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Motor responses evoked by stimulating the spinal cord percutaneously between the T11 and T12 spinous processes were studied in eight human subjects during walking and running. Stimulation elicited responses bilaterally in the biceps femoris, vastus lateralis, rectus femoris, medial gastrocnemius, soleus, tibialis anterior, extensor digitorum brevis and flexor digitorum brevis. The evoked responses were consistent with activation of Ia afferent fibres through monosynaptic neural circuits since they were inhibited when a prior stimulus was given and during tendon vibration. Furthermore, the soleus motor responses were inhibited during the swing phase of walking as observed for the soleus H-reflex elicited by tibial nerve stimulation. Due to the anatomical site and the fibre composition of the peripheral nerves it is difficult to elicit H-reflex in leg muscles other than the soleus, especially during movement. In turn, the multisegmental monosynaptic responses (MMR) technique provides the opportunity to study modulation of monosynaptic reflexes for multiple muscles simultaneously. Phase-dependent modulation of the MMR amplitude throughout the duration of the gait cycle period was observed in all muscles studied. The MMR amplitude was large when the muscle was activated whereas it was generally reduced, or even suppressed, when the muscle was quiescent. However, during running, there was a systematic anticipatory increase in the amplitude of the MMR at the end of swing in all proximal and distal extensor muscles. The present findings therefore suggest that there is a general control scheme by which the transmission in the monosynaptic neural circuits is modulated in all leg muscles during stepping so as to meet the requirement of the motor task.
Modulation of reflexes by the central nervous system is critical for successful movement. The stretch reflex has been shown to contribute up to 40–60% to the total torque produced around the target joint during static motor tasks (Toft et al. 1991; Mrachacz-Kersting & Sinkjaer, 2003). This implies that the central nervous system has to modulate the strength of muscle reflexes throughout the movement as required to perform the specific task, i.e. task- and phase-dependent reflex modulation (Capaday & Stein, 1986, 1987; Crenna & Frigo, 1987; Dyhre-Poulsen et al. 1991; Edamura et al. 1991; Sinkjaer et al. 1996; Lavoie et al. 1997; Faist et al. 1999; Schneider et al. 2000; Mrachacz-Kersting et al. 2004). The Hoffman (H)-reflex technique uses direct electrical stimulation to the motor nerve to evoke the short latency component of the stretch reflex in dynamic conditions (Andersen & Sinkjaer, 1999). The H-reflex bypasses the fusimotor regulation system as well as the mechanical stimulus on muscle spindles, and therefore directly assesses the central modulation of synaptic efficacy of afferent input from large-diameter fibres onto motor neurons (Schieppati, 1987). The amplitude of the soleus H-reflex is strongly modulated during human locomotion with high amplitude during the stance phase and suppression during the swing phase (Capaday & Stein, 1986; Simonsen & Dyhre-Poulsen, 1999). Furthermore, the soleus H-reflex amplitude is significantly lower during walking and running compared with standing (Edamura et al. 1991; Simonsen & Dyhre-Poulsen, 1999; Ferris et al. 2001). Such task- and phase-dependent changes of the input–output properties of the soleus H-reflex neural circuitry reflect an adaptive modulation of the functional connectivity within the spinal motor infrastructure to meet the biomechanical and physiological demands of locomotion (Stein & Capaday, 1988; Simonsen et al. 2002).
The neural control of locomotion conceivably requires the central nervous system to modulate reflex excitability not only in the soleus muscle, but in all leg muscles. Knowledge on the modulation of synaptic efficacy of proprioceptive afferents from multiple leg muscles would facilitate the understanding of the neural control properties of human locomotion and allow investigation of these systems following neurological disorders. For example, extensor and flexor muscles as well as proximal versus distal muscles respond very differently to a given neuromotor impairment (Harkema, 2001; Dietz & Muller, 2004; Courtine et al. 2005b). However, only a few muscles are accessible using the H-reflex technique and the H-reflex cannot be studied simultaneously in multiple muscles during walking (Dietz et al. 1990b; Toft et al. 1991; Faist et al. 1999; Christensen et al. 2001; Mrachacz-Kersting et al. 2004).
In this study, we show that percutaneous stimulation applied between T11 and T12 spinous processes elicit multisegmental monosynaptic responses (MMR) in multiple leg muscles bilaterally (Minassian et al. 2007). We used this methodology to study the modulation of the monosynaptic neural circuits from a number of flexor and extensor motor pools innervating proximal and distal leg muscles during locomotion. We investigated the modulation of MMR during walking and running to assess whether the task-dependent reflex modulation observed for the soleus monosynaptic neural circuit could be generalized to the other leg muscles.