Arm sway holds sway: Locomotor-like modulation of leg reflexes when arms swing in alternation

Neuroscience 258 (2014) 34–46

ARM SWAY HOLDS SWAY: LOCOMOTOR-LIKE MODULATION OF LEG REFLEXES WHEN ARMS SWING IN ALTERNATION F. MASSAAD, a* O. LEVIN, a P. MEYNS, a D. DRIJKONINGEN, a S. P. SWINNEN a AND J. DUYSENS a,b

humans (Grillner, 1975). Some authors have suggested that alternated arm swing may enable humans to save energy while others debated that it can affect gait stability (Ortega et al., 2008; Bruijn et al., 2010). This arm swing could be a passive reaction to the leg motions (Gerdy, 1829; Pontzer et al., 2009) or rather a manifestation of an active control by the neural system because arm muscles contract rhythmically even if arm swing is prevented (Elftman, 1939; Ballesteros et al., 1965). However, it is not obvious what purpose this alternated rhythmic muscle contraction serves, in fact it could be regarded as wasteful (Jackson, 1983). This neural control could be an evolutionary remnant of quadrupedal locomotion where movements in the upper limbs are partly coordinated with the hindlimbs through propriospinal pathways that connect cervical and lumbar spinal circuits such as central-patterngenerators (CPGs) activated rhythmically in alternation (Dietz, 2002; Juvin et al., 2012). If this is still the case in humans, one would expect that connections between the circuits involved in arm and leg movements would favor conditions that are ‘‘locomotor-like’’ (i.e. alternating or anti-phase arm swing). However, the arms have become specialized to perform skilled movements in humans, and in-phase movements such as hand clapping are usually more accurate and stable (Swinnen, 2002). Soleus H-reflexes have been used to investigate interlimb connections. Reflex changes during a given task (task-dependent modulation), or during a movement phase within this task (phase-dependent modulation), were used to probe a possible CPG’s contribution when this modulation was independent from the electromyographic (EMG) background activity (Burke, 1999; Zehr and Duysens, 2004). Previous studies showed that soleus H-reflex decreased during all rhythmic arm movements, such as arm swing or cycling, indicating a persistence of neural coupling between upper and lower limbs (Hiraoka, 2001; Hiraoka and Iwata, 2006; Knikou, 2007; De Ruiter et al., 2010). However, phase-dependent modulation was not always assessed during arm swing, and methodological concerns were raised in some of the previous studies because EMG background was not always controlled. Furthermore, since some features of leg movements are mainly controlled by the spinal automatism of the stepping limb movement whereas others depend on other limb movements (Shik and Orlovsky, 1976), it remained unclear if locomotor-like alternated arm movements would induce similar soleus H-reflex

a

Research Center for Movement Control and Neuroplasticity, Department of Kinesiology, Faculty of Kinesiology and Rehabilitation Sciences, K.U. Leuven, Heverlee, Belgium b

Department of Research, Development and Education, Sint Maartenskliniek, Nijmegen, The Netherlands

Abstract—It has been argued that arm movements are important during human gait because they affect leg activity due to neural coupling between arms and legs. Consequently, one would expect that locomotor-like alternating arm swing is more effective than in-phase swing in affecting the legs’ motor output. Other alternating movements such as trunk rotation associated to arm swing could also affect leg reflexes. Here, we assessed how locomotor-like movement patterns would affect soleus H-reflexes in 13 subjects performing arm swing in the sagittal plane (ipsilateral, contralateral and bilateral in-phase versus locomotor-like anti-phase arm movements) and trunk rotation with the legs stationary, and leg stepping with the arms stationary. Findings revealed that soleus H-reflexes were suppressed for all arm, trunk or leg movements. However, a marked reflex modulation occurred during locomotor-like anti-phase arm swing, as was also the case during leg stepping, and this modulation flattened out during in-phase arm swing. This modulation had a peculiar bell shape and showed maximum suppression at a moment where the heel-strike would occur during a normal walking cycle. Furthermore, this modulation was independent from electromyographic activity, suggesting a spinal processing at premotoneuronal level. Therefore, trunk movement can affect legs’ output, and a special neural coupling occurs between arms and legs when arms move in alternation. This may have implications for gait rehabilitation. Ó 2014 Published by Elsevier Ltd. on behalf of IBRO.

Key words: H-reflex, arm movement, locomotion, central pattern generators.

INTRODUCTION ‘‘Why do quadrupeds move their legs crisscross?’’ This fundamental question was already raised by Aristotle in the first known manuscript on locomotion (Aristotle, 350 BC). Indeed, the diagonal nature of interlimb coordination is striking even in free-arm bipedal gait of *Corresponding author. E-mail address: fi[email protected] (F. Massaad). Abbreviations: CPG, central-pattern-generator; EMG, electromyography; M-max, maximal M-wave; RMS, root mean square. 0306-4522/13 $36.00 Ó 2014 Published by Elsevier Ltd. on behalf of IBRO. http://dx.doi.org/10.1016/j.neuroscience.2013.10.007 34

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modulation as during walking or stepping in place. Another limitation was that the former studies did not sufficiently consider that alternated trunk rotation is closely associated to arm swing during walking (Bruijn et al., 2008). Therefore, the question emerges whether trunk movement may also determine leg motor output. Trunk movements could be important as soleus Hreflexes are not anymore significantly depressed during arm cycling if the head and trunk are immobilized (Hiraoka and Taniguchi, 2010). We investigated the modulation pattern of the soleus H-reflex during rhythmical anti-phase arm movements versus unilateral, bilateral in-phase arm movements and trunk rotation while the legs were stationary. In addition, leg stepping movements were tested while the arms were stationary. We hypothesized that soleus H-reflex modulation would be more pronounced during antiphase arm movements. This modulation should contribute to the well-known soleus modulation previously described during walking if connections between the circuits involved in arm and leg movements would favor conditions that are ‘‘locomotor-like’’.

EXPERIMENTAL PROCEDURES Subjects Thirteen subjects (six men and seven women) aged 25 ± 3 years (mean ± SD) participated in the present study. None of the subjects reported any neurological deficit, low back pain, or other musculoskeletal disorders. All the subjects were right-handed and rightfooted. The experimental procedures were approved by the Ethics Committee of Biomedical Research at the KU-Leuven University (Leuven, Belgium). Informed consent was signed by every participant prior to testing. The experiments were conducted in accordance with the Helsinki Declaration. Apparatus and task The subjects were instructed to perform rhythmic movements with their upper limbs, lower limbs or trunk (Fig. 1). The H-reflex was elicited by stimulating the tibial nerve of the right leg in the following test conditions: (1) ipsilateral right arm flexion/extension in the sagittal plane, (2) contralateral left arm flexion/extension, (3) anti-phase flexion/extension of both arms, (4) in-phase flexion/extension of both arms, and (5) trunk rotation in the transverse plane. Data collection was conducted while the subjects performed these five conditions in a sitting position. In standing condition, only one condition was performed where H-reflex was elicited during stepping in place (6). During all trials, subjects were asked to look straight ahead and to restrain from unwanted head, trunk or leg movements during a given trial of the experiment. During the sitting position, the subjects had their back supported and their hip, knee, and ankle angles were set at approximately 90°, 110°, and 90°, respectively, and they were asked to maintain the legs stationary and produce a controlled activation

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of the right soleus by constantly pushing the right foot onto a pedal to produce low-level tonic contractions (around 10% maximum voluntary contraction) using an online visual feedback of their EMG on an oscilloscope. This minimal voluntary sustained contraction of the homonymous muscle is a way to maintain stable motoneuron excitability and minimize postsynaptic effects (Stein and Thompson, 2006; Knikou, 2008). Indeed, the state of excitability of the motoneuron pool plays a significant role in determining the H-reflex magnitude which may vary within and across subjects. The chair and foot pedal were fixed in a similar position during all movements and the subjects were asked to press the pedal while keeping the same position so that the segment positions of their lower limbs remain stable, which was continuously verified by the experimenter. The maximum voluntary contraction was determined by asking the seated subject at the beginning of the experiment to perform a maximum tonic contraction of the soleus while pushing on the pedal three times for 5 s and the average of the muscle contraction amplitude during these three trials was calculated. For arm flexion/extension conditions, participants were asked to move the arms in the sagittal plane and maintain the elbows comfortably extended and to perform a movement as large as possible from maximum possible extension up to around 70–80° flexion. These positions were chosen because previous studies demonstrated significant effects of similar shoulder positions on reflex excitability in the legs (Delwaide et al., 1973, 1977; Eke-Okoro, 1994; Frigon et al., 2004; Knikou, 2007). For trunk rotation, the subjects were asked to cross their arms on the chest and perform a rotation movement of the trunk with the head looking straight ahead and not rotating along with the trunk. For the ‘stepping in place’ condition, the subjects were asked to make walking movements in place with the legs and without changing the position of the body in space. An alternated flexion of the hips and knees in the sagittal plane was then performed. We visually verified that the movement amplitudes were not exaggerated with the feet stepping in the same place and the arms relaxed along the body. During a given movement the subjects heard double-tone auditory signals (high versus low pitch), which provided pacing for the movements at a frequency of 1 Hz (i.e. a full movement cycle beginning and ending at the same position was performed during the period of 1 s separating two consecutive high pitch sounds). This frequency was chosen as it is close to that seen during gait at common intermediate speeds of 4 km/h (Donker et al., 2001, 2005; Huang et al., 2010). The subjects were asked to match the high pitch sound either with the maximum extension position of their right arm for the ipsilateral flexion/extension arm movement (condition 1 in Fig. 1), or the maximum flexion position of the left arm for contralateral, anti-phase and in-phase flexion/extension arm movements (conditions 2–4), or with the maximum rotation position of the trunk to the right for the trunk rotation (i.e. left shoulder forward)

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Fig. 1. The illustrative explanation of the experimental set up while the subject is performing the different task conditions. The limb involved in a given task is depicted in black. For each condition, the frame is depicted at the moment where the subject would hear the high pitch. The arrow shows the movement direction that subjects would perform after this high pitch. The right part of the figure depicts above a panel of the different phases where the stimulations are sent with respect to the metronome pace for the anti-phase arm movement as an example. Below this panel we can see a typical trace of the subject’s kinematics during the anti-phase flexion/extension (positive/negative values respectively) of the arms as a function of 1 s in time. At the bottom part, the soleus H-reflexes elicited during this arm movement cycle. The stimulations sweep the cycle every 0.125 s with a stimulation every 3 to 6 s.

(condition 5), or with the maximum flexion position of the left leg for the stepping in place (condition 6). The low pitch matched half of the movement cycle. The six task conditions were presented in a random order and short breaks were allowed between trials to avoid fatigue. Before and after each trial, we measured the H-reflex in a control condition where the subject was asked to maintain the arms stationary and relaxed along the body in a vertical position. During both control and arm movement conditions in sitting, subjects were asked to maintain the legs stationary and produce a controlled activation of the right soleus (around 10% maximum voluntary contraction). All participants were sufficiently trained before the experiment to be able to perform the prescribed coordination patterns at a paced frequency. Movement frequency was monitored and, if required, verbal feedback was offered to the participant to maintain the appropriate frequency. Trials where the subject could not properly follow the pace were discarded. Stimulation procedure In all subjects, the tibial nerve was stimulated with a single square-wave 1-ms pulse. This stimulation elicited an H-reflex that was recorded according to procedures

described previously in detail (Pierrot-Deseilligny and Mazevet, 2000; Zehr, 2002; Misiaszek, 2003; PierrotDeseilligny and Burke, 2005; Tucker et al., 2005; Knikou, 2008). The posterior tibial nerve over the right popliteal fossa was stimulated by using a stainless steel monopolar electrode as a probe connected to a stimulator (Grass S88 stimulator connected in series with an SIU5 isolator and a CCU1 constant current unit, Grass Instruments, Warwick, USA). First we established the optimal site of stimulation that delivered clear and stable M-waves and H-reflexes. This corresponded to the site that the M-wave had a similar shape to that of the H-reflex, and where an H-reflex could be evoked without an M-wave (at the lowest stimulus intensity) (Knikou et al., 2011). Then the stimulation and the electrode amplifiers were firmly fixed with tapes. A bandage covered the stimulation site to stabilize it. Mwave/H-reflex recruitment curves were constructed while the subject was producing a 10% maximum voluntary contraction of the soleus in sitting position. From these data the maximal M-wave (M-max) was calculated as the mean of the five largest M-wave values. The stimulation was set at an intensity where the soleus Hreflex was around 70% of H-max (20–40% of M-max) and the corresponding M-waves (which were monitored online) between 5% and 10% of the M-max (Knikou,

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2008). EMG activity was recorded from the right soleus and tibialis anterior muscles with Ag–AgCl surface electrodes (with a diameter of 1 cm) (Electrodes Blue Sensor, Ambu, Penang, Malaysia). The tibialis anterior muscle was recorded to assess the effects of reciprocal inhibition from antagonist muscles which may affect the overall amplitude of the H-reflexes (Nielsen and Kagamihara, 1992, 1993). A pair of recording electrodes was placed over the muscle belly with an interelectrode distance of 2 cm and parallel with the muscle fibers and at a location recommended by previous references (Perotto, 1994; Winter, 2005). Care was taken not to place the electrodes over the muscles’ edges to minimize EMG cross talk. The electrodes were placed after hair shaving, skin abrasion and application of alcohol and ether. The EMG signal was amplified, filtered then sampled at 5000 Hz (CED Power 1401, Cambridge Electronic Design, UK) and stored on a PC for offline analysis. Signal Software (4.0 Version, Cambridge Electronic Design, UK) was used to control stimulus triggering and data acquisition. M-wave amplitudes were continually monitored to ensure constancy in the stimulation and recording procedures. Current intensity was occasionally adjusted between conditions if needed to maintain constant M-wave amplitudes.

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Data analysis Kinematics. Motion data were low-pass filtered (second-order Butterworth with cut-off frequency at 4 Hz, with zero-lag). To evaluate if the subject kinematics matched with the metronome pace, the time delay was calculated between the high pitch and the peak arm, trunk or leg excursions. To measure the coordination between the arms, the relative phase was calculated following a methodology previously described in detail by Kurz and Stergiou (2004). To calculate relative phase, the phase angles were calculated from the phase portrait trajectories of the shoulder’s angular position versus its angular velocity in the sagittal plane. Then we subtracted the phase angle of the shoulder movement of one arm from that of the other arm for each data point of the time-normalized movement cycle. Relative phase values that are zero degrees suggest that the two oscillating arms are in phase, while relative phase values that approach 180° are considered out of phase.

Tasks were presented in a random order with the subjects performing the five arm swing or trunk rotation conditions in sitting position or stepping in place in standing position. Each condition was tested in five trials. A trial lasted 40 s with a rest time of around 1 min between every trial. Subjects were required to produce one complete movement cycle for each beat of the metronome. Hreflex was delivered at one of eight equidistant time phases of 0.125 s (i.e., at 0, 0.125, 0.25, 0.375, 0.5, 0.625, 0.75 and 0.875 s) (Fig. 1 right part). The order of stimulations was randomly set with an interval of 3–6 s between two consecutive stimulations (i.e. every 3 to 6 cycles). The high pitch was synchronized with the first stimulation (at 0 s). A control trial with the arms stationary was repeated before and after each condition. During this trial a total of 10 stimulations was delivered. The average H-reflex of the control trial before and after a given condition was considered as the ‘‘control value’’ for this condition.

Electromyography. The peak-to-peak amplitudes of the soleus H-reflex and M-wave were determined using custom-written Matlab software. We measured the EMG background activity of soleus and tibialis anterior muscles by taking the root mean square (RMS) (fullwave rectification and band-pass filtering at 20–500 Hz) of the corresponding muscle activity in a stimulation free cycle that preceded the stimulated one. The EMG background activity was calculated at the same time and duration (around 30 ms) where the H-reflex should otherwise occur during a movement cycle. This was done by using the preceding movement cycle and calculating the time corresponding to the onset of the Hreflex. The EMG background was then evaluated in a window of 30 ms following this time point. For each phase and for every condition the H-reflex amplitude, Mwave amplitude, and the RMS of the soleus EMG background were averaged and expressed as a percentage of the corresponding control values. The control values for a given condition were again the ones measured when the arms were stationary before and after that condition. Soleus H-reflexes accompanied by an M-wave or soleus EMG background whose amplitudes was not within ±2 SD of the mean control values were excluded from data analysis.

Kinematics

Statistics

Kinematics data were acquired with an opto-electronic motion-analysis system (Optotrak 3020) and synchronized with the H-reflex stimulation software. Ten markers (infrared-emitting diodes) were attached to the elbows, shoulders, trunk and feet to measure the segmental motion. Custom software (Optrotrak Data Analysis Package) was used to express the motion of each joint according to an anatomical coordinate system. The marker displacements were recorded at 100 Hz in parallel with EMG data collection and were stored on a PC for an off-line analysis.

Statistical analysis was conducted on amplitudes of Hreflex, M-wave and EMG background which were expressed as a percentage of control values. For a given condition, the control value was calculated as the mean of the control trial preceding and following this condition. Data obtained during the five trials for a given task condition were averaged, and the mean values were used for statistical analysis. A two-way repeated measures was run with the factors: Task (6 task conditions)  Phase (8 stimulation times). After comparing each condition with respect to each other, a comparison was done

Tasks

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between each condition and the averaged control value (e.g., anti-phase in sitting position versus control condition) and phase modulation within a task (e.g., phase 1 versus any of the other seven remaining time phases in anti-phase and control condition). When a significant overall difference was found between a given task and the control value, post hoc significance was tested with Bonferroni’s tests to assess in which phase this difference was significant (SigmaStat version 2.0, SPSS Sciences Software GmbH, Erkrath, Germany). The assumption of normality and homoscedasticity was verified for all the comparisons and the significance level alpha was set at 0.05. Then Pearson’s correlation and simple linear regression analysis were used for each condition to assess the correlation between Hreflex amplitude and EMG background activity.

amplitudes of this subject are depicted in Fig. 2B in two formats. One plot on the left was made to illustrate the proper positioning of the intensity used with respect to M-max, the other on the right to emphasize the reflex suppression with respect to the control condition. When expressed in terms of M-max, it is clear that the amplitude of the H-reflex was in the proper range (around 20–40% of the M-max) while the M-wave was stable for the different phases around 5% M-max (Knikou, 2008). When expressed in terms of the % of control values, arm movements caused an overall suppression of soleus H-reflexes throughout the arm movement cycle. Data such as shown for one subject in Fig. 2B right plot were obtained for the various conditions at different stimulation time phases, averaged for the whole population and used later in the figures with respect to control conditions (no arm movements) at a value of 100%.

RESULTS Kinematics

Phase modulation: soleus H-reflex

The kinematics data of the subjects are summarized in Table 1. The movement’s amplitude and period were on average similar for all arm and trunk movements (on average about 80° during 0.95 s). During stepping in place, arm movement and trunk rotation were negligible. The relative phase shift was on average 168.2° for antiphase and 3.8° for in-phase arm movements, and the delay between the high pitch sound and the maximum reached position (‘‘delay’’ in Table 1) was 0.12 s (i.e. around 10% overshoot with respect to the high pitch sound for a movement pace of 1 s). This suggests that the subjects followed the instructions.

The H-reflex modulation at different stimulation time phases were averaged for the whole population for each movement condition and illustrated in Fig. 3. A significant phase interaction was seen for contralateral (p = 0.001) and anti-phase arm movements (p = 0.006) and in the stepping condition (p < 0.001). It can also be seen that the most pronounced interaction and H-reflex modulation were in anti-phase arm movements and stepping movements. In anti-phase arm movements, H-reflex modulation showed a bell shape with maximum suppression at the beginning and end of the arm cycle when the ipsilateral right arm was in maximum extension and the contralateral left arm in maximum flexion (Fig. 3, left column, third panel). The least suppression occurred when the arms were in the opposite position. Indeed, soleus H-reflex suppression was lower during the middle phases (from 0.250 to 0.625 s) than for the phases at the beginning and end of the arm cycle. We have to acknowledge that not all subjects showed this clear bell shape modulation as on the average trace. Nonetheless, around half of the subjects showed a clear modulation similar to the typical trace in Fig. 2. A similar

Typical trace of soleus H-reflex modulation Soleus H-reflexes are shown for a typical subject in Fig. 2 performing an anti-phase arm movement while seated versus control value with arms stationary (Fig. 2A). While M-wave amplitude remained constant the Hreflex amplitude decreased at the transition phases (onset and end of the arm movement cycle) (Fig. 2B). The average peak-to-peak H-reflex and M-wave Table 1. Kinematics data

Ipsilateral Arm Flex/Ext Contralateral Arm Flex/Ext Anti-phase Arm Flex/Ext In-phase Arm Flex/Ext Trunk rotation Stepping in place Averages during Arm movements a b c

Arm Flex/ Ext Amplitude (°)

Trunk Rotation Amplitude (°)

Max Arm Flexion (°)

Max Arm Extensiona (°)

Movement period (s)

Relative phase shiftb (°)

Delayc (s)

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Mean

SD

80.9 80.1 74.0 79.4 – 2.9 78.6

13.2 7.2 8.3 7.6 – 1.7 3.2

6.5 6.9 9.3 3.2 79.8 11.8 6.5

5.7 3.4 8.9 1.1 12.9 3.9 2.5

70.9 70.6 67.2 71.3 – 1.8 70.0

11.4 10.9 10.0 10.3 – 2.4 1.9

10.0 9.5 6.8 8.1

8.6 8.0 5.7 9.2 – 3.9 1.4

0.92 0.94 0.93 0.94 0.95 – 0.93

0.01 0.01 0.01 0.01 0.01 – 0.01

– – 168.2 3.8 – – 86.02

– – 4.0 1.1 – – 116.23

0.11 0.13 0.14 0.10 0.02 0.15 0.12

0.10 0.11 0.11 0.09 0.07 0.16 0.02

– 1.1 8.6

Extension with respect to the vertical arm position. Relative phase shift between ipsi- and contralateral arms. Delay between the high pitch sound and the maximum reached position (see experimental procedures).

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Fig. 2. Typical trace of H-reflex and M-wave during anti-phase arm movements. (A) This figure shows the soleus reflexes at 0 s as compared to control reflexes for this condition (stippled lines). We can see the decrease in the soleus H-reflex amplitude while the M-wave is stable with respect to control. (B) This figure shows the average data of the soleus H-reflexes (open circles) and M-waves (black squares) as function of different stimulation phases (symbols are average of five trials for the anti-phase arm movements and vertical bars depict standards deviations). H-reflexes and M-waves amplitudes are depicted with respect to % M-max in the left panel and % control in the right panel. Soleus H-reflexes show a bellshaped modulation as a function of different phases while M-waves remain stable for all phases.

modulation was not seen for the in-phase arm movements although both arms were moving at similar amplitude and frequency. In stepping, the H-reflex during foot contact with the ground was higher (from 0 to 0.375 s) than during the phases after the middle of a stepping cycle (which corresponds to the aerial phase). Thus, the soleus Hreflex showed the largest suppression when the ipsilateral right leg was in flexion and the contralateral left leg was in extension and the least suppression when the legs were in the opposite position. Phase modulation: Soleus M-wave and EMG As it is known that reflex amplitudes depend heavily on the excitability of the corresponding motoneurons, we investigated the modulation of soleus M-wave and EMG background in detail. Soleus M-wave and EMG background were constant and did not show any significant phase modulation in sitting while a significant phase modulation was seen in stepping (Fig. 3). Note that four subjects had difficulties to control EMG background activity while performing trunk rotation and showed a significant decrease in EMG background activity during some phases of the trunk rotation. Therefore, these trials were removed from the current analysis to ensure that EMG activity level during trunk rotation was not different from those in the control trial. We then addressed the question whether the changes in H-reflexes were correlated with the EMG background.

Soleus H-reflex, M-wave and EMG for each task We then averaged the group results across all time phases for each condition in Fig. 4 and compared each condition with respect to each other and to control values. No significant differences were seen between different conditions in soleus H-reflex (p = 0.406) or M-wave (p = 0.548). EMG background was also not significantly different between conditions except for stepping which was higher in comparison to other conditions (p < 0.001). When compared with respect to control values, soleus H-reflex was significantly suppressed in all conditions (up to 63% in stepping, 66% in anti-phase arm movements and trunk rotation, 69% in ipsilateral, 76% in in-phase and 77% in contralateral movements, p < 0.001). Soleus M-waves did not differ from controls demonstrating that the stimulus conditions were not altered, and EMG background activity was similar to control values except during stepping (p = 0.037).

Correlation of soleus H-reflex with soleus EMG background Fig. 3 (right column) shows the correlation between soleus H-reflex and EMG background. A significant positive correlation was found between soleus H-reflex and EMG background during ipsilateral arm movements (r2 = 0.51, p = 0.047), but no significant correlation was found in anti-phase arm movements

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Fig. 3. Soleus H-reflex, M-wave, and EMG background activity as % of control values are depicted from left to right respectively as a function of a movement cycle period for arms and trunk movements and stepping. Vertical bars show standard deviations and horizontal lines depict 100% of control values. Black circles show where phase interaction was significant in H-reflexes. The right column shows the correlation between the soleus H-reflex versus soleus EMG background activity as % of control values. Linear regressions are fitted across the points for the different conditions. Stars show when the correlation was significant.

(r2 = 0.01, p = 0.868). Indeed, the EMG background did not show a bell-shaped modulation with respect to movement phases and was relatively constant, which means that the bell-shaped modulation in soleus Hreflex was not explained by similar modulation in background EMG.

Correlation of soleus H-reflex with tibialis anterior EMG background Another possible source of the modulation in soleus Hreflex seen during anti-phase arm movements could be caused by reciprocal inhibition from antagonist muscle

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Fig. 4. Total average of soleus H-reflex, M-wave, and EMG background activity as % of control values are depicted from left to right respectively for each condition averaged across all the eight phases for arm and trunk movements and stepping. X and Y axis labels are removed when they are the same as the bottom or left ones respectively. Vertical bars show standard deviations and horizontal dotted lines depict 100% of control values. Asterisks indicate significant differences with respect to 100% control values or significant linear correlation in the correlation column. Black symbols indicate where there are significant phase interactions with these dark symbols significantly different from the other light symbols.

activation (tibialis anterior). The EMG background activity of the tibialis anterior during anti-phase arm movements is depicted in Fig. 5 with respect to control values. No significant difference was found between the tibialis anterior activity at different movements phases and the 100% control value (107.96 ± 10.3%, p = 0.559). The phase interaction was also not significant (p = 0.620), indicating an absence of significant modulation in the tibialis anterior activity. In addition, no significant correlation was found between the soleus H-reflex and the tibialis anterior EMG background (r2 = 0.04, p = 0.619). This suggests that the modulation of the soleus H-reflex seen in anti-phase arm movements was not simply related to modulation in the tibialis anterior muscle. Comparison between individual unilateral and simultaneous bilateral arm movements The previous data showed that soleus H-reflex during bilateral anti-phase arm movements depicts a bellshaped modulation that was not correlated to soleus background EMG activity and this modulation was not

seen with in-phase movements. One would think that a part of this modulation could be explained by the simple combined effect of each arm. Indeed, maximum soleus H-reflex suppression was around maximum shoulder extension during ipsilateral arm swing and maximum shoulder flexion during contralateral arm swing (Fig. 3). Therefore, the modulation during bilateral anti-phase movement or lack of modulation during in-phase movements could simply be related to combined effects of both arms since the moment of maximum suppression would occur simultaneously during antiphase or on the contrary cancel each other out during in-phase movements. If this was the case, the modulation caused by the bilateral arm movements would be similar to the effects seen when the two single arm movements are averaged. If, on the other hand, the bell-shaped modulation during anti-phase movement is peculiar to the simultaneous alternating arm movements then one would expect that averaging the effect of both arm movements would yield a different pattern of Hreflex modulation. To evaluate the added value of moving the arms simultaneously we first compared the soleus H-reflex

Fig. 5. Tibialis anterior EMG background activity as % of control values is depicted as a function of a movement cycle period for anti phase arm movements (open circles). Vertical bars show standard deviations and horizontal dotted line depicts 100% of control values. The right panel shows the correlation between the soleus H-reflex and tibialis anterior EMG background activity both expressed as % of control values. Linear regression is fitted across the points.

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during bilateral anti-phase arm movements versus that expected if we averaged unilateral ipsi- and contralateral arm movements (Fig. 6, left column). To perform such analysis one has to order the data of the two unilateral conditions in such a way that when combined the resulting combined movement resembles the bilateral condition (upper panels). Flexion phases

1,2,3,and 4 for ipsilateral arm movement were respectively averaged with extension phases 1,2,3,4 for contralateral arm movement. Extension phases 5,6,7,8 for ipsilateral arm movement were respectively averaged with flexion phases 5,6,7,8 for the contralateral arm movement (left upper panel). The soleus H-reflex, resulting for the average of each arm

Fig. 6. Comparison of the numeric average of the soleus H-reflex depression as % of control arising from the individual unilateral flexion–extension of the ipsi- and contralateral arms versus that occurring during bilateral simultaneous anti-phase (left column) and in-phase flexion–extension of the arms (right column) across the eight phases of a movement cycle. Data of the H-reflex are taken from Fig. 3. Upper panels show the subject performing unilateral and contralateral arm movements (8 phases for each movement). It can be seen that in both traces there are periods of flexion (1, 2, 3, 4) and extension phases (5, 6, 7, 8) for ipsi- or contralateral side. To calculate the combined effect of flexion for instance we averaged the reflexes in these two periods phase by phase to obtain the averaged flexion effect. Middle panels compare the numeric average of the soleus Hreflex for unilateral ipsi- and contralateral arm movements from the upper panels (black circles) versus bilateral anti-phase and in-phase arm movements (open circles in left and right columns respectively). The shaded area in the middle panel represents the difference between the numeric average of the H-reflex during unilateral ipsi- and contralateral arm movements (black circles) and the H-reflex during bilateral anti-phase and inphase arm movements respectively (open circles). These differences are plotted for clarity in the lower panels. Asterisks indicate when these differences are statistically significant (Bonferroni’s post hoc test).

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segment moving separately (black symbols), showed a slight bell-shaped modulation with minimum at the beginning and end (left middle panel). However, antiphase arm movements (while symbols) showed a significantly larger depression of the H-reflex (65.58 ± 5.9 versus 73.06 ± 3.3% respectively, p = 0.049), and the bell shape during anti-phase movement was more marked especially at the beginning and end of the movement (i.e. around 10–15% depression difference as calculated in the left bottom panel). This means that a part of the bell shape could be due to the combined effect of each limb, but another part is related to the alternation as such. The same question was now addressed for the inphase arm movements (Fig. 6, right column). Here we shuffled the phase order in the ipsilateral arm movement so that extension phases (phases 5,6,7,8) were respectively averaged with extension phases (phases 1,2,3,4) for contralateral arm movement. Flexion phases 1,2,3,4 for ipsilateral arm movement were respectively averaged with flexion phases 5,6,7,8 for the contralateral arm movement (right upper panel). The numeric average of the soleus H-reflex during individual flexion–extension of the ipsi- and contralateral arms was 73.06 ± 2% which was not significantly different from inphase arm movements 75.84 ± 4.3% (p = 0.36) (right middle panel). Interestingly, the difference between the individual average and the in-phase movements at the right bottom panel was systematically negative. That means that in-phase movements induced less inhibition than what would have been predicted from the combined effect of unilateral arm movements, which clearly contrasted with what was seen in the anti-phase mode.

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maximum suppression when the contralateral arm is around maximum flexion while the ipsilateral arm somehow shows the opposite pattern (Hiraoka, 2001; Hiraoka and Iwata, 2006). Our findings support such non-specific inhibition but show that there is a high specificity in the phase-dependent modulation during alternated bilateral movements that reinforce the effects of individual arms. Interestingly, arm cycling movements also produced a non-specific reflex suppression during unilateral or bilateral movements, but the phasic modulation during anti-phase arm cycling also showed a bell-shaped profile with the greatest suppression around contralateral shoulder flexion and ipsilateral shoulder extension (Frigon et al., 2004; Loadman and Zehr, 2007; De Ruiter et al., 2010). This confirms that the underlying central neural control mechanisms are conserved across different rhythmic tasks despite different kinematics (Zehr et al., 2007). This also suggests that the alternated movement pattern is crucial and is likely controlled by neural circuits organized to move limbs in alternation such as CPG centers. Alternating arm swing and spinal processing

Our results showed that (1) soleus H-reflex was suppressed for all rhythmic arm movements, (2) a marked reflex modulation occurred during anti-phase arm swing, as was also the case during leg stepping, and this modulation flattened out during in-phase arm swing, (3) trunk rotation induced a major reflex suppression independent of arm or leg movements. This confirms a strong neural coupling between the arms and legs in humans especially during locomotor-like alternated movements.

Reflex changes during a task or a movement phase were used to probe a possible CPG contribution if this change was independent from the EMG background activity (Burke, 1999; Duysens et al., 2004; Zehr and Duysens, 2004). All rhythmic arm movements suppressed the soleus H-reflex although the stimulus intensity and the soleus EMG background were similar. However, only locomotor-like anti-phase arm movements induced a significant phase-dependent modulation independent from the soleus EMG background, which indicates a possible output from CPGs gating the leg reflex activity. Reciprocal inhibition from the tibialis anterior muscle was also unlikely to be the source of the bell-shape modulation as demonstrated in Fig. 5. However, further research is required to better assess reciprocal inhibition as investigated by previous studies (Petersen et al., 1999; Zehr and Stein, 1999; Pierrot-Deseilligny and Burke, 2005). Another potential candidate for the modulation is the cortex, i.e. the supplementary motor area or pre-motor cortex which is known to be involved in coordinating upper and lower limbs (Debaere et al., 2001; Byblow et al., 2007).

Arm swing in alternation is meaningful

Significance of arm swing in alternation

Although all arm movements suppressed the H-reflexes, we found a clear difference between anti-phase and inphase arm movements. Moving both arms in an alternated manner magnified the individual effects of each arm on the reflex modulation whereas moving them in-phase was detrimental to this modulation as both arms canceled each other out. Previous studies showed that during rhythmic arm swing, the soleus H-reflex is indeed depressed regardless of the movements (unilateral or bilateral) (Knikou, 2007). Other studies pointed out that soleus Hreflex is modulated during unilateral arm swing with

The reflex depression arising from averaging unilateral arm movements was less than that arising from both arms moving alternately. Therefore, there appears to be some kind of ‘‘over-insurance’’ against inadvertent expression of the reflex when the arms move together alternately. This over-insurance is most prominent when the legs would usually be at the critical transition between swing and stance phases with the right arm in extension and the left arm in flexion at right heel-strike. This is meaningful since a sudden plantar flexion would interfere with the normal landing. Indeed, hindering the arm movements increases the soleus H-reflex during

DISCUSSION

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walking predominantly at heel-strike (Krauss and Misiaszek, 2007). Furthermore, this suppression of Hreflexes coincides with the period when subjects are least sensitive to foot stimulation (Duysens et al., 1995) and show suppression of foot cutaneous reflexes (Duysens et al., 1990; Yang and Stein, 1990). Nonetheless, the potential role of the anti-phase arm movements may not be limited to heel-strike but throughout a walking cycle. Indeed, soleus H-reflex during walking increases from heel strike to late stance phase and decreases during swing phase (Capaday and Stein, 1987; Simonsen and Dyhre-Poulsen, 1999; Andersen and Sinkjaer, 1999). The ipsilateral arm moves forward with the contralateral arm moving backward during the stance phase and the opposite arm movement occurs during swing phase (Ballesteros et al., 1965; Hinrichs, 1990). The reflex increasing during stance and decreasing during swing would therefore follow the bell-shaped modulation. Although a special coupling between upper and lower limbs occurs during alternated arm movements, it remains unclear if this coupling is functionally very important as humans can walk without any arm movements. However, it is reasonable to think that this special coupling with the legs stationary would still hold – or even get magnified – during walking. Indeed, previous studies pointed out that interlimb reflex responses showed a task-dependent neural coupling that can become gated by the activity of the CPG’s during walking (Dietz et al., 2001, Dietz, 2002). Arm and leg alternations act in concert to modulate leg motor output Soleus H-reflex modulation during stepping showed an increase during the stance phase and suppression during the aerial phase, which is similar to previous studies (Crenna and Frigo, 1987). The most suppression occurred around ipsilateral leg flexion and contralateral leg extension. This is opposite to what occurred during anti-phase arm movements. That means if the arm alternation (distant movement) crisscrosses with leg alternation (local movement) so that the ipsilateral arm is in extension (maximum inhibition) when the ipsilateral leg is in flexion (maximum inhibition), the arm alternation would assist the leg alternation. This means that anti-phase movements of arms and legs may have an added advantage. In this line, previous studies showed that leg rhythmic movements can also affect arm reflexes (Dietz et al., 2001, Dietz, 2002). Other studies provided evidence for this interlimb coupling by assessing the effect of separate or combined arms and leg rhythmic movements on neuronal excitability of a stationary leg (Mezzarane et al., 2011). This further emphasizes an interlimb coupling between cervical and lumbar circuits in gating the excitability of reflex pathways to a leg muscle likely through long propriospinal neurons that connect cervical and lumbar enlargements (Nathan et al., 1996). In mammalian species, experiments have indeed shown the existence of propriospinal coupling between lumbar and cervical generators. Activity in one

generator is influenced by activity in the other, thus mediating interlimb coordination (Juvin et al., 2005). Trunk movement affects leg motor output Although major changes have occurred from undulatory to quadrupedal and bipedal locomotion, a similar basic organization of axial neuronal networks that control trunk muscles has been found across the vertebrate range (Falgairolle et al., 2006). It was thus questioned whether this movement may also determine leg motor output or is merely a by-product of gait mechanics. Intriguingly, trunk rotation produced nearly as much suppression as did anti-phase movement or stepping. The suppression was likely due to a presynaptic inhibition as EMG background was not significantly changed (Stein, 1995). This indicates that human walking may have a basic reptilian origin when limbs had not yet evolved (Grillner, 2011). It also echoes recent findings suggesting that rhythmic arm movement such as arm cycling would not significantly depress the soleus H-reflex if the head and trunk are immobilized (Hiraoka and Taniguchi, 2010). Limitations of the study While the aim of the paper was to assess the effect of the locomotor-like alternated movement pattern, the movement amplitudes were greater than the ones that usually occur during walking (e.g. anti-phase arm movements were 75° versus around 40° in normal walking, Hinrichs, 1990). We used greater amplitude to compare with previous studies that assessed the effect of arm movements or posture on soleus H-reflexes (Delwaide et al., 1973, 1977; Eke-Okoro, 1994; Frigon et al., 2004; Knikou 2007). However, we suspect that the modulation we saw may still hold at lower amplitudes because previous studies showed that soleus H-reflexes facilitation and inhibition are maximal at 45° arm posture, and decrease at greater values (Delwaide et al., 1977). Finally our study may have implication in walking rehabilitation as it emphasizes the importance of alternated arm movements to gate leg motor output and the use of trunk rotation to affect leg output in patients where arms are impaired.

CONCLUSIONS The present results revealed a special neural coupling between arm and legs when arm movements are performed in a locomotor-like alternated pattern which seems to highlight the activity of possible neural circuits organized to move limbs rhythmically in alternation such as CPG centers. Trunk rotation had also a major effect on soleus H-reflexes. This indicates that not only are the legs important in bipedal humans but also the alternated arm and trunk movements.

AUTHOR CONTRIBUTIONS Conception and design of the experiments: Firas Massaad, Oron Levin, Pieter Meyns, Jacques Duysens.

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Collection, analysis and interpretation of data: Firas Massaad, Oron Levin, Pieter Meyns, Jacques Duysens. Drafting the article or revising it critically for important intellectual content: Firas Massaad, Oron Levin, Pieter Meyns, David Drijkoningen, Stephan P. Swinnen, Jacques Duysens. All authors approved the final version of the manuscript.

CONFLICT OF INTEREST The authors declared no conflicts of interest with respect to the authorship and/or publication of this article. Acknowledgments—This project was supported by a grant from ‘bijzonder onderzoeksfonds’ Katholieke Universiteit Leuven, Belgium (OT/08/034). F.M. was supported by a Postdoctoral fellowship of the Research Foundation – Flanders (FWO), a Krediet aan Navorsers from the FWO (1.5.129.13N), an F+ fellowship from KU Leuven and a scientific prize of the Foundation Van Goethem-Brichant, Belgium. J.D. was supported by FWO grant (G.0901.11).

REFERENCES Andersen JB, Sinkjaer T (1999) The stretch reflex and H-reflex of the human soleus muscle during walking. Motor Control 3:151–157. Aristotle (350 BC) On the gait of animals [Translated by A.S.L. Farquharson (2004)]. UK: Kessinger Publishing. Ballesteros ML, Buchthal F, Rosenfalck P (1965) The pattern of muscular activity during the arm swing of natural walking. Acta Physiol Scand 63:296–310. Bruijn SM, Meijer OG, Beek PJ, van Diee¨n JH (2010) The effects of arm swing on human gait stability. J Exp Biol 213:3945–3952. Bruijn SM, Meijer OG, van Diee¨n JH, Kingma I, Lamoth CJ (2008) Coordination of leg swing, thorax rotations, and pelvis rotations during gait: the organisation of total body angular momentum. Gait Posture 27:455–462. Burke RE (1999) The use of state-dependent modulation of spinal reflexes as a tool to investigate the organization of spinal interneurons. Exp Brain Res 128:263–277. Byblow WD, Coxon JP, Stinear CM, Fleming MK, Williams G, Mu¨ller JF, Ziemann U (2007) Functional connectivity between secondary and primary motor areas underlying hand–foot coordination. J Neurophysiol 98:414–422. Capaday C, Stein RB (1987) Excitability of the soleus H-reflex arc during walking and stepping in man. Exp Brain Res 66:49–60. Crenna P, Frigo C (1987) Difference in the amplitude of the human soleus H reflex during walking and running. J Physiol (Lond) 392:513–522. De Ruiter GC, Hundza SR, Zehr EP (2010) Phase-dependent modulation of soleus H-reflex amplitude induced by rhythmic arm cycling. Neurosci Lett 475:7–11. Debaere F, Swinnen SP, Be´atse E, Sunaert S, Van Hecke P, Duysens J (2001) Brain areas involved in interlimb coordination: a distributed network. Neuroimage 14:947–958. Delwaide PJ, Figiel C, Richelle C (1973) Influence de la position du membre superieur sur l’excitabilite de l’arc soleaire. Electromyogr Clin Neurophysiol 13:515–523. Delwaide PJ, Figiel C, Richelle C (1977) Effects of postural changes of the upper limb on reflex transmission in the lower limb. Cervicolumbar reflex interactions in man. J Neurol Neurosurg Psychiatry 40:616–621. Dietz V (2002) Do human bipeds use quadrupedal coordination? Trends Neurosci 25:462–467.

45

Dietz V, Fouad K, Bastiaanse CM (2001) Neuronal coordination of arm and leg movements during human locomotion. Eur J Neurosci 14:1906–1914. Donker SF, Beek PJ, Wagenaar RC, Mulder T (2001) Coordination between arm and leg movements during locomotion. J Mot Behav 33:86–102. Donker SF, Daffertshofer A, Beek PJ (2005) Effects of velocity and limb loading on the coordination between limb movements during walking. J Mot Behav 37:217–230. Duysens J, Tax AA, Nawijn S, Berger W, Prokop T, Altenmu¨ller E (1995) Gating of sensation and evoked potentials following foot stimulation during human gait. Exp Brain Res 105:423–431. Duysens J, Trippel M, Horstmann GA, Dietz V (1990) Gating and reversal of reflexes in ankle muscles during human walking. Exp Brain Res 82:351–358. Duysens J, Bastiaanse CM, Smits-Engelsman BC, Dietz V (2004) Gait acts as a gate for reflexes from the foot. Can J Physiol Pharmacol 82:715–722. Eke-Okoro ST (1994) Evidence of interaction between human lumbosacral and cervical neural networks during gait. Electromyogr Clin Neurophysiol 34:345–349. Elftman H (1939) The function of arms in walking. Hum Biol 11:529–535. Falgairolle M, de Seze M, Juvin L, Morin D, Cazalets JR (2006) Coordinated network functioning in the spinal cord: an evolutionary perspective. J Physiol (Paris) 100:304–316. Frigon A, Collins DF, Zehr EP (2004) Effect of rhythmic arm movement on reflexes in the legs: modulation of soleus H-reflexes and somatosensory conditioning. J Neurophysiol 91:1516–1523. Gerdy PN (1829) Memoires sur le mecanisme de la marche le l’homme. J Pysiol Exp Path 9:1–28. Grillner S (1975) Locomotion in vertebrates: central mechanisms and reflex interaction. Physiol Rev 55:247–304. Grillner S (2011) Human locomotor circuits conform. Science 334:912–913. Hinrichs R (1990) Whole body movement: coordination of arms and legs in walking and running. In: Winters J, Woo SLY, editors. Multiple muscle systems: biomechanics and movement organization. New York: Springer-Verlag. p. 694–705. Hiraoka K (2001) Phase-dependent modulation of the soleus H-reflex during rhythmical arm swing in humans. Electromyogr Clin Neurophysiol 41:43–47. Hiraoka K, Iwata A (2006) Cyclic modulation of H-reflex depression in ipsilateral and contralateral soleus muscles during rhythmic arm swing. Somatosens Mot Res 23:127–133. Hiraoka K, Taniguchi Y (2010) The effect of immobilization of the head and trunk on arm-cycling-induced depression of soleus motoneuron pool excitability. Somatosens Mot Res 27:28–33. Huang Y, Meijer OG, Lin J, Bruijn SM, Wu W, Lin X, Hu H, Huang C, Shi L, van Diee¨n JH (2010) The effects of stride length and stride frequency on trunk coordination in human walking. Gait Posture 31:444–449. Jackson KM (1983) Why the upper limbs move during human walking. J Theor Biol 105:311–315. Juvin L, Le Gal JP, Simmers J, Morin D (2012) Cervicolumbar coordination in mammalian quadrupedal locomotion: role of spinal thoracic circuitry and limb sensory inputs. J Neurosci 32:953–965. Juvin L, Simmers J, Morin D (2005) Propriospinal circuitry underlying interlimb coordination in mammalian quadrupedal locomotion. J Neurosci 25:6025–6035. Knikou M (2007) Neural coupling between the upper and lower limbs in humans. Neurosci Lett 416:138–143. Knikou M (2008) The H-reflex as a probe: pathways and pitfalls. J Neurosci Methods 171:1–12. Knikou M, Hajela N, Mummidisetty CK, Xiao M, Smith AC (2011) Soleus H-reflex phase-dependent modulation is preserved during stepping within a robotic exoskeleton. Clin Neurophysiol 122:1396–1404. Krauss EM, Misiaszek JE (2007) Phase-specific modulation of the soleus H-reflex as a function of threat to stability during walking. Exp Brain Res 181:665–672.

46

F. Massaad et al. / Neuroscience 258 (2014) 34–46

Kurz MJ, Stergiou N (2004) Applied dynamic systems theory for the analysis of movement. In: Stergiou N, editor. Innovative analyses of human movement. Champaign, Illinois: Human Kinetics. Loadman PM, Zehr EP (2007) Rhythmic arm cycling produces a nonspecific signal that suppresses soleus H-reflex amplitude in stationary legs. Exp Brain Res 179:199–208. Mezzarane RA, Klimstra M, Lewis A, Hundza SR, Zehr EP (2011) Interlimb coupling from the arms to legs is differentially specified for populations of motor units comprising the compound H-reflex during ‘‘reduced’’ human locomotion. Exp Brain Res 208:157–168. Misiaszek JE (2003) The H-reflex as a tool in neurophysiology: its limitations and uses in understanding nervous system function. Muscle Nerve 28:144–160. Nathan PW, Smith M, Deacon P (1996) Vestibulospinal, reticulospinal and descending propriospinal nerve fibres in man. Brain 119:1809–1833. Nielsen J, Kagamihara Y (1992) The regulation of disynaptic reciprocal Ia inhibition during co-contraction of antagonistic muscles in man. J Physiol 456:373–391. Nielsen J, Kagamihara Y (1993) The regulation of presynaptic inhibition during co-contraction of antagonistic muscles in man. J Physiol 464:575–593. Ortega JD, Fehlman LA, Farley CT (2008) Effects of aging and arm swing on the metabolic cost of stability in human walking. J Biomech 41:3303–3308. Perotto AO (1994) Anatomical guide for the electromyographer. The limbs and trunk. Springfield: Charles C. Thomas. Petersen N, Morita H, Nielsen J (1999) Modulation of reciprocal inhibition between ankle extensors and flexors during walking in man. J Physiol 520:605–619. Pierrot-Deseilligny E, Burke D (2005). The circuitry of the human spinal cord. Its role in motor control and movement disorders, vol. 1. New York: Cambridge Univ. Press.

Pierrot-Deseilligny E, Mazevet D (2000) The monosynaptic reflex: a tool to investigate motor control in humans. Interest and limits. Neurophysiol Clin 30:67–80. Pontzer H, Holloway 4th JH, Raichlen DA, Lieberman DE (2009) Control and function of arm swing in human walking and running. J Exp Biol 212:523–534. Shik ML, Orlovsky GN (1976) Neurophysiology of locomotor automatism. Physiol Rev 56:465–501. Simonsen EB, Dyhre-Poulsen P (1999) Amplitude of the human soleus H reflex during walking and running. J Physiol (Lond) 515:929–939. Stein RB (1995) Presynaptic inhibition in humans. Prog Neurobiol 47:533–544. Stein RB, Thompson AK (2006) Muscle reflexes in motion: how, what, and why? Exerc Sport Sci Rev 34:145–153. Swinnen SP (2002) Intermanual coordination: from behavioural principles to neural-network interactions. Nat Rev Neurosci 3:348–359. Tucker KJ, Tuncer M, Tu¨rker KS (2005) A review of the H-reflex and M-wave in the human triceps surae. Hum Mov Sci 24:667–688. Winter DA (2005) Biomechanics and motor control of human movement. 3rd ed. New Jersey: John Wiley & Sons. Yang JF, Stein RB (1990) Phase-dependent reflex reversal in human leg muscles during walking. J Neurophysiol 63:1109–1117. Zehr EP (2002) Considerations for use of the Hoffmann reflex in exercise studies. Eur J Appl Physiol 86:455–468. Zehr EP, Balter JE, Ferris DP, Hundza SR, Loadman PM, Stoloff RH (2007) Neural regulation of rhythmic arm and leg movement is conserved across human locomotor tasks. J Physiol (Lond) 582:209–227. Zehr EP, Duysens J (2004) Regulation of arm and leg movement during human locomotion. Neuroscientist 10:347–361. Zehr EP, Stein RB (1999) Interaction of the Jendra´ssik maneuver with segmental presynaptic inhibition. Exp Brain Res 124:474–480.

(Accepted 4 October 2013) (Available online 18 October 2013)