Influence of muscle compression on dynamic muscle performance

Abstract

During daily activities such as sitting, carrying loads or wearing orthoses, skeletal muscles are compressed. Surrounding muscles and bones can also induce force leading to compression of the muscle. The aim of the present study was to examine the influence of varying unidirectional transversal muscle loading on contraction dynamics. We performed isometric experiments on isolated rat M. gastrocnemius medialis (n = 10) without and with five different transversal loads (0.64, 1.13, 1.62, 2.11, 2.60 N). The muscles were loaded by a custom-made plunger in transversal direction. The muscle force was measured at the distal tendon and the vertical movement of the plunger was captured with a high-speed camera during supramaximal muscle stimulation. Increasing transversal muscle force resulted in an almost linear decrease in maximum isometric muscle force (Fim) from 5.0 ± 1.5 % Fim to 13.1 ± 2.1 % Fim. Compared with an unloaded isometric contraction, the rate of force development (RFD) decreased from 20.7 ± 4.1 % RFD to 35.2 ± 5.8 % RFD. The lifting height of the plunger decreased from 1.7 ± 0.2 mm at the lowest transversal load to 0.6 ± 0.2 mm at the maximum load applied. Unidirectional transversal compression depresses longitudinal force development in skeletal muscles and, thus, should be considered in musculo-skeletal models simulating the interaction of muscles with their mechanical environment, e.g. in impact biomechanics. The interactions between the muscle and one unidirectional transversal load could be explained reasonably well with a simple model including a hill-type muscle model, a geometric model transferring transversal forces into a longitudinal direction and a viscoelastic model representing the characteristics of passive muscle tissue. Data presented in this study may be important to develop and validate muscle models which enable simulation of muscle contractions under compression and reveal the mechanisms behind.