Source-Specific Photobiomodulation Regulates Mitochondrial Bioenergetics, Redox Signaling, and Functional Outputs in C2C12 Myoblasts Across Replicative Aging

Age-related muscle decline is associated with impaired mitochondrial bioenergetics, altered redox signaling, and reduced myogenic capacity, yet how photobiomodulation (PBM) source characteristics shape these processes under replicative aging remains unclear. Here, we investigated source-specific PBM responses in C2C12 myoblasts using a 660 nm light-emitting diode (LED) and an 830 nm near-infrared (NIR) laser across fluence ranges and replicative stages. Single-cell screening performed at passage 25 identified 5 J/cm² as the optimal fluence for both sources, producing biphasic increases in mitochondrial membrane potential and ROS. Population-level assays in young (≤5 passages) and old (≥30 passages) cells revealed divergent downstream outcomes. LED irradiation elicited stronger metabolic activation and ATP production, particularly in aged cells, whereas NIR irradiation robustly enhanced myogenic fusion in both age groups and partially rescued differentiation deficits in aged myoblasts. Bulk ROS increased significantly after PBM independent of source, while extracellular vesicle release displayed age-dependent source sensitivity, with NIR favoring canonical small EV populations in young cells and LED inducing greater particle release in aged cells. Together, these findings demonstrate that PBM engages conserved mitochondrial signaling while source-specific delivery and wavelength differentially directs metabolic, paracrine, and myogenic outputs under replicative aging conditions.