Swimming Training Alters Immune Cell Spatiotemporal Distribution to Mitigate Fat Infiltration and Enhance Muscle Regeneration

Skeletal muscle injury repair involves dynamic immune cell regulation, yet how exercise modulates this process remains unclear. This study investigated the impact of swimming training on immune cell spatiotemporal dynamics, fat infiltration, and muscle regeneration in glycerol-injured mouse gastrocnemius muscles. Swimming training accelerated functional recovery (grip strength, rotarod endurance, gait mechanics) and reduced serum creatine kinase activity, indicating mitigated muscle damage. Histological analysis revealed that swimming decreased collagen deposition and fat infiltration while promoting myofiber regeneration. Mechanistically, swimming induced early (days 1–3 post-injury) increases in natural killer cells, granulocytes, monocytes, macrophages, and M1 macrophages, initiating inflammatory responses for necrotic tissue clearance. From days 3–7, M2 macrophage numbers rose significantly, facilitating satellite cell differentiation. Concurrently, swimming promoted recruitment of regulatory T (Treg) cells and fibro-adipogenic progenitors (FAPs), particularly IL-33⁺FAPs, driving the immune microenvironment from pro-inflammatory to anti-inflammatory states. Cytokine profiling showed transient elevation of pro-inflammatory factors (TNF-α, IL-6) in the acute phase, followed by anti-inflammatory IL-10 upregulation during repair. Transcriptomic analysis validated spatiotemporal gene expression shifts, including enhanced Treg cell chemotaxis and suppressed adipogenesis. These findings demonstrate that swimming training optimizes muscle repair by orchestrating immune cell dynamics and cytokine networks, highlighting its potential as a therapeutic strategy to mitigate fat infiltration and enhance regeneration in injured muscle.