Modelling of muscular force induced by non-isometric contraction

The main objective of the study was to investigate and simulate skeletal muscle force production during and after isometric contractions, active muscle lengthening and active muscle shortening. The motivation behind this work was to improve the dominant model of muscle force generation based on the theories of Hill from 1938. Effects of residual force enhancement and force depression were observed after concentric and eccentric contractions, and also during stretch-shortening cycles. It was shown that this force modification is not related to lengthening/shortening velocity, but instead the steady-state force after non-isometric contractions can be well described by an initial isometric force to which a modification is added. The modification is evaluated from the mechanical work performed by and on the muscle during length variations. The time constants calculated for isometric force redevelopment appeared to be in certain relations with those for initial isometric force development, an observation which extended our basis for muscle modelling. A macroscopic muscular model consisting of a contractile element, and parallel and series elastic elements was supplemented with a history component and adopted for mouse soleus muscle experiments. The parameters from the experiment analysis, particularly the force modification after non-isometric contractions and the time constants, were reproduced by the simulations. In a step towards a general implementation, the history modification was introduced in the muscluloskeletal model of OpenSim software, which was then used for simulations of full body movements.