A hill-type muscle model expansion accounting for effects of varying transverse muscle load.

Recent studies demonstrated that uniaxial transverse loading (FG) of a rat gastrocnemius medialis muscle resulted in a considerable reduction of maximum isometric muscle force (ΔFim). A hill-type muscle model assuming an identical gearing G between both ΔFim and FG as well as lifting height of the load (Δh) and longitudinal muscle shortening (ΔlCC) reproduced experimental data for a single load. Here we tested if this model is able to reproduce experimental changes in ΔFim and Δh for increasing transverse loads (0.64 N, 1.13 N, 1.62 N, 2.11 N, 2.60 N). Three different gearing ratios were tested: (I) constant Gc representing the idea of a muscle specific gearing parameter (e.g. predefined by the muscle geometry), (II) Gexp determined in experiments with varying transverse load, and (III) Gf that reproduced experimental ΔFim for each transverse load. Simulations using Gc overestimated ΔFim (up to 59%) and Δh (up to 136%) for increasing load. Although the model assumption (equal G for forces and length changes) held for the three lower loads using Gexp and Gf, simulations resulted in underestimation of ΔFim by 38% and overestimation of Δh by 58% for the largest load, respectively. To simultaneously reproduce experimental ΔFim and Δh for the two larger loads, it was necessary to reduce Fim by 1.9% and 4.6%, respectively. The model seems applicable to account for effects of muscle deformation within a range of transverse loading when using a linear load-dependent function for G.