Abstract Multichannel surface EMG recordings of amultiheaded skeletal muscle during cyclic locomotioncombined with cineradiography were analysed in achronic experiment. The resulting detailed two-dimen-sional activation pattern from the long and lateral tricepsbrachii heads of the rat during treadmill locomotion werecombined with gait characteristics and fibre typing of themuscle. Shortly before ground contact of the forelimb,maximum muscle activity was found in the proximal partof the long head of the muscle. During the stance phasemaximum activity was observed in the proximal part ofthe lateral head. The frequency dependent behaviour ofcross-covariance functions over both muscle heads con-firmed this selective shift in activation. In the lateral tri-ceps brachii head of the investigated rats, exclusivelytype II fibres were found. In the long head the frequencyof type I fibres was the highest in the deep muscle lay-ers, proximally more than distally, whereas type II fibreswere dominant in more superficial muscle layers. Acombination of physiological and histological findingssupports an anticipating mechanism whereby fine-tuningof the vertical foot down manoeuvre is mainly achievedby the (type I fibre dominated) proximal deep compart-

ment of the biarticular long triceps brachii head andforce generation is predominantly executed by the mono-articular lateral triceps brachii head.

Keywords Triceps brachii · Topographic surface EMG ·Selective muscle activation · Muscle fibre distribution ·Rat

Introduction

In a number of skeletal muscles discrete regions can beidentified which are innervated by the primary branchesof the nerve to the muscle. These regions are termedneuromuscular compartments (English and Letbetter1982). Often, these compartmentalised muscles haverather complex morphologies and functions such as themasseter or the gastrocnemius muscle. Analysis of motorunits using the glycogen depletion technique in the catlateral gastrocnemius (LG) muscle indicated that singlemotor units are distributed in the same muscle areas thatare innervated by the primary branches of the nerve tothe LG muscle (English and Weeks 1984). These authorssuggested that this neuromuscular compartmentalisationcould be “a major organizational principle in the designof skeletal muscles”. An electromyographic analysis (bi-polar fine wire electrodes) of the four compartments ofthe cat LG muscle (English 1984) demonstrated that dur-ing unrestrained locomotion the compartments were se-lectively activated with different activation patterns.These patterns were generally consistent with an orderlyactivation of motor units as described by Henneman etal. (1965). Spatial and size distributions of motoneuronsinnervating the compartments of the cat LG muscle (ret-rograde characterisation by horseradish peroxidase) alsoconfirmed such a selective muscle activation (Weeks andEnglish 1985). Distributed activation was also describedin investigations of the human masseter muscle studiedwith scanning electromyography (Stalberg and Eriksson1987). Surface EMG (SEMG) mapping techniques re-

H.Ch. Scholle (✉ ) · N.P. Schumann · F. BiedermannD.F. Stegeman · R. Graßme · K. RoeleveldMotor Research Group, Institute of Pathophysiology,Friedrich-Schiller-University, Erfurter Str. 35, 07740 Jena,Germanye-mail: hscho@moto.uni-jena.deTel.: +49-3641-637373, Fax: +49-3641-637377

N. Schilling · M.S. FischerInstitute of Systematic Zoology and Evolutionary Biology,Friedrich-Schiller-University, Jena, Germany

D.F. StegemanDepartment of Clinical Neurophysiology, Institute of Neurology,University Medical Centre Nijmegen, The Netherlands

K. RoeleveldDepartment of Sport Sciences,Norwegian University of Science andTechnology (NTNU) Trondheim, Norway

Exp Brain Res (2001) 138:26–36DOI 10.1007/s002210100685

R E S E A R C H A RT I C L E

H.Ch. Scholle · N.P. Schumann · F. BiedermannD.F. Stegeman · R. Graßme · K. RoeleveldN. Schilling · M.S. Fischer

Spatiotemporal surface EMG characteristics from rat triceps brachiimuscle during treadmill locomotion indicate selective recruitmentof functionally distinct muscle regionsReceived: 30 August 2000 / Accepted: 17 January 2001 / Published online: 2 March 2001© Springer-Verlag 2001

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vealed a non-homogeneous muscle activation in otherskeletal muscles as well (Masuda and Sadoyama 1988,1989; Scholle et al. 1994; Schumann et al. 1994; Roele-veld 1998a).

The functional relevance of these neuromuscularcompartments is not generally established. There is evi-dence that the diversity in the architecture of muscle fi-bre groups and compartments (pennation and complexin-parallel or in-series arrangements of muscle fibres), aswell as the existence of distributed origins and inser-tions, requires a sophisticated control of muscles via aselective activation of different types of motor units per-mitting fine-tuning of movements and body postures (seereview by Windhorst et al. 1989). The result of a long-term study on Ia pathways is that “the elbow extensors(are) in the centre of the locomotion related Ia connec-tions” (Caicoya et al. 1999) and that the evolution of arecurrent inhibitory system as stiffness control is due tothe different posture in therian locomotion (Illert et al.1996). It is especially noteworthy that postural musclesin therians are three headed. In the case of the tricepsbrachii muscle the medial and lateral head of the tricepsare monoarticular and the long head is biarticular. Ac-cording to Jacobs et al. (1993), distinct functional differ-ences between mono- and biarticular muscles or musclecompartments can be expected.

In a chronic animal approach by Chanaud et al.(1991b), the spatiotemporal aspect of regional muscleactivation was considered by using patch electrodes withusually two to four pairs of bipolar electrodes wrappedaround the muscle belly to differentiate the electrical ac-tivity of superficial, middle and deep muscle parts.These EMG results were correlated with the muscle fibredistribution and musculoskeletal architecture to analysethe various morphofunctional relationships with respectto motor control. In a further study by these authors(Chanaud et al. 1991a), the neuromuscular compartmentsof the cat biceps femoris were characterised, after dissec-tion of the hindlimb, by nerve branch stimulation andhigh density two-dimensional surface EMG recordings(240 electrodes) of deep and superficial muscle parts.

To gain more insight into the intramuscular differenti-ation of complex movement control processes in vivo,we investigated the topography and temporal course ofseveral SEMG parameters of the triceps brachii musclein rats during defined treadmill locomotion using a 16-electrode array fixed under the skin close to the fascia oflateral and long muscle heads. The derivation techniquechosen (monopolar with “inactive” reference electrode)permits a sufficient quantitative estimation of the activa-tion of different areas of the muscle during static and dy-namic load situations (SEMG interference pattern map-ping; see Scholle et al. 1992, 1994; Schumann et al.1992, 1994). Moreover, it allows differentiation betweensuperficial and deeper activities of the long and lateralmuscle heads with the application of post hoc spatial andtemporal filtering schemes as described by Graßme et al.(2000). That is quite possible because the spread of theactivity of any individual motor unit over the outer mus-

cle surface can be modulated both by spatial (Reucher etal. 1987a, 1987b; Disselhorst-Klug et al. 1997) and bytemporal filtering schemes (Graßme et al. 2000). Rela-tive to the depth of the specific source of activity, the ex-tent of this spread (Roeleveld et al. 1997a, 1997b) andthe frequency range of this activity (Graßme et al. 2000)vary.

Despite the lower number of electrodes used in thisexperiment as compared with the approach of Chanaudet al. (1991a), the electrode matrix covered the largerpart of the lateral surface of the lateral and long heads oftriceps brachii muscle. The alternative method of usingfine wire recordings (Cohen and Gans 1975) is method-ologically limited with respect to representativeness anda reproducible quantification of the activity within eachstep cycle.

The electrophysiological results are compared withthe three-dimensional muscle fibre distribution and fibretyping, determined in both cross-sectional and longitudi-nal directions. The fact that the electrode array is appliedover the muscle and not intramuscularly guarantees thatthis post hoc histological and histochemical analysis isnot influenced by procedural muscle lesions.

In summary, considering the aim of our investigationswas to correlate the three-dimensional muscle fibre dis-tribution of the long and lateral heads of triceps brachiiwith the activation patterns of these muscle heads duringrat locomotion to obtain a better understanding of the in-tramuscular control processes, the electrode array ap-plied and the monopolar lead-off principle have some es-sential advantages over previous methods.

Preliminary results of these investigations have beenpresented in several conference abstracts (Roeleveld etal. 1998b; Biedermann et al. 1999; Scholle et al. 1999;Schumann et al. 1998, 1999).

Materials and methods

Ten rats (Rattus norvegicus, Hannover Wistar) were investigated.They were trained by positive conditioning to move on a horizon-tal motor driven treadmill within a Perspex enclosure(100×45×11 cm) for several weeks. The analysis of SEMG map-ping data, the core result of this study, was performed in six ofthese rats. Further investigation results (muscle fibre distribution)were obtained from the four other rats as well.

Matrix electrode and surgical technique

The electrode matrix contained 16 active globular silver electrodeswith a diameter of 0.4 mm embedded in a silicon matrix with aninterelectrode distance of 3 mm (Fig. 1a) fixed surgically by foursutures at its corners on the fascia of long and lateral triceps heads(Fig. 1b). Thus, the approximately equal surfaces of long and lat-eral heads were nearly covered. The uncovered parts mostly con-sisted of tendons. The influence of matrix electrode fixation byfascia sutures on rat's locomotion was not significant (no statisti-cally significant differences in kinematic parameters in animalsbefore and after matrix electrode implantation; Fischer 1999). Asdemonstrated in a pre-experiment, no additional (high) spatial fre-quency topographic characteristics are observed when using anelectrode array with a smaller interelectrode distance of 1.5 mm(Biedermann et al. 2000). Reference and ground electrode both

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were silver wire eyelets (diameter 3 mm) fixed underneath thenape and connected by silver strand with a microconnector. The16 wires of the matrix electrode were also soldered to the micro-connector. Surgical invasion was achieved under isoflurane anaes-thesia given by mask. Measurements started from the second post-operative day. For further technical details, see Biedermann et al.(2000).

All experiments were done in accordance with the animal wel-fare regulations and guidelines of Germany.

EMG recording

During treadmill locomotion at a relatively constant speed foreach animal (individually tuned treadmill velocity: that means thespeed could vary interindividually; mean speed of the electromyo-graphically investigated rats: 0.5±0.162 m/s), we simultaneously

recorded EMG from the chronically implanted matrix electrode,3D high-speed videometry (150–200 frames/s), and from time totime cineradiography (150 frames/s). The 16 monopolarly record-ed EMG channels (referenced to the nape) were amplified, filteredbetween 10 and 1000 Hz, and AD converted at a rate of4000/s/channel with a resolution of 2.44 µmV/bit. Additional digi-tal high-pass filtering at 20 Hz was performed. From these mono-polar recordings, all further (digital) manipulations as discussed inthe following were derived. The root mean square (RMS) of themonopolar SEMG signals was calculated using a samplewiseshifted data window of 21 ms. Triggers indicating the beginningand end of the stance phases were derived from the high-speedvideometry. Occasionally, also X-ray cinematographic recordings(150 frames/s) were made. These data were used, among others, tovalidate the use of the videometry to assess the subdivision ofsteps in swing and stance phase. Two frames from such a record-ing are presented in Fig. 2 to show the stability of the implantedelectrode during a step. The triggers were used to time normalisethe RMS data for the duration of each stance phase by resampling.Subsequently, the normalised RMS profiles were averaged, firstover the steps and then over the animals.

Three-dimensional muscle fibre distribution

The muscle fibre distribution in triceps brachii of the electromyo-graphically investigated rats was analysed three-dimensionally(see Mering and Fischer 1999; Fischer 1999; Gorb and Fischer2000). Sections of skinned, quick-frozen forelimbs were processedfor oxidative capacity and muscle fibre typing using the alkalinecombination reaction (Ziegan 1979). The combination consists offirst testing the activity of NADH-TR, and afterwards the myofi-brillar ATPase activity. Using this technique three usual fibretypes could be differentiated: type I (SO fibres), type IIa (FOG fi-bres), and type IIb (FG fibres).

Cross-covariance function analysis

To determine spatiotemporal relations between different signals,the calculation of covariances is an appropriate method (Graßmeet al. 2000). After the selection of a specific phase (pre-stance orstance) in the step cycle, cross-covariance functions of a selectedreference (bipolar montage) channel with all other bipolar (proxi-mal-distal) SEMG segments were calculated and averaged over allthe steps in the experiment. Filtering of these cross-covariancefunctions can then be performed to determine in what frequencyrange the specific activity spreads over the muscle. This frequencycontent is expected to give an indication of from what depth theactivity originates. Mainly guided by the duration of the pre-stance phases, SEMG segment lengths between 14 and 19 ms perindividual step phase were used for this cross-covariance analysis.

Fig. 1a, b The electrode grid. a Numbering and arrangement ofthe monopolarly linked electrodes (circles), bipolar derivations(and their numbering –x–) obtained by subtraction of signals ofthe neighbouring electrode positions in a proximal-distal musclefibre direction. b Localisation of a 16-electrode array on the leftrat triceps brachii muscle

Fig. 2 X-ray cinematographicrecordings (image frequency:150 frames/s) of the beginning(a) and the end (b) of a stancephase during treadmill locomo-tion of a rat (arrows mark theelectrode array)

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Results

Topography of surface EMG interference patterns

An example of monopolar 16-channel surface EMG (re-cordings 1–16) of the left triceps brachii muscle isshown in Fig. 3, representing one step of a rat on thetreadmill (trot, 0.6 m/s, the stance phase is marked by thearrow). Shortly before the ground contact of the leftforelimb, the proximal area of the long triceps brachii

head (electrodes 2, 3, 7, 8) shows a higher activity thanwith other muscle parts. At the beginning of the stancephase, the activation of the lateral head is dominant andincreases up to the middle of the stance phase. Simulta-neously, the activity of the long head also increasesagain, but at relatively lower amplitudes. During the lastthird of the stance phase, the EMG activity of both headsdecreases to the baseline level.

Bipolar EMG recordings are calculated by subtractingthe monopolarly recorded surface EMG signals from

Fig. 3 Example of monopolar16-channel surface EMG re-cordings of the left rat tricepsbrachii muscle long and lateralheads (a trails 1–16). BipolarEMG montages of this examplewere calculated by subtractingthe monopolar recorded signalsfrom subsequent electrodes in aproximodistal muscle fibre di-rection (a trails 17–28, arrowstance phase; b zooming the bi-polar montages, arrow footdown). See the phase reversalbetween bipolar montages12/11 and 13/12 as well as 7/6and 8/7

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subsequent electrodes in a proximodistal muscle fibre di-rection (trails 17–28 in Fig. 3a). This bipolar EMG activ-ity distribution on both muscle heads is similar to that ofthe monopolar montages, but the difference in activityduring the stance phase between both heads is even morepronounced. In a zoomed representation (Fig. 3b), apropagation of action potentials in two opposite direc-tions starting at electrodes 12 and 7 can be found. Thebidirectional propagation (conduction velocity approxi-mately 5 m/s) and the phase reversal of EMG potentialsat the bipolar derivations 12/11 and 13/12 as well as at7/6 and 8/7 indicate an end plate position around elec-trodes 12 and 7. The phenomena shown in Fig. 3 are typ-ical for all rats studied.

Data from six rats (mean speed: 0.5±0.162 m/s) witha stance duration of 100–150 ms were taken into account(total of 96 steps) for the RMS profiles (Fig. 4). Such astance duration range is usually found during faster trot(see also Cohen and Gans 1975). The differences in theactivation profiles between stance and swing phase are inaccordance with the example presented above (Fig. 3).During the stance phase, the overall EMG activity isgenerally higher than during the swing phase (Fig. 4b).Furthermore, the localisation of the maximum activityshifts between the regions of lateral and long tricepsheads. The maximum activation in the proximal part ofthe long head is found shortly before ground contact ofthe left forelimb (electrodes 7 and 8: the bold white linesin Fig. 4a). The lateral head shows distinctly more activi-ty during the stance phase. With clear-cut dominance, the

region just proximal to the central region of the lateralhead (electrodes 11, 12, 13, 15, 16: bold black lines inFig. 4a) is the loci of highest activity occurring duringthe first half of the stance phase. Triceps brachii activa-tion decreases at all electrode positions during the sec-ond half of the stance phase, but a relatively higher ac-tivity persists in the lateral head (Fig. 4a). During the lastthird of the stance phase the myoelectrical activity is aslow as during the swing phase. There was a similarcourse of RMS profiles for a stance duration of151–200 ms (slower trot).

Frequency dependent behaviour of cross-covariancefunctions

Cross-covariance functions of bipolar surface EMG seg-ments were calculated and averaged during the pre-stance phase and during the maximum of the stancephase. Steps out of the stance duration ranges between100 and 150 ms as well as between 151 and 200 ms wereconsidered (n=6 rats, mean speed: 0.5±0.162 m/s). Thus,all stance phase duration values during trot were in-volved. In Fig. 5, the cross-covariance functions of bothanalysed stance duration ranges of rat 13 were averaged(n=21 steps), because the characteristics of these cross-covariance functions were very similar and typical for allsix rats studied. During the pre-stance phase, the bipolarchannel 8–7 was used as reference channel. Cross-cova-riance functions were calculated between this reference

Fig. 3b Legend see page 29

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channel and all other proximal-distal bipolar channels(Fig. 5a–c). Unfiltered cross-covariance functions fromthe pre-stance phase are shown in Fig. 5a. A clear rela-tion mainly based on active propagation (velocity5.8 m/s) over the long head can be seen (bipolar chan-nels 8–7 to 5–4 and channel 3–2). Volume conductionmediated spread of activity is present in the channelsclose to midline over the lateral head (channels 13–12 to10–9) and possibly in channel 2–1 over the long head. Ifthese cross-covariance functions are high-pass filteredwith a lower frequency of 430 Hz (Fig. 5b), their previ-ous structure (propagation and spread) in the data is pre-served. If the lower cut-off frequency is further increasedto 860 Hz, the structure (especially the propagation as-pect) disappears (Fig. 5c).

Bipolar channel 10–9, situated over the area of thedistal lateral muscle head, is used as reference channel inFig. 5d, e, in which case unfiltered cross-covariancefunctions during the maximum stance phase are present-ed in Fig. 5d. A distinct correlation on the basis of actionpotential propagation is evident between the referencechannel and the other ones along the same line across thelateral muscle head (bipolar channels 11–10 to 13–12). Aphase reversal and propagation in the opposite directionscan be seen between the bipolar channels 12–11 and11–10, indicating the presence of the end plate region ofthis head, which is in agreement with our anatomical ob-servations. Again, the volume conducted spread of thesephenomena is present in channels 16–15 and 15–14 andto a much lesser extent in the channels over the long

Fig. 4 a Mean RMS profiles oftime-normalised swing andstance phase of six rats (onlyconsidering a stance phase du-ration of 100–150 ms; total of96 steps); black lines electrodepositions on the lateral head,white lines electrode positionson the long head, bold blackand white lines proximal elec-trode positions. b Topographyof mean RMS in left rat tricepsbrachii muscle correspondingto electrode localisations of theused array

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Fig. 5a–e Stepwise high-passfiltered cross-covariance func-tions of bipolar surface EMGsegments obtained from 16-channel monopolar recordingsof rat triceps brachii (rat no.13/averaging 21 steps of thestance duration ranges between100 and 150 ms as well as be-tween 151 and 200 ms; rightmargin combinations of mono-polar derivations for the calcu-lation of the bipolar montages).Bipolar reference channel: 8–7(a) 20-Hz high-pass filteredcross-covariance functions ofthe pre-stance phase; b 430-Hzhigh-pass filtered cross-covari-ance functions of the pre-stancephase; c 860-Hz high-pass fil-tered cross-covariance func-tions of the pre-stance phase.Bipolar reference channel:10–9 d 20-Hz high-pass fil-tered cross-covariance func-tions during the maximumstance phase; e 860-Hz high-pass filtered cross-covariancefunctions during the maximumstance phase

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head. On the basis of the shifts in the distal and proximaldirections, a mean propagation velocity of 5.2 m/s can beestimated. The average cross-covariance functions dur-ing the maximum stance phase presented in Fig. 5e arehigh-pass filtered with a lower cut-off frequency of860 Hz. In contrast to the example in Fig. 5c, the charac-teristics of propagating action potentials (time shift, signreversal) in the cross-covariance functions between bipo-lar channels over the lateral muscle head are not changedwith this high frequency cut-off in comparison to the re-sults of Fig. 5d.

Three-dimensional rat triceps brachii muscle fibredistribution

Muscle fibre typing involved determination of types Iand II only. In the lateral triceps brachii head of all ratsof the Hannover Wistar type (n=10), exclusively type IIfibres could be found. In contrast, the long head wascharacterised by a deep distinct type I fibre region. Incross-section, fibre type ratios (type I: type II) were:40:60 in the deep region, 20:80 in the middle, and 10:90in the superficial region at the very proximal part of thelong head. Further distally, on the level of the proximalelectrodes, these ratios were: 30:70 (deep) and 0:100 (su-perficial). In the middle of the cross-section the propor-tions of oxidative fibres decreased continuously from20:80 to 2:98. Distally, fibre type ratios were: 15:85(deep) and 0:100 (superficial).

Discussion

Methodological aspects

In previous chronic in vivo studies, functional character-istics of muscle heads or neuromuscular compartmentswere investigated by one to four pairs of bipolar elec-trodes per muscle, and mostly in cats. In some studiesthese were arranged in patches (e.g., Hoffer and Loeb1980; English 1984; Chanaud et al. 1991b). A detailedtwo-dimensional surface EMG analysis (deep and super-ficial part of cat biceps femoris) was only performed invitro after electrical stimulation of nerve branches acti-vating single muscle compartments of the prepared catbiceps femoris (Chanaud et al. 1991a). Rat triceps bra-chii muscle has been investigated previously by bipolarwire electrodes which were replaced for each experi-ment, whereby it is difficult to optimally reproduce themuscle areas studied (see Cohen and Gans 1975). Thus,we think we have shown for the first time in a chronicexperiment the detailed two-dimensional activation pat-tern of a skeletal muscle with several heads during a de-fined dynamic motor task.

Interelectrode distances of less than 3 mm did not im-prove the spatial detail of the SEMG interference patternmaps (Biedermann et al. 2000). Furthermore, simulta-neous intramuscular recordings with wire electrodes in-

Fig. 5d–e

serted into the muscle to a depth of 1 mm in the proxi-mal, distal, and middle regions of the long as well as lat-eral triceps head gave very similar results regarding theactivity of corresponding array electrodes (Biedermannet al. 2000). Thus, the electrode grid with an interelec-trode distance of 3 mm is expected to document suffi-ciently the electrical sources within the investigated tri-ceps brachii heads. The topographic in vitro analysis ofthe neuromuscular cat biceps femoris compartments byChanaud et al. (1991a) also found an interelectrode dis-tance of 3 mm as being appropriate. Likewise, from amorphological point of view it can be assumed that aninterelectrode distance of 3 mm covers the region of arat's motor unit in an adequate manner (Fuentes et al.1998).

The kinematic analysis of the forelimb movementsbefore and after implantation of the array electrodeshowed no statistically significant differences (Fischer1999). Consequently, it could be concluded that the im-planted grid did not alter the locomotion of the rats onthe treadmill.

Topography of the surface EMG interference pattern

Monopolar 16-channel SEMG recordings and meanRMS profiles of the long and lateral triceps brachii heads(Figs. 3, 4) demonstrated that shortly before ground con-tact of the investigated left forelimb the proximal part ofthe long triceps head has the maximum activation, andthat during the following entire stance phase the lateralhead is more activated. More precisely, the maximum ac-tivation could be found in the proximal part of the lateralhead. At the end of the first two-thirds of the stancephase the activity of the lateral and long head decreasedto the activity level of the swing phase.

These results are only partially in agreement with therat EMG results of Cohen and Gans (1975) recordedwith wire electrodes, the main contradiction being thataccording to their study the long triceps head seemed toplay a decisive role during the stance phase to generatethe forces counteracting gravity, and that the measuredlow-level activity of the lateral head of the triceps bra-chii and brachialis muscles reflected their provision ofonly an additional stabilising influence to prevent lateralbuckling at the elbow. For the most part differing experi-mental methods and analysis techniques can account forthe divergent findings. Cohen and Gans (1975) used onlyone pair of wire electrodes per muscle head, which werereplaced for each experiment, meaning a small, not al-ways identical recording area. Furthermore, the load sit-uations were different (activity wheel driven by therats/mild electrical shocks: Cohen and Gans 1975; in ourstudy: treadmill/arbitrary locomotion of rats). Finally,differences in the signal analysis procedures were pres-ent: Cohen and Gans (1975) evaluated “the number ofstrides, or the number of times the muscle began orceased firing relative to event indicated”. In the presentstudy, the data from 16 recording positions could be

quantitatively evaluated in different ways, leading tonew, convergent notions of the underlying processes.

Topographic surface EMG characteristics comparedwith three-dimensional muscle fibre distribution

Muscle fibre types and their distribution in the tricepsbrachii of the rat have been described recently (Fuenteset al. 1998); however, the authors did not deal separatelywith the triceps heads. In order to compare the topogra-phy of SEMG characteristics and the triceps brachii mus-cle fibre distribution in detail, it is necessary to considerthe three-dimensional muscle fibre distribution of thewhole muscle.

The muscle fibre distribution in triceps brachii of theelectromyographically investigated rats was three-dimensionally analysed. In the lateral head, exclusivelytype II fibres were found. In the long head, type II fibresdominated in the surface layers. The frequency of type Ifibres increased in the deep muscle layers and in theproximal direction. The maximum activation shortly be-fore ground contact of the forelimb is limited to theproximal part of the long triceps brachii head. During thepre-stance phase the cross-covariance function analysispoints to a distinct correlation between the bipolar chan-nel 8–7 and the bipolar channels which were located onthe long head region up to lower cut-off frequencies of430 Hz (Fig. 5). For a higher cut-off frequency (seeFig. 5c; 860 Hz), this covariance structure disappeared.This behaviour of the cross-covariances suggests that theactivity comes from the deeper muscle layers. Thus, dur-ing the pre-stance of the investigated trot of rats the deeptype I fibres of the long triceps brachii head seem re-sponsible for the fine-tuning of forelimb in anticipationof ground contact. The fact that the relative activity fromthe bipolar traces from the long head in the stance phaseis lowered compared to the activity from monopolar trac-es (Fig. 3a) indicates that long head activity originatesfrom deeper regions in the muscle also during the latterphase. Simultaneous recordings from wire electrodes in-serted into the muscle to a depth of 1 mm confirm thelow activity of the superficial long head (Biedermann etal. 2000).

During the stance phase, maximum activation shifts tothe proximal part of the lateral triceps head. The cross-covariance function analysis in Fig. 5d, e showed that thebipolar reference channel 10–9 correlated to the other bi-polar channels located in the lateral head region. Thiscorrelation was still observable after high-pass filteringwith a lower frequency of 860 Hz. Therefore, thesesources of activation are expected to be situated in moresuperficial muscle layers. The lateral head exclusivelycontains fatigable type II muscle fibres which are espe-cially suited for generating forces during the step phasewhen higher forces must be produced to balance the highmidstance torques (Fischer 1999; Witte et al. 2001).

The shifts of the cross-covariance peaks are in accor-dance with the usual action potential propagation veloci-

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ties (Linssen et al. 1993). The positions of innervationzones detected electrophysiologically (Fig. 5d, e) werealso in agreement with our anatomical results, becausethe branch of the radial nerve supplying the triceps bra-chii muscle enters mainly proximally near the middle ofmuscle, except for an additional smaller entry distally.

Thus, it can be concluded that there is a generalagreement between the morphological characteristics ofthe muscle and the neurophysiological results in thestride activity studied. The long head of the triceps bra-chii seems to be to some extent responsible for fine-tun-ing tasks; the lateral head generates higher forces.

Further interpretation of results in termsof biomechanical function differentiation

Fischer (1999) points out that in small mammals the basictherian design of forelimbs is represented by a three-seg-mented limb which is mainly displaced in the scapularfulcrum during locomotion while the more distal limbjoints contribute only to a lesser degree to propulsion.Therefore, the function of these more distal joints shouldbe rather to avoid vertical fluctuations of the body's cen-tre of gravity evoked by gait changes or ground uneven-ness. This fine-tuning of the position of the body's centreof gravity is realised by the biarticular extensors counter-acting the gravity (Jacobs et al. 1993). That means themain task of these antigravity muscles is to prevent ex-cessive flexion in the limb joints during the stance phaseof locomotion (Goslow et al. 1981; Fischer 1994). Mus-cles with tricepity as the triceps brachii always consist ofat least one bi- and one monoarticular muscle head, andoxidative type I fibres dominate in one monoarticularhead. This head is usually covered by the second monoar-ticular head with mainly fast twitch fibres. In deep areasof the long triceps brachii head (biarticular part) there aredistinct regions with oxidative slow twitch fibres (type I).The superficial layers consist of glycolytic fast twitch fi-bres (type II). A hypothesis about the biomechanicalfunction of this tricepity is that fine-tuning of the joint isperformed by the biarticular head whereas the monoartic-ular heads are responsible for force generation (Jacobs etal. 1993; van Ingen Schenau et al. 1995; Fischer 1999).An anticipating activity of the postulated fine-tuningmuscle part in triceps brachii (long head) is necessary inthe pre-stance period to react with a certain electrome-chanical delay to ground unevenness. The monoarticular(lateral) muscle head studied is strongly activated to pre-vent flexion above a certain limit caused by gravity andto resist higher torques in the limb joints. Therefore, ourresults also appear in agreement with this hypothesis onthe biomechanical tasks of different functions of mono-and biarticular muscles. Considering the different musclefascicle orientation in the anterior and posterior parts ofthe long head and the different fascicle arrangements ofthe long and lateral heads along the longitudinal body ax-is (Gorb and Fischer 2000), further motor functions ofthese heads in small mammals can be assumed.

Acknowledgement This study was supported by the DeutscheForschungsgemeinschaft: Innovationskolleg “Bewegungssysteme”,Universität Jena (Teilprojekt A2, A1).

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