Linkingin vivomuscle dynamics toin situforce-length and force-velocity reveals that guinea fowl lateral gastrocnemius operates at shorter than optimal lengths

Abstract

ABSTRACTForce-length (F-L) and force-velocity (F-V) properties characterize skeletal muscle’s intrinsic properties under controlled conditions, and it is thought that these properties can inform and predictin vivomuscle function. Here, we map dynamicin vivooperating range and mechanical function during walking and running, to the measuredin situF-L and F-V characteristics of guinea fowl (Numida meleagris) lateral gastrocnemius (LG), a primary ankle extensor. We usein vivopatterns of muscle tendon force, fascicle length, and activation to test the hypothesis that muscle fascicles operate at optimal lengths and velocities to maximize force or power production during walking and running. Our findings only partly support our hypothesis:in vivoLG velocities are consistent with optimizing power during work production, and economy of force at higher loads. However, LG does not operate at lengths on the force plateau (±5% Fmax) during force production. LG length was near L0at the time of EMG onset but shortened rapidly such that force development during stance occurred almost entirely on the ascending limb of the F-L curve, at shorter than optimal lengths. These data suggest that muscle fascicles shorten across optimal lengths in late swing, to optimize the potential for rapid force development near the swing-stance transition. This may provide resistance against unexpected perturbations that require rapid force development at foot contact. We also found evidence of passive force rise (in absence of EMG activity) in late swing, at lengths where passive force is zeroin situ, suggesting that dynamic history dependent and viscoelastic effects may contribute toin vivoforce development. Direct comparison ofin vivowork loops and physiological operating ranges to traditional measures of F-L and F-V properties suggests the need for new approaches to characterize dynamic muscle properties in controlled conditions that more closely resemblein vivodynamics.