Meeting Abstract

42.5  Wednesday, Jan. 5  Force-length properties, pinnation angle and mechanical advantage of the goat medial and lateral gastrocnemius in relation to in vivo contractile function and muscle models. DE BOEF MIARA, M.*; LEE, S.M.; WAKELING, J.M.; BIEWENER, A.A.; Harvard University, Cambridge, MA; Simon Fraser University, Burnaby, BC; Simon Fraser University, Burnaby, BC; Harvard University, Cambridge, MA mdeboef@fas.harvard.edu

Muscles are able to actively produce the greatest force when operating at intermediate lengths along their force-length (F-L) curve. In vivo recordings of the operating lengths of the goat medial gastrocnemius (MG) and lateral gastrocnemius (LG) have previously been made, but it is unknown how these lengths correspond to the muscles’ F-L properties, how pinnation angles changes with changing fascicle length or how mechanical advantage changes with changing joint angle. Consequently, in situ measures of MG and LG muscle fascicle lengths, muscle pinnation angles, muscle forces and joint angles were obtained during fixed–end contractions following in vivo treadmill and jumping trials. Muscle fascicle lengths and pinnation angles were obtained using sonomicrometry crystals and muscle forces via tendon buckles, with the muscles stimulated by nerve cuffs. Both muscles exhibited stereotypical force-length properties: forces increased with increasing fascicle length to an optimum, then decreased with further increases in length. In some cases, there were large increases in force over very small increases in fascicle length on the ascending limb of the curve. These were accompanied by changes in pinnation angle that allowed for continuous shortening of the overall muscle even when fascicle lengths changed very little. In vivo fascicle length measures showed that these muscles operated at approximately 85% of optimal length during walking and increased to optimal length (maximum isometric force) with increasing speed or inclination. However, during jumping, when the greatest forces were produced these muscles surprisingly operated at lengths much shorter than optimal. (Supported by NIH AR055648).