It depends on the case; there is no single guideline.
Meta-analysis (18 trials): contrast therapy with cold and hot water reduced soreness and strength loss after damaging exercise compared with passive recovery. NOTE: high risk of bias, and cold can blunt adaptations if you are trying to build muscle (use it to recover, not after training for hypertrophy).
View the study (meta-analysis) →⏱ 2 min read
Contrast therapy consists of repeated alternation between heat and cold exposure (typically hot and cold water baths, contrast showers, alternating between a sauna and a cold plunge) in sessions of 15-30 minutes, a practice with a long tradition in Scandinavian, Japanese, and Russian culture, and with growing use in sports medicine and recovery; the proposed mechanism is the rapid alternation between vasodilation (heat) and vasoconstriction (cold), which produces a peripheral vascular "pump" with effects on circulation, post-exercise muscle recovery, autonomic modulation, and inflammation.
The most studied indication with clinical support is muscle recovery after intense exercise: several small trials have documented significant reductions in delayed-onset muscle soreness (DOMS), markers of muscle damage, and subjective improvements in recovery with contrast therapy after intense exercise, compared with passive recovery or lukewarm water alone; the effects are modest and comparable to other recovery strategies (cold-water immersion alone, active recovery with light exercise).
Some small studies have documented favorable modulation of immune markers (especially reduced inflammatory markers and improvements in lymphocyte function) with regular contrast practices; the data are preliminary but consistent with the mechanistic profile.
Combining heat exposure (with its documented cardiovascular benefits) and cold exposure (with effects on brown adipose tissue, autonomic modulation, and mental well-being) may provide synergy, although the specific data on cardiovascular contrast therapy are more limited than those for each modality separately.
Typical guidelines are: alternate between heat exposure (3-4 minutes in a sauna at 80-90°C or hot water at 38-40°C) and cold exposure (30-60 seconds in cold water at 10-15°C or a cold shower), 3-5 cycles per session; start and end with heat; after exercise, begin the therapy with heat to reduce muscle "stiffness."
Safety considerations are important: people with ischemic heart disease, arrhythmias, poorly controlled arterial hypertension, severe Raynaud's phenomenon, or significant circulatory disorders should consult beforehand; pregnant women should avoid prolonged exposure to very cold or very hot temperatures; people with peripheral neuropathy (especially diabetic) should use caution with very hot water because of the risk of unnoticed burns; in general, gradual introduction and attention to individual response are the best safety guides.
Information, not medical advice. Scientific evidence reflects the quality of the studies.