Meeting Abstract
Skeletal muscles are arranged in flexor-extensor groups to produce opposing joint movements and enable variations in cycle frequency (gait), duty-factor, and movement mechanics (operation as a motor, strut, or brake). However, it is unclear if flexors and extensors, which often vary in fiber architecture and composition, have equal scope for modulating power and work across movement conditions. To address this, we systematically varied stimulus phasing and cycle frequency parameters in the work-loop paradigm to dissect the modulatory scope of power and work output from two mouse hindlimb muscles; a fast-fibered, slightly pennate dorsiflexor (tibialis anterior; TA), and a mixed-slow fibered, plantarflexor (soleus; SOL) with near-parallel fiber architecture. We used literature values for mouse in vivo stride frequencies and duty factors for walk, trot, and gallop. Three stimulation phases were calculated to simulate muscle motor, spring, and brake performance. Our data suggest that SOL mostly operates as a brake (in line with available in vivo data) and TA as a motor or strut. SOL has the greatest modulatory scope and a clearer response to stimulus phasing than to variations in cycle frequency on power and work. However, comparing only the active work-loop portions reveal clear similarities in function between these muscles, consistent with the conserved contractile properties of vertebrate skeletal muscle. Perhaps unsurprisingly, muscle type appears to profoundly influence the modulatory scope of muscle performance. Our ability to measure from more than one muscle in a given individual provides a powerful framework for forthcoming evaluations of how mechanical insults and dietary variation shape muscle adaptability and performance.
