ten-dons may be taken to represent that amountof the movement which is due to themechanical stimulation of the muscle. Ifthis is true the extent of the normal kneejerk depends largely upon the reflex in-fluence, the mechanical stimulation beingresponsible for only the initial stage of themovement.
LIST OF RECORDS*
1. Record of five experiments under normalconditions in Group I for Subject M,right leg.
2. Record of five experiments under normalconditions in Group IV for Subject M,right leg. Eighty-five experiments wereperformed between Groups I and IV.
3. Record of one experiment under normalconditions in Group IV for Subject H,right leg.
4. Record of three experiments for Subject H,right leg in Group I.
5. Record of five experiments with motor aug-mentation in Group I for Subject M,right leg.
6. Record of five experiments with motor aug-mentation in Group IV for Subject M,right leg.
7. Record of one experiment with motor aug-mentation in Group IV for Subject H,right leg.
8. Record of one experiment with motor aug-mentation in Group IV for Subject H,right leg.
* The eight records appear on pp. 38-39 of thethesis but are omitted here.