Meeting Abstract

43.3  Saturday, Jan. 5  The effect of muscle fibre recruitment on force-velocity properties and the implications for Hill-type models HOLT, NC*; MIARA, M; WAKELING, JM; BIEWENER, AA; Concord Field Station; Concord Field Station; Simon Fraser University; Concord Field Station natalie.c.holt@gmail.com

Hill-type muscle models, which are broadly used in human applications, provide a simple way to predict muscle forces based on fibre length and force-velocity properties. However, muscles are not, as these models assume, homogenous and maximally activated. Instead, they are composed of mechanically distinct fibre types; a subset of which is recruited to meet mechanical demand. Therefore, a muscle’s force-velocity properties depend on its instantaneous activation state. Failure to account for this is likely to lead to errors in the prediction of dynamic forces. However, no data exist to show the effect of recruitment of fast and slow-twitch fibres on muscle force-velocity properties. This study aims to determine the force-velocity properties of a muscle with selective activation of different muscle fibre types and to evaluate the effect of accounting for recruitment in Hill-type models. The plantaris muscles of anaesthetized rats were stimulated, via the sciatic nerve, to activate all muscle fibres or, selectively activate slow-twitch fibres. Isometric and after-loaded isotonic tetani were performed, muscle force and muscle length change recorded, and electromyography performed. Preliminary data show that activation of only slow muscle fibres resulted in lower maximum isometric force (1.98 Ncm^-2 vs. 4.49 Ncm^-2) and maximum shortening velocity (1.11 Ls^-1 vs. 4.10 Ls^-1) reflecting the lower number of active fibres and their mechanical properties. These data will be incorporated into a Hill-type model which accounts for muscle fibre recruitment and the predictions of this model validated against in vivo measures of muscle force. (Supported by NIH AR055648)