Architecture-based force-velocity models of load-moving skeletal muscles

A predictive model of muscle force-velocity relationships is presented based on functional architectural variables. The parameters of Hillʹs equation describing muscle force-velocity relationship of nine muscles were estimated by their relationships with variables extracted from the whole-muscle length-force relationship and the percentage of slow-twitch fibres. Specifically, the maximal unloaded velocity (Vo) was estimated through multiple linear regression, from each muscleʹs fibre composition and the shortening range through which each muscle could produce active force. The maximal isometric force (Po) was also extracted from each muscleʹs length-force relationship. The ratio of Hillʹs dynamic constant a to Po and b to Vo, which determines the degree of curvature of the relation, was determined solely by the percent of slow-twitch fibres. This model was verified by fitting it to experimental force-velocity curves of nine different muscles in the catʹs hindlimb. It was found that reasonable fits of force-velocity curves would be obtained with correlation coefficient in the range of 0.61 to 0.92, with an average of 0.82. The model predicted that muscles with relatively long shortening ranges would achieve higher maximal velocity, and that muscles with higher percentage of slow-twitch fibres had less pronounced curvature and lower maximal velocity in their force-velocity relationships.