MODELING FUNCTIONAL-EFFECTS OF MUSCLE GEOMETRY

Muscle architecture is an important aspect of muscle functioning. Henc e, geometry and material properties of muscle have great influence on the force-length characteristics of muscle. We compared experimental r esults for the gastrocnemius medialis muscle (GM) of the rat to model results of simple geometric models such as a planimetric model and thr ee-dimensional versions of this model. The capabilities of such models to adequately calculate muscle geometry and force-length characterist ics were investigated. The planimetric model with elastic aponeurosis predicted GM muscle geometry well: maximal differences are 6, 1, 4 and 6% for fiber length, aponeurosis length, fiber angle and aponeurosis angle respectively. A slanted cylinder model with circular fiber cross -section did not predict muscle geometry as well as the planimetric mo del, whereas the geometry results of a second slanted cylinder model w ere identical to the planimetric model. It is concluded that the plani metric model is capable of adequately calculating the muscle geometry over the muscle length range studied. However, for modelling of force- length characteristics more complex models are needed, as none of the models yielded results sufficiently close to experimental data. Modell ed force-length characteristics showed an overestimation of muscle opt imum length by 2 mm with respect to experimental data, and the force a t the ascending limb of the length force curve was underestimated. The models presented neglect important aspects such as non-linear geometr y of muscle, certain passive material properties and mechanical intera ctions of fibers. These aspects may be responsible for shortcomings in the modelling. It is argued that, considering the inability to adequa tely model muscle length-force characteristics for an isolated maximal ly activated (in situ) muscle, it is to be expected that prediction wi ll fail for muscle properties in conditions of complex movement with m any interacting factors. Therefore, modelling goals should be limited to the heuristic domain rather than expect to be able to predict or ev en approach medical or biological reality. However, the increased unde rstanding about muscular mechanisms obtained from heuristic use of suc h simple models may very well be used in creating progress in, for exa mple, clinical applications. (C) 1998 Elsevier Science Ltd. All rights reserved.