Mayfield, D. L. | Search bioRxiv

bioRxiv2024

Does force depression resulting from shortening against series elasticity contribute to the activation dependence of optimum length?

The optimum length for force generation (L0) increases as activation is reduced, challenging classic theories of muscle contraction. Although the activation dependence of L0 is seemingly consistent with length-dependent Ca2+ sensitivity, this mechanism cant explain the apparent force dependence of L0, or the effect of series compliance on activation-related shifts in L0. We have tested a theory proposing that the activation dependence of L0 relates to force depression resulting from shortening against series elasticity. This theory predicts that significant series compliance would cause tetanic L0 to be shorter than the length corresponding to optimal filament overlap, thereby increasing the activation dependence of L0. We tested this prediction by determining L0 and maximum tetanic force (P0) with (L0_spring, P0_spring) and without added compliance in bullfrog semitendinosus muscles. The activation dependence of L0 was characterised with the addition of twitch and doublet contractions. Springs attached to muscles gave added fixed-end compliances of 11-39%, and this added compliance induced force depression for tetanic fixed-end contractions (P0_spring/P0 < 1). We found strong, negative correlations between spring compliance and both P0_spring (r2 = 0.89-91) and L0_spring (r2 = 0.60-63; P < 0.001), while the activation dependence of L0 was positively correlated to added compliance (r2 = 0.45, P = 0.011). However, since the compliance-mediated reduction in L0 was modest relative to the activation-related shift reported for the bullfrog plantaris muscle, additional factors must be considered. Our demonstration of force depression under novel conditions adds support to the involvement of a stress-induced inhibition of cross-bridge binding.

physiology↗

bioRxiv2023

Linking in vivo muscle dynamics to in situ force-length and force-velocity reveals that guinea fowl lateral gastrocnemius operates at shorter than optimal lengths

Force-length (F-L) and force-velocity (F-V) properties characterize skeletal muscles intrinsic properties under controlled conditions, and it is thought that these properties can inform and predict in vivo muscle function. Here, we map dynamic in vivo operating range and mechanical function during walking and running, to the measured in situ F-L and F-V characteristics of guinea fowl (Numida meleagris) lateral gastrocnemius (LG), a primary ankle extensor. We use in vivo patterns 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 vivo LG 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 L0 at 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 zero in situ, suggesting that dynamic history dependent and viscoelastic effects may contribute to in vivo force development. Direct comparison of in vivo work 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 resemble in vivo dynamics.

physiology↗