Journal of Neurophysiology

Robotic-assisted stepping modulates monosynaptic reflexes in forearm muscles in the human

Tsuyoshi Nakajima, Taku Kitamura, Kiyotaka Kamibayashi, Tomoyoshi Komiyama, E. Paul Zehr, Sandra R. Hundza, Kimitaka Nakazawa

Abstract

Although the amplitude of the Hoffmann (H)-reflex in the forelimb muscles is known to be suppressed during rhythmic leg movement, it is unknown which factor plays a more important role in generating this suppression—movement-related afferent feedback or feedback related to body loading. To specifically explore the movement- and load-related afferent feedback, we investigated the modulation of the H-reflex in the flexor carpi radialis (FCR) muscle during robotic-assisted passive leg stepping. Passive stepping and standing were performed using a robotic gait-trainer system (Lokomat). The H-reflex in the FCR, elicited by electrical stimulation to the median nerve, was recorded at 10 different phases of the stepping cycle, as well as during quiet standing. We confirmed that the magnitude of the FCR H-reflex was suppressed significantly during passive stepping compared with during standing. The suppressive effect on the FCR H-reflex amplitude was seen at all phases of stepping, irrespective of whether the stepping was conducted with body weight loaded or unloaded. These results suggest that movement-related afferent feedback, rather than load-related afferent feedback, plays an important role in suppressing the FCR H-reflex amplitude.

  • flexor carpi radialis
  • Hoffmann reflex
  • soleus muscle
  • passive stepping
  • stepping-related feedback

during rhythmic arm cycling, Hoffmann (H)-reflex amplitude in the soleus muscle (Sol) is strongly suppressed in humans (Frigon et al. 2004; Hundza and Zehr 2009; Loadman and Zehr 2007). Interestingly, leg cycling also leads to suppression of the H-reflex amplitude in the forearm muscles (Zehr et al. 2007). These results suggest a reciprocally organized pattern-generating system activated by descending locomotor commands and afferent feedback that modulates reflex excitability in remote muscles (Zehr and Duysens 2004; Zehr et al. 2009). Currently, though, the neural mechanisms producing this organization remain unclear.

Loadman and Zehr (2007) suggested that central commands for rhythmic arm cycling were a major source of modulation, because differences in the range of arm motion (i.e., range of muscle-length change) did not alter H-reflex amplitude in stationary leg muscles. A central source for the modulation is also suggested by the observation that active arm cycling induces phase-dependent modulation of the Sol H-reflex (de Ruiter et al. 2010). In addition, Hundza et al. (2008) suggested that suppression of the Sol H-reflex during passive arm cycling was small compared with during active arm cycling. These reports suggest that central drive for rhythmic arm movement is important in suppressing H-reflexes in the ankle extensor muscles. Considerably less is known about the modulation of H-reflexes in arm muscles during passive leg movement, and the underlying mechanisms require further clarification. Importantly, the extent to which modulation of reflex excitability within a limb muscle can be fully accounted for by central drive or different sources of afferent feedback is unclear. Answering this means determining the relative contributions of movement-related afferent feedback, load-related bias, and central motor commands in the amplitude modulation of H-reflexes in forearm muscles.

Phase-dependent modulation of H-reflex amplitude in arm muscles during rhythmic leg movement remains an uncertain area. Phasic modulation of the forearm flexor H-reflex was seen with isolated, rhythmic foot movement (Baldissera et al. 1998) but not with rhythmic leg cycling (Zehr et al. 2007). Currently, there are no comparable data from arm muscles during walking-based driven gait orthosis (DGO) stepping and passive movement of the leg. Since passive DGO leg movement resembles “passive” cycling as a whole-leg rhythmic locomotor movement, we hypothesized that the passive stepping would suppress H-reflex amplitudes in the forearm, irrespective of body loading and without phase-dependent modulation.

We demonstrated previously that phase-dependent, cutaneous reflex modulation was absent in leg muscles during unloaded stepping, during passive stepping, suggesting that load-related afferents were important for generating phasic modulation of the cutaneous reflex in leg muscles (Nakajima et al. 2008). However, a similar approach did not affect Sol H-reflex modulation, suggesting that the effect of loading during passive stepping strongly depends on excitability in reflex circuitry (Kamibayashi et al. 2010). This parallels the earlier suggestion of Zehr et al. (2001), showing a differential modulation of cutaneous and H-reflexes in leg muscles depending on motor output and loading during leg cycling. However, it is not known whether lumbar load-related afferent feedback modulates the excitability of monosynaptic H-reflex circuits in the cervical spinal cord. We hypothesized that although we can expect a modulation in H-reflex amplitude in the remote limb, loading should have only a small effect on phase-modulation during DGO passive stepping. Absence of a modulatory effect of load-related afferent feedback on H-reflex excitability would implicate movement-related afferents in contributing to the pattern of H-reflex modulation observed during passive leg movement.

MATERIALS AND METHODS

Subjects

Participants were 16 healthy males, aged 22–32 yrs. All gave informed, written consent prior to participation in the experiments. The protocol was approved by the local ethics committee of the National Rehabilitation Center for Persons with Disabilities (Saitama, Japan) and is in accordance with the guidelines set out in the Declaration of Helsinki (1964).

General Procedure

The recently developed DGO system, Lokomat (Hocoma AG, Volketswil, Switzerland), was used to produce “passive stepping”, defined as stepping movements driven by the DGO system while relaxing the leg muscles. In this paradigm, there may be some very low-level, incidental muscle activation, but it is involuntary (see Kamibayashi et al. 2009). A detailed description of the Lokomat can be found elsewhere (Colombo et al. 2001). Briefly, this system consists of a treadmill, a body-weight support system, and two robotic actuators that are attached to each subject's legs. The Lokomat is fully programmable, including the control of knee and hip kinematic trajectories during different types of stepping, with and without body-weight loading.

The right forearm, wrist, and hand were fixed to a rigid platform to minimize any unwanted movement of the arm. A brace was worn to restrict arm movement and was fixed on the elbow and wrist positions at 90° and 0°, respectively, in all experiments. All trials were performed when the flexor carpi radialis (FCR) muscle was quiescent. Unloading of body weight was accomplished by suspending the subject's body with a harness connected to an overhead crane. For all passive stepping conditions, the treadmill speed was kept constant at 2.0 km/h for all subjects. During passive stepping, the subjects were instructed to relax and allow the lower-limb movements to be imposed by the DGO. Dorsiflexion of the ankle joint during the stepping condition was achieved by passive foot lifters (spring-assisted elastic straps) to prevent foot drop at the swing phase (see Kamibayashi et al. 2010).

Experimental Tasks

To explore the effect of passive leg stepping on the upper arm H-reflex amplitude, subjects participated in three experiments using the Lokomat systems: 1) phase-modulation of the FCR H-reflex during passive stepping (n = 10); 2) determination of the H-reflex and muscle response (M-wave; H-M) recruitment curves during passive stepping (n = 8; all participated in experiment 1); and 3) effects of loading on the FCR H-reflex during passive stepping (n = 10). Of these last 10, four participated in one or both of the other experiments, and six participated only in this experiment. When a given subject participated in two or three experiments, data from each experiment were collected on different days. Based on a previous study (Javan and Zehr 2008), the intervening time interval is sufficient for residual suppression to disappear; thus our results across experiments were not affected by residual suppression. During experiments to investigate phase-modulation and the effect of loading on FCR H-reflex amplitudes, experimental conditions (phases or load conditions) were pseudo-randomly executed for each subject.

Effect of stepping-related afferent feedback on FCR H-reflex.

In 10 subjects, the effect of leg stepping-related afferent feedback on the FCR H-reflex amplitude was investigated during robotic-assisted passive stepping and standing conditions (40% unloading of body weight). The subjects performed passive stepping on the treadmill with the arms at rest. Electrical stimulations to elicit the H-reflex were pseudo-randomly delivered at 10 different phases of the stepping tasks (see FCR H-reflexes below).

Effect of load-related afferent feedback on FCR H-reflex during DGO stepping.

To investigate the effect of load-related afferent feedback on the FCR H-reflex during passive stepping, 10 subjects performed passive stepping and standing in separate trials under two conditions: 1) unloaded condition (100% of body-weight support) above the treadmill belt and 2) loaded condition (40% unloading of body weight) on the treadmill (Kamibayashi et al. 2009, 2010). Reflexes were evoked at the midstance phase and during standing, with and without loading of body weight for all trials.

FCR H-Reflexes

FCR H-reflexes in the right arm were evoked by stimulating (rectangular pulse, 0.5-ms duration) the median nerve with a constant current electrical stimulator (SEN-7023, Nihon Kohden, Tokyo, Japan). Bipolar stimulus electrodes were placed just proximal to the medial epicondyle of the humerus, near the cubital fossa (cf. Zehr et al. 2007). To examine the effect of leg position on FCR H-reflex in the passive stepping, the step-cycle duration was set at ∼2.0 s by the Lokomat system and divided into 10 phases. The step duration was chosen to ensure subjects' safety and effectively avoid unintentional electromyographic (EMG) activity of the leg muscles. The electrical stimulations were pseudo-randomly delivered at various times during the stepping tasks (0, 200, 400, 600, 800, 1,000, 1,200, 1,400, 1,600, and 1,800 ms after the trigger signal), defined by predetermined hip-joint angles. The trigger signal was made by hip angles coincident with the timing of heel contact. These stimuli were delivered randomly once every two to three step cycles. The stimulus intensity was adjusted to ∼10% of the maximal amplitude of the direct motor response (Mmax) in each phase (cf. Kamibayashi et al. 2010; Simonsen and Dyhre-Poulsen 1999). These stimulation intensities were confined to H-reflex amplitudes evoked on the ascending limb of the recruitment curve of the H-reflex. The consistency of the test stimulus was confirmed by examining the shape and peak-to-peak amplitude of the M-wave. As controls, Mmax (n = 5 sweeps) and H-reflex (n = 12 sweeps) amplitudes were measured during standing and at each of the 10 stepping phases.

In additional control experiments, the H-M recruitment curves were recorded in eight subjects during quiet standing and at the stance and swing phases of passive stepping. The stimulus intensity was increased gradually from below the threshold of the H-reflex to supramaximum stimulation of the M-wave. Five responses were recorded at each of the stimulus intensities.

To investigate the effect of load-related afferent feedback on the FCR H-reflex amplitude during passive stepping, recruitment curves were also recorded during standing and at the stance phase of passive stepping, with and without loading of body weight in 10 subjects. All trials were performed when FCR was quiescent.

EMG Recording

EMG activity was recorded from the FCR, extensor carpi radialis (ECR), rectus femoris (RF), biceps femoris (BF), tibialis anterior (TA), and Sol muscles on the right side. EMG signals were obtained with surface electrodes (SS-2096, Nihon Kohden) over the belly of each muscle after reducing skin impedance (below 10 kΩ) by light abrasion and alcohol cleaning. All EMG signals were amplified (×1,000) and band-pass filtered between 15 Hz and 3 kHz via a bioamplifier system (MEG-6108, Nihon Kohden). All EMG and angular signals were converted to digital data with an analog-to-digital converter card [Micro1401, Cambridge Electronic Design (CED), Cambridge, UK] and stored on a hard disk with a sampling rate of 5 kHz using Spike2 software (CED).

Data Analysis

Peak-to-peak amplitudes of M-waves and H-reflexes were normalized to the respective Mmax amplitudes recorded during standing and at each phase of stepping.

Analysis of H-M recruitment curve.

H-reflex amplitudes from the standing control curves were compared with those from the same values induced by electrical stimulation on the curves conditioned by stepping at midswing and midstance (Klimsta and Zehr 2008; Mezzarane et al. 2011; Zehr et al. 2007). Stimulus intensities for eliciting the maximum H-reflex (Hmax) and ∼50% Hmax values were defined from the recruitment curves obtained during the standing control. For the H-M recruitment curve, means and SDs were calculated and plotted with respect to the intensity of electrical stimulation (recruitment curve; see Figs. 5 and 7).

Reflex amplitudes of the standing, midswing, and midstance phases of stepping, obtained by two stimulus strengths for eliciting ∼100% and ∼50% of the Hmax at control, were compared using two-way repeated measures (RM) ANOVA (three conditions × two stimulus intensities). Two-way RM ANOVA was also used to examine modulation of the H-reflex amplitudes, with and without loading at the stance phase of passive stepping and during standing (two load conditions × two tasks).

Analysis of the FCR H-reflex amplitude at the 10 stepping phases and during standing.

The H-reflex, M-wave in the FCR and background (BG) EMG activities in the FCR, ECR, RF, BF, Sol, and TA were compared using a one-way RM ANOVA [factors; standing control and passive stepping (10 phases of step cycle)]. The BG EMG activity was calculated as the root mean square value of the EMG signal for 50 ms before the electrical stimulation. Multiple comparisons were performed using the Bonferroni post hoc test. The data were expressed as means ± SEM. Significant differences were recognized at P < 0.05 in all cases. All statistical tests were performed using SPSS software version 11.0 (SPSS, Chicago, IL). The F values and degrees of freedom were obtained after Greenhouse-Geisser correction when appropriate.

RESULTS

EMG and Kinematic Patterns during Passive Stepping

Figure 1 shows typical recordings of EMG activities and joint angles during passive stepping (40% unloading of body weight) for a single subject. Because the hip- and knee-joint trajectories were controlled by the robotic-assisted DGO system, joint movements were highly reproducible. Also, the trajectory of the ankle joint was modulated during passive stepping. EMG activities of the FCR, ECR, BF, and TA were quiescent during passive stepping, whereas those of the Sol and RF EMG activities were slightly visible in several stepping phases of this subject.

Fig. 1.

Typical averaged recordings of joint angle and electromyographic (EMG) activity in the flexor carpi radialis (FCR), extensor carpi radialis (ECR), biceps femoris (BF), rectus femoris (RF), soleus (Sol), and tibialis anterior (TA) muscles obtained from a single subject during driven gait orthosis (DGO) passive stepping. EMG data were full-wave rectified and averaged (12 sweeps). Tracings of the motion of the lower limbare, which were synchronized to the stepping cycle and the joint and EMG data, are shown at the top of the figure.

Figure 2 illustrates the group means of the BG EMG activities obtained from 10 subjects during passive stepping and static conditions. Although the amplitudes of the FCR, ECR, BF, RF, and TA did not change and did not differ significantly across phases and standing conditions [one-way ANOVA: FCR, F(10,90) = 1.298, P > 0.05; ECR, F(10,90) = 1.119, P > 0.05; BF, F(10,90) = 1.179, P > 0.05; RF, F(10,90) = 0.639, P > 0.05; TA, F(10,90) = 0.708, P > 0.05], the mean amplitude of the Sol EMG was significantly larger at phase 5 than during other phases [one-way ANOVA: Sol, F(10,90) = 3.055, P < 0.05; Bonfferoni post hoc: phases 2, 7, 8, 9, and 10, P < 0.05]. However, there was no significant difference between standing and phase 5 (Bonfferoni post hoc, P > 0.05).

Fig. 2.

Grand means (±SEM) of the amplitudes of background EMG activity in the FCR, ECR, BF, RF, Sol, and TA muscles obtained from 10 subjects during standing (open circles) and DGO passive stepping (10 phases; filled circles) conditions (40% unloading of body weight). *P < 0.05; **P < 0.01.

Modulation of FCR H-Reflex Amplitude during Passive Stepping

Figure 3 shows representative recordings of the FCR H-reflex during standing and at different phases of passive stepping obtained from a single subject. The amplitude of the FCR H-reflex was suppressed strongly during stepping compared with the standing condition. The suppressive effect on the FCR H-reflex amplitude was seen at all phases of stepping, with little difference based on phase. Figure 4, A and B, illustrates pooled data for the amplitudes of the FCR H-reflex and M-wave, respectively, obtained from 10 subjects during passive stepping and static standing. Although the amplitudes of the M-wave and BG EMG activities did not differ significantly across phases [compare Fig. 4B with Fig. 2A; one-way ANOVA: M-wave, F(10,90) = 0.621, P > 0.05], the mean amplitude of the FCR H-reflex during passive stepping was significantly smaller than those during standing (Fig. 4A; Bonferroni test, P < 0.001). The one-way RM ANOVA of H-reflex amplitudes revealed a significant main effect for conditions [F(10,90) = 13.152, P < 0.001]. However, there was no significant difference in H-reflex amplitudes across the stepping phases (Bonferroni test, P > 0.05). Mmax amplitude did not change significantly during the static standing condition and the 10 stepping phases [one-way ANOVA: F(10,90) = 0.26, P > 0.05].

Fig. 3.

Typical superimposed recordings (12 sweeps) of FCR Hoffmann (H)-reflex waveforms (H) with a muscle response (M-wave; M) size of ∼10% maximal amplitude of the direct motor response (Mmax) at standing and at 10 phases of DGO passive stepping in 1 subject. Thick, vertical solid and dashed lines indicate stance and swing phases, respectively.

Fig. 4.

Grand means (±SEM) of the magnitude of the H-reflex (A) and M-wave (B) in the FCR muscle obtained from 10 subjects during standing (open circles) and DGO passive stepping (10 phases; filled circles) conditions (40% unloading of body weight). *P < 0.01.

Figure 5 shows the H-M recruitment curves during standing and at the stance and swing phases of passive stepping obtained from a single subject. It is notable that passive stepping reduced H-reflex amplitudes in the FCR across a wide range of stimulus strengths. Similar results were obtained from eight subjects. Interestingly, the extent of the H-reflex suppression did not depend on the size of the control H-reflex. Figure 6 illustrates the group means of the H-reflex amplitude (n = 8) obtained with two different stimulus strengths (i.e., for eliciting the Hmax amplitude and ∼50% of Hmax during the standing condition). There was a significant suppression of H-reflex amplitude during both the stance and swing phases (Bonferroni test, P < 0.05). The two-way RM ANOVA showed a significant main effect and interaction [condition: F(2,14) = 21.008, P < 0.001; stimulus intensity: F(1,7) = 31.360, P < 0.001; condition × stimulus intensity: F(2,14) = 6.718, P < 0.01].

Fig. 5.

H-reflex and M-wave (H-M) recruitment curves during standing (A) and at the stance (B) and swing (C) phases of stepping from a single subject. Each plot (H-reflexes, closed circles; M-waves, open circles) shows the mean value (+SD) of 5 responses at each stimulus intensity. Abscissa shows the intensity of the electrical stimulation with respect to the M-wave threshold.

Fig. 6.

Group means of the H-reflex amplitudes during standing and the stance and swing phases of passive stepping. Two different intensities of electrical stimulation were used—the intensity that elicited the maximal H-reflex (Hmax) amplitude (black bars) and that elicited an amplitude of ∼50% of Hmax (gray bars) during standing. *, +P < 0.05, significantly different from the respective control (standing) values.

Effect of Load-Related Afferent Feedback on FCR H-Reflex Amplitude during Passive Stepping

We recorded FCR H-reflexes during the stance phase of passive stepping and standing with load (40% unloading of body weight) and with full body-weight support (100% unloading) in 10 subjects. Figure 7, A–D, depicts the H-M recruitment curves obtained from a single subject. In both the loaded and unloaded conditions, passive stepping reduced H-reflex amplitudes in the FCR across a wide range of stimulus strengths. Figure 7, E and F, shows the group means of the amplitudes of H-reflexes during the loaded and unloaded conditions, elicited by two different stimulus strengths—one for eliciting the Hmax amplitude and the other for ∼50% of the Hmax at control standing. The FCR H-reflex amplitudes at the stance phase, with and without load, were significantly suppressed compared with the respective standing controls (Bonferroni test, P < 0.05). The two-way RM ANOVA showed a significant main effect of condition [load: F(1,9) = 0.095, P > 0.05; condition: F(1,9) = 31.386, P < 0.001; load × condition: F(1,9) = 0.717, P > 0.05]; however, there was no significant difference in the reflex amplitudes between the loaded and unloaded conditions.

Fig. 7.

Effect of loading on the FCR H-reflexes during standing and DGO passive stepping in 10 subjects. H-M recruitment curves at standing (A and C) and passive stepping (B and D), with load (40% unloading of body weight; A and B) and without load (100% unloading; C and D) obtained from a single subject. E and F: group means of H-reflex amplitudes with (black bars) and without (gray bars) load. Two different stimulus strengths were used to elicit the Hmax amplitude (E) and ∼50% of Hmax (F) while standing. *, +P < 0.05, significantly different from values during the respective standing conditions.

DISCUSSION

In the present study, we demonstrated that the magnitude of the FCR H-reflex was strongly suppressed during robotic-assisted, passive stepping, compared with that elicited during standing. The suppressive effect on the FCR H-reflex amplitude was seen at all phases of stepping; however, there were no significant phase-dependent differences. Furthermore, the H-reflex amplitudes were suppressed during stepping tasks, both with and without load, with no significant effect of loading itself. These findings suggest that movement-related afferent feedback, rather than load-related afferent feedback, plays a key role in modulating the FCR H-reflex amplitude.

Methodological Considerations

In the current study, the amplitude of the direct M-wave elicited in the FCR was used as an indication of the constancy of the afferent test volley. Similar M-wave amplitudes maintained for all conditions (∼10% of maximal M-wave amplitude) are an indication that the activated afferent volley evoked by the various test conditions also remains constant (Fukushima et al. 1982). In fact, there were no significant differences in the M-wave amplitudes among the 10 stepping phases and the standing condition (see Fig. 4B).

Although the H-reflex and M-wave amplitudes were normalized to the Mmax amplitude to mitigate intersubject variability, there is a possibility that the Mmax amplitude itself differed among step phases (Simonsen and Dyhre-Poulsen 1999) and over the time course of an experiment (Crone et al. 1999). Therefore, the Mmax in the FCR was recorded at each stepping phase, and predetermined M-wave amplitudes were checked and adjusted carefully with respect to the Mmax amplitude in each phase. We confirmed that there were no significant differences in the normalized M-wave amplitude during tasks. Thus the suppression of the H-reflex amplitude during passive stepping was not due to changes in the efficacy of the electrical stimulation delivered to the median nerve.

In the present study, intensity of the test electrical stimulation to the median nerve was maintained at ∼10% Mmax in each subject. However, this procedure inevitably required us to use different sizes of test H-reflexes depending on the subject. As it was reported that the degree of the conditioning effect on the H-reflex is proportional to the size of the test H-reflex, it is possible that the responses were affected by the size of the test H-reflex (cf. Crone et al. 1990). However, the H-reflex amplitudes evoked by a wide range of stimulus intensities were suppressed during both the stance and swing phases of passive stepping compared with those during quiet standing (see Fig. 5). Furthermore, when two different stimulus intensities (∼50% Hmax and Hmax during control standing) to the median nerve were investigated, there was a significant suppression of H-reflex amplitudes at both the stance and swing phases of passive stepping for both stimulus intensities (see Fig. 6). Thus it is likely that the extent of amplitude suppression did not depend on the size of the test H-reflex (cf. Crone et al. 1990).

Reciprocal inhibitory effects arising from the forearm extensor muscles may possibly also affect the amplitude of the FCR H-reflex (Day et al. 1984). However, the amplitude of the ECR EMG activity was kept to a minimum (see Fig. 2B), and there were no significant differences across conditions during our DGO stepping and static standing. Thus suppression of the FCR H-reflex during passive stepping cannot be ascribed to a change in antagonist muscle activity.

Possible Sources of the FCR H-Reflex Suppression during Passive Stepping

Our finding that passive stepping suppressed the magnitude of the forearm H-reflex is well in line with previous reports (Frigon et al. 2004; Loadman and Zehr 2007; Zehr et al. 2007) of conditioning by remote rhythmic movement. However, one possible discrepancy between our study and previous ones is the possible substantial contribution of the voluntary drive to maintain rhythmic leg movements. Although subjects were instructed to relax and allow the DGO to drive lower-limb movements, complete passive stepping was difficult to achieve. In fact, slight Sol EMG activities [∼1–2% of maximal voluntary contraction (MVC)] in the late stance phase were found and were significantly larger than those during other phases (see Figs. 1 and 2). These small Sol EMG activities during loaded stepping were also observed in our recent studies (Kamibayashi et al. 2009, 2010), even though the subjects were asked to relax. In addition, the ankle joint was held by foot lifters (spring-assisted elastic straps) to prevent foot drop and thereby restricting the trajectory of the ankle joint. Under this situation, an increase in EMG activity in the Sol at phase 5 might be a stretch-induced muscle activity, signifying that they are involuntary in nature. During normal walking, generally, it has been demonstrated that the peak EMG value of the Sol muscle was above 80% of MVC (Arsenault et al. 1986; Nishijima et al. 2010). In our situation, leg EMG activities (∼0.5–2% of MVC) during DGO stepping were extremely low compared with during normal walking. Also, reciprocal EMG activity in TA was barely discernible (see Figs. 1 and 2). Therefore, it may be that the contribution of descending commands to suppression of the FCR H-reflex during DGO-driven stepping was extremely small compared with during normal walking. Furthermore, even while performing isolated knee or hip passive movements of the ipsilateral leg with the same DGO system, the magnitude of the FCR H-reflex did not show any suppression compared with during quiet standing (T. Nakajima and T. Kitamura, unpublished observations). Delwaide and Toulouse (1981) previously demonstrated that stretch reflex amplitudes in the leg muscle were not suppressed by passive, discrete wrist movement in the remote muscle. Taking all of these observations into consideration, we favor the explanation that stepping-related afferents arising from combined joint movements in both legs play a key role in generating suppression of the FCR H-reflex amplitudes during our DGO stepping.

We further investigated the effect of load-related afferent feedback on the FCR H-reflex amplitude during the stance phase of passive stepping and standing. During locomotion, inputs from load-related receptors are important for the neural control of locomotion (Dietz et al. 2002; Duysens et al. 2000; Pearson and Collins 1993; Shoji et al. 2005; Stephens and Yang 1999; Van de Crommert et al. 1998). The potential mechanoreceptors include those in muscles, skin, and joints from both legs (Duysens et al. 2000). In fact, we have observed strong facilitation of the cutaneous reflex in the TA muscle during the late-stance to early-swing phase of passive loaded stepping but not during passive unloaded stepping (Nakajima et al. 2008). More recently, we investigated the effect of body load on the amplitude of the H-reflex in the Sol muscle using the same DGO system (Kamibayashi et al. 2010) and found that the Sol H-reflex was equally suppressed by passive stepping, both with and without body loading. This was also true in the current study for the suppression of the FCR H-reflex during passive stepping (see Fig. 6). In addition, load-related afferent feedback during rhythmic arm movement was shown to have no influence on Sol H-reflex suppression in the stationary leg (S. R. Hundza, G. C. de Ruiter, and E. P. Zehr, unpublished observations).

Based on these findings, it is unlikely that load-related afferent feedback contributed to the suppression of FCR H-reflex amplitude during passive stepping in our experimental situations [e.g., our population of subjects and number of subjects (n = 10)].

Lack of Phasic Reflex Modulation in the FCR during Passive Stepping

We found that movement-related afferent feedback from both legs does not produce a phase-dependent modulation of the FCR H-reflex (see Figs. 3 and 4). Indeed, the specific source of the modulation is not certain, although these findings may indicate that the phasic afferent inputs arising from passive leg movements are conveyed to the ascending, long propriospinal neurons responsible for the inhibition of the monosynaptic reflex arc in the other segments of the spinal cord (Alstermark et al. 1987; Cheng et al. 1998; Dietz 2002; Frigon et al. 2004; Loadman and Zehr 2007; Misiaszek et al. 1998; Zehr and Duysens 2004). More recently, de Ruiter et al. (2010) reported that suppression of the Sol H-reflex amplitude was dependent on the phase of movement during active arm cycling. Probably descending commands accompanying active arm cycling generate phasic modulation of the H-reflex during remote rhythmic movement. Based on these and our findings, it is likely that afferent information for stepping plays an important role in generating tonic suppression of the H-reflex amplitude in remote muscles (Cheng et al. 1998; Misiaszek et al. 1998; Sasada et al. 2010). These features of the general suppression of forelimb reflex excitability during passive leg stepping can be explained by the afferent-induced presynaptic inhibition on the Ia terminals of the FCR H-reflex circuitry during locomotor activity (Cheng et al. 1998; Frigon et al. 2004; Misiaszek et al. 1998; Sasada et al. 2010; Zehr and Duysens 2004). This discussion is based on indirect evidence in humans (Frigon et al. 2004; Zehr et al. 2007), and further study is needed to determine the possible contribution of presynaptic inhibition on suppression of the H-reflex pathway during passive movement of the remote limb. In addition, further investigation is needed to elucidate the functional implication of walking-related afferent signals on remote H-reflex suppression during walking.

It has been suggested that it is possible to regain locomotor abilities after spinal cord injury with intense stepping training on a treadmill (Dietz et al. 2002; Van de Crommert et al. 1998). As a translational implication for rehabilitation, our findings suggest that there may be a potential therapeutic use for passive stepping in the management of spasticity in remote muscles after spinal cord injury and stroke (cf. Hundza et al. 2009; Zehr and Duysens 2004; Zehr et al. 2009). In fact, a relationship has been seen between spasticity and hyperexcitable reflexes (e.g., H-reflexes; Levin and Hui-Chan 1993); however, specific studies designed to test this hypothesis are needed.

GRANTS

This work supported by grants from the Japan Society for the Promotion of Science (JSPS) for Young Scientists.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the author(s).

ACKNOWLEDGMENTS

We thank Dr. Tania Lam for helpful comments on an earlier version of the manuscript.

REFERENCES

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