Muscle lengthening contractions lead to physiological responses such as strength loss, soreness, immune cell infiltration, and CK production, along with elevated heat shock proteins (HSPs), which protect and regenerate cells. Passive heating before or after exercise can aid in recovery by restoring strength and reducing soreness, requiring a core temperature of over 38.5°C for 20 minutes to be effective. Studies show that combined mechanical and thermal stress induces a cumulative HSP response, enhancing cellular protection. Thermal stress alone peaks at 16- 24 hours, whereas mechanical stress shows a biphasic response, with peaks at 3-6 hours and around 39 hours. Thus, combining heat with exercise could optimize HSP production, improving recovery by increasing and accelerating the HSP response. However, more research is needed to determine the best timing for thermal stress in human applications, especially in sports and therapy contexts.

This study aimed to compare the effects of hot-water immersion administered before versus after muscle-lengthening exercise on muscle function and HSP70 expression in healthy young athletes.

Inclusion Criteria:

• Sex: Healthy, athletic men practicing track and field (10 to 12 hours/week)
• Age: 19 to 25 years old
• Social security coverage
• Signed informed consent

Exclusion Criteria:

• Age over 26 years
• Low level of physical activity (less than 8 hours per week)
• Recent sprain (type 1, 2, or 3) or ligament rupture in the lower limb (within the last 8 months)
• Exposure to an altitude of 2500m within 15 days prior to the protocol
• Presence of arterial hypotension:
• Systolic blood pressure below 90 mmHg
• Diastolic blood pressure below 60 mmHg
• A drop in systolic blood pressure greater than 20 mmHg upon moving from a lying to a standing position.
• A drop in diastolic blood pressure greater than 10 mmHg upon moving from a lying to a standing position.
• Presence of dizziness, lightheadedness, or blurred vision upon moving from a lying to a standing position.
• Participant under guardianship/conservatorship or deprived of liberty.