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 (Delta F-im). A hill-type muscle model assuming an identical gearing G between both Delta F-im and F-G as well as lifting height of the load (Delta h) and longitudinal muscle shortening (Delta l(cc)) reproduced experimental data fora single load. Here we tested if this model is able to reproduce experimental changes in Delta F-im and Delta h 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 Delta F-im, for each transverse load. Simulations using G(c) overestimated Delta F-im, (up to 59%) and Delta h (up to 136%) for increasing load. Although the model assumption (equal G for forces and length changes) held for the three lower loads using Gem, and G(f), simulations resulted in underestimation of Delta F-im, by 38% and overestimation of Delta h by 58% for the largest load, respectively. To simultaneously reproduce experimental Delta F-im, and [SA 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. (C) 2017 Elsevier Ltd. All rights reserved.