Skeletal muscle has fiber architectures ranging from simple to complex, alongside variation in fiber-type and neuro-anatomical compartmentalization. However, the functional implications of muscle subdivision into discrete functional units remain poorly understood. The rat medial gastrocnemius has well-characterized regions with distinct architectures and fiber type composition. Here, force-length and force-velocity contractions were performed for two stimulation intensities (supramaximal and submaximal) and for three structural units (whole muscle belly, proximal region and distal region) to assess the effect of muscle compartmentalization on contractile force-length-velocity relations and optimal speed for power production. Additionally, fiber strain, fiber rotation, pennation, and architectural gearing were quantified. Our results suggest that the proximal and distal muscle regions have fundamentally different physiological function. During supramaximal activation, the proximal region has shorter (8.4±0.8mm vs 10.9±0.7mm) fibers and steeper (28.7±11.0° vs 19.6±6.3°) fiber angles at optimum length, and operates over a larger (17.9 ± 3.8% vs 12.6 ± 2.7%) range of its force-length curve. The proximal region also exhibits larger changes in pennation angle (5.6 ± 2.2°/mm vs 2.4 ± 1.5°/mm muscle shortening) and architectural gearing (1.82 ± 0.53 vs 1.25 ± 0.24); whereas, the distal region exhibits greater peak shortening speed (96.0mm/s vs 81.3mm/s) and 18-27% greater optimal speed. Overall, similar patterns were observed during submaximal activation. These regional differences in physiological function with respect to the whole muscle highlight how variation in motor recruitment could fundamentally shift regional functional patterns within a single muscle, which likely has important implications for whole muscle force and work output in vivo.

中文翻译：

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肌肉刺激强度对结构复杂肌肉内区域异质功能的影响

骨骼肌具有从简单到复杂的纤维结构，以及纤维类型和神经解剖分区的变化。然而，肌肉细分为离散功能单元的功能含义仍然知之甚少。大鼠内侧腓肠肌具有特征明确的区域，具有不同的结构和纤维类型组成。在这里，对两种刺激强度（超最大和次最大）和三个结构单元（整个肌肉腹部、近端区域和远端区域）进行力-长度和力-速度收缩，以评估肌肉划分对收缩力-长度-的影响。速度关系和电力生产的最佳速度。此外，还量化了纤维应变、纤维旋转、羽状排列和建筑传动装置。我们的研究结果表明，近端和远端肌肉区域具有根本不同的生理功能。在超大激活期间，近端区域在最佳长度处具有较短（8.4±0.8mm 对 10.9±0.7mm）纤维和更陡峭（28.7±11.0°对 19.6±6.3°）纤维角度，并且在更大的（17.9±3.8%与其力-长度曲线的 12.6 ± 2.7%) 范围相比。近端区域也表现出较大的羽状角变化（5.6 ± 2.2°/mm vs 2.4 ± 1.5°/mm 肌肉缩短）和结构传动装置（1.82 ± 0.53 vs 1.25 ± 0.24）；而远端区域表现出更大的峰值缩短速度（96.0mm/s 对 81.3mm/s）和 18-27% 的最佳速度。总体而言，在次最大激活期间观察到类似的模式。