Joint Department of Biomedical Engineering, University of North Carolina, Chapel Hill, NC 27599-7575, USA.
Proc Natl Acad Sci U S A. 2012 Jan 17;109(3):977-82. doi: 10.1073/pnas.1107972109. Epub 2012 Jan 4.
Humans walk and run over a wide range of speeds with remarkable efficiency. For steady locomotion, moving at different speeds requires the muscle-tendon units of the leg to modulate the amount of mechanical power the limb absorbs and outputs in each step. How individual muscles adapt their behavior to modulate limb power output has been examined using computer simulation and animal models, but has not been studied in vivo in humans. In this study, we used a combination of ultrasound imaging and motion analysis to examine how medial gastrocnemius (MG) muscle-tendon unit behavior is adjusted to meet the varying mechanical demands of different locomotor speeds during walking and running in humans. The results highlighted key differences in MG fascicle-shortening velocity with both locomotor speed and gait. Fascicle-shortening velocity at the time of peak muscle force production increased with walking speed, impairing the ability of the muscle to produce high peak forces. Switching to a running gait at 2.0 m·s(-1) caused fascicle shortening at the time of peak force production to shift to much slower velocities. This velocity shift facilitated a large increase in peak muscle force and an increase in MG power output. MG fascicle velocity may be a key factor that limits the speeds humans choose to walk at, and may explain the transition from walking to running. This finding is consistent with previous modeling studies.
人类以极高的效率在很宽的速度范围内行走和奔跑。为了实现稳定的运动，以不同的速度移动需要腿部的肌肉-肌腱单元来调节肢体在每一步中吸收和输出的机械功率量。个体肌肉如何调整其行为以调节肢体功率输出，已经通过计算机模拟和动物模型进行了研究，但在人类的体内尚未进行研究。在这项研究中，我们使用超声成像和运动分析相结合的方法，研究了在人类行走和跑步时，不同的运动速度如何调整内侧腓肠肌（MG）肌肉-肌腱单元的行为以满足不同的机械需求。结果突出了 MG 肌束缩短速度与运动速度和步态的关键差异。在肌肉力量产生的峰值时，肌束缩短速度随着步行速度的增加而增加，从而降低了肌肉产生高峰值力量的能力。当以 2.0 m·s(-1) 的速度切换到跑步步态时，肌肉力量峰值时的肌束缩短速度会转移到更慢的速度。这种速度的转移促进了肌肉力量的大幅增加和 MG 功率输出的增加。MG 肌束速度可能是限制人类选择行走速度的关键因素，并且可能解释了从行走到跑步的转变。这一发现与之前的建模研究一致。
