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 <i>in vivo</i> 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）的丰度在物种内部及跨物种间均存在显著异质性，但目前学界对该差异的机制性成因与后果仍缺乏清晰认知。此前有研究将肌梭丰度作为肌肉功能的关联指标，暗示该差异存在力学基础，但此类假说尚未得到实验验证。本研究通过整合医学成像技术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、肌肉架构与体内（in vivo）肌肉行为之间的关联。分析结果显示，肌梭数量与肌纤维束（muscle fascicle）长度、肌纤维束绝对长度变化量、纤维拉长速度以及行走过程中的主动肌肉力量呈显著相关。研究还发现了功能指标与肌梭丰度之间的全新关联：丰度较高的肌肉在行走过程中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、拉长速度与纤维力量是中枢神经系统调控运动的关键生理信号，而肌梭丰度或许与肌肉做功的方式密切相关。上述研究结论可结合神经力学与运动控制的机器人学研究，进一步厘清肌梭组成的功能性驱动因素。