A comparison of models of force production during stimulated isometric ankle dorsiflexion in humans.

In this paper, we compare seven models on their ability to fit isometric muscle force. We stimulated the ankle dorsiflexors of eight subjects at seven ankle angles (85 degrees-120 degrees). Three different stimulation patterns (twitch, triangular, and random) were applied at all ankle angles. Four additional patterns (doublets, steady rates, "catch property," and walking-like) were applied at 95 degrees. Parameter values were optimized for each model at each angle. Parameters for the general linear model were calculated using a novel least-squares algorithm. A linear, second-order critically damped model gave the poorest fits (average root mean square (rms) error: 15 N). The models of Ding et al. (2002) and Bobet and Stein (1998) gave the best fits (average rms errors: 9.2 and 9.4 N). The other models (general linear second-order model, Wiener model, Zhou et al. (1995) model, general linear model) gave intermediate results. Results were similar at all ankle angles. We conclude that the Ding and Bobet-Stein models are the best overall for isometric contractions, that no linear model of any kind will give an error less than 9% of maximum force, and that the models tested are consistent across lengths.