Motorized assessment of the stretch reflex is instrumental to gain understanding of the stretch reflex, its physiological origin and to differentiate effects of neurological disorders, like spasticity. Both short-latency (M1) and medium-latency (M2) stretch reflexes have been reported to depend on the velocity and acceleration of an applied ramp-and-hold perturbation. In the upper limb, M2 has also been reported to depend on stretch duration. However, wrong conclusions might have been drawn in previous studies as the interdependence of perturbation parameters (amplitude, duration, velocity, acceleration) possibly created uncontrolled, confounding effects. We disentangled the duration-, velocity- and acceleration-dependence and their interactions of the M1 and M2 stretch reflex in the ankle plantarflexors. To disentangle the parameter interdependence, forty-nine unique ramp-and-hold joint perturbations elicited reflexes in ten healthy volunteers during a torque control task. Linear mixed model analysis showed M1 depended on acceleration, not velocity or duration, whereas M2 depended on acceleration, velocity and duration. Simulations of the muscle spindle Ia afferents coupled to a motoneuron pool corroborated these experimental findings. Additionally, this simulation model did show a nonlinear M1 velocity- and duration-dependence for perturbation parameters outside the experimental scope. Concluding, motorized assessment of the stretch reflex or spasticity using ramp-and-hold perturbations should be systematically executed and reported. Our systematic motorized and simulation assessments showed that M1 and M2 depend on acceleration, velocity and duration of the applied perturbation. The simulation model suggested that these dependencies emerge from: muscle-tendon unit and muscle cross-bridge dynamics, Ia sensitivity to force and yank, and motoneuron synchronization.

中文翻译：

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解开脚踝跖屈肌中短潜伏期和中潜伏期牵张反射的加速度、速度和持续时间依赖性

牵张反射的机动评估有助于了解牵张反射、其生理起源以及区分神经系统疾病（如痉挛）的影响。据报道，短潜伏期 (M1) 和中潜伏期 (M2) 牵张反射取决于施加的斜坡和保持扰动的速度和加速度。据报道，在上肢，M2 也取决于拉伸持续时间。然而，在先前的研究中可能得出了错误的结论，因为扰动参数（幅度、持续时间、速度、加速度）的相互依赖性可能会产生不受控制的混杂效应。我们解开了踝跖屈肌中 M1 和 M2 牵张反射的持续时间、速度和加速度依赖性及其相互作用。为了解开参数相互依赖性，在扭矩控制任务期间，49 个独特的斜坡和保持关节扰动在 10 名健康志愿者中引发了反射。线性混合模型分析表明 M1 取决于加速度，而不是速度或持续时间，而 M2 取决于加速度、速度和持续时间。与运动神经元池耦合的肌梭 Ia 传入神经的模拟证实了这些实验结果。此外，该模拟模型确实显示了实验范围之外的扰动参数的非线性 M1 速度和持续时间依赖性。最后，应系统地执行和报告使用斜坡和保持扰动对牵张反射或痉挛的机动评估。我们系统的机动化和模拟评估表明，M1 和 M2 取决于加速度，施加扰动的速度和持续时间。仿真模型表明，这些依赖关系来自：肌肉肌腱单元和肌肉跨桥动力学、Ia 对力和拉力的敏感性以及运动神经元同步。