肌细胞收缩效能的影响因素T ension (肌张力)：The force exerted on an object by a contracting muscle Load （负荷）：The force exerted on the muscle by an object (usually its weight)骨骼肌的收缩•Isometric ContractionWhen the two ends of a muscle are held at fixed points, stimulation causes the development of force (tension ) without change in muscle length .等长收缩: 肌肉收缩时只有张力的增加而无长度的缩短。
主要维持人体的位置和姿势•Isotonic ContractionStimulation of a muscle causes a shortening while developing a constant force (tension ).等张收缩：肌肉收缩时只有长度的缩短而无张力的变化。
人体骨骼肌收缩大多数情况下是混合式的，而且总是张力增加在前（等长收缩），长度缩短在后（等长收缩）。
•Preload 前负荷•Afterload 后负荷•Contractility 肌肉收缩能力•Size summation 肌纤维总和•Frequency summation 频率总和影响横纹肌收缩效能的因素•Preload: the initial stretching of the muscle fiber prior to contraction•Initial length of muscle determines the preload↑ length = ↑ PreloadEffect of Preload on Force of Contraction 前负荷(preload)：肌肉收缩前所承受的负荷初长度：肌肉收缩之前的长度，可以衡量前负荷的大小等长收缩条件下，肌肉存在着一个最适前负荷和与之相对应的最适初长度，在这样的初长度情况下进行收缩，产生的主动张力最大1.5µm0.650.650.350.350.20M肌小节ZZ骨骼肌体内所处自然长度肌小节2.0-2.2µm粗细肌丝理想重叠/收缩Effect of Afterload on Muscle Contraction•Afterload is the tension or stress developed in the muscular during contraction.后负荷：肌肉在收缩过程中所承受的负荷When the load has been increased toequal the maximum force that themuscle can exert, the velocity ofcontraction becomes zero and nocontraction results, despite activation ofthe muscle fiber.Force-Velocity Relationship后负荷增大，产生张力也大，但缩短出现迟，初速度小，缩短总长度也小负荷对横桥周期的影响前负荷及后负荷与肌肉收缩的关系实质在于：肌肉收缩所能产生的张力由每一瞬间与肌动蛋白结合的横桥数决定；肌肉收缩的速度取决于横桥周期的长短。
