Abstract
Although individual heads of triceps surae, soleus (SO) and medial gastrocnemius (MG) muscles, are often considered close functional synergists, previous studies have shown distinct activity patterns between them in some motor behaviors. The goal of this study was to test two hypotheses explaining inhibition of slow SO with respect to fast MG: (1) inhibition occurs at high movement velocities and mediated by velocity-dependent sensory feedback and (2) inhibition depends on the ankle-knee joint moment combination and does not require high movement velocities. The hypotheses were tested by comparing the SO EMG/MG EMG ratio during fast and slow motor behaviors (cat paw shake responses vs. back, straight leg load lifting in humans), which had the same ankle extension-knee flexion moment combination; and during fast and slow behaviors with the ankle extension-knee extension moment combination (human vertical jumping and stance phase of walking in cats and leg load lifting in humans). In addition, SO EMG/MG EMG ratio was determined during cat paw shake responses and walking before and after removal of stretch velocity-dependent sensory feedback by self-reinnervating SO and/or gastrocnemius. We found the ratio SO EMG/MG EMG below 1 (p < 0.05) during fast paw shake responses and slow back load lifting, requiring the ankle extension-knee flexion moment combination; whereas the ratio SO EMG/MG EMG was above 1 (p < 0.05) during fast vertical jumping and slow tasks of walking and leg load lifting, requiring ankle extension-knee extension moments. Removal of velocity-dependent sensory feedback did not affect the SO EMG/MG EMG ratio in cats. We concluded that the relative inhibition of SO does not require high muscle velocities, depends on ankle-knee moment combinations, and is mechanically advantageous for allowing a greater MG contribution to ankle extension and knee flexion moments.
Highlights
Skeletal muscles have diverse morphological properties and can differ substantially in muscle fiber type composition, fascicle length, physiological cross-sectional area, pennation, tendon length and thickness, etc., (Ariano et al, 1973; Johnson et al, 1973; Sacks and Roy, 1982; Wood et al, 1989; Cutts et al, 1991; Ward et al, 2009)
The goal of this study was to test the two possible explanations emerged from the literature for the differential activation of SO and GA—(1) SO inhibition occurs at high movement velocities and mediated by velocity-dependent sensory feedback and (2) SO inhibition depends on the ankle-knee joint moment combination and does not require high movement velocities
The hypotheses were tested by comparing the SO EMG/medial gastrocnemius (MG) EMG ratio during fast and slow motor behaviors—cat paw shake responses vs. back, straight leg load lifting in humans, which had the same ankle extension-knee flexion moment combination (De Looze et al, 1993; Prilutsky et al, 1998, 2004; Klishko et al, 2011); and during fast and slow behaviors with the ankle extension-knee extension moment combination—human vertical jumping (Bobbert and Van Ingen Schenau, 1988; Kurokawa et al, 2001), stance phase of walking in cats (Gregor et al, 2006) and leg load lifting in humans (De Looze et al, 1993)
Summary
Skeletal muscles have diverse morphological properties and can differ substantially in muscle fiber type composition, fascicle length, physiological cross-sectional area, pennation, tendon length and thickness, etc., (Ariano et al, 1973; Johnson et al, 1973; Sacks and Roy, 1982; Wood et al, 1989; Cutts et al, 1991; Ward et al, 2009). E.g., triceps surae, quadriceps, hamstrings and triceps brachii, consist of individual muscle heads that differ from each other in their morphological properties, and in the number of joints they cross. The fact that GA can contribute to both ankle extension and knee flexion moments gives this muscle head a mechanical advantage over SO in tasks that require this combination of joint moments; this is true for two-joint heads of the quadriceps, hamstrings and triceps brachii muscles mentioned above (Wells and Evans, 1987; Prilutsky, 2000a)
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