Schwaner M J, Mayfield D L, Azizi E, Daley M A
Department of Ecology and Evolutionary Biology, University of California, Irvine, CA United States.
Department of Evolution, Ecology, and Organismal Biology, University of California, Riverside, CA, United States.
bioRxiv. 2023 Oct 16:2023.10.11.561922. doi: 10.1101/2023.10.11.561922.
Force-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 predict muscle function. Here, we map dynamic operating range and mechanical function during walking and running, to the measured F-L and F-V characteristics of guinea fowl () lateral gastrocnemius (LG), a primary ankle extensor. We use 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: 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 L 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 , suggesting that dynamic history dependent and viscoelastic effects may contribute to force development. Direct comparison of 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 dynamics.
力-长度（F-L）和力-速度（F-V）特性表征了骨骼肌在受控条件下的内在特性，人们认为这些特性可以为肌肉功能提供信息并进行预测。在此，我们将行走和跑步过程中的动态工作范围和力学功能，与珍珠鸡外侧腓肠肌（LG）（主要的踝关节伸肌）测量得到的F-L和F-V特性进行映射。我们使用肌腱力、肌束长度和激活模式来检验这一假设，即肌束在行走和跑步过程中以最佳长度和速度运行，以最大限度地提高力或功率的产生。我们的研究结果仅部分支持我们的假设：LG的速度与在工作产生过程中优化功率以及在较高负荷下的力经济性一致。然而，在产生力的过程中，LG并非在力平台（±5%Fmax）上的长度运行。LG在肌电图开始时的长度接近L，但迅速缩短，以至于在站立期的力发展几乎完全发生在F-L曲线的上升支上，长度短于最佳长度。这些数据表明，肌束在摆动后期缩短至最佳长度，以优化在摆动-站立转换附近快速产生力的潜力。这可能为抵抗在足部接触时需要快速产生力的意外扰动提供阻力。我们还发现了在摆动后期被动力上升（在无肌电图活动的情况下）的证据，此时被动力为零，这表明动态历史依赖性和粘弹性效应可能有助于力的发展。将工作环和生理工作范围与传统的F-L和F-V特性测量进行直接比较，表明需要新的方法来表征在更接近动态的受控条件下的动态肌肉特性。
