Askew G N, Marsh R L
Department of Biology, Northeastern University, Boston, MA 02115, USA.
J Exp Biol. 1998 May;201(Pt 10):1527-40. doi: 10.1242/jeb.201.10.1527.
The force-velocity relationship has frequently been used to predict the shortening velocity that muscles should use to generate maximal net power output. Such predictions ignore other well-characterized intrinsic properties of the muscle, such as the length-force relationship and the kinetics of activation and deactivation (relaxation). We examined the effects of relative shortening velocity on the maximum net power output (over the entire cycle) of mouse soleus muscle, using sawtooth strain trajectories over a range of cycle frequencies. The strain trajectory was varied such that the proportion of the cycle spent shortening was 25, 50 or 75 % of the total cycle duration. A peak isotonic power output of 167 W kg-1 was obtained at a relative shortening velocity (V/Vmax) of 0.22. Over the range of cyclical contractions studied, the optimal V/Vmax for power production ranged almost fourfold from 0.075 to 0.30, with a maximum net power output of 94 W kg-1. The net power output increased as the proportion of the cycle spent shortening increased. Under conditions where the strain amplitude was high (i.e. low cycle frequencies and strain trajectories where the proportion of time spent shortening was greater than that spent lengthening), the effects of the length-force relationship reduced the optimal V/Vmax below that predicted from the force-velocity curve. At high cycle frequencies and also for strain trajectories with brief shortening periods, higher rates of activation and deactivation with increased strain rate shifted the optimal V/Vmax above that predicted from the force-velocity relationship. Thus, the force-velocity relationship alone does not accurately predict the optimal V/Vmax for maximum power production in muscles that operate over a wide range of conditions (e.g. red muscle of fish). The change in the rates of activation and deactivation with increasing velocity of stretch and shortening, respectively, made it difficult to model force accurately on the basis of the force-velocity and length-force relationships and isometric activation and deactivation kinetics. The discrepancies between the modelled and measured forces were largest at high cycle frequencies.
力-速度关系常常被用于预测肌肉为产生最大净功率输出应采用的缩短速度。此类预测忽略了肌肉的其他已充分表征的固有特性，比如长度-力关系以及激活和失活（松弛）的动力学特性。我们使用一系列循环频率下的锯齿形应变轨迹，研究了相对缩短速度对小鼠比目鱼肌最大净功率输出（在整个周期内）的影响。改变应变轨迹，使得缩短过程在整个周期时长中所占比例为25%、50%或75%。在相对缩短速度（V/Vmax）为0.22时，获得了167 W kg-1的峰值等张功率输出。在所研究的周期性收缩范围内，产生功率的最佳V/Vmax从0.075到0.30几乎相差四倍，最大净功率输出为94 W kg-1。随着缩短过程在周期中所占比例增加，净功率输出也增加。在应变幅度较高的条件下（即低循环频率以及缩短时间所占比例大于延长时间的应变轨迹），长度-力关系的影响使得最佳V/Vmax低于从力-速度曲线预测的值。在高循环频率以及缩短期较短的应变轨迹情况下，随着应变率增加，激活和失活速率更高，使得最佳V/Vmax高于从力-速度关系预测的值。因此，仅有力-速度关系并不能准确预测在广泛条件下工作的肌肉（如鱼类的红色肌肉）产生最大功率时的最佳V/Vmax。随着拉伸和缩短速度增加，激活和失活速率分别发生变化，这使得难以基于力-速度和长度-力关系以及等长激活和失活动力学准确模拟力。在高循环频率下，模拟力与测量力之间的差异最大。
