Sustained skeletal muscle power for cardiac assist devices: implications of metabolic constraints

A device to harness power from skeletal muscle contracting in a linear configuration is under development. This application requires a sustained level of power that is dependent upon muscle mechanics and metabolic properties. A biomechanical muscle model and a metabolic model constructed from experimental data were used to predict maximum power available in a sustainable region of loading and stimulation conditions. Latissimus dorsi (LD) of four goats were evaluated in vivo after a 10 week in situ conditioning protocol with an implanted Telectronics myostimulator. The LD insertion was reconnected to a hydraulic loading system, allowing isometric and isotonic contractions for biomechanical characterization. Metabolic utilization was measured by a thermister based myothermic technique. Brief fatigue tests of working isotonic contractions revealed stimulation conditions associated with sustained power. The results show metabolic utilization was dependent on contraction duration, rate, force, and stroke. The region of sustainable contractions was found for a range of durations of 0.1 to 0.6 sec and rates of 10 to 120 bpm. The boundary for the sustainable power region was well approximated by a constant value of metabolic utilization. A constant duty cycle (contraction to cycle duration ratio) also approximated the sustained power but differed by as much as 30% during the shorter contraction durations. The results demonstrate that a mechanical muscle model can predict maximum sustained power when the operating conditions are constrained to a sustainable range determined by a metabolic model. Furthermore, metabolic constraints influence the optimum conditions for sustained power needed in the design of skeletal muscle powered assist devices.