Passive characteristics of conditioned skeletal muscle for ventricular assistance

Systems to drive a ventricular assist device (VAD) with power from skeletal muscle have been proposed and are under development. During VAD filling, these systems must counter passive muscle force to control the precontraction length and optimize power output. To determine how muscle conditioning with electrical stimulation alters basic biomechanical characteristics and influences available power, goat latissimus dorsi were evaluated in vivo after an 8 week training protocol with an implanted myostimulator. Conditioned muscles displayed increased passive stiffness. After conditioning, the slope of the exponential passive force-length relation, at a passive force of 10 N, significantly increased from 5.1 to 7.6 N/cm (p = 0.003). Similarly, for a passive force of 10 N, the length relative to the zero developed force length decreased from 5.5 to 4.2 cm (p < 0.014). The linear relationship between slope (dF/dL) and force (F) also demonstrated a significant intercept shift. The latter relationship is independent of absolute length. Consistent with other studies, conditioning also resulted in fatigue resistance, fiber type transformation, and reductions in maximum developed force and shortening velocity. In the context of available power for cardiac assist, the results demonstrate that the influence of passive characteristics is accentuated after conditioning and has a substantial effect on available power.