Stiffness of the fastrocnemius medialis muscle-tendon complex and isolated tendon of the rat measured by sinusoidal length changes

Abstract

STFFNESS OF THB GASTROCNBMIUS MEDIALIS MUSCLE-TBNDGN COMPLBX AND ISGLA‘IED TENDON OF THE RAT MEASURED BY SINUSOIDAL LENGTH CHANGES Gertjau J.C. Ettema and Peter A. Huijing Faculteit der Beweglngswetenschappen Vxije Uulverslteit Amsterdam, Van der Boechorststraat 9,108l BT Amsterdam, The Netherlands The aim of the present investigation was to determine at which frequencies of small sinusoidal leugth changes the. muscle-tendon complex acts within it’s short range stiffness. Furthermore, the relation between force aud stiffness for the muscle-tendon complex as well as for the isolated ten&n was studied. Sinusoidal length changes (frequency: 10 120 Hz, amplitude: 0.05 0.1 mm, and stepwlse length changes (+0.2 and -0.2 mm) were imposed on four GM muscle-tendon complexes of the Wlstar rat during isometric tetaulc contractions at several muscle lengths. Isometric fonze levels ranged from maximal isometric force (Fo) to about 5% Fo. Sinusoidal length changes were also imposed on two isolated teudous (GM aud Tibialis Anterior). All measurements were done at 27O C. Elastic stiffness (ES) (Butler, D.L. et al. in Handbook of engiueetiug in medicine and biology, pp 279-314,1978) of the muscle-tendon complex was compared to series elastic stiffhess as determined by means of step-wise length changes (SS). For the 80 and 120 Hz movements ES was similar to SS, whereas for the lower frequencies (10 60 Hz) much lower ES values are found, indicating leugth changes of the coutmctlle machinery. Force responses of both muscle-tendon complex and isolated tendon were merely elastic of nature. only at extreme low force levels a considerable visco-elastlc behaviour was found. The stiffness of the muscle-tendon complex as well as the stiffness of the isolated tendon continued to increase with force, even at force levels near Fo. This is in contradiction with the generally accepted opinion that tendluous stiffness is constaut at force levels above 30% Fo. The current results indicate that using sinusoidal movements of small amplitudes a frequency of 80 to 120 Hz is sufficient for an acamte determination of the series elastic (short range) stiffness of the rat GM.