Mechanisms of coordination between one- and two-joint synergist muscles
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Major muscle groups (e.g. triceps surae, quadriceps, hamstrings, triceps brachii) contain synergist muscles that cross either one or two joints; they are called one- and two-joint muscles. The functional significance of this musculoskeletal design, extensively studied in the past, has been suggested to increase the economy and efficiency of movement. Much less attention has been paid to the mechanisms responsible for the differential activation, or coordination, between one- and two-joint synergists. The understanding of these mechanisms will not only add to the basic knowledge of neural control of movement but also contribute to prevention and therapeutic interventions of muscle injuries that often occur in two-joint muscles. Previous work has suggested that mechanical intermuscular interactions, resultant muscle moment requirements at the adjacent joints, movement speed, and muscle length-velocity related sensory feedback can affect this coordination. Additionally, the comparison of motoneuronal and muscle activity patterns between fictive and real locomotion in cats suggests a greater influence of motion related sensory feedback on activity of proximal two-joint muscles (i.e., rectus femoris and hamstrings) compared to one-joint muscles and distal two-joint muscles (medial and lateral gastrocnemius). Therefore, the first goal of this work was to test the possible contribution of mechanical intermuscular interactions between one- and two-joint ankle extensors in the cat. The second goal was to examine the role of joint moment requirements, movement speed and length-velocity related feedback in distinct activation of distal one- and two-joint muscles (soleus and gastrocnemius). The third goal was to investigate the effect of removal of length-velocity sensory feedback from proximal one- and two-joint muscles (vastii and rectus femoris) on coordination of these muscles. To address the above goals, an array of motor tasks with different speeds and combinations of joint moments were studied in cats and humans. The tasks included level, downslope and upslope walking and paw shake response in cats, as well as back and leg load lifting and jumping in humans. Motion capture and force plate data were recorded to analyze kinematics and joint moments, sonomicrometry was used to measure muscle fascicle length in cats, and electromyography (EMG) was used to quantify muscle activity. Length-velocity related sensory feedback was removed in cats by muscle self-reinnervation. Results show that mechanical intermuscular interactions via myofascial force transmission should be considered in the coordination between adjacent one- and two-joint synergist muscles in certain pathological conditions leading to increased muscle lengths. Coordination between distal and proximal one- and two-joint synergists depends on joint moment requirements, and the differential inhibition of soleus and excitation of gastrocnemius does not depend on movement speed or length-velocity related sensory feedback. Removal of length-velocity related sensory feedback has a strong effect on coordination between the studied proximal synergist pair (vastii and rectus femoris) but not on coordination of the distal synergist pair (soleus and gastrocnemius). Findings presented here expand on understanding the role of mechanical interactions, sensory feedback and feedforward control in the coordination between one- and two-joint muscles. These findings have potential implications for developing targeted rehabilitation strategies/treatment and implementation of new control strategies for robotics and prosthetics to improve movement efficiency.