A force-similarity model of the activated muscle is able to predict primary locomotor functions.

Biomechanical macroscopic models of the muscle organ as whole are conceptually limited in explaining muscle function in relation to structure. The examples are Hill-type and rheological muscle models where elastic properties of the muscle's contractible element are approached by a spring arranged in series and parallel, respectively. A new scaling model of the activated muscle powering a particular function is proposed. This model is based on the physical similarity suggested between the action-production muscle force and resulting reaction elastic muscle forces. Considered at a macroscopic scale, this force similarity provides four patterns of constraints in development of muscle architecture in different-sized animals. As the result, the analytical modeling predicts the primary motor, brake, strut and spring functions of individual muscles revealed earlier in work-loop experiments and now provided in terms of the scaling exponents for muscle cross-sectional area and fiber length. The model reliability is tested via literature available from muscle allometric data. The conceptual outcome of the study is that the architecture design of skeletal muscles is likely effected by the powering contractions of last fibers known as having higher myofibril volume than slow fibers.