Bobbert Maarten Frank, Casius L J Richard, Van Soest Arthur J
MOVE Research Institute Amsterdam, Faculty of Human Movement Sciences, VU University Amsterdam, Amsterdam, THE NETHERLANDS.
Med Sci Sports Exerc. 2016 May;48(5):869-78. doi: 10.1249/MSS.0000000000000845.
Relationships between tangential pedal force and crank angular velocity in sprint cycling tend to be linear. We set out to understand why they are not hyperbolic, like the intrinsic force-velocity relationship of muscles.
We simulated isokinetic sprint cycling at crank angular velocities ranging from 30 to 150 rpm with a forward dynamic model of the human musculoskeletal system actuated by eight lower extremity muscle groups. The input of the model was muscle stimulation over time, which we optimized to maximize average power output over a cycle.
Peak tangential pedal force was found to drop more with crank angular velocity than expected based on intrinsic muscle properties. This linearizing effect was not due to segmental dynamics but rather due to active state dynamics. Maximizing average power in cycling requires muscles to bring their active state from as high as possible during shortening to as low as possible during lengthening. Reducing the active state is a relatively slow process, and hence must be initiated a certain amount of time before lengthening starts. As crank angular velocity goes up, this amount of time corresponds to a greater angular displacement, so the instant of switching off extensor muscle stimulation must occur earlier relative to the angle at which pedal force was extracted for the force-velocity relationship.
Relationships between pedal force and crank angular velocity in sprint cycling do not reflect solely the intrinsic force-velocity relationship of muscles but also the consequences of activation dynamics.
短距离自行车冲刺时切向踏板力与曲柄角速度之间的关系往往呈线性。我们试图弄清楚为何它们不像肌肉的内在力-速度关系那样呈双曲线关系。
我们使用由八个下肢肌肉群驱动的人体肌肉骨骼系统的正向动力学模型，模拟了曲柄角速度在30至150转/分钟范围内的等速短距离自行车冲刺。模型的输入是随时间变化的肌肉刺激，我们对其进行了优化，以在一个周期内最大化平均功率输出。
发现峰值切向踏板力随曲柄角速度下降的幅度比基于肌肉固有特性预期的要大。这种线性化效应并非由于节段动力学，而是由于激活状态动力学。在骑行中最大化平均功率要求肌肉在缩短过程中将其激活状态从尽可能高降至在拉长过程中尽可能低。降低激活状态是一个相对缓慢的过程，因此必须在拉长开始前的一定时间就启动。随着曲柄角速度上升，这段时间对应着更大的角位移，所以相对于为了力-速度关系而提取踏板力的角度，关闭伸肌肌肉刺激的瞬间必须更早发生。
短距离自行车冲刺中踏板力与曲柄角速度之间的关系不仅反映了肌肉的内在力-速度关系，还反映了激活动力学的影响。
