Abstract / Summary
Photobiomodulation outcomes depend on how light delivery, tissue heating, and cellular energy-related activity change together. This study develops a three-dimensional computational framework that combines light transport, heat transfer, and a phenomenological model of adenosine triphosphate-related activity in layered skin, fat, and skeletal muscle. The model compares wavelengths of 810, 850, and 900 nm and evaluates the effects of optical power, beam radius, and a 180 s exposure period. Under the common optical condition, 900 nm produces the highest average and maximum light levels in muscle, whereas 850 nm provides an intermediate response and is retained as a representative wavelength for the coupled analyses. Increasing the beam radius markedly reduces local light concentration near the tissue surface while producing little change in the average light level within muscle. At 850 nm with a Gaussian \(1/e^2\) beam radius of \(w=4\) mm, the modeled muscle-averaged ATP-related response increases at low optical power, reaches a near-flat maximum of approximately 14.73% between 0.50 and 0.75 W, and then gradually decreases to approximately 12.29% at 2.50 W. In contrast, maximum tissue temperature and cumulative thermal exposure increase continuously with optical power. At 0.75 W, the maximum temperature is 38.584 \(^\circ \textrm{C}\) , while at 2.50 W it rises to 45.857 \(^\circ \textrm{C}\) . A local sensitivity analysis further shows that the magnitude and location of the modeled response maximum depend on the selected inhibition threshold. The results show that greater light delivery does not necessarily produce a greater modeled cellular energy response and may instead increase thermal loading. The framework supports comparison of coupled optical, thermal, and biological trends, but its predictions remain conditional on the selected tissue properties, geometry, and phenomenological parameters. Experimental measurements and higher-fidelity light-transport models are required before quantitative physiological or clinical interpretation.