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Hybrid fibers co\u2010expressing 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.<\/jats:p><\/jats:sec><jats:sec><jats:title>Methods<\/jats:title><jats:p>Twenty\u2010two sedentary healthy elderly men and women underwent a 16\u2010week supervised endurance exercise intervention. Eighteen endurance\u2010trained age\u2010 and gender\u2010matched volunteers served as controls. Fiber type distribution was determined by immunohistochemistry on <jats:italic>vastus lateralis<\/jats:italic> muscle biopsies pre\u2010intervention and post\u2010intervention.<\/jats:p><\/jats:sec><jats:sec><jats:title>Results<\/jats:title><jats:p>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 (<jats:italic>P<\/jats:italic>\u00a0=\u00a00.0301) and Type IIx fibers (<jats:italic>P<\/jats:italic>\u00a0=\u00a00.0328). Hybrid fibers represented almost 5% of total muscle fibers in both groups. There was no significant difference between groups (I\u2013IIa, <jats:italic>P<\/jats:italic>\u00a0=\u00a00.6719 and IIa\u2013IIx, <jats:italic>P<\/jats:italic>\u00a0=\u00a00.0998). Intervention triggered qualitative dynamics towards an increase in Type I, and decrease in Type II fibers, paralleled by an increase in I\u2013IIa hybrids (<jats:italic>P<\/jats:italic>\u00a0=\u00a00.0301).<\/jats:p><\/jats:sec><jats:sec><jats:title>Conclusions<\/jats:title><jats:p>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\u2010trained state. Exercise intervention increased Type I\u2013IIa hybrid fibers along with shift dynamics in other fiber types suggesting the contribution of hybrid fiber to a fast\u2010to\u2010slow 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.<\/jats:p><\/jats:sec>","DOI":"10.1002\/jcsm.12410","type":"journal-article","created":{"date-parts":[[2019,3,25]],"date-time":"2019-03-25T09:11:56Z","timestamp":1553505116000},"page":"687-695","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":27,"title":["Hybrid fiber alterations in exercising seniors suggest contribution to fast\u2010to\u2010slow muscle fiber shift"],"prefix":"10.1002","volume":"10","author":[{"given":"Maxime","family":"Moreillon","sequence":"first","affiliation":[{"name":"Aging and Muscle Metabolism Lab, Department of Physiology &amp; Institute of Sport Sciences University of Lausanne  Lausanne Switzerland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Sonia","family":"Conde Alonso","sequence":"additional","affiliation":[{"name":"Aging and Muscle Metabolism Lab, Department of Physiology &amp; Institute of Sport Sciences University of Lausanne  Lausanne Switzerland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Nicholas T.","family":"Broskey","sequence":"additional","affiliation":[{"name":"Aging and Muscle Metabolism Lab, Department of Physiology &amp; Institute of Sport Sciences University of Lausanne  Lausanne Switzerland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Chiara","family":"Greggio","sequence":"additional","affiliation":[{"name":"Aging and Muscle Metabolism Lab, Department of Physiology &amp; Institute of Sport Sciences University of Lausanne  Lausanne Switzerland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Cyril","family":"Besson","sequence":"additional","affiliation":[{"name":"Sports Medicine Unit Lausanne University Hospital and Lausanne University  Lausanne Switzerland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Valentin","family":"Rousson","sequence":"additional","affiliation":[{"name":"Division of Biostatistics and Quantitative Methods, Institute of Social and Preventive Medicine University of Lausanne  Lausanne Switzerland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Francesca","family":"Amati","sequence":"additional","affiliation":[{"name":"Aging and Muscle Metabolism Lab, Department of Physiology &amp; Institute of Sport Sciences University of Lausanne  Lausanne Switzerland"},{"name":"Sports Medicine Unit Lausanne University Hospital and Lausanne University  Lausanne Switzerland"},{"name":"Service of Endocrinology, Diabetology and Metabolism, Department of Medicine Lausanne University Hospital and Lausanne University  Lausanne Switzerland"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2019,3,25]]},"reference":[{"key":"e_1_2_9_2_1","doi-asserted-by":"publisher","DOI":"10.1007\/BF00493174"},{"key":"e_1_2_9_3_1","doi-asserted-by":"publisher","DOI":"10.1177\/40.4.1552189"},{"key":"e_1_2_9_4_1","doi-asserted-by":"publisher","DOI":"10.1007\/s00421-004-1104-7"},{"key":"e_1_2_9_5_1","doi-asserted-by":"publisher","DOI":"10.1152\/jappl.2001.91.5.1955"},{"key":"e_1_2_9_6_1","doi-asserted-by":"publisher","DOI":"10.1152\/jappl.1999.86.2.455"},{"key":"e_1_2_9_7_1","doi-asserted-by":"publisher","DOI":"10.1113\/jphysiol.2004.062166"},{"key":"e_1_2_9_8_1","doi-asserted-by":"publisher","DOI":"10.1007\/s00424-011-0943-4"},{"key":"e_1_2_9_9_1","doi-asserted-by":"publisher","DOI":"10.1249\/01.MSS.0000053735.99344.C0"},{"key":"e_1_2_9_10_1","doi-asserted-by":"publisher","DOI":"10.1249\/01.mss.0000170123.27209.e1"},{"key":"e_1_2_9_11_1","doi-asserted-by":"publisher","DOI":"10.1152\/japplphysiol.01595.2005"},{"key":"e_1_2_9_12_1","doi-asserted-by":"publisher","DOI":"10.1249\/mss.0b013e3181453546"},{"key":"e_1_2_9_13_1","doi-asserted-by":"publisher","DOI":"10.1007\/BF00370757"},{"key":"e_1_2_9_14_1","doi-asserted-by":"publisher","DOI":"10.1055\/s-2002-35067"},{"key":"e_1_2_9_15_1","doi-asserted-by":"publisher","DOI":"10.1111\/j.1748-1716.1990.tb08975.x"},{"key":"e_1_2_9_16_1","doi-asserted-by":"publisher","DOI":"10.1002\/jcsm.12139"},{"key":"e_1_2_9_17_1","doi-asserted-by":"publisher","DOI":"10.1002\/(SICI)1097-4598(199904)22:4<449::AID-MUS4>3.0.CO;2-2"},{"key":"e_1_2_9_18_1","doi-asserted-by":"crossref","unstructured":"LexellJ.Human aging muscle mass and fiber type composition. 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