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

Recent studies demonstrated that uniaxial transverse loading (F _G ) of a rat gastrocnemius medialis muscle resulted in a considerable reduction of maximum isometric muscle force (ΔF _im ). A hill-type muscle model assuming an identical gearing G between both ΔF _im and F _G as well as lifting height of the load (Δh) and longitudinal muscle shortening (Δl _CC ) reproduced experimental data for a single load. Here we tested if this model is able to reproduce experimental changes in ΔF _im 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 G _c representing the idea of a muscle specific gearing parameter (e.g. predefined by the muscle geometry), (II) G _exp determined in experiments with varying transverse load, and (III) G _f that reproduced experimental ΔF _im for each transverse load. Simulations using G _c overestimated ΔF _im (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 G _exp and G _f , simulations resulted in underestimation of ΔF _im by 38% and overestimation of Δh by 58% for the largest load, respectively. To simultaneously reproduce experimental ΔF _im and Δh for the two larger loads, it was necessary to reduce F _im 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.
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