Comparison of the validity of Hill and Huxley muscle tendon complex models using experimental data obtained from rat m. soleus in situ

Abstract

The relationship between mechanical and metabolic behaviour in the widely used Hill muscle-tendon complex (MTC) model is not straightforward, while this is an integral part of the Huxley model. In this study we assessed to what extent Huxley and Hill type MTC models yield adequate predictions of mechanical muscle behaviour during stretch-shortening cycles (SSC). In fully anaesthetized male Wistar rats (N=3), m. soleus was dissected completely free, except for the insertion. Cuff electrodes were placed over the n. ischiadicus. The distal end of the tendon was connected to a servo motor, via a force transducer. The setup allowed for full control over muscle stimulation and length, while force was measured. Quick release and isovelocity contractions (part 1), and SSC (part 2) were imposed. Simulations of part 2 were made with both a Hill and a Huxley MTC model, using parameter values determined from part 1. A modification to the classic two-state Huxley model was made to incorporate series elasticity, activation dynamics and active and passive force-length relations. Results were similar for all rats. Fitting of the free parameters to data of part 1 was near perfect (R2 > .97). During SSC, predicted peak force and force during relaxation deviated from the experimental data, for both models. Overall, both models yielded similarly adequate predictions of the experimental data. We conclude that Huxley and Hill MTC models are equally valid with respect to mechanical behaviour.