Short read
Ahokas and colleagues at the University of Jyväskylä and Mid Sweden University ran a randomised crossover trial, published in Biology of Sport in 2023. 16 male basketball players from two teams of one club, aged 18.9 on average, completed the same 60-minute strength and plyometric session twice, a week apart, with squats, Nordic hamstring curls and leg press at 90 to 95 % of their one-repetition maximum. Once the session was followed by 20 minutes in an infrared cabin measured at 43 degrees (set to 50, 21 % relative humidity), once by 20 minutes of passive sitting at room temperature. Measurements ran until 14 hours after the session.
Compared with passive sitting, the infrared session came with a smaller loss in jump height at 14 hours: 1.1 % against 5.0 % (p = 0.009). Soreness on a scale from 0 to 12 was 3.0 against 5.3 right after recovery and 2.9 against 5.2 at 14 hours (p = 0.005), and perceived recovery on a scale from 1 to 5 was 3.2 against 2.6 (p = 0.007). Sprint over 20 m, isometric leg strength, creatine kinase, myoglobin, sleep duration, sleep quality and night-time heart rate variability did not differ between the two conditions.
The modality was an infrared cabin at 43 degrees, not a Finnish sauna. Blinding is not possible with heat. The study covers 16 men from one club, a single session and a horizon of 14 hours, and heart rate variability data were missing for several participants. The authors themselves limit the transfer of their findings to short recovery periods.
The result is narrower than the headline claim that heat after lifting relieves soreness: soreness and jump height moved, while strength, sprint and the muscle damage markers did not.