LINEAR DAMPER DOES NOT FULLY EXPLAIN PASSIVE LOSSES IN SKELETAL MUSCLE

Abstract

Viscous forces within skeletal muscle resist passive length change. These forces have been modeled as a linear damper, but are often considered trivial. PURPOSE To establish the validity of passive muscle loss models and determine if these losses are indeed trivial. METHODS Passive and active forces produced by in situ male Syrian Golden hamster gastrocnemius were recorded during sinusoidal length changes at 1, 2, 4, and 8Hz during length excursions of 1, 2, 3, and 4mm. Instantaneous force and power, and work and power over the cycle were calculated. Equations for instantaneous force and power, work lost over a cycle, and power absorbed over a cycle were derived based on a linear damper model. Multiple regression analysis was used to determine if the modeling equations accurately predicted force, work, and power losses or if other factors (shortening velocity, cycle frequency, excursion length) independently contributed to the observed losses. RESULTS Regression analyses demonstrated that a linear model accurately predicted instantaneous force (R2=.933) and power (R2=.996) at. Work was significantly predicted by the linear model (R2=.81), however, the addition of an excursion length term significantly increased the predictive accuracy (R2=.98). Similary, power over a cycle was significantly predicted by the linear model (R2=.96) and the addition of velocity term significantly increased the predictive accuracy (R2=.999). When compared to active work, viscous work losses were negligible at low frequencies and muscle length displacements (0.4%). At increased velocities, due to the concomitant effects of decreased muscle force and increased energy loss, hysteresis accounted for up to 37% of active work. CONCLUSION Passive viscous losses at l are well predicted by a linear damper model. Over a complete cycle, however, an additional length effect was observed, which may reflect passive activation or rheological phenomena. At high cycle frequencies, viscous losses represent a substantial portion of active work. Supported by ALA-RGD13-N