Modelling of muscular force induced by non-isometric contraction

Abstract: The main objective of the study was to investigate and simulate skeletal muscleforce production during and after isometric contractions, active muscle lengtheningand active muscle shortening. The motivation behind this work was to improve thedominant 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 wasshown that this force modification is not related to lengthening/shortening velocity, butinstead the steady-state force after non-isometric contractions can be well describedby an initial isometric force to which a modification is added. The modification isevaluated from the mechanical work performed by and on the muscle during lengthvariations. The time constants calculated for isometric force redevelopment appearedto 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 paralleland series elastic elements was supplemented with a history component and adoptedfor mouse soleus muscle experiments. The parameters from the experiment analysis, particularly the force modification after non-isometric contractions and the timeconstants, were reproduced by the simulations. In a step towards a general implementation, the history modification was introduced in the muscluloskeletal model ofOpenSim software, which was then used for simulations of full body movements.