Meeting Abstract
Most physiological analyses and musculoskeletal models assume all fascicles of a muscle to have equal biomechanical properties. The pennate rat medial gastrocnemius (MG) is a compartmentalized muscle with proximal fascicles that are shorter and more obtusely angled than the distal fascicles, which draws this assumption into question. Using sonomicrometry in combination with in situ ergometry, we evaluate how stimulation magnitude and phase influence the net work done by three MG units: the whole muscle as well as the proximal and distal compartments. Cyclic length changes at 3.5 Hz were imposed, with each cycle starting with a 3 mm stretch, followed by 6 mm shortening and 3 mm re-lengthening. Recruitment level (maximal, submaximal) and stimulation phase (-9%, 0%, +4.5% relative to initial stretch) were varied with excitation for 35% of the cycle. Mass specific net work was calculated for each unit based on their velocity and estimates of compartment mass. At supramaximal activation, increasing stimulation phase affected whole muscle work (-8.9 J/kg to +5.6 J/kg) and distal fascicle work (-7.5 J/kg to +6.3 J/kg) to a similar extent, while work done by proximal fascicles changed from -20.2 J/kg to +0.7 J/kg. Submaximal activation resulted in negative work for all stimulation phases and less variable work output across stimulation phases by whole muscle (-6.5 J/kg to -1.8 J/kg), proximal fascicles (-6.0 J/kg to -1.3 J/kg) and distal fascicles (-7.7 J/kg to -3.5 J/kg). These results suggest that effects of stimulation phase on work output depend on muscle activation levels. They also highlight differences in work output between compartments within a muscle, which may be relevant for musculoskeletal models when estimating the mechanical output of compartmentalized muscles. Funded by NIH AR055648 to A.B.
