ESM Raw data from Skeletal muscle function underpins muscle spindle abundance

Muscle spindle abundance is highly variable within and across species, but we currently lack any clear picture of the mechanistic causes or consequences of this variation. Previous use of spindle abundance as a correlate for muscle function implies a mechanical underpinning to this variation, but these ideas have not been tested. Herein, we use integrated medical imaging and subject-specific musculoskeletal models to investigate the relationship between spindle abundance, muscle architecture and in vivo muscle behaviour in the human locomotor system. These analyses indicate that muscle spindle number is tightly correlated with muscle fascicle length, absolute fascicle length change, velocity of fibre lengthening and active muscle forces during walking. Novel correlations between functional indices and spindle abundance are also recovered, where muscles with a high abundance predominantly function as springs, compared to those with a lower abundance mostly functioning as brakes during walking. These data demonstrate that muscle fibre length, lengthening velocity and fibre force are key physiological signals to the central nervous system and its modulation of locomotion, and that muscle spindle abundance may be tightly correlated to how a muscle generates work. These insights may be combined with neuromechanics and robotic studies of motor control to help further tease apart the functional drivers of muscle spindle composition.

肌梭（muscle spindle）丰度在物种内部及跨物种间均存在显著差异，但目前学界对该差异的机制成因与影响尚缺乏清晰认知。既往将肌梭丰度作为肌肉功能关联指标的研究，暗示该差异存在力学基础，但相关假说尚未得到验证。本研究采用整合医学成像与个体特异性肌肉骨骼模型，探究人类运动系统中肌梭丰度、肌肉架构与活体肌肉行为之间的关联。本分析结果显示，行走过程中肌梭数量与肌束（muscle fascicle）长度、肌束绝对长度变化量、纤维拉长速度及主动肌肉力均呈显著相关。本研究还揭示了功能指标与肌梭丰度间的全新关联：行走过程中，肌梭丰度较高的肌肉主要发挥弹簧样功能，而丰度较低的肌肉则多充当制动器。本研究数据表明，肌纤维长度、拉长速度与纤维力是中枢神经系统调控运动的关键生理信号，而肌梭丰度或与肌肉做功方式密切相关。上述研究结论可与神经力学及运动控制机器人学研究相结合，以进一步厘清肌梭组成的功能驱动因素。