Abstract
Skeletal muscles change length and develop force both passively and actively. Gearing allows muscle fiber length changes to be uncoupled from those of the whole muscle-tendon unit. During active contractions this process allows muscles to operate at mechanically favorable conditions for power or economical force production. Here we ask whether gearing is constant in passive muscle; determining the relationship between fascicle and muscle-tendon unit length change in the bi-articular medial gastrocnemius and investigating the influence of whether motion occurs at the knee or ankle joint. Specifically, the same muscle-tendon unit length changes were elicited by rotating either the ankle or knee joint whilst simultaneously measuring fascicle lengths in proximal and distal muscle regions using B-mode ultrasound. In both the proximal and distal muscle region, passive gearing values differed depending on whether ankle or knee motion occurred. Fascicle length changes were greater with ankle motion, likely reflecting anatomical differences in proximal and distal passive tendinous tissues, as well as shape changes of the adjacent mono-articular soleus. This suggests that there is joint-dependent dissociation between the mechanical behavior of muscle fibers and the muscle-tendon unit during passive joint motions that may be important to consider when developing accurate models of bi-articular muscles.
Highlights
Skeletal muscles are an organ whose primary function is to produce force and movement
To test whether muscle-tendon unit (MTU) gearing in MG was joint dependent, a combination of simulation and experimental work was completed with methods selected to provide: (i) estimation of MTU lengths that occurred within a range of both knee and ankle joint angles; and (ii) measures of fascicle geometric properties that occurred for equivalent MTU lengths within the defined joint angle ranges of motion
The purpose of this study was to test whether the extent and velocity of fascicle length change, and MTU gearing, within proximal and distal regions of the passive MG muscle varied depending on the joint at which the motion occurred
Summary
Skeletal muscles are an organ whose primary function is to produce force and movement They are a structurally organized, composite material containing multiple contractile elements, internal and external connective tissues as well as nerve and vascular tissues. As the muscle belly shortens, the fascicles both shorten and rotate to greater pennation angles Such fiber rotation means that the fiber velocity (Vf) can be uncoupled from that of the muscle tendon unit (VMTU): a process termed MTU gearing (Wakeling et al, 2011)
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