Muscle Mechanics: Adaptations with Exercise‐Training
Muscle Mechanics: Adaptations with Exercise‐Training
复制标题
DOI：
10.1249/00003677-199600240-00016
复制
发表时间：
1996-01
影响因子：
5.7
通讯作者：
R. Fitts;J. Widrick
中科院分区：
文献类型：
--
作者：
R. Fitts;J. Widrick
基于MHC同种型模式，成年哺乳动物肢体骨骼肌含有两种，在某些物种中，三种类型的快纤维（IIa型，IIx型和IIb型）和一种慢纤维（I型）。慢肌，如比目鱼肌，主要含有慢I型纤维，而快肌主要由快肌球蛋白同工酶的混合物组成。力的产生涉及跨桥相互作用和从弱结合、低力状态（AM-ADP-P（i））到强结合、高力状态（AM-ADP）的转变。这种转变被认为是速率限制的dP/dt，高力状态是主导的跨桥形式在峰值等长收缩。完整的快骨骼肌和慢骨骼肌产生大约相同量的峰值力（Po），在200和250 kN.m-2之间。然而，从低到高的力状态的过渡率显示Ca 2+的敏感性，是7倍高，在快收缩，慢收缩，骨骼肌纤维相比。纤维Vo或最大跨桥循环速率与肌球蛋白或肌原纤维ATP酶的比活性高度相关，并被认为依赖于肌球蛋白或肌原纤维ATP酶的比活性。Vo的层次结构是IIb > IIx > IIa > I型。快速纤维类型的这种功能差异解释了在主要的IIb型SVL中观察到的Vo高于混合的快速IIa型和IIb型EDL肌肉。Vo与物种大小的关系图表明Vo与体重之间存在反比关系。从做功能力的观点来看，重要的性能是功率输出。个别肌肉的分析表明，峰值功率是在负荷大大低于50%的Po。具有高百分比的快缩纤维的个体在给定的速度下比具有主要慢缩纤维的个体产生更大的扭矩和更高的功率。在人类中，IIb、IIa和I型纤维的平均峰值功率比例为10：5：1。在人体肌肉中的扭矩-速度关系和Vmax的体内测量是困难的，因为所使用的设备中固有的限制和无法独立地研究大的肢体肌肉。然而，体内扭矩-速度关系与动物体外测量的相似。这一观察结果表明，几乎不存在中枢神经系统抑制，并且健康受试者能够实现肌肉的最大激活。虽然峰值等长张力不依赖于纤维类型分布，但在中等至高等速角速度下，快纤维的百分比与峰值扭矩输出之间存在正相关。因此，峰值功率输出在具有快纤维优势的受试者中显著更大。慢肌和快肌的机械特性确实适应于有规律的锻炼计划。耐力运动训练已被证明可以增加20%的慢比目鱼肌的Vo。这种增加可能是由于所有或大部分纤维的少量增加，或者是由于少数纤维从慢到快的转换。最近，这种增加被证明是由前者引起的，因为比目鱼肌的单个慢I型纤维显示Vo增加了20%，但快纤维的百分比几乎没有变化。Vo的增加与纤维ATP酶的增加相关，并可能由其引起。我们推测ATP酶和跨桥循环速度的增加可能是由于I型慢纤维中快速MLCs表达的增加（图14.10）。这一假设是基于这样的事实，即轻链已被证明参与动力冲程，轻链的去除会降低力和速度。定期耐力运动训练对纤维大小没有影响，但随着日常训练持续时间的延长，它会降低Po和峰值功率。当训练持续较长时间时，它甚至可能诱导慢I型和快IIa型纤维的萎缩。（摘要截断）
Based on the MHC isoform pattern, adult mammalian limb skeletal muscles contain two and, in some species, three types of fast fibers (Type IIa, IIx, and IIb), and one slow fiber (Type I). Slow muscles, such as the soleus, contain primarily the slow Type I fiber, whereas fast-twitch muscles are composed primarily of a mixture of the fast myosin isozymes. Force generation involves cross-bridge interaction and transition from a weakly bound, low-force state (AM-ADP-P(i)) to the strongly bound, high-force state (AM-ADP). This transition is thought to be rate limiting in terms of dP/dt, and the high-force state is the dominant cross-bridge form during a peak isometric contraction. Intact fast and slow skeletal muscles generate approximately the same amount of peak force (Po) of between 200 and 250 kN.m-2. However, the rate of transition from the low- to high-force state shows Ca2+ sensitivity and is 7-fold higher in fast-twitch, as compared to slow-twitch, skeletal muscle fibers. Fiber Vo or the maximal cross-bridge cycle rate is highly correlated with and thought to be dependent on the specific activity of the myosin or myofibrillar ATPase. The hierarchy for Vo is the Type IIb > IIx > IIa > I. This functional difference for the fast fiber types explains the higher Vo observed in the predominantly Type IIb SVL vs. the mixed fast Type IIa and IIb EDL muscle. A plot of Vo vs. species size demonstrates that an inverse relationship exists between Vo and body mass. From the standpoint of work capacity, the important property is power output. An analysis of individual muscles indicates that peak power is obtained at loads considerably below 50% of Po. Individuals with a high percentage of fast-twitch fibers generate a greater torque and higher power at a given velocity than those with predominantly slow-twitch fibers. In humans, mean peak power occurred in a ratio of 10:5:1 for the Type IIb, IIa, and I fibers. The in vivo measurement of the torque-velocity relationship and Vmax in human muscle is difficult because of limitations inherent in the equipment used and the inability to study the large limb muscles independently. Nevertheless, the in vivo torque-velocity relationships are similar to those measured in vitro in animals. This observation suggests that little central nervous system inhibition exists and that healthy subjects are able to achieve maximal activation of their muscles. Although peak isometric tension is not dependent on fiber type distribution, a positive correlation exists between the percentage of fast fibers and peak torque output at moderate-to-high angular isokinetic velocities. Consequently, peak power output is substantially greater in subjects possessing a predominance of fast fibers. The mechanical properties of slow and fast muscles do adapt to programs of regular exercise. Endurance exercise training has been shown to increase the Vo of the slow soleus by 20%. This increase could have been caused by either a small increase in all, or most, of the fibers, or to a conversion of a few fibers from slow to fast. Recently, the increase was shown to be caused by the former, as the individual slow Type I fibers of the soleus showed a 20% increase in Vo, but there was little or no change in the percentage of fast fibers. The increased Vo was correlated with, and likely caused by, an increased fiber ATPase. We hypothesize that the increased ATPase and cross-bridge cycling speed might be attributable to an increased expression of fast MLCs in the slow Type I fibers (Fig. 14.10). This hypothesis is based on the fact that light chains have been shown to be involved in the power stroke, and removal of light chains depresses force and velocity. Regular endurance exercise training had no effect on fiber size, but with prolonged durations of daily training it depressed Po and peak power. When the training is maintained over prolonged periods, it may even induce atrophy of the slow Type I and fast Type IIa fibers. (ABSTRACT TRUNCATED)