The Effect of Architecture On Skeletal Muscle Performance: a Simple Planimetric Model

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Abstract
A simple planimetric model predicting the geometry of skeletal muscle as well as its force at all physiological fibre lengths, is presented. The model is verified against the experimental data of the m. gastrocnemius, caput mediale, of three Wistar rats. It is concluded that the model allows prediction of the behaviour of unipennate muscle under isometric conditions. The experimental data show that unipennate muscle becomes 0.4% thicker for every percent decrease of fibre length and that the angle of the fibres, with regard to the line of pull of the muscle, increases as a consequence, especially at lower fibre lengths. For a given shortening of the fibres the travel of the tendon, in the direction of the line of pull of the muscle, is greater for a unipennate muscle than the length change of its fibres and thus also greater than the corresponding length change of a parallel fibred muscle of equal fibre length. According to the model this is due to the increasing fibre angles of the shortening unipennate muscle. The normalized length force relation of unipennate muscle is compared to that of its fibres and of a parallel fibred muscle, of equal fibre length, relatively narrow as well as asymmetrical. The model predicts that the maximal work deliverable by a unipennate muscle may be greater than that of a parallel fibred muscle of equal physiological cross section and fibre length.

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