Trappe Todd
Human Performance Laboratory, Ball State University, Muncie, IN 47306, USA.
Appl Physiol Nutr Metab. 2009 Jun;34(3):459-64. doi: 10.1139/H09-041.
Understanding the quantitative and qualitative changes in skeletal muscle that control changes in function is crucial in the development of countermeasures to the loss of skeletal muscle function observed with real and simulated microgravity exposure (i.e., unloading) and with aging in humans. Qualitative changes that could influence the force and power producing properties of skeletal muscle are changes in the distribution of the 3 isoforms of the main motor protein myosin heavy chain (MHC), as well as the abundance of MHC, actin (the other main contractile protein), and the force distributing the connective tissue network. Numerous studies have examined quantitative and qualitative changes in skeletal muscle, from the whole muscle to the single myofiber from individuals undergoing real and simulated space flight for a few weeks to several months, as well as from aging men and women. When considering the relative content of the main functional and structural elements (i.e., myosin, actin, collagen), it appears that human muscle appropriately scales changes in size of 10%-40% induced over a relatively short time period (1-3 months) and over the lifespan (in humans 20 to 90+ years old). The main qualitative change with unloading and aging is a redistribution of the 3 MHC isoforms, which have vastly different contractile characteristics. It is now known that the response of skeletal muscle to unloading is muscle and gender specific. In summary, changes in muscle mass (quantity) combined with the alterations in MHC distribution (quality) are the primary determinants of changes in muscle function with unloading and aging. These parameters are the key components of muscle that should be targeted with countermeasures for conditions related to muscle loss, along with considerations for muscle- and gender-specific responses.
了解骨骼肌中控制功能变化的定量和定性变化，对于制定应对措施以对抗在真实和模拟微重力暴露（即失重）以及人类衰老过程中观察到的骨骼肌功能丧失至关重要。可能影响骨骼肌产生力量和功率特性的定性变化包括主要运动蛋白肌球蛋白重链（MHC）的三种同工型的分布变化，以及MHC、肌动蛋白（另一种主要收缩蛋白）的丰度，以及分布力量的结缔组织网络的变化。许多研究已经考察了骨骼肌的定量和定性变化，这些研究对象包括经历了数周至数月真实和模拟太空飞行的个体，以及老年男性和女性，涉及从整块肌肉到单个肌纤维的层面。当考虑主要功能和结构元件（即肌球蛋白、肌动蛋白、胶原蛋白）的相对含量时，似乎人类肌肉能够在相对较短的时间段（1至3个月）以及整个生命周期（人类年龄从20岁到90多岁）内，适当应对10%至40%的大小变化。卸载和衰老导致的主要定性变化是三种MHC同工型的重新分布，它们具有截然不同的收缩特性。现在已知骨骼肌对卸载的反应具有肌肉和性别特异性。总之，肌肉质量（数量）的变化与MHC分布的改变（质量）相结合，是卸载和衰老过程中肌肉功能变化的主要决定因素。这些参数是肌肉的关键组成部分，在针对与肌肉流失相关的状况制定应对措施时应将其作为目标，同时还要考虑肌肉和性别特异性反应。
