Hessel Anthony L, Lindstedt Stan L, Nishikawa Kiisa C
Department of Biological Sciences, Center for Bioengineering Innovation, Northern Arizona University Flagstaff, AZ, USA.
Front Physiol. 2017 Feb 9;8:70. doi: 10.3389/fphys.2017.00070. eCollection 2017.
When active muscles are stretched, our understanding of muscle function is stretched as well. Our understanding of the molecular mechanisms of concentric contraction has advanced considerably since the advent of the sliding filament theory, whereas mechanisms for increased force production during eccentric contraction are only now becoming clearer. Eccentric contractions play an important role in everyday human movements, including mobility, stability, and muscle strength. Shortly after the sliding filament theory of muscle contraction was introduced, there was a reluctant recognition that muscle behaved as if it contained an "elastic" filament. Jean Hanson and Hugh Huxley referred to this structure as the "S-filament," though their concept gained little traction. This additional filament, the giant titin protein, was identified several decades later, and its roles in muscle contraction are still being discovered. Recent research has demonstrated that, like activation of thin filaments by calcium, titin is also activated in muscle sarcomeres by mechanisms only now being elucidated. The mutation in mice appears to prevent activation of titin, and is a promising model system for investigating mechanisms of titin activation. Titin stiffness appears to increase with muscle force production, providing a mechanism that explains two fundamental properties of eccentric contractions: their high force and low energetic cost. The high force and low energy cost of eccentric contractions makes them particularly well suited for athletic training and rehabilitation. Eccentric exercise is commonly prescribed for treatment of a variety of conditions including sarcopenia, osteoporosis, and tendinosis. Use of eccentric exercise in rehabilitation and athletic training has exploded to include treatment for the elderly, as well as muscle and bone density maintenance for astronauts during long-term space travel. For exercise intolerance and many types of sports injuries, experimental evidence suggests that interventions involving eccentric exercise are demonstrably superior to conventional concentric interventions. Future work promises to advance our understanding of the molecular mechanisms that confer high force and low energy cost to eccentric contraction, as well as signaling mechanisms responsible for the beneficial effects of eccentric exercise in athletic training and rehabilitation.
当活跃的肌肉被拉伸时，我们对肌肉功能的理解也得到了拓展。自从肌丝滑行理论问世以来，我们对向心收缩分子机制的理解有了显著进展，而离心收缩过程中力量增加的机制直到现在才逐渐明晰。离心收缩在日常人类活动中起着重要作用，包括移动性、稳定性和肌肉力量。在肌肉收缩的肌丝滑行理论提出后不久，人们勉强认识到肌肉的表现就好像它包含一根“弹性”细丝。让·汉森和休·赫胥黎将这种结构称为“S-细丝”，尽管他们的概念并未得到广泛认可。几十年后，这种额外的细丝——巨大的肌联蛋白被确定，其在肌肉收缩中的作用仍在不断被发现。最近的研究表明，就像钙对细肌丝的激活一样，肌联蛋白在肌肉肌节中也通过目前才被阐明的机制被激活。小鼠中的这种突变似乎会阻止肌联蛋白的激活，是研究肌联蛋白激活机制的一个很有前景的模型系统。肌联蛋白的刚度似乎会随着肌肉力量的产生而增加，这提供了一种机制来解释离心收缩的两个基本特性：高力量和低能量消耗。离心收缩的高力量和低能量消耗使其特别适合运动训练和康复。离心运动通常被用于治疗多种病症，包括肌肉减少症、骨质疏松症和肌腱病。离心运动在康复和运动训练中的应用激增，涵盖了对老年人的治疗，以及在长期太空旅行中对宇航员肌肉和骨密度的维持。对于运动不耐受和多种类型的运动损伤，实验证据表明，涉及离心运动的干预措施明显优于传统的向心干预措施。未来的工作有望增进我们对赋予离心收缩高力量和低能量消耗的分子机制的理解，以及负责离心运动在运动训练和康复中产生有益效果的信号机制的理解。
