Strength Training in the Heat Amplifies Muscle Growth Signals
New research finds heated resistance exercise activates temperature-sensitive pathways that accelerate protein synthesis and strength gains.
Combining strength training with heat exposure might be the next evolution in strength and hypertrophy programming. That is, at least for the more pedantic among us, for whom regular old picking up a barbell now seems far too unevolved, no matter its effectiveness. This review by Pryor and colleagues examined heated resistance exercise (HRE), a training method where heat is applied to muscles before, during, or after lifting weights, showing promising mechanisms for accelerated muscle growth and performance improvements.
Aim
The review synthesised evidence from human studies on HRE, examining how this training approach affects neuromuscular function, hormone activity, and cellular signalling pathways that drive muscle hypertrophy and strength development. The authors sought to bridge the gap between laboratory discoveries in cell and rodent models and practical applications for athletes and fitness people.
Methods
This narrative review focused primarily on translational studies using human subjects, examining both acute responses to single HRE sessions and chronic adaptations from training programs. The authors analysed studies that applied heat through various methods, including heated training environments (30-40°C), heating pads, hot water immersion (up to 50°C), microwave therapy, and sauna exposure (up to 100°C). They examined muscle temperature responses, hormonal changes, molecular signalling pathways, neuromuscular performance, and training adaptations spanning 2-13 weeks.
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Results
Molecular Mechanisms
Heat exposure combined with resistance exercise triggers multiple anabolic pathways in skeletal muscle. Most notably, the mTOR signalling cascade—a master regulator of protein synthesis—responds to heat in a temperature-dependent manner, with effects beginning when muscle temperature reaches 38°C and intensifying up to 42°C.
In one study, young men received 20 minutes of microwave therapy before knee extensions, raising muscle temperature to 41°C. This resulted in increased phosphorylation of Akt and p70S6K, key proteins in the muscle-building pathway, one hour after exercise. The mechanism appears to involve heat-induced calcium release through temperature-sensitive channels (TRPV-1), which activate calcineurin and stimulate growth-promoting signals.
Heat stress also increases the expression of heat shock proteins, which protect cells from damage and may suppress muscle breakdown pathways. Studies in immobilised or denervated rats showed that heat therapy reduced muscle atrophy by 3-5% compared to controls.
Hormonal Responses
The hormonal response to HRE shows mixed results depending on training program design. Human growth hormone (hGH) increased following HRE in some studies, particularly when environmental temperatures reached 30-35°C, though individual variation was substantial. In older women, combining heating sheets applied to the quadriceps 8 hours daily, three times weekly for 12 weeks with low-intensity leg extensions (3 sets of 25 reps at 40% 1RM) elevated both hGH and IGF-1 while producing similar strength and size gains as moderate-intensity training (3 sets of 15-18 reps at 60% 1RM) without heat.
Testosterone responses proved context-dependent. In one recent study using a high-volume protocol designed to stimulate testosterone secretion, levels increased in temperate conditions (20°C) but not in the heated condition (40°C), with heat stress appearing to blunt the response. Cortisol, the stress hormone that can inhibit muscle growth, increased significantly during HRE when environmental stress was high or when training volume was elevated.
Neuromuscular Performance
Moderately elevated muscle temperature (approximately 38-39°C) enhances neuromuscular function through faster nerve conduction, improved calcium handling in muscle fibers, and accelerated energy production. These changes improve force production and rate of force development by 2-5% per 1°C increase in muscle temperature, with effects more pronounced in fast-twitch muscle fibers.
Following a strength and power-focused HRE session in 30°C, competitive athletes showed improved upper body power and vertical jump height in males, while female athletes improved countermovement jump and strength. However, when core body temperature becomes excessive (≥39°C), voluntary activation declines and neuromuscular performance suffers. One study found that recreationally trained subjects showed decreased deadlift velocity at 60% 1RM after a high-volume hypertrophy session in 40°C heat.
Training Adaptations
The majority of longer-term studies (up to 13 weeks) demonstrated strength gains following HRE. In young resistance training-naive subjects, applying a 75°C heating pad for 20 minutes before low-intensity elbow extensions (3 sets of 8 reps at 30% 1RM) three days weekly for 6 weeks increased both strength and cross-sectional area compared to controls. Professional rugby players training in 35°C for 3 weeks improved both upper and lower body strength within the heat group.
However, two studies failed to show benefits, likely because muscle temperature increases were insufficient. One achieved only a 1°C difference between groups, while another peaked at 37.6°C, below the threshold needed to stimulate protein synthesis. The evidence suggests muscle temperature must reach at least 38°C for at least 90 minutes to promote muscle retention or growth.
Critical Heating Parameters
The “heating impulse”, the combination of temperature magnitude, duration, and frequency, determines HRE effectiveness. Studies that failed to increase muscle temperature adequately or maintain elevation for sufficient duration showed no benefits. Preheating or heating during exercise appears superior to post-exercise application because it preconditions muscle cells, initiates protective protein expression, reduces exercise-induced damage, and primes neuromuscular function.
Whole-body heat exposure may provide greater benefits than local heating. One hour of sauna exposure that increased muscle temperature to 38.8°C produced greater heat shock protein expression and Akt/mTOR activation in the quadriceps compared to heating only one leg to 38.1°C.
Takeaways
Heat stress combined with resistance training offers a scientifically supported method to potentially enhance muscle growth and strength development, particularly when muscle temperature reaches 38°C or higher for at least 90 minutes. The mechanisms involve temperature-dependent activation of the mTOR pathway, increased calcium signalling, heat shock protein expression, and enhanced neuromuscular function when core temperature remains compensable.
For lifters and fitness folks, practical applications include training in heated environments (30-40°C), using heating pads or garments before or during sessions, or applying sauna or hot water immersion before workouts. Strength and power training blocks appear to benefit more from HRE than hypertrophy-focused programs, while muscular endurance training may be counterproductive, given the additional cardiovascular strain.
However, exercisers should be heat-adapted and have at least intermediate training experience before attempting HRE to ensure session completion and minimise cardiovascular stress. Proper hydration before, during, and after exercise is also vital for a safe and effective experience during HRE, and indeed, any activity in such environments and conditions. The research field remains in its infancy, particularly regarding well-trained athletes, and many questions about optimal protocols, individual responses, and long-term adaptations require further investigation.
Reference
Pryor, J. Luke; Sweet, Daniel; Rosbrook, Paul; Qiao, JianBo; Hess, Hayden W; Looney, David P. Resistance Training in the Heat: Mechanisms of Hypertrophy and Performance Enhancement. Journal of Strength and Conditioning Research 38(7):p 1350-1357, July 2024. | DOI: 10.1519/JSC.0000000000004815
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