Variable series elasticity accounts for Fenn effects of skeletal and cardiac muscles.

The Fenn effect differs between skeletal and cardiac muscles in the magnitude of energy consumption of shortening contraction relative to isometric contraction at the same preload. The former is typically greater than the latter in the skeletal muscle, whereas the former is smaller than the latter in the cardiac muscle. The present theoretical study examined whether the different Fenn effects could be accounted for by different compliances of the series elasticity (SE) in different muscles. A two-element model consisting of an idealized contractile element (CE) and an SE was used. The compliance of the SE was assumed to be variable. Results show that the skeletal Fenn effect can be simulated when SE is stiff and the cardiac Fenn effect can be simulated when SE is compliant. Moreover, when SE is compliant the total work of CE approximates for force-length area, which has been proposed as a measure of the total mechanical energy and shown to correlate linearly with myocardial oxygen consumption.