Aiko K. Thompson

H-reflex modulation in the human medial and lateral gastrocnemii during standing and walking

Introduction: The soleus H‐reflex is dynamically modulated during walking. However, modulation of the gastrocnemii H‐reflexes has not been studied systematically. Methods: The medial and lateral gastrocnemii (MG and LG) and soleus H‐reflexes were measured during standing and walking in humans. Results: Maximum H‐reflex amplitude was significantly smaller in MG (mean 1.1 mV) or LG (1.1 mV) than in soleus (3.3 mV). Despite these size differences, the reflex amplitudes of the three muscles were positively correlated. The MG and LG H‐reflexes were phase‐ and task‐dependently modulated in ways similar to the soleus H‐reflex. Conclusions: Although there are anatomical and physiological differences between the soleus and gastrocnemii muscles, the reflexes of the three muscles are similarly modulated during walking and between standing and walking. Our findings support the hypothesis that these reflexes are synergistically modulated during walking to facilitate ongoing movement. Muscle Nerve 45: 116–125, 2012

Interlimb coordination during locomotion: Finding available neural pathways and using them for gait recovery

Locomotion is a complicated movement. It requires the wellcoordinated activation of many muscles in all four limbs. Yet, once acquired, this skill is largely automated, and thus, is often taken for granted. However, if one recalls how the infant develops locomotor skills (i.e., crawling and stepping) and how difficult it is to regain normal gait after central nervous system (CNS) damage (e.g., stroke or spinal cord injury (SCI)), it is not difficult to appreciate the complexity that lies in neural control of locomotion. Locomotion is generated and regulated through the interaction of supraspinal inputs, spinal rhythm generating networks (i.e., central pattern generators (CPG)), and sensory feedback from the periphery. While supraspinal descending inputs activate the CPG, it is CPG activity together with peripheral feedback that regulates the basic pattern of locomotion (Brooke et al., 1997; Zehr and Duysens, 2004). Over the last several decades, the existence of a locomotor CPG in humans and the quadrupedal nature of human locomotion have become clear (Dietz, 2002; Dietz et al., 1994; Dietz and Harkema, 2004; Grillner, 1981; Lamb and Yang, 2000; Pang and Yang, 2001; Wernig et al., 2000; Yang et al., 2004, 1998). The isolated human spinal cord is capable of generating coordinated locomotor EMG activity (Dietz, 1995; Dietz et al., 1994; Maegele et al., 2002). In infants, in whom the descending cortical connections (especially the corticospinal tract) is immature, the regulation of stepping is similar to that in adult non-primate quadrupeds (Lamb and Yang, 2000; Pang and Yang, 2000, 2001; Yang et al., 2004, 1998). Recently, Patrick et al. (2009) described quadrupedal coordination of the four limbs in crawling human adults and infants, thus providing evidence for locomotor CPG and quadrupedal interlimb coordination in the intact mature human CNS. Furthermore, Ferris and his colleagues (Ferris et al., 2006; Huang and Ferris, 2004) showed in normal subjects that upper limb movement influences the recruitment of lower limb motoneurons during rhythmic actions. The upper and lower limb linkage also exists in the opposite direction; lower limb movement affects upper limb muscle activation (Sakamoto et al., 2007). While the presence of interlimb coupling can be detected by measuring the effects of movement in one limb on another limb’s muscle activity (i.e., EMG), it is difficult to quantify the extent of interlimb coupling or its phasic effects from EMG alone. This problem can be addressed by using spinal reflexes to examine the modulation in neural activity and excitability that is associated with interlimb coupling (Zehr and Duysens, 2004; Zehr et al., 2009; Zehr