The origin of activity in the biceps brachii muscle during voluntary contractions of the contralateral elbow Xexor muscles

Transcription

1 Exp Brain Res (2006) 175: DOI /s z RESEARCH ARTICLE The origin of activity in the biceps brachii muscle during voluntary contractions of the contralateral elbow Xexor muscles Inge Zijdewind Jane E. Butler Simon C. Gandevia Janet L. Taylor Received: 3 April 2006 / Accepted: 22 May 2006 / Published online: 22 August 2006 Springer-Verlag 2006 Abstract During strong voluntary contractions, activity is not restricted to the target muscles. Other muscles, including contralateral muscles, often contract. We used transcranial magnetic stimulation (TMS) to analyse the origin of these unintended contralateral contractions (termed associated contractions). Subjects (n = 9) performed maximal voluntary contractions (MVCs) with their right elbow-xexor muscles followed by submaximal contractions with their left elbow Xexors. Electromyographic activity (EMG) during the submaximal contractions was matched to the associated EMG in the left biceps brachii during the right MVC. During contractions, TMS was delivered to the motor cortex of the right or left hemisphere and excitatory motor evoked potentials (MEPs) and inhibitory (silent period) responses recorded from left biceps. Changes at a spinal level were investigated using cervicomedullary stimulation to activate corticospinal paths (n = 5). Stimulation of the right hemisphere produced silent periods of comparable duration in associated and voluntary contractions (218 vs 217 ms, respectively), whereas left hemisphere stimulation caused a depression of EMG but no EMG silence in either contraction. Despite matched EMG, MEPs I. Zijdewind (&) Department Medical Physiology, University Medical Center Groningen, University of Groningen, A. Deusinglaan 1, 9713 AV Groningen, The Netherlands i.zijdewind@med.umcg.nl J. E. Butler S. C. Gandevia J. L. Taylor Prince of Wales Medical Research Institute, Barker Street, Randwick, NSW, 2031, Australia elicited by right hemisphere stimulation were» times larger during associated compared to voluntary contractions (P < 0.005). Similar inhibition of the associated and matched voluntary activity during the silent period suggests that associated activity comes from the contralateral hemisphere and that motor areas in this (right) hemisphere are activated concomitantly with the motor areas in the left hemisphere. Comparison of the MEPs and subcortically evoked potentials implies that cortical excitability was greater in associated contractions than in the matched voluntary evorts. Keywords Voluntary contractions Transcranial magnetic stimulation Associated activity Silent period Cortical excitability Introduction During strong voluntary contractions activity is not restricted to the target muscles. Other ipsilateral and contralateral muscles also become active (Curschmann 1906; Pinneo 1961; Cernacek 1961; Todor and Lazarus 1986; Gandevia et al. 1993; Armatas et al. 1994; Durwen et al. 1997; Mayston et al. 1999; Zijdewind and Kernell 2001; Aranyi and Rosler 2002; Shinohara et al. 2003). This associated activity is not always accompanied by overt movement and therefore is often not noticed by the subject. Interaction between the two cerebral hemispheres is essential for coordinated movements with the limbs of the two sides of the body. From several lines of experiment both facilitatory and inhibitory evects from one hemisphere to the other are known in humans (Hess et al. 1986; Zwarts 1992; Meyer et al. 1995; Tinazzi and

2 Exp Brain Res (2006) 175: Zanette 1998; Stedman et al. 1998; Muellbacher et al. 2000; Liepert et al. 2001; Hortobagyi et al. 2003; Carson et al. 2004), and it is thought that most of these evects cross via the corpus callosum. Dynamic, lowforce contractions are reported to suppress the excitability of the ipsilateral motor cortex (Leocani et al. 2000; Liepert et al. 2001; Sohn et al. 2003), whereas during high force contractions facilitation is more frequently observed (Hess et al. 1986; Zwarts 1992; Meyer et al. 1995; Tinazzi and Zanette 1998; Stedman et al. 1998; Muellbacher et al. 2000; Hortobagyi et al. 2003). However, this facilitation is often accompanied by changes in excitability at spinal levels (Hess et al. 1986; Zwarts 1992; Muellbacher et al. 2000). Similar facilitation is also seen in patients with callosal agenesis (Meyer et al. 1995), which suggests that the evect need not be mediated through the main callosal pathways. In contrast, other experiments have shown that the coupling between forces produced with the left and right Wnger Xexor muscles (Diedrichsen et al. 2003) is reduced in split-brain subjects. Furthermore, data from Hortobagyi et al. (2003) showed that strong voluntary contractions can increase excitability in the ipsilateral motor cortex (larger TMS-evoked potentials) without a change in spinal excitability. These results suggest that facilitation must occur at a supraspinal level under some circumstances. The evect of muscle activation on the other side of the body is usually analysed in a resting muscle. However, muscle contraction is often accompanied by associated contralateral activity (Samii et al. 1997; Tinazzi and Zanette 1998; Muellbacher et al. 2000). This suggests that to ensure muscle relaxation in the experimental setting subjects may need to inhibit their muscles actively and may thereby reduce changes at the cortical level. We have therefore chosen to compare the TMS-evoked responses at the same intensity of muscle activation during deliberate voluntary activation and during activation associated with strong contractions of the other arm. During the contraction of the other arm, the unintended associated contraction was measured and was later given as a target for a voluntary contraction. By matching the contraction level the evect of a strong voluntary contraction on the excitability of the ipsilateral motor cortex could be measured without the potential inxuence of inhibition generated to ensure muscle relaxation. We used this setup to investigate the origin of the associated contraction. There are several potential routes by which activity in the contralateral muscle can be generated (see also Carson 2005). Figure 1 shows a schematic representation of these potential pathways. These include routes from the ipsilateral cortex acting bilaterally or unilaterally via corticospinal axons, and routes from the contralateral cortex which in turn might be driven via the ipsilateral hemisphere or directly by higher centres. To test the possible pathways involved in the associated activity we used cortical stimulation to disrupt activity in the ipsilateral or contralateral motor cortex. In voluntary contractions, the silent period follows TMS over the contralateral motor cortex. This silent period consists of an early phase which is induced by spinal mechanisms and a later phase that is induced by intracortical inhibition (Inghilleri et al. 1993; Brasil-Neto et al. 1995; Chen et al. 1999). The intracortical inhibition which follows strong TMS has been used as a temporary lesion to disrupt activity in various cortical areas (e.g. Cohen et al. 1991; Maccabee et al. 1991; Pascual-Leone et al. 2000). Thus, for associated contractions the presence of a long silent period should indicate that output from the stimulated hemisphere is driving the unintended muscle activity. Methods Nine healthy right-handed subjects (25 59 years old; 5 women) participated in this study. All subjects understood the experimental procedures and gave their informed consent before the studies. Five subjects participated in two separate experiments (see Additional experiments). Set-up Subjects were seated with both arms in isometric myographs. Their elbows were Xexed with their forearms vertical. Both forearms were supinated and strapped at the wrist to the myographs (Xtran, Melbourne, Australia). For each subject the height of the chair was adjusted to a comfortable position. Feedback of elbow Xexion force from the right arm was visible on an LED display in front of the subject. During maximal contractions subjects were verbally encouraged throughout the contractions to perform maximally and visual feedback was provided (Gandevia 2001). Surface EMG-recordings of the left and right biceps brachii, and the left triceps muscles were made in a belly-tendon montage (9 mm diameter Ag/AgCl electrodes, ca 10 cm inter-electrode diverence). A bandshaped earth electrode was Wxed high around the upper arm. EMG-signals were ampliwed (200 1,000 ) and Wltered with a band-pass Wlter (30 1,000 Hz). In addition, EMG-recordings of the left biceps muscle were online rectiwed and smoothed (time constant 100 ms) and

3 528 Exp Brain Res (2006) 175: higher centres A B C D motor cortex motoneuron pool muscle left right left right left right left right Fig. 1 Schematic illustration of hypothetical pathways involved in generation of associated activity. The right muscle is the operant muscle showing volitional activity while the left muscle is the muscle showing the associated activity. The pathways involved in the volitional activity are shown in black lines; the proposed pathways involved in associated activity are shown by interrupted lines. a The associated activity comes from a projection of the normal pathway at subcortical levels to muscles ipsilateral to the target hemisphere. b The associated activity comes from an ipsilateral projection from the target motor cortex, this projection is activated simultaneously with the corticospinal pathway to the right target muscle. c The target hemisphere projects via interhemispheric connections to the contralateral motor area. d The associated activity comes from the contralateral corticospinal pathway; the contralateral hemisphere is concomitantly activated with the target hemisphere made visible to the subject by a second LED display. All EMG signals, as well as elbow Xexion force from both arms were stored on a computer with the use of an A/D converter (Cambridge electronic design; CED 1401, sampling frequency: 5,000 and 500 Hz, for the EMG and force recordings, respectively). Stimulation Responses were evoked by transcranial magnetic stimulation (TMS) over the motor cortex of the left or right hemisphere, stimulation of the brachial plexus (Erb s point) and in Wve subjects, additional responses were evoked by cervicomedullary stimulation. Transcranial magnetic stimuli were delivered via a Wgure-eight coil (9 cm diameter) over the motor cortex of the appropriate hemisphere (Magstim 200). The coil was oriented with the handle pointing postero-laterally at around 45 to the midline with the induced current in the cortex Xowing posterior to anterior across the motor strip. The optimal location was dewned as the position in which TMS evoked the largest MEP in the contralateral biceps brachii and was determined for each subject. The TMS coil together with the head of the subject was held by one of the experimenters. Stimulus intensity was adjusted for each subject so that a silent period of > 160 ms followed the MEP in maximally contracting muscles. Long silent periods were evoked to ensure that intracortical inhibition, and not spinal mechanisms, was responsible for changes in silent period duration (Inghilleri et al. 1993; Brasil- Neto et al. 1995; Chen et al. 1999). This stimulation intensity was used in the voluntary and associated contractions. Subjects were instructed to regain torque as fast as possible after the stimulus. Lower stimulus intensities, set to evoke MEPs of»10% of the maximal M-wave, were used in the additional experiments. Single electrical stimuli were delivered to the left brachial plexus (100 μs duration, constant current, Digitimer DS7). The stimulation electrode (cathode 9 mm Ag/AgCl electrode) was positioned in the supraclavicular fossa with a second electrode (anode) on the acromion. The intensity of the stimulation was set to at least 110% of that needed to produce a maximal compound action potential (M-wave) in biceps brachii. In Wve subjects additional motor responses were evoked by cervicomedullary stimulation. Two electrodes were Wxed over the left (anode) and right (cathode) mastoid process. Single electrical stimuli were delivered by a constant-voltage stimulator (100 μs duration; Digitimer D180) and the intensity of the stimulator was set to produce a response (cervicomedullary motor evoked potential, CMEP) in the preactivated left biceps (»5 10% MVC) of about 10% of the maximal M-wave. Care was taken to ensure that there was no direct activation of peripheral motor axons (Taylor and Gandevia 2004). For comparison, the stimulus intensity to the cortical stimulation was set to produce a MEP of similar size (»10% of the maximal M-wave). General experimental procedures At the start of the experiment, all subjects performed three brief maximal voluntary contractions (MVCs; 3 5 s) of the elbow Xexors with the right and left arms separately with 1 min rest between contractions. To test whether the associated activity is transmitted via the contralateral (Fig. 1c, d) or ipsilateral cortex (Fig. 1a, b) we used two experimental protocols.

4 Exp Brain Res (2006) 175: Protocol I Subjects performed ten sets of contractions. A set of contractions consisted of: (1) a brief maximal voluntary elbow Xexion with the right arm (3 5 s) and (2) a submaximal voluntary elbow Xexion with the left arm (3 5 s); all contractions were separated by 30 s rest. During the MVC, the amplitude of the associated activity (rectiwed and smoothed EMG) of the left arm was measured and was used as a target for the subsequent voluntary contraction of the left arm (Fig. 2). Thus, we aimed for similar EMG activity of the left biceps in the two contractions. During both contractions TMS was delivered to the right hemisphere to evoke an initial excitatory response (MEP) and then a silent period in the left biceps brachii. If associated activity is generated by the contralateral cortex (Fig. 1c, d), then the silent period in the left biceps brachii after stimulation of the right hemisphere should be similar during the associated and voluntary contractions. Thus, in Protocol 1 we tested whether the associated activity is driven by the contralateral hemisphere (hypotheses 1c, d). Protocol II In the second study, subjects performed ten brief MVCs (3 5 s) with their right elbow Xexors; contractions were separated by 1 min rests. During each MVC, TMS was delivered to the left hemisphere to test whether the associated activity in the left arm could be interrupted. If associated activity is generated directly or indirectly by the ipsilateral cortex (Fig. 1a c), stimulation over this cortex might be expected to result in a silent period. For comparison, the evect of TMS over the left hemisphere on EMG in the left biceps brachii was examined while subjects performed brief bilateral submaximal voluntary contractions that were matched to the mean associated EMG activity in the left biceps as measured in Protocol 1. Bilateral contractions were used to engage any population of neurones which innervated muscles bilaterally (as represented in Fig. 1b) and which might be preferentially activated by bilateral activity. Additional experiments Five of the subjects performed a second experiment on another day. In this experiment subjects repeated the A B MVC Right Submax Left L biceps a 20% MVC magnetic cortical stimulation right force feedback L force b 20% MVC cervicomedullary stimulation brachial plexus stimulation biceps brachii EMG left EMG feedback L biceps R force c d 50% MVC 1 mv R biceps e 5 mv 5 s Fig. 2 a Schematic illustration of the experimental setup. b Records of a rectiwed and smoothed EMG of the left biceps, b elbow Xexion force of the left arm, c elbow Xexion force of the right arm, d EMG of left biceps brachii and e EMG of right biceps brachii during a maximal voluntary contraction of the right elbow Xexor muscles (associated activity in the left biceps brachii muscle) followed by a submaximal contraction of the left elbow Xexor muscles (voluntary activation of the left biceps brachii). In a the interrupted line indicates the amount of associated EMG that was matched during the voluntary submaximal contraction. The vertical interrupted lines denote the start and the end of the MVC of the right elbow Xexor muscles and the submaximal contraction of the left elbow Xexor muscle

5 530 Exp Brain Res (2006) 175: combination of an MVC with the right elbow Xexors followed by a voluntary submaximal contraction with the left elbow Xexors. The size of the submaximal contraction was again matched with the EMG activity during the associated contraction. Subjects performed Wve blocks of ten contractions (Wve MVCs alternated with Wve submaximal contractions); in the Wrst and third block, contractions were combined with TMS to the right hemisphere; the left biceps being the muscle of interest. In the second and fourth block of contractions, cervicomedullary stimulation was delivered. Stimulus intensities for both TMS and cervicomedullary stimulation were set to evoke responses of»10% of the maximal M-wave. In the Wfth block the contractions were combined with electrical stimulation of the brachial plexus in order to check for potential changes within the muscle. Data analysis and statistics The area and peak-to-peak amplitude of the MEPs, M- waves and CMEPs were measured with customized software. The area was determined between cursors that encompassed the potentials evoked by all stimuli of a particular type in each experiment. The duration of the silent period was taken as the interval from the stimulus artefact to the return of continuous EMG. The amplitude of the ongoing EMG was calculated as the mean rectiwed EMG for a period of 100 ms before the stimuli (TMS, electrical stimuli); the elbow Xexion force was measured for the same time period. Because the depression of EMG which followed ipsilateral TMS (ipsilateral silent period, isp) was generally diycult to determine we have used the analysis as described by Chen and collegues (2003). For each subject and each condition, the EMG was rectiwed and averaged. The mean and standard deviation of the ongoing EMG activity was determined for a period of 100 ms before the TMS stimuli. The onset of the isp was dewned as the time-point when the EMG dropped more than one SD below the mean and the end of the isp when the EMG returned through this level. The depth of the EMG depression was calculated from the mean EMG activity during the isp divided by the amplitude of the pre-stimulus EMG. In the text, data are expressed as means (SD). Statistical analysis focussed on possible diverences between the evoked response in the voluntary contraction and the associated contractions and was performed by paired t-tests (SPSS-12, SPSS-Inc, Chicago, USA). Statistical analysis was performed for individual subjects and for the group. Statistical signiwcance was set at P <0.05. Results In all subjects, brief MVCs of the right elbow Xexors were accompanied by involuntary associated activity in the left elbow Xexors. The amplitude of the rectiwed EMG varied between 2 and 20% of the EMG values during MVC (mean % MVC). The peak Xexion force during the associated contraction was % MVC. Often subjects (n =7) showed an associated contraction in extension rather than in the Xexion direction; for the calculation of the mean force values we designated this to be a Xexion force of zero. During the associated contractions, the left triceps brachii muscle also showed signiwcant activity, as was also suggested by the force direction. In contrast, during the voluntary contraction, the activity was not signiwcantly diverent from the noise level. Mean EMG in left triceps brachii after subtraction of the mean noise was mv in voluntary submaximal elbow Xexions but was mv during associated contractions (diverent from noise, P <0.05). Protocol I: Responses to stimulation of the right motor cortex Figure 3a and b shows typical responses to stimulation of the right motor cortex. TMS was delivered during MVCs of the right elbow Xexor muscles and submaximal voluntary contractions of the left Xexor muscles. In both contractions, MEPs and silent periods were evoked in the left biceps brachii (same TMS intensity during both contractions; intensity varied between 75 and 90% of maximal output). Since the area of the MEP and the duration of the silent period are inxuenced by background excitation, ongoing activity was matched in the two contractions; mean EMG before the delivery of TMS was similar in the two contractions (mean % MVC and % MVC, for the associated and voluntary contractions, respectively; P > 0.18). The main Wnding was that the duration of the silent period was similar during the associated contraction (mean ms, range ms) and the voluntary contraction ( ms, range ms; P > 0.8; Fig. 3b, c). In addition, although the MEPs were large because of the high stimulus intensities, both MEP amplitude and area were signiwcantly larger during the associated contraction (MEP amplitude associated contraction: mv, MEP amplitude voluntary contraction: mv; P < 0.005; Fig. 3a, c). Despite matching EMG activity in biceps brachii during the associated and voluntary contractions a signiwcantly lower Xexion force was measured (mean

6 Exp Brain Res (2006) 175: A VC AC 10 mv 10 ms B VC AC 10 mv 0.1 s C associated / voluntary contraction * * Area Amplitude Silent period mean EMG Fig. 3 a Motor evoked potentials (MEP) recorded from one subject during a submaximal voluntary contraction (VC left panel) and an associated contraction (AC right panel) at matched EMG levels. Five traces are superimposed. b The same recordings as in a with a longer time scale. The silent period was 210 ms in VC and 201 ms in AC. c Ratio (SE) between values obtained during the associated contraction and the volitional contraction. Data were obtained after stimulation of the right hemisphere while subjects contracted the left biceps brachii muscle at matched EMG levels. Interrupted line indicates a ratio of 1; the values obtained during the voluntary contraction are equal to the values obtained during the associated contraction. Asterisks denotes a signiwcant diverence between associated and volitional contractions force over 100 ms before the TMS stimulus) during the associated contractions (associated contraction % MVC and voluntary % MVC; P < 0.005). Protocol II: Response to stimulation of the left motor cortex To investigate the possibility of a direct ipsilateral inxuence on the associated contractions we stimulated the left motor cortex during brief MVCs of the right elbow Xexors and looked for responses in the left biceps brachii (stimulation intensity % output). No overt silent period occurred. However, EMG-activity was depressed to 55 10% of pre-stimulus values after the stimulation. In one subject, no EMG depression could be identiwed. The period of reduced EMG (from to ms after the stimulus) was much shorter than the values usually observed for a silent period induced by stimulation of the contralateral cortex during voluntary activity. In one of the 9 subjects we found a MEP (35% of the amplitude of the contralateral MEP) at the usual latency (ca 10 ms). In three other subjects there was a small MEP (mean 9% contralateral MEP) at longer latencies (21 25 ms). In the matched bilateral submaximal contractions, a depression of EMG in left biceps brachii (58 9% of pre-stimulus EMG and from to ms) similar to that during the associated contractions was seen. Two subjects showed an MEP at 12 ms in the biceps brachii ipsilateral to stimulation. Response to cervicomedullary stimulation In Wve subjects we repeated the Wrst protocol using stimulation over the right motor cortex, cervicomedullary stimulation and brachial plexus stimulation. This experiment conwrmed the increased MEP during the associated contraction (MEP area during associated contractions % M-max, range % M-max; MEP area during voluntary contractions % M-max, range % M-max; P < 0.05; Fig. 4). In four subjects the individual values also showed a signiwcant MEP increase during the associated

7 532 Exp Brain Res (2006) 175: contractions (all P values < 0.01). To investigate whether this increase was due to changes at cortical or spinal levels we repeated the set of contractions and delivered cervicomedullary stimulation to activate the corticospinal tract. The mean CMEP for the group of subjects did not diver signiwcantly between the voluntary and the associated contractions (associated contraction % M-max; voluntary contraction % M-max; P > 0.4; Fig. 4). However, two subjects showed a signiwcant increase in CMEP (P <0.01) while two other subjects showed a signiwcant decrease in CMEP (P < 0.01) during the associated contractions compared to the voluntary contraction. Nevertheless, none of the subjects showed as large an increase in CMEP area as in MEP area during associated contractions. Thus, these results are consistent with an increase in cortical excitability. Voluntary contraction A MEP CMEP M-wave Associated contraction 1 mv 1 mv 10 mv 25 ms Discussion Our data showed that strong voluntary activation of the right elbow Xexor muscles results in associated activity in the left elbow Xexor muscles. This unintentional activation of the contralateral biceps brachii muscle increased the evoked potential after cortical magnetic stimulation (MEP) compared to MEPs elicited in voluntary contractions with matched levels of background EMG. In contrast, the silent period was equally long in associated and voluntary contractions. The interpretation of these Wndings in terms of the possible pathways generating associated activity is discussed below. In voluntary contractions, the latter part of the silent period produced by strong stimulation of the motor cortex largely results from inhibition of corticospinal output (Fuhr et al. 1991; Inghilleri et al. 1993) and is probably due to a long-lasting GABAergic inhibition of corticofugal activity (Kang et al. 1994). Thus, our main observation that stimulation of the contralateral motor cortex evoked a similar silent period in associated and matched voluntary contractions suggests that this activity is driven in a similar way by corticospinal output from the contralateral motor cortex. This observation suggests that the associated activity involves activation of the contralateral motor cortex; it makes unlikely any pathway for generation of associated activity that does not include the contralateral motor cortex (rules out Fig. 1a, b). The lack of a clear silent period in the associated contractions following stimulation of their ipsilateral motor cortex complements this observation. Although the cortical area in which contralateral or ipsilateral MEPs are evoked can diver B associated / voluntary contraction * area background EMG MEP CMEP Mmax Fig. 4 a Recordings from the left biceps brachii muscle after transcranial magnetic stimulation (MEP top row), corticospinal stimulation (CMEP second row) and brachial plexus stimulation (M-wave third row) during submaximal voluntary contractions and associated contractions at matched EMG levels. Five traces are superimposed. b Ratio (SE) between values obtained during associated contractions and voluntary contractions (white bars) in the left biceps brachii muscle during cortical stimulation of the right hemisphere (MEP), corticospinal stimulation (CMEP) and brachial plexus stimulation (M-max). During the voluntary contractions subjects were asked to match the amount of EMG activity that was recorded during the associated contraction; the ratio between the mean EMG level during the associated and voluntary contraction is displayed as black bars. Interrupted line indicates a ratio of 1; the values obtained during the voluntary contraction are equal to the values obtained during the associated contraction. Asterisks denotes a signiwcant diverence between associated and volitional contractions (Ziemann et al. 1999), as can the preferred current direction for evoking them, for the silent period the current direction (see Fig. 5 in Zieman et al. 1999) and stimulated cortical area (Wassermann et al. 1991;

8 Exp Brain Res (2006) 175: Wassermann et al. 1994; Chen et al. 2003) are less crucial, especially at high stimulus intensities. Therefore, the lack of a clear silent period conwrms that the associated activity is not produced by direct ipsilateral corticospinal drive to the motoneurones. Indeed, stimulation over the ipsilateral motor cortex produces a similar»40% depression of EMG in both associated and matched voluntary contractions. This depression is consistent with previously described interhemispheric inhibition which probably occurs via a transcallosal pathway (Wassermann et al. 1991; Ferbert et al. 1992). There is an additional implication of cortical stimulation producing similar evects in the ipsilateral muscle whether the subject was producing a voluntary or an associated contraction with this muscle. This Wnding suggests that output from the strongly activated motor cortex does not provide continuous drive to the other hemisphere and thus, indirectly generate the associated activity (as in Fig. 1c). If the ipsilateral motor cortex was the source of the activity in the contralateral hemisphere (Fig. 1c) one would expect to Wnd a clear silent period that is an interruption of the EMG activity during the associated contraction. Our results suggest that the ipsilateral motor cortex is not the main origin of the associated activity. Taken together the results concerning the silent period after contralateral and ipsilateral stimulation suggest that associated muscle activity is driven by output from the contralateral motor cortex and that the ipsilateral motor cortex plays little part in its generation (Fig. 1d). Furthermore, our force data showed that besides elbow Xexor muscles, extensor muscles were also active and thus, that the associated activity is not restricted to the homologous muscles. Our experimental observations directly prove the suggestions made by Mayston et al. (1999) and Carson et al. (2004), that the contralateral and not the ipsilateral motor cortex is important for the associated activity. The Wnding that the contralateral motor cortex is responsible for the associated activity is also important for the evaluation of functional imaging studies in which ipsilateral activation during unilateral contractions is often reported (e.g. Kim et al. 1993; Singh et al. 1998). A common proposal is that this ipsilateral activation is directly involved in the target contraction, however, our data shows that ipsilateral activation could instead be due to low levels of associated activity. Facilitation of MEPs by activation of the contralateral homologous muscle has been described in relaxed muscles (Hess et al. 1986; Zwarts 1992; Meyer et al. 1995; Tinazzi and Zanette 1998; Stedman et al. 1998; Muellbacher et al. 2000; Liepert et al. 2001; Hortobagyi et al. 2003; Carson et al. 2004). Some authors (Samii et al. 1997) have proposed that the facilitation could be due to unintentional activity in the muscle under investigation. The observation that associated activity is often seen during activation of the homologous muscle suggests that it can be diycult to prevent (Samii et al. 1997; Tinazzi and Zanette 1998; Muellbacher et al. 2000). Other studies have shown increased responses to cortical stimulation despite unchanged excitability of the motoneuron pool when tested directly by stimulation of descending corticospinal paths (Hortobagyi et al. 2003; Carson et al. 2004). This implies that either contraction of the homologous muscle has provided extra excitation to the stimulated motor cortex but this is subthreshold for the production of extra ongoing excitatory corticospinal output, or the increased activity to the motoneuron pool is counteracted by a simultaneously increased inhibition. In the current study, MEPs elicited during activity associated with the MVC of the contralateral homologous muscle were larger than those elicited during matched voluntary activity. This is surprising as production of the same EMG activity suggests that activity within the motoneuron pool is similar. This in turn suggests that net excitatory input to the motoneurons is similar and, unless averent input is altered, descending drive should be similar. If descending input is largely from the contralateral motor cortex in both conditions, as the silent period results suggest, it is diycult to understand where the diverences lie that cause the larger MEP in associated contractions. One possibility is a change in the gain of the motoneuron pool. That is, during an associated contraction the same input evokes more output than during a voluntary contraction. Even when the net input to the motoneuron pool is similar, diverences in the amount of excitation and inhibition to a pool can have strong evects on its input-output relations (Kernell and Hultborn 1990). Related to the diverence in gain is a diverence in motoneuron recruitment. DiVerent motoneurons could be active during associated and voluntary contractions. Although individual subject s responses to subcortical stimulation of the corticospinal tract changed variably between associated and voluntary contractions, there were no consistent alterations at a segmental level. Another possibility is an alteration in the balance between averent and descending excitation. For example, the triceps muscle activity during the associated but not the voluntary contractions could increase reciprocal inhibition onto the biceps brachii motoneuron pool. In addition, during strong contractions of the wrist Xexors, there is presynaptic inhibition of the H-reXex in the relaxed contralateral muscle (Hortobagyi et al. 2003; Carson et al. 2004). With decreased muscle spindle

9 534 Exp Brain Res (2006) 175: excitation to the motoneurons, more descending drive would be required to produce the same muscle activity and motor cortical excitability might be increased. However, it is unclear whether the presynaptic inhibition acts when relaxation is not complete. By contrast, one study showed increased H-reXexes during strong contralateral contractions (Tinazzi and Zanette 1998). More descending drive from the contralateral cortex could also be required if the motoneuron pool receives descending inhibition. GerloV and colleagues (1998) found an ipsilateral inhibition of the motoneuron pool that was partly mediated at subcortical levels. A Wnal possibility is that the increased MEP in the associated contractions represents an increase in cortical excitability which does not produce extra descending drive (i.e. a change in gain of the cortical output cells). This option is also suggested to explain taskrelated diverences in MEPs in a hand muscle (Wrst dorsal interosseous) during contractions in which this muscle is a prime mover versus contractions in which it acts as a synergist (Datta et al. 1989; Flament et al. 1993; Tinazzi et al. 2003). In summary, our results using TMS to interrupt the activity within the ipsilateral and contralateral cortex and stimulation of descending corticospinal paths favour an obligatory role for the contralateral cortex in generation of the unintended muscle contraction associated with strong volitional evorts (Fig. 1d). This Wnding also has important implications for the interpretation of the ipsilateral cortical activation often seen with unilateral contractions during functional imaging studies. Acknowledgments This research was supported by the National Health and Medical Research Council (Australia). I. Zijdewind was supported by a travel grant of The Netherlands Organization for ScientiWc Research (NWO). References Aranyi Z, Rosler KM (2002) EVort-induced mirror movements. A study of transcallosal inhibition in humans. Exp Brain Res 145:76 82 Armatas CA, Summers JJ, Bradshaw JL (1994) Mirror movements in normal adult subjects. J Clin Exp Neuropsychol 16: Brasil-Neto JP, Cammarota A, Valls-Sole J, Pascual-Leone A, Hallett M, Cohen LG (1995) Role of intracortical mechanisms in the late part of the silent period to transcranial stimulation of the human motor cortex. Acta Neurol Scand 92: Carson RG (2005) Neural pathways mediating bilateral interactions between the upper limbs. Brain Res Brain Res Rev 49: Carson RG, Riek S, Mackey DC, Meichenbaum DP, Willms K, Forner M, Byblow WD (2004) Excitability changes in human forearm corticospinal projections and spinal rexex pathways during rhythmic voluntary movement of the opposite limb. J Physiol 560: Cernacek J (1961) Contralateral motor irradiation cerebral dominance. Its changes in hemiparesis. Arch Neurol 4: Chen R, Lozano AM, Ashby P (1999) Mechanism of the silent period following transcranial magnetic stimulation. Evidence from epidural recordings. Exp Brain Res 128: Chen R, Yung D, Li JY (2003) Organization of ipsilateral excitatory and inhibitory pathways in the human motor cortex. J Neurophysiol 89: Cohen LG, Bandinelli S, Sato S, Kufta C, Hallett M (1991) Attenuation in detection of somatosensory stimuli by transcranial magnetic stimulation. Electroencephalogr Clin Neurophysiol 81: Curschmann H (1906) Beitrage zur physiologie und pathologie der kontralateralen mitbewegungen. Deutsche Zeitschrift Nervenheilkunde 31:1 52 Datta AK, Harrison LM, Stephens JA (1989) Task-dependent changes in the size of response to magnetic brain stimulation in human Wrst dorsal interosseous muscle. J Physiol 418:13 23 Diedrichsen J, Hazeltine E, Nurss W, Ivry R (2003) The role of the corpus callosum in the coupling of bimanual isometric force pulses. J Neurophysiol 90: Durwen HF, Herzog AG, Falk A, Calabrese P, Gehlen W (1997) Motorische spiegelphanomene; Spielgelbildliche mitbewegungen und mitaktivierungen. Neurol Rehabil 2:69 75 Ferbert A, Priori A, Rothwell JC, Day BL, Colebatch JG, Marsden CD (1992) Interhemispheric inhibition of the human motor cortex. J Physiol 453: Flament D, Goldsmith P, Buckley CJ, Lemon RN (1993) Task dependence of responses in Wrst dorsal interosseous muscle to magnetic brain stimulation in man. J Physiol 464: Fuhr P, Agostino R, Hallett M (1991) Spinal motor neuron excitability during the silent period after cortical stimulation. Electroencephalogr Clin Neurophysiol 81: Gandevia SC (2001) Spinal and supraspinal factors in human muscle fatigue. Physiol Rev 81: Gandevia SC, MaceWeld VG, Bigland-Ritchie B, Gorman RB, Burke D (1993) Motoneuronal output and gradation of evort in attempts to contract acutely paralysed leg muscles in man. J Physiol 471: GerloV C, Cohen LG, Floeter MK, Chen R, Corwell B, Hallett M (1998) Inhibitory inxuence of the ipsilateral motor cortex on responses to stimulation of the human cortex and pyramidal tract. J Physiol 510: Hess CW, Mills KR, Murray NM (1986) Magnetic stimulation of the human brain: facilitation of motor responses by voluntary contraction of ipsilateral and contralateral muscles with additional observations on an amputee. Neurosci Lett 71: Hortobagyi T, Taylor JL, Petersen NT, Russell G, Gandevia SC (2003) Changes in segmental and motor cortical output with contralateral muscle contractions and altered sensory inputs in humans. J Neurophysiol 90: Inghilleri M, Berardelli A, Cruccu G, Manfredi M (1993) Silent period evoked by transcranial stimulation of the human cortex and cervicomedullary junction. J Physiol 466: Kang Y, Kaneko T, Ohishi H, Endo K, Araki T (1994) Spatiotemporally diverential inhibition of pyramidal cells in the cat motor cortex. J Neurophysiol 71: Kernell D, Hultborn H (1990) Synaptic evects on recruitment gain: a mechanism of importance for the input output relations of motoneurone pools? Brain Res 507:

10 Exp Brain Res (2006) 175: Kim SG, Ashe J, Georgopoulos AP, Merkle H, Ellermann JM, Menon RS, Ogawa S, Ugurbil K (1993) Functional imaging of human motor cortex at high magnetic Weld. J Neurophysiol 69: Leocani L, Cohen LG, Wassermann EM, Ikoma K, Hallett M (2000) Human corticospinal excitability evaluated with transcranial magnetic stimulation during diverent reaction time paradigms. Brain : Liepert J, Dettmers C, Terborg C, Weiller C (2001) Inhibition of ipsilateral motor cortex during phasic generation of low force. Clin Neurophysiol 112: Maccabee PJ, Amassian VE, Cracco RQ, Cracco JB, Rudell AP, Eberle LP, Zemon V (1991) Magnetic coil stimulation of human visual cortex: studies of perception. Electroencephalogr Clin Neurophysiol 43(Suppl): Mayston MJ, Harrison LM, Stephens JA (1999) A neurophysiological study of mirror movements in adults and children. Ann Neurol 45: Meyer BU, Roricht S, GraWn von EH, Kruggel F, Weindl A (1995) Inhibitory and excitatory interhemispheric transfers between motor cortical areas in normal humans and patients with abnormalities of the corpus callosum. Brain 118: Muellbacher W, Facchini S, Boroojerdi B, Hallett M (2000) Changes in motor cortex excitability during ipsilateral hand muscle activation in humans. Clin Neurophysiol 111: Pascual-Leone A, Walsh V, Rothwell J (2000) Transcranial magnetic stimulation in cognitive neuroscience virtual lesion, chronometry, and functional connectivity. Curr Opin Neurobiol 10: Pinneo LR (1961) The evects of induced muscle tension during tracking on level of activation and on performance. J Exp Psychol 62: Samii A, Canos M, Ikoma K, Wassermann EM, Hallett M (1997) Absence of facilitation or depression of motor evoked potentials after contralateral homologous muscle activation. Electroencephalogr Clin Neurophysiol 105: Shinohara M, Keenan KG, Enoka RM (2003) Contralateral activity in a homologous hand muscle during voluntary contractions is greater in old adults. J Appl Physiol 94: Singh LN, Higano S, Takahashi S, Kurihara N, Furuta S, Tamura H, Shimanuki Y, Mugikura S, Fujii T, Yamadori A, Sakamoto M, Yamada S (1998) Comparison of ipsilateral activation between right and left handers: a functional MR imaging study. Neuroreport 9: Sohn YH, Dang N, Hallett M (2003) Suppression of corticospinal excitability during negative motor imagery. J Neurophysiol 90: Stedman A, Davey NJ, Ellaway PH (1998) Facilitation of human Wrst dorsal interosseous muscle responses to transcranial magnetic stimulation during voluntary contraction of the contralateral homonymous muscle. Muscle Nerve 21: Taylor JL, Gandevia SC (2004) Noninvasive stimulation of the human corticospinal tract. J Appl Physiol 96: Tinazzi M, Zanette G (1998) Modulation of ipsilateral motor cortex in man during unimanual Wnger movements of diverent complexities. Neurosci Lett 244: Tinazzi M, Farina S, Tamburin S, Facchini S, Fiaschi A, Restivo D, Berardelli A (2003) Task-dependent modulation of excitatory and inhibitory functions within the human primary motor cortex. Exp Brain Res 150: Todor JI, Lazarus JA (1986) Exertion level and the intensity of associated movements. Dev Med Child Neurol 28: Wassermann EM, Fuhr P, Cohen LG, Hallett M (1991) EVects of transcranial magnetic stimulation on ipsilateral muscles. Neurology 41: Wassermann EM, Pascual-Leone A, Hallett M (1994) Cortical motor representation of the ipsilateral hand and arm. Exp Brain Res 100: Ziemann U, Ishii K, Borgheresi A, Yaseen Z, Battaglia F, Hallett M, Cincotta M, Wassermann EM (1999) Dissociation of the pathways mediating ipsilateral and contralateral motorevoked potentials in human hand and arm muscles. J Physiol 518: Zijdewind I, Kernell D (2001) Bilateral interactions during contractions of intrinsic hand muscles. J Neurophysiol 85: Zwarts MJ (1992) Central motor conduction in relation to contraand ipsilateral activation. Electroencephalogr Clin Neurophysiol 85: