Adipose Levels Within 10 Individual Thigh Skeletal Muscles With Aerobic Cycle Exercise Training in Older Men

Objective and Hypothesis: Adipose accumulation within skeletal muscles (i.e., intramuscular myosteatosis) has been implicated in contributing to decrements in skeletal muscle metabolic health and function in aging individuals. The objective of this study was to investigate intramuscular adipose levels of older adults in response to aerobic cycle exercise training, as this type of exercise relies heavily on adipose (i.e., lipid) oxidation for energy. We hypothesized that aerobic cycle exercise training would reduce intramuscular adipose levels across the 10 individual skeletal muscles of the thigh in relation to the relative involvement of each muscle with cycling exercise. Methods: Intramuscular (rectus femoris, vastii, adductor longus, adductor magnus, gracilis, sartorius, biceps femoris-long head, biceps femoris-short head, semimembranosus, semitendinosus) adipose levels were measured via magnetic resonance imaging (MRI) before and after 12-weeks of progressive aerobic cycle exercise training (3-4d/wk, 60-180min/wk, 60-80% heart rate reserve; VO2max: +13%) in six older men (74±8y; 177±4cm; pre-training: 82.3±10.4kg, post-training: 81.5±6.9kg). Results: Absolute (mm2) and relative (%) intramuscular adipose levels varied across the 10 thigh muscles before exercise training, with all of the muscles containing a relatively low average adipose content (< 4%; rectus femoris: 5±6mm2, 0.5±0.7%; vastii: 46±29mm2, 0.9±0.6%; adductor longus: 1±3mm2, 0.2±0.4%; adductor magnus: 19±23mm2, 0.7±0.9%; gracilis: 0.3±0.7mm2, 0.2±0.4%; sartorius: 0±0mm2, 0±0%; biceps femoris-long head: 10±8mm2, 0.9±0.8%; biceps femoris-short head: 10±10mm2, 2±2%; semimembranosus: 34±25mm2, 3±3%; semitendinosus: 0.4±0.7mm2, 0.1±0.1%). Absolute and relative intramuscular adipose levels were unchanged (p >0.05) from pre- to post-training in all 10 thigh muscles. Conclusions: These data suggest that intramuscular adipose levels are unchanged in older men following short term (i.e., 12 weeks) aerobic exercise training, which has been shown to significantly improve muscle fatty acid oxidation enzyme levels in these same individuals (vastus lateralis β-HAD: +400%; PMID: 23873965). This lack of intramuscular adipose level responsiveness to aerobic exercise training is likely related to the relatively low levels of intramuscular adipose at the onset of training and the relatively good overall health status of the aging individuals. Funding: This research was supported by the National Institutes of Health Grant AG032127 and AG038576, NASA Grant NNJ06HF59G, and the Ball State University Human Bioenergetics Program. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.