A comparison of four different muscle pennation models and their effects on predictions in peak fiber force and operating range of fiber length

In this study, the effects of various pennation models on predictions of total fiber force and fiber length were compared. Four different pennation models were assumed, and musculotendon dynamics was performed with each pennation model for two different simulations: (1) isokinetic contraction at angular velocities of 1, 50, 100, and 500°/s with fully-activated tibialis anterior (TA) muscle; and (2) isometric contraction at -30°, -15°, 0°, and 15° with altered TA muscle activation. Total fiber force and fiber length obtained from each model were compared. Results from the isokinetic simulation showed that errors in the peak normalized fiber force were approximately ±1%, and the operating range of the normalized fiber length was the narrowest in the realistic model. Pennation angles from the realistic model showed a considerable variation compared with the variable model in isometric simulation; moreover, the realistic model predicted the largest forces and the narrowest operating range of the normalized fiber length. From this study, the variable pennation model was good for the condition of fixed muscle activation, but rather underestimated the force when the muscle activation varied. In addition, it was confirmed that the changes in pennation angle mainly result in the operating range of the fiber length.