After one session of photobiomodulation on one leg, with a sham emitting no light on the other, mitochondrial respiration did not differ significantly between legs in skin (2.7 ± 0.8 vs 2.6 ± 0.7 pmol/s/mg wet weight, P = 0.66) or in the muscle beneath (474 ± 114 vs 467 ± 81 pmol/s/mg dry weight, P = 0.71) in 12 healthy adults, though it warmed the skin by 8.0 °C; a resting mechanistic study of the proposed mechanism, no exercise or performance outcome

Aussieker and colleagues tested the claim that photobiomodulation directly raises mitochondrial activity in skin and muscle in living humans. Twelve healthy adults (6 men, 6 women) received PBM on one randomly assigned leg and a sham treatment with no light emitted on the other. Each leg got three cycles of 5 min 16 s, with 5.6 kJ of light energy per cycle on the PBM leg.

Skin temperature was measured before and after, and skin and muscle samples were then taken from both legs. Mitochondrial respiration was measured in permeabilised muscle fibres and minced skin with an Oroboros Oxygraph-O2k, and 91 metabolic genes were assessed in muscle. Skin temperature rose only on the PBM leg, by 8.0 °C.

Maximal complex I+II-linked respiration did not differ between legs in skin or in muscle, muscle mitochondrial ADP sensitivity did not differ, and only 3 of 91 genes differed in expression. The authors conclude that a single PBM session does not increase mitochondrial respiration in skin or underlying muscle, and does not modulate muscle gene expression measured ex vivo.