reduced short and long latency reflexes during voluntary tracking movement of the human wrist joint

Reduced short and long latency re¯exes during voluntary

tracking movement of the human wrist joint

P . B A W A 1 and T . S I N K J á R 2

1 School of Kinesiology, Simon Fraser University, Burnaby, BC, Canada

2 Center for Sensory±Motor Interaction, Aalborg University, Aalborg, Denmark

ABSTRACT

In six healthy human subjects we compared changes in the strength of Hoffmann (H), short latency

(30±55 ms) and long latency (55±100 ms) stretch reflexes of flexor carpi radialis (FCR) muscle during

movement and isometric contractions. In one set of experiments, stretches were imposed to the

wrist during voluntarily tracked sinusoidal movement and during matched isometric contractions to

compare short and long latency stretch reflex responses. In the second set, H-reflexes were

compared during similar matched conditions. All reflexes decreased significantly (P < 0.05) during

the voluntary tracking movement. The H-re¯ex was reduced during the wrist ¯exion, on average, by

33% of its value obtained during the isometric condition. Compared with their values during isometric

conditions, the short latency stretch re¯ex and long latency stretch re¯ex during movement were

reduced by 52 and 40%, respectively. From the pattern changes of the stretch re¯exes and the

H-re¯ex, a movement-induced presynaptic inhibition combined with pronounced muscle spindle

unloading is proposed to play an important role in decreasing the strength of the stretch re¯exes

during the tracking task as compared with a matched isometric contraction.

Keywords H-re¯ex, human wrist, movement, short and long latency stretch re¯exes, spindle

unloading.

Received 24 February 1999, accepted 31 August 1999

During imposed stretches of isometrically contracting

muscles, human studies have demonstrated clear short

and long latency re¯ex responses in ®nger (Carter et al.

1990), wrist (e.g. Lee & Tatton 1982, Gielen & Houk

1987), elbow (Bennett et al. 1994, Stein et al. 1995), and

ankle (e.g. Gottlieb & Agarwal 1979, Toft et al. 1991)

muscles. These re¯exes increase the stiffness of the

joint, which, in turn, prevents too large an unwanted

displacement to take place. For example, at the wrist

(Sinkjñr & Hayashi 1989) and elbow (Bennett et al.

1994), large unexpected displacements were nearly

halved when re¯exes were present compared with no

re¯ex feedback. During a movement, changes in

peripheral input and central input to the muscle are

likely to in¯uence the gain of the stretch re¯ex as has

been shown by MacKay et al. (1983) and Doemges &

Rack (1992). Capaday & Stein (1986) suggested a

centrally mediated task-dependent presynaptic modu-

lation of the stretch re¯ex gain. Changes in peripheral

input also seem to play an important role. For example,

muscle spindle unloading during concentric movements

was suggested to play an important role in explaining a

decreased stretch re¯ex (Andersen & Sinkjñr 1999).

The following study was carried out to compare the

relative importance of peripheral and central factors

mediating modulation of re¯exes during tracking cyclic

movement of the forearm. The study compared the

stretch re¯exes and H-re¯exes of the wrist ¯exors

elicited during voluntarily tracked sinusoidal move-

ments with those elicited during isometric contractions,

which were matched for stretch amplitudes, joint

positions and background EMG levels.

MATERIALS AND METHODS

The reported data are from experiments done on six

subjects in the age range 23±40 years (two females and

four males, including one of the authors). Two subjects

participated twice in the experiment. None of the

subjects had any known neuro-muscular problems. All

experiments were done on the right forelimb irrespec-

tive of the subjects' handiness. The Ethics Committee

Correspondence: Prof. Thomas Sinkjñr, PhD, Dr med., Center for Sensory±Motor Interaction, Department of Medical Informatics and Image

Analysis, Aalborg University,Fredrik Bajers Vej 7D-3, 9220 Aalborg, Denmark.

Acta Physiol Scand 1999, 167, 241±246

Ó 1999 Scandinavian Physiological Society 241

on Human Experiments at Simon Fraser University

approved these experiments.

The details of equipment and subject preparation are

described in Calancie & Bawa (1985). Brie¯y, the subject

held a vertical handle attached to the shaft of a precision

torque motor (Aero¯ex TQ 82W) with a horizontal bar

carrying strain gauges. When the shaft was immobilized,

force of ¯exion or extension against the vertical handle

measured the isometric force. When it was free to move,

a potentiometer coupled to the shaft measured the

angular position of the wrist. Surface electromyographic

(EMG) activity was recorded from wrist ¯exor and

extensor muscles. Two 9-mm AG/AgCl disc electrodes

were placed 2±3 cm apart on the belly of the muscle

along the long axis. For ¯exors, electrodes were placed

on ¯exor carpi medialis and for extensors, on extensor

carpi radialis. Extensor EMG was recorded for inter-

pretation of the data. The timing of its onset was

essential. When the subject held the vertical handle, the

background EMG activity of either the extensors or the

¯exors could be increased by applying an appropriate

preload to the torque motor (TM). Subjects were

instructed to keep their ®ngers relaxed. It was possible

to carry out the sinusoid without ®nger involvement

because of the ¯exor load which kept the handle pressed

against the palm of the hand. To elicit stretch re¯exes,

wrist extensors or ¯exors could be stretched by applying

a transient load to the TM. Square pulses, 100 ms in

duration from Grass S88 stimulator, drove the power

ampli®er for load pulses.

Stretch re¯exes

At the beginning of the stretch re¯ex paradigm, the

subject was asked to hold his/her wrist in a slightly

¯exed position, and a load pulse of »0.5 N m was

applied to obtain a clear re¯ex response from wrist

¯exors. The pro®le of the angular position of the

wrist from this run was stored as the `control

perturbation' on a digital oscilloscope for later

matching of stretch perturbations imposed during

other conditions. The perturbation typically extended

the wrist through »15° in 110 ms. It was the initial

perturbation in the ®rst 50 ms (»3°) which was used

for matching later pro®les. The main experimental

paradigm was divided into `dynamic' and `static' parts.

For each condition eliciting stretch re¯exes, the

initial 50 ms of perturbation were ascertained to be

identical to the control perturbation throughout one

experiment. Adjusting the strength of the load pulse

for each condition attained this.

For the `dynamic' part of the paradigm, the subject

was given a sinusoidal template (between 0.6 and 0.7 Hz;

40±45° peak to peak ¯exion±extension cycle) on the oscillo-

scope screen. He/she was asked to execute sinusoidal

wrist extension and ¯exion to match the template. While

the subject executed the sinusoidal movement, load

pulse was applied at a predetermined phase of the

sinusoid. The amplitude of the load pulse was adjusted so

that the position trace matched the control perturbation

in the ®rst 50 ms. When the correct load pulse was

attained, 15±20 responses were collected with this load

and phase of sinusoid. During the dynamic part with

respect to the predetermined trigger, the onset of the

stretch always took place later than during the static part.

This we attributed to a higher inertial component during

movement. For the `static' part of the paradigm, the

sinusoidal movement was stopped, and the shaft of the

TM was ®xed at the angular position of the wrist where

the subject received the torque pulse during the dynamic

phase. Thus the length of the forearm muscles, at which

the perturbation was applied, was similar under the two

conditions. The subject was asked to ¯ex his/her ¯exors

isometrically so that the background EMG of wrist

¯exors during this static condition matched the back-

ground EMG of ¯exors at the time when the perturba-

tion was applied during the dynamic phase. When the

background EMG and load pulse were appropriate to

elicit matched perturbation, 15±20 responses were

collected for this `matched' static condition.

H-re¯exes

As above, H-re¯exes were recorded during dynamic

and static conditions. The median nerve at the cubital

fossa was stimulated to elicit H-re¯exes in FCR.

Subjects were chosen only if an H-re¯ex was elicited

in FCR before the appearance of an M-wave. For

control condition, the strength of the stimulus was

adjusted so that the H-re¯ex was large, but on the

ascending limb of the M- and H-curves. Such stimulus

intensity provided a measurable M-wave, which was

used as the control value throughout the experiment.

H-re¯exes were recorded during 5±6 phases of the

sinusoids (15±20 responses for each condition).

Corresponding to each dynamic condition, equivalent

responses for static conditions were recorded

by matching background ¯exor EMG activity and

M-waves.

Analysis

Data were analysed using SIGAVG software and

1401plus data acquisition system from Cambridge

Electronics Design (CED, Cambridge, UK).

Re¯exes

For each condition, that is, for each phase of the

sinusoid and dynamic/static condition, 10±15

242 Ó 1999 Scandinavian Physiological Society

Stretch re¯exes during wrist movement � P Bawa and T Sinkjñr Acta Physiol Scand 1999, 167, 241±246

responses were averaged. Angular wrist position,

tension, recti®ed ¯exor, and extensor EMGs were

averaged on four channels with digitization rates of

2 kHz. Each response was displayed on the computer

screen so that inappropriate runs could be deleted.

Inappropriateness included runs in which the trigger

was at the wrong phase of the sinusoid, or the ¯exor

background EMG was either too high or too low. For

EMG, the aim was to match the background EMGs

during the static runs to the equivalent dynamic runs.

The background (±25 to 20 ms) EMG for the static run

was assured to be either the same or less than that

during the dynamic average. In addition to the back-

ground, the average M-wave in the static average for

the H-re¯ex experiments was also assured to be equal

or less than that for the equivalent dynamic average.

The reason for these parameters (for the static condi-

tions to be kept equal to or less than those during the

equivalent dynamic conditions) is described in the

Results section.

For each averaged response, mean value (i.e. area

per unit time) of the background EMG (Bgnd) activity,

H-re¯ex response, short and long latency re¯ex (SLR

and LLR) responses for the stretch re¯exes were

computed for each of the dynamic and static conditions

and at each of the phases of the sinusoid. From each of

the mean re¯ex responses, the corresponding mean

Bgnd activity was subtracted to obtain the net re¯ex

activity (H, SLR, and LLR) above background. When

the background was high and the re¯ex was small, the

difference was sometimes negative during movement

(e.g. Fig. 2b). Negative values could result from (i) a

decrease in voluntary ¯exor activity after the onset of

stimulus pulse, and (ii) start of inhibition of ¯exors at

this phase of the cycle. Around 0.8T there is onset of

extension EMG which may cause inhibition of ¯exor

activity. It is not visible in the position trace. For each

stretch re¯ex experiment, the maximum (SLR or LLR)

above background was used to normalize all EMG

values including the background. As in the H-re¯ex

experiments, maximum H (averaged) was used to

normalize all background and re¯ex values.

The M-wave area was not measured because the

beginning of the M-wave was never clear owing to

stimulus artefact. Hence only the peak value was

measured in millivolts. The maximum value of the M-

wave for each experiment was used to normalize other

M-wave values. Therefore the maximum normalized

M-wave amplitude was 1.0 for each experiment. Typical

ratios of maximum H to maximum M for our subjects

were 0.5±0.6.

Figure 1 illustrates an averaged record (10 triggers)

with angular position of the wrist in the top panel,

tension records in the second panel, recti®ed surface

EMG in the third (¯exors) and fourth (extensors)

panels for a period of 8 s. Peak of the position record

indicates maximum extension (start of ¯exion) of the

sinusoidal movement, and the trough indicates

maximum ¯exion (start of extension). The time

between these two extremes was T. Most of the

perturbations arrived during the ¯exion phase. In order

to normalize values of T between subjects, the time of

perturbation was calculated as a fraction of T with 0.0T

at the top (maximim extension) and 1.0T at the bottom

(maximum ¯exion).

Statistics

A two-tailed paired t-test with a level of signi®cance at

P < 0.05 was applied. The intra-subject variation of the

two subjects, whom we have included twice, made it

impossible from the eight experiments to point out

which data sets were taken from the same persons. For

that reason we treated the eight experiments indepen-

dently in the statistics.

RESULTS

Results are presented from eight experiments on stretch

re¯exes and from eight experiments on H-re¯exes. The

torque motor imposed stretch re¯exes in human wrist

Figure 1 Averaged record (10 triggers) of collected data. Panel 1

(top) is the angular position of the wrist, and panel 2 is tension. Panels

3 and 4 are recti®ed surface EMGs from wrist ¯exor and extensor

muscles, respectively. Peak of the position record (Panel 1) indicates

maximum extension (start of ¯exion) of the sinusoidal movement,

and the trough indicates maximum ¯exion (start of extension). The

time between these two extremes was de®ned as T. The time of

perturbation was calculated as a fraction of T with 0.0T at the top

(max. extension) and 1.0T at the bottom (max. ¯exion). Calibration

bars: ch. 1±42°; ch. 2±0.45 N m; ch. 3 and 4±50 lV. Tension record

showed an instantaneous increase with the onset of load pulse, the

position record could show a delay of 5±15 ms, depending on the

magnitude of the load and background ¯exor force. In this ®gure

peak to peak sinusoidal movement was 42°, the extension imposed at

0.012T was 17° which was attained in 100 ms.


Acta Physiol Scand 1999, 167, 241±246 P Bawa and T Sinkjñr � Stretch re¯exes during wrist movement

muscles which comprise two main components: the

short latency re¯ex (SLR, »30±55 ms after the onset of

the torque pulse) and the long latency re¯ex (LLR, 55±

100 ms; Calancie & Bawa 1985). The exact start and

end of each component depend on the subject. In

general, the background EMG activity and M-wave

were less during the static condition than during the

dynamic condition. Yet both the stretch re¯ex (SLR

and LLR) and H-re¯ex activities were higher during the

static conditions implying that re¯ex gains are reduced

during movement. Figure 2 shows raw data from a

single subject comparing H-re¯exes (Fig. 2a) and

stretch re¯exes (Fig. 2b) at a single phase during

movement and during matched isometric conditions.

A clear reduction of re¯exes during movement is

demonstrated in this subject at the chosen times.

The averaged re¯ex activities during various values

of T for the same subject are shown in Fig. 3. For

H-re¯exes (Fig. 3a), background activities were

comparable for the two conditions, and M-wave values

were consistently lower during the static phase. Yet,

H-re¯ex values are either larger or the same during

isometric conditions. As for the stretch re¯exes

(Fig. 3b), the average background activity during

isometric conditions is not higher, yet both the SLR

and LLR components of the re¯ex are much larger

during the isometric conditions compared with re¯ex

values during the equivalent dynamic phases.

The averages of all eight experiments were

computed. Figure 4(a) shows population data for

H-re¯exes. On average, the background activity was

slightly smaller during the static isometric conditions,

and the same applied for the M-wave activity.

Bgnd(dynamic) � 0.157 � 0.041 and Bgnd(static) �0.140 � 0.022. A two-tailed paired t-test showed that

the latter was less at the level of P � 0.011. Similarly,

M(dynamic) � 0.855 � 0.027 and M(static) � 0.790 �

0.26, with M(static) < M(dynamic) at P � 3.5 ´ 10±6.

Although both the Bgnd and M-wave values were less

for static conditions, the mean H-value for the popu-

lation was much higher for the static conditions

Figure 2 Example of H-re¯ex (a) and stretch re¯exes (b) in the wrist

¯exor of a single subject during the wrist ¯exion movement (dotted

lines) and during matched isometric (full lines) conditions. The data

are normalized and averaged as described in Materials and Methods.

Figure 2(a) shows the raw EMG with a large stimulus artefact at time

zero (stimuli not aligned with zero because the computer triggers on

the negative phase) followed by the M-wave from 5 to 20 ms (satu-

rated in the ®gure, but not on the recorded data) and the H-re¯ex.

Figure 2(b) shows the recti®ed and normalized EMG data during a

stretch in an isometric contraction and a tracking movement. The

stretch was elicited at time zero. EMG and rest position were matched

prior to stretch. Ten responses were averaged in each of the four

seconds shown. Calibration bars: (a) 50 lV; (b) 180 lV.

Figure 3 Re¯exes during various

times of the wrist ¯exion movement in

one subject. (s) Responses from the

movement (termed `dynamic'); (j)

responses from the isometric situation

(termed `static') at different fractions

of the normalized time T (see Fig. 1).

The three top ®gures (Fig. 3a) give

results from the H-re¯ex measure-

ments, and the three bottom ®gures

(Fig. 3b) from the stretch re¯ex

measurements. The negative H-re¯ex

in the last time slot (0.75T ) is

explained by the subtraction of the

large background EMG from the

elicited H-re¯ex (see Materials and

Methods).



[H(static) > H(dynamic) at P � 6.1 ´ 10±6]. The mean

population values were H(dynamic) � 0.466 � 0.061

and H(static) � 0.704 � 0.052. Compared with their

values during isometric conditions, H-re¯exes for the

population were reduced by 33% during movement. It

should be noted that H-re¯ex modulation is maximal at

the beginning of ¯exion (0.1T) and is insigni®cant

towards maximum ¯exion.

For stretch re¯exes (Fig. 4b), the mean background

(Bgnd) during dynamic condition was 0.448 � 0.049

(mean � SEM), and during static conditions it was

0.380 � 0.040. A two-tailed paired t-test showed that

Bgnd(static) < Bgnd(dynamic) at P � 0.003. Yet, both

SLR and LLR were higher for static conditions. The

values for SLR were 0.247 � 0.028 (SLR dynamic),

0.512 � 0.044 (SLR static), with SLR(sta-

tic) > SLR(dynamic) at P � 3.2 ´ 10±10. The values for

LLR were 0.344 � 0.047 (LLR dynamic) and

0.569 � 0.042 (LLR static) with LLR(static) >

LLR(dynamic) at P � 0.0003. Thus during the ¯exion

phase, the average stretch re¯ex values were lower

during movement than during matched isometric

conditions. Compared with their values during

isometric conditions, the SLR and LLR during move-

ment were reduced by 52 and 40%, respectively. The

trends in modulation during various points of ¯exion

are different for LLR and SLR. Modulation of LLR is

non-existent at the start of ¯exion and is maximal

towards the end of ¯exion.

DISCUSSION

Stretch re¯exes and H-re¯exes of the wrist ¯exors

were all reduced during the voluntary tracking

movement as compared with the matched isometric

conditions.

H-re¯ex and SLR are both primarily an Ia mediated

monosynaptic re¯ex. The LLR re¯ex is also mediated

by group Ia afferents, but likely through a transcortical

re¯ex pathway (Cheney & Fetz 1984). If one looks at

the population curves (Fig. 4), the H-re¯exes are

reduced the most at the beginning of ¯exion with very

little reduction at maximal ¯exion. The LLR compo-

nent (Fig. 4b, right), on the other hand, is not affected

at the start of ¯exion, but is maximally reduced during

maximal ¯exion. For SLR, there is no clear pattern, but

there is a clear tendency that it is decreased during the

¯exion movement.

Spindle unloading is likely to be maximal when the

muscle is maximally shortened at 1.0T and minimally

shortened at 0.0T (see, e.g. Fig. 3c in Burke et al. 1978).

This factor would reduce stretch re¯exes maximally at

1.0T. This may explain the reduction of both SLR and

LLR during movement at maximal ¯exion. Also, the

amplitude of LLR is instruction (or central set)

dependent (Sciarretta & Bawa 1990) and can be in¯u-

enced by other afferents, such as cutaneous (e.g. Chez

& Pisa 1972). At the start of ¯exion (0.0T ), the

movement is like the `compensate' instruction that

increases the gain of the re¯ex enormously, whereas

towards 1.0T, the movement is ready to enhance

re¯exes for the extensors. Thus, the nature of move-

ment (central set) and spindle unloading both reduce

LLR maximally at 1.0T. Instruction does not affect

SLR, but the effect of spindle unloading reduces it.

An unloading of the spindles will argue against a

tightly coupled alpha±gamma co-activation. Prochazka

et al. (1985) showed that the conscious cat fusimotor

action on one and the same muscle spindle Ia afferent

can change from largely static action during a volun-

tarily driven movement to largely dynamic action during

imposed movements. Thus the CNS appears to set the

Figure 4 The averages of all eight

experiments were computed as in

Fig. 3. (a) Population data for

H-re¯exes, and (b) Population data for

stretch re¯exes.


Acta Physiol Scand 1999, 167, 241±246 P Bawa and T Sinkjñr � Stretch re¯exes during wrist movement

levels of gamma activity independent of the alpha

activity. The present experiments will be consistent

with the fact that the muscle spindles receive a nearly

constant gamma (static) drive during a concentric ¯exor

movement, which in practice will decrease the sensi-

tivity of the spindles as the ¯exors are being continu-

ously shortened. Burke et al. (1978) suggested that one

of the roles of the fusimotor action during a slow or a

loaded shortening contraction is to give spindle endings

in the contracting muscle the background discharge

necessary to encode the unloading that occurs when the

speed of movement suddenly increases.

The H-re¯ex is reduced maximally at the beginning.

Both instruction and spindle unloading are not likely to

affect H-re¯exes. A different mechanism is reducing

spinal re¯exes at the beginning of the ¯exion phase,

affecting both H and SLR components. This may, at least

partly, be attributed to a presynaptic effect on the

terminals of ¯exor Ia afferents by the afferents of

the stretched wrist extensors (Berardelli et al. 1987). As

the background EMG builds up during the ¯exion

movement (Fig. 4, top), the post-synaptic facilitation of

the ¯exor motor neurone pool will diminish this effect on

the H-re¯ex. The decreased SLR at the beginning of the

movement can be explained by this pre-synaptic effect.

A movement-induced pre-synaptic inhibition

combined with pronounced muscle spindle unloading is

proposed to play an important role in explaining the

decreased stretch re¯exes during movement. This

points to an important peripheral in¯uence on re¯ex

strength during a voluntary wrist movement.

Funded by Natural Science and Engineering Research Council of

Canada, and The Danish National Research Foundation.

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reduced short and long latency reflexes during voluntary tracking movement of the human wrist joint

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