Muscle efficiency: the controversial role of elasticity and mechanical energy conversion in stretch-shortening cycles

The analysis of muscle efficiency was performed on a variety of simulated muscle stretch-shortening cycles of in situ rat gas rocnemius muscle. The processes of biochemical energy conversions (phosphorylation and contraction-coupling) and mechanical conversions (internal work to external work) were incorporated in the efficiency calculations. Metabolic cost was determined usi g a simple linear model. Special attention was drawn to the interacting roles of series elastic compliance and contraction dynamics. The results showed that series elastic compliance affected the efficiency of muscle contraction to a great extent. Sti f muscle was well designed to perform efficient contractions in which muscle merely shortened while active. Compliant muscle performed best in true stretch-shortening contractions utilising the storage and release of series elastic energy effectively. owever, both a stiff and compliant series elastic element showed similar optimal muscle efficiency values in shortening contractions and stretch-shortening contractions, respectively. The findings indicate that the storage and re-utilisation of series lastic energy does not enhance overall muscle efficiency, but that optimal efficiency is obtained by a proper design of the muscle with regard to the dynamics of the movement task. Furthermore, it was found that although biochemical efficiency determi ed the feasible range of muscle efficiency, mechanical work conversions had the strongest impact on the exact value of overallmuscle efficiency in stretch-shortening contractions.