Prediction of the In Vivo Force–Velocity Relationship of Slow Human Skeletal Muscle from Measurements in Myofibers

Abstract

Direct measurement of the in vivo contractile properties of an individual muscle cannot be made in humans. The objective of this study was to predict the force-velocity (F-V) properties of slow human skeletal muscle for the in vivo temperature of 37 °C from F-V measurements in type I myofibers. Specifically, to quantitatively link myofiber measurements, which must be conducted at relatively low temperatures, to in vivo properties, the temperature-dependence of contractile properties must be modeled. We estimated the kinetic parameters of a crossbridge model within 15-30 °C from F-V measurements recorded in the myofibers of one subject, extrapolated their values at 37 °C, and then predicted the in vivo shortening and lengthening F-V curves. The prediction for maximal shortening velocity was 2.2 ± 0.2 fiber lengths per second and that for saturation force during lengthening was 2.3 ± 0.2 times isometric force. These estimates agree with previously reported in vivo measurements but are substantially different than those used in muscle models for many musculoskeletal simulations. The results from this study indicate that during low levels of muscle activation when slow motor units are primarily recruited, musculoskeletal models should consider having F-V properties that reflect the contractile properties of type I myofibers.