The University of Queensland, School of Human Movement and Nutrition Sciences, Centre for Sensorimotor Performance , Brisbane, Queensland , Australia.
J Appl Physiol (1985). 2018 Apr 1;124(4):993-1002. doi: 10.1152/japplphysiol.00356.2017. Epub 2018 Jan 4.
The maximum force-generating capacity of a muscle is dependent on the lengths and velocities of its contractile apparatus. Muscle-tendon unit (MTU) length changes can be estimated from joint kinematics; however, contractile element length changes are more difficult to predict during dynamic contractions. The aim of this study was to compare vastus lateralis (VL) MTU and fascicle level force-length and force-velocity relationships, and dynamic muscle function while cycling at a constant submaximal power output (2.5 W/kg) with different cadences. We hypothesized that manipulating cadence at a constant power output would not affect VL MTU shortening, but significantly affect VL fascicle shortening. Furthermore, these differences would affect the predicted force capacity of the muscle. Using an isokinetic dynamometer and B-mode ultrasound (US), we determined the force-length and force-velocity properties of the VL MTU and its fascicles. In addition, three-dimensional kinematics and kinetics of the lower limb, as well as US images of VL fascicles were collected during submaximal cycling at cadences of 40, 60, 80, and 100 rotations per minute. Ultrasound measures revealed a significant increase in fascicle shortening as cadence decreased (84% increase across all conditions, P < 0.01), whereas there were no significant differences in MTU lengths across any of the cycling conditions (maximum of 6%). The MTU analysis resulted in greater predicted force capacity across all conditions relative to the force-velocity relationship ( P < 0.01). These results reinforce the need to determine muscle mechanics in terms of separate contractile element and connective tissue length changes during isokinetic contractions, as well as dynamic movements like cycling. NEW & NOTEWORTHY We demonstrate that vastus lateralis (VL) muscle tendon unit (MTU) length changes do not adequately reflect the underlying fascicle mechanics during cycling. When examined across different pedaling cadence conditions, the force-generating potential measured only at the level of MTU (or joint) overestimated the maximum force capacity of VL compared with analysis using fascicle level data.
肌肉产生最大力的能力取决于其收缩装置的长度和速度。可以根据关节运动学来估计肌肉-肌腱单位（MTU）的长度变化；然而，在动态收缩过程中，更难以预测收缩成分的长度变化。本研究的目的是比较股外侧肌（VL）MTU 和肌束水平的力-长度和力-速度关系，以及在以恒定次最大功率输出（2.5 W/kg）下以不同的踏频进行动态收缩时的肌肉功能。我们假设在恒定功率输出下改变踏频不会影响 VL MTU 的缩短，但会显著影响 VL 肌束的缩短。此外，这些差异会影响肌肉的预测力能力。我们使用等速测力计和 B 型超声（US）来确定 VL MTU 及其肌束的力-长度和力-速度特性。此外，在 40、60、80 和 100 转/分钟的踏频下进行亚最大强度自行车运动时，还收集了下肢的三维运动学和动力学以及 VL 肌束的 US 图像。超声测量结果显示，随着踏频的降低，肌束缩短明显增加（所有条件下增加 84%，P < 0.01），而在任何自行车运动条件下，MTU 长度均无显著差异（最大为 6%）。与力-速度关系相比，MTU 分析得出在所有条件下都具有更大的预测力能力（P < 0.01）。这些结果强调了在等速收缩以及像骑自行车这样的动态运动中，需要根据单独的收缩成分和结缔组织长度变化来确定肌肉力学。
我们证明，股外侧肌（VL）肌肉-肌腱单位（MTU）的长度变化不能充分反映骑自行车时的肌束力学。在检查不同的踩踏节奏条件时，仅在 MTU（或关节）水平测量的产生力的潜力与使用肌束水平数据的分析相比，高估了 VL 的最大力能力。
