Aging and Muscle Metabolism Lab, Department of Physiology & Institute of Sport Sciences, University of Lausanne, Lausanne, Switzerland.
Sports Medicine Unit, Lausanne University Hospital and Lausanne University, Lausanne, Switzerland.
J Cachexia Sarcopenia Muscle. 2019 Jun;10(3):687-695. doi: 10.1002/jcsm.12410. Epub 2019 Mar 25.
Human skeletal muscle is composed of a functional and metabolic continuum of slow (Type I) and fast fibers (IIa and IIx). Hybrid fibers co-expressing different myosin heavy chains are also present and seem to be more prominent in aging muscle. Their role is debated; hybrid fibers were reported either in a transitional state, between slow and fast fibers, or as fixed individual entities. This study examined the fate of hybrid fibers with an endurance exercise intervention in an elderly sedentary population.
Twenty-two sedentary healthy elderly men and women underwent a 16-week supervised endurance exercise intervention. Eighteen endurance-trained age- and gender-matched volunteers served as controls. Fiber type distribution was determined by immunohistochemistry on vastus lateralis muscle biopsies pre-intervention and post-intervention.
A total of 13840 fibers were analyzed. At baseline, a Type II dominant fiber profile was observed compared with the control group, with more Type IIa (P = 0.0301) and Type IIx fibers (P = 0.0328). Hybrid fibers represented almost 5% of total muscle fibers in both groups. There was no significant difference between groups (I-IIa, P = 0.6719 and IIa-IIx, P = 0.0998). Intervention triggered qualitative dynamics towards an increase in Type I, and decrease in Type II fibers, paralleled by an increase in I-IIa hybrids (P = 0.0301).
The present study is, to our knowledge, the first to examine hybrid muscle fiber type adaptations to an endurance exercise intervention in the elderly. Hybrid fiber proportions did not differ between chronic sedentary state and chronic endurance-trained state. Exercise intervention increased Type I-IIa hybrid fibers along with shift dynamics in other fiber types suggesting the contribution of hybrid fiber to a fast-to-slow fiber type transition, eventually serving as intermediate reservoir from one monomorphic myosin heavy chain expressing fiber type to another. This finding favours the transitional theory regarding hybrid muscle fibers and exercise, crucial to understanding reversible mechanisms of sarcopenia and development of prevention measures.
人类骨骼肌由具有不同功能和代谢特性的慢肌（I 型）和快肌（IIa 和 IIx）纤维组成。还存在同时表达不同肌球蛋白重链的杂交纤维，且其在衰老肌肉中似乎更为突出。关于其功能存在争议；杂交纤维或处于慢肌和快肌之间的过渡状态，或作为固定的个体实体。本研究在久坐的老年人群中通过耐力运动干预检查了杂交纤维的命运。
22 名久坐的健康老年男女接受了 16 周的监督耐力运动干预。18 名耐力训练的年龄和性别匹配的志愿者作为对照组。在干预前和干预后，通过免疫组织化学方法对股外侧肌活检确定纤维类型分布。
共分析了 13840 根纤维。在基线时，与对照组相比，观察到 II 型纤维占主导地位的纤维谱，IIa 型（P=0.0301）和 IIx 型纤维（P=0.0328）更多。杂交纤维在两组中几乎占总肌纤维的 5%。两组之间无显著差异（I-IIa，P=0.6719 和 IIa-IIx，P=0.0998）。干预引发了定性动态变化，导致 I 型纤维增加，II 型纤维减少，同时 I-IIa 杂交纤维增加（P=0.0301）。
本研究是首次检查了耐力运动干预对老年人的杂交肌纤维类型适应性。在慢性久坐状态和慢性耐力训练状态下，杂交纤维比例无差异。运动干预增加了 I-IIa 杂交纤维，同时其他纤维类型的转变动态表明，杂交纤维对快肌向慢肌纤维类型转变的贡献，最终成为从一种单型肌球蛋白重链表达的纤维类型到另一种纤维类型的中间储备库。这一发现支持了关于杂交肌纤维和运动的过渡理论，这对理解肌少症的可逆机制和预防措施的发展至关重要。
