Muscle disuse caused by bed rest and immobilization is associated with muscle atrophy and insulin resistance, which might be related to increases in intramuscular fat (IntraMAT) and intermuscular fat (InterMAT) accumulation. In this systematic review, we compiled evidence on the effects of bed rest and unilateral lower-limb immobilization on IntraMAT and InterMAT in healthy adults. Following PRISMA guidelines, we searched PubMed, Scopus, Web of Science, and Cochrane Library from inception until July 2025. Included studies were clinical trials involving healthy adults (≥18 years old) undergoing bed rest or immobilization, with IntraMAT and InterMAT as outcomes (CRD420251112427). Two reviewers extracted data and assessed bias using RoB2 and ROBINS-I tools for randomized and non-randomized studies, respectively. Data were synthesized using a narrative synthesis. Nine studies met the inclusion criteria. Bed rest increased IntraMAT in the lumbar multifidus, erector spinae, and quadratus lumborum by 18.7%, 5.4% and 4.5%, respectively (P < 0.01), whereas thigh-related changes were inconsistent. Immobilization produced heterogeneous effects: calf muscles showed both small decreases (-0.5%) and increases (+0.3%), whereas thigh IntraMAT increased by 12.8% after 28 days. InterMAT increased in the lumbar spine (1.2-5.3%) and immobilized thigh (+14.5%) and calf (+20%) muscles. Muscle disuse from bed rest or immobilization can promote site-specific increases in IntraMAT and InterMAT, particularly in lumbar regions, highlighting muscle fat infiltration as a potential key adaptation to unloading.
Prokopidis et al. (2026) studied this question.