One of the main potential sources of error when recording reflex responses during dynamic muscle actions is the spatial relationship between the nerve and the stimulating electrode. A common technique in H-reflex studies is to record a small M-wave concurrently with the H-reflex, the amplitude of which is used as a calibration to ensure consistent effective stimulus intensity. However, as can be seen in the recruitment curves of Fig. 1, at low stimulus intensities the M-wave is insensitive to small changes in stimulation. Therefore, reflexes were recorded at a stimulus intensity of 50 %Mmax where the resulting M-wave lies on the steep ascending limb of the recruitment curve and is sensitive to small changes in stimulus intensity. The consistent M-wave amplitudes in this condition indicate that passive muscle actions did not alter the spatial relationship between the stimulating electrode and the tibial nerve. Based on these findings, it is unlikely that the effective stimulus intensity was altered during trials using 50 %Hmax stimulation.
It should be noted that, at stimulus intensities sufficient to elicit an M-wave, the H-reflex response cannot be considered to arise exclusively from Ia afferent input. Furthermore, at a stimulus intensity of 50 %Mmax the resulting H-reflex had either reached a plateau or lay on the descending limb of the recruitment curve and, consequently, would probably be subjected to considerable antidromic collision. Despite the possible contamination of reflex responses, the H-reflex was modulated in a similar manner at the two stimulation intensities. Therefore, it appears as though most of the reflex modulation could be attributed to alterations in the Ia afferent pathway projecting onto the SOL motoneurone pool.
During dynamic trials there is the potential that the recording electrodes may move relative to underlying muscle, thereby influencing the amplitude of the recorded reflex response. This was not considered a problem in the present study, however, as all reflex responses were recorded at the same ankle angle (90 deg) and, presumably, at the same muscle length.
Possible inhibitory mechanisms
Just as the reflex response to a given stimulus is influenced by the composition and mechanical characteristics of the homonymous muscle, a number of neural mechanisms are also known to influence motor output in response to sensory stimuli. Mechanisms involving presynaptic inhibition (either centrally or peripherally induced), homosynaptic post-activation depression, reciprocal inhibition from antagonist activation, Ib afferents, group II afferents and cutaneous mechanoreceptors, as well as plateau potentials, have all been implicated in modulating reflex responses.
The group II afferents from lengthening plantar flexors are considered to have little influence on the reflex modulation seen in the present study. Group II afferents are predominantly regarded as indicators of static length changes; therefore, the influence of muscle spindle discharge on reflex modulation was considered to arise from predominantly Ia afferents. Likewise, the influence of Ib afferents was not considered to be the likely cause of reflex modulation as, although Ib afferents are sensitive to very small changes in muscle tension (Houk & Henneman, 1967), they are more influential during active muscle actions than when the movement is performed passively (Burg et al. 1973).
It is possible that afferent responses from cutaneous mechanoreceptors and joint receptors may influence the resultant H-reflex amplitude. However, Hultborn et al. (1996) applied a tourniquet at the ankle, thereby inducing an ischaemic block of these sensory inputs without altering the reflex depression following passive dorsiflexion. When the tourniquet was applied just distal to the knee, reflex depression was abolished and the origin of reflex inhibition was attributed to the large diameter Ia afferents located between the ankle and knee. Based on these findings, cutaneous mechanoreceptors and joint receptors were not considered to be responsible for the reflex inhibition seen in the present study.
The fact that H-reflex amplitudes during passive lengthening actions were significantly less than the isometric H-reflex amplitude when the latency from movement onset to stimulation was at its shortest (57 ms) indicates that inhibition of the H-reflex amplitude can occur in the absence of supraspinal influences. However, reflex modulation at a latency of 57 ms was not consistent, with H-reflexes being facilitated in some subjects while inhibited in others (see Fig. 5). The unpredictable nature of this modulation would not appear to reflect a centrally controlled functional reflex modulation, at least at latencies as short as 57 ms, and as such, is most probably influenced by the variability of Ia afferent activity generated from the muscle spindles.
Although some subjects displayed considerably more stretch-induced EMG activity than others, the amplitude of stretch reflexes and H-reflexes varied substantially both between and within subjects. This variability may reflect differences in muscle spindle output or the susceptibility of the motoneurone pool to Ia input. Muscle spindle output is influenced by factors such as muscle stiffness, gamma drive as well as thixotropic affects on both intrafusal and extrafusal muscle fibres. Small plantar flexor efforts were performed before the start of each passive movement in an attempt to minimize any thixotropic influences on spindle discharge. However, differences in muscle stiffness and gamma drive were not evaluated in the present study and may account for some of the differences in muscle spindle output. As this study utilized passive movements and subjects were unaware of the direction of the following movement, the excitability of the motoneurone pool was not considered to be altered by higher motor centres. Differences in the susceptibility of motoneurones to Ia input cannot, however, be excluded as an explanation for the observed variations in stretch reflex responses.
Depending on the timing of transmission within the Ia afferents, muscle spindle discharge may either inhibit or facilitate H-reflex responses. Modulation of the H-reflex by submaximal stimulation of homononymous Ia afferents has been reported by Fukushima et al. (1982). A conditioning stimulus at an intensity subthreshold for the H-reflex produced short latency inhibition of the test H-reflex for conditioning-test intervals of 1.5-3.0 ms, which was attributed to the refractoriness of Ia afferent fibres. Subsequently, the H-reflex was facilitated up to 15 ms as a result of temporal summation of excitatory postsynaptic potentials (EPSPs) at the motoneurone pool. Therefore, depending on the temporal relationship between the discharge from muscle spindle afferents and delivery of the H-reflex stimulus, the H-reflex recorded at the shortest latency during passive lengthening actions may have been inhibited due to refractory Ia afferents or facilitated as a result of EPSP summation at the motoneurone pool.
Interestingly, at latencies longer than 57 ms, H-reflexes were strongly inhibited even though the muscle spindle discharge was not sufficient to elicit stretch reflexes. It is possible that despite the weaker strength of the spindle output, the longer time frame over which the muscle was lengthening resulted in a greater total release of neurotransmitter from the presynaptic terminal and, hence, a greater influence of homosynaptic post-activation depression. However, even a latency of 165 ms seems very short for neurotransmitter depletion and one must question the functional significance of such an inhibitory mechanism. Alternatively, when assessing reflex amplitudes at longer latencies the influence of higher motor centres cannot be ruled out. It is possible that the strong inhibition seen at these latencies was a result of centrally or peripherally induced presynaptic inhibition.
As all plantar flexor muscles were lengthened simultaneously, the influence of presynaptic inhibition arising from heteronymous Ia afferents cannot be ignored (Barnes & Pompeiano, 1970; Brooke et al. 1995a). Inhibition of the MG monosynaptic reflex induced by sinusoidal changes in the length of the lateral gastrocnemius- soleus (LGS) in cats was reported by Barnes & Pompeiano (1970). This reflex depression was attributed to primary afferent depolarization as indicated by dorsal root potentials recorded in MG Ia fibres. However, depression of the monosynaptic reflex only became evident once the vibration was removed. During vibration this inhibition was overshadowed by a strong facilitation of Ia afferents also induced by LGS vibration. Therefore, although presynaptic inhibition from heteronymous Ia afferents may have occurred during lengthening actions in the present study, it is possible for this inhibition to be overshadowed by temporal facilitation of EPSPs at the motoneurone pool.
Given the complexity of neural inputs projecting onto the motoneurone pool, it is difficult to definitively identify the neural mechanism responsible for reflex modulation. It is also likely that the resultant reflex amplitude reflects the integration of a number of influential neural mechanisms and no single mechanism can fully account for the observed reflex modulation. For example, based on the time course of reflex recovery, Voigt & Sinkjaer (1998) suggest that reflex inhibition during lengthening actions is the result of the quickly recovering presynaptic inhibition (≈500 ms) and the longer lasting homosynaptic post-activation depression (≈4 s). Furthermore, it is likely that the predominant mechanism responsible for reflex modulation varies depending on the demands of the task and caution must be taken when interpreting the reflex modulation evident during different functional tasks.
In the study of Schneider et al. (2000) H-reflex inhibition was closely associated with activation of the antagonist, tibialis anterior (TA), and the authors suggested that the reflex modulation was predominantly regulated by centrally produced reciprocal inhibition. Although TA activation is known to have a strong inhibitory influence on the SOL H-reflex (Crone, 1987), Yang & Whelan (1993) have shown that depression of the SOL H-reflex during gait can occur in the absence of TA activation. In the present study reciprocal inhibition was not considered a contributing factor to the H-reflex depression during passive lengthening actions as no activity from TA was detected in any trials. However, it is possible that TA Ia afferents were activated during passive plantar flexion movements, particularly at the faster velocities. Afferent activity from TA muscle spindles, although insufficient to elicit a stretch reflex response, would have an inhibitory influence on the plantar flexor motoneurones via Ia inhibitory interneurones. The potential for such reciprocal inhibition may explain why H-reflexes at the faster shortening velocities were significantly less than those at the slower shortening velocity (see Fig. 3), although it cannot account for the facilitation of the H-reflex during shortening actions when compared with the isometric H-reflex.
Facilitation of the shortening H-reflex may, however, be related to activity within homonymous Ia afferents. It is possible that during the passive shortening actions the muscle spindles fell silent, allowing recovery of neurotransmitters that may have been depleted during the previous lengthened position. A similar scenario was reported by Wood et al. (1996) who recorded muscle spindle discharge and H-reflexes during dynamic muscle actions in the cat. Once the muscle was shortened from the lengthened position there was a large recovery of the H-reflex amplitude in the absence of spindle discharge. This recovery does not explain the observed increase in reflex amplitude above the isometric level. However, it is possible that, in the present study, isometric trials were associated with a tonic firing of muscle spindles that may have elicited homosynaptic post-activation depression and consequently reduced the isometric H-reflex amplitude. Interestingly, during the short latency experiments, shortening H-reflexes were not significantly different from the isometric H-reflex. This may indicate that at these latencies, there was insufficient time for complete recovery following neurotransmitter depletion. The consistency of shortening H-reflexes at the three latencies argues against supraspinal facilitation of the shortening H-reflexes.
It was interesting to note in the present study that the SOL and MG muscles, which have distinctly different fibre-type compositions, showed similar patterns of H-reflex modulation. Moritani et al. (1990) demonstrated differences in modulation of reflex responses in SOL and MG during a functional hopping movement. However, as the movements performed in the present study were not considered ‘functional’ tasks, it might be considered less likely to find significant differences in H-reflex modulation between these two muscles. During functional movements, Ia afferents can discharge at very high frequencies and the resultant EPSP has been shown to depend on the properties of the motoneurone pool (Koerber & Mendell, 1991). Changes in EPSP amplitude have also been shown to vary depending on the pattern of stimulation, which reflects a task-dependent distribution of synaptic input to the motoneurone (Koerber & Mendell, 1991). These findings may account for some of the discrepancy between SOL H-reflex amplitude and the underlying activation during phases of the gait cycle.
Differences in reflex modulation between SOL and MG may also be related to the Ia afferent motoneurone connection. It is suggested that Ia afferents predominantly depolarize slow twitch, low threshold motoneurones (Messina & Cotrufo, 1976). The predominance of these motoneurones in SOL would explain the larger Hmax: Mmax ratio recorded from SOL than from MG in the present study. Despite their different magnitudes, the Hmax: Mmax ratios of SOL and MG were modulated in the same manner during passive lengthening and shortening actions. This suggests the H-reflex modulation was not related to muscle composition or motor unit recruitment order. However, it must also be noted that, during the 50 %Hmax condition, the relative stimulus intensities were different for both SOL and MG (see ‘Passive reflex modulation’ in Methods). As indicated by Crone et al. (1990), the susceptibility to facilitation or inhibition varies depending upon the size of the control reflex, with smaller reflexes being more susceptible to inhibitory influences than larger reflexes. Therefore, differences in H-reflex modulation in each muscle due to Ia afferent- motoneurone connections may be obscured by the effect of different stimulus intensities.
It is important to note that the movements performed in this study are not considered functional tasks. However, the H-reflex modulation observed in this study probably reflects changes in the afferent pathways that project onto the SOL motoneurone pool. Passive reflex modulation provides information regarding peripheral inputs during dynamic activities in the absence of strong input from higher motor centres. These data may assist in the interpretation of H-reflex modulation during more functional tasks such as walking, running and cycling.