Short read
Ahokas and colleagues at the University of Jyväskylä and Mid Sweden University ran a pair-matched controlled study over 6 weeks, published in Frontiers in Sports and Active Living in 2025. 40 female team-sport athletes from basketball, futsal, ice hockey and American football, aged 22, took part and 36 were analysed after dropouts. They were paired by sport and performance level and assigned without randomisation. Both groups followed the same strength and sport training; the infrared group also sat 10 minutes in an infrared cabin at 50 degrees after each session, 3 times a week, 18 sessions in all.
Measured against the control group of 18 athletes with identical training and no infrared, the heat added nothing to muscle growth: vastus lateralis cross-section rose from 21.21 to 22.81 square centimetres with infrared and from 23.17 to 24.12 without, with no interaction between group and time. Isometric leg strength showed neither a group nor an interaction effect. Countermovement jump height rose only in the infrared group, from 19.5 to 21.0 cm (p = 0.006), while the control group stayed flat (interaction p = 0.002). The morning cortisol response after the infrared sessions decreased over the six weeks.
The modality was an infrared cabin at 50 degrees for 10 minutes, not a Finnish sauna. Allocation was pair-matched, not randomised, and muscle temperature was not measured. Trained athletes with a high starting level adapt little in six weeks, and the authors themselves consider 10 minutes possibly too short for an effect on muscle size.
The study counts against the claim that heat after lifting builds more muscle. Its one positive signal, jump height, stands next to that. It is also the only training study in this collection with women only.