Short read
Paul Hafen, Robert Hyldahl and colleagues at Brigham Young University in Provo immobilised one leg of young adults for ten days, published in the Journal of Applied Physiology in 2019. During that time the muscle received two hours of deep local heating per day from pulsed shortwave diathermy, which raised muscle temperature by about 4 degrees. The other leg received a sham treatment and served as the comparison. The modality was a local device applied to one muscle, not a sauna.
In the sham condition, immobilisation led to clear losses in mitochondrial respiratory capacity and in the proteins of all 5 respiratory complexes. Compared with that sham condition, the heat treatment prevented this loss and attenuated muscle wasting. Heat shock proteins 70 and 90 increased, as did PGC-1alpha, a regulator of mitochondrial biogenesis.
The sample is small, the participants were young and healthy, and the study lasted only ten days. Local diathermy is not whole-body heat. Carrying the result over to sauna use is an assumption, not a finding. Further figures on heat shock proteins in the entry come from a related study by the same group and are not reported here as results of this trial.
The study transfers findings from animal experiments on heat and muscle wasting to humans for the first time and opens the question of heat during injury and immobilisation. Mechanistically it is strong; in its modality it is far from the sauna, and it does not show that sauna use protects muscle during immobilisation.