Meeting Abstract

P3-106    Effect of recruitment level on architectural gearing in young and old muscle Mayfield, DL*; Holt, NC; Azizi, E; University of California, Riverside; University of California, Riverside; University of California, Irvine dean.mayfield@uqconnect.edu.au

Skeletal muscle undergoes dynamic shape changes during contraction to accommodate the radial expansion of shortening fibers. In pennate muscle, fibers may rotate during contraction such that muscle shortening speed exceeds fascicle shortening speed. Architectural gearing, the ratio between muscle and fiber shortening, is thought to be dependent on contractile force; being high during low-force contractions and decreasing as contractile force increases. The loss of variable gearing with age has been interpreted as evidence that this mediation of gearing by force is due to the interaction between contractile force and the resistance to muscle shape change imparted by connective tissue stiffness. However, this idea appears to be contradicted by recent findings showing gearing in young muscle to be similar for maximal and submaximal recruitment levels when force is normalized to recruitment. If gearing was entirely dependent on contractile force, we would expect higher gear ratios at lower activation levels. This study aims to clarify whether gearing is modulated by recruitment level and to extend this work by examining the interactive effect of age and recruitment. Gearing was quantified for the medial gastrocnemius muscle of young and old rats with supramaximal and submaximal stimulation (i.e. 50% of maximum isometric force). Preliminary results indicate that, in young and old muscle, the relationship between gear ratio and force is affected by recruitment level. In submaximal contractions, gearing is lower at higher forces. Interestingly, when force was normalized to recruitment level, variable gearing was similar for maximal and submaximal conditions, as reported previously. These findings suggest that resistance to shape change is not solely a consequence of the interaction between contractile force and connective tissue resistance but influenced by the proportion of active muscle fibers.