Sauna and Muscle Loss: What the Research Shows
Key insights
- Ten days of single-leg immobilisation cost 7.6 per cent of whole-muscle cross-sectional area in untreated limbs, against 4.5 per cent in limbs given two hours of daily deep-tissue heating — and at the level of individual myofibres, 10.8 per cent against 5.8 per cent 1.
- Heat protected function as well as size: coupled myofibre respiratory capacity fell 27 per cent over those ten days without treatment and only 8 per cent with it, so the mitochondria were largely spared 1.
- Six consecutive days of the same heating protocol in freely moving legs raised HSP70 by 45 per cent and HSP90 by 38 per cent and increased PGC-1α along with electron transport chain complexes I and V — an adaptation profile that normally requires training 2.
- Whole-body heat looks like the stronger signal. One hour at 44–50°C, enough to push core temperature to 39.1°C, activated the Akt/mTOR growth pathway and phosphorylated FOXO1 and FOXO3a, the transcription factors that switch on muscle breakdown; a local heat treatment reaching a similar muscle temperature but leaving core temperature untouched moved none of it 3.
- The ceiling is real. Eight weeks of infrared sauna in healthy older adults raised muscle fibre capillarisation by roughly a third but left basal and postprandial muscle protein synthesis rates unchanged 4, and six weeks of localised heating in already-active adults altered neither muscle size nor strength 5.
The question arrives at a predictable moment: a torn ligament, a cast, a surgery date, a fortnight of enforced stillness. Muscle begins to disappear almost immediately, and the person losing it wants to know whether anything short of training can slow it down. Heat is an appealing candidate, partly because it is passive and partly because saunas have accumulated a reputation for doing more or less everything.
There is a real literature here, and it is more encouraging than most wellness claims. But it comes with a specific and important caveat: the strongest muscle-preservation data does not come from saunas. It comes from targeted deep-tissue heating of a single limb, delivered by a physiotherapy device, for two hours a day. Whether a sauna session delivers the same stimulus is a separate question, and one the evidence answers only partially.
So the honest version is a two-part answer. Heat does appear to blunt disuse atrophy, with a plausible mechanism and a controlled human trial behind it. And whole-body heating, the sauna's actual stimulus, activates the relevant signalling more strongly than local heating does — while stopping short of the outcomes people most want to see. Here is what each part of the evidence actually says.
The immobilisation trial
The central study comes from Brigham Young University. Twenty-three healthy volunteers, eleven of them women, had one leg immobilised for ten days. Half received a daily two-hour treatment of pulsed shortwave diathermy to the quadriceps — a device that heats deep tissue directly — and half received an identical-feeling sham. Muscle biopsies and MRI scans were taken before and after 1.
Ten days of disuse is enough to do measurable damage, and it did: whole-muscle cross-sectional area fell 7.6 per cent in the sham group. In the heated group it fell 4.5 per cent. At the level of single myofibres the gap was wider still, 10.8 per cent against 5.8 per cent. Heat did not prevent atrophy. It roughly halved it 1.
The mitochondrial result is arguably the more interesting one, because loss of oxidative capacity is what makes returning from an injury feel so disproportionately hard. Coupled respiratory capacity dropped 27 per cent in untreated limbs and 8 per cent in heated ones; uncoupled capacity followed the same pattern. The proteins making up all five mitochondrial respiratory complexes were preserved in the heated leg and lost in the other 1.
Why heat does anything at all
Muscle mass is a balance between synthesis and breakdown, and both sides are under signalling control. The Akt/mTOR pathway drives protein synthesis. The FOXO transcription factors do the opposite: when active, they switch on the atrogenes that tag muscle protein for degradation. Disuse tips the balance by suppressing the first and releasing the brake on the second.
Heat appears to intervene on both. In the same research group's earlier work, twenty participants received two hours of daily diathermy for six consecutive days, raising muscle temperature by about 3.9°C. HSP70 rose 45 per cent and HSP90 38 per cent — chaperone proteins that stabilise and repair other proteins under stress. Alongside them came increases in PGC-1α, the master regulator of mitochondrial biogenesis, and in respiratory complexes I and V, with measurable gains in respiratory capacity 2. This was the first demonstration that passive heat alone could produce a mitochondrial adaptation in human muscle. We have written separately on what heat exposure does to heat shock protein levels and how much heat it takes.
Whole-body versus local heat: the distinction that decides everything
None of the above involved a sauna, which raises the obvious objection. A group at Aspetar in Doha ran the comparison directly. Nine active men underwent, on separate occasions, either sixty minutes of whole-body passive heating at 44–50°C or sixty minutes of single-leg heating via a water-perfused suit. Muscle temperature ended up statistically similar in the two conditions — 38.8°C against 38.1°C. Core temperature did not: it reached 39.1°C with whole-body heating and did not move at all with the local treatment 3.
The molecular results split along the same line. Whole-body heating raised the phosphorylation of Akt, mTOR, S6K1, rpS6 and eIF4E; increased HSP72, HSP90 and HSP25 messenger RNA; increased markers of mitochondrial biogenesis including NRF1 and NRF2; and increased phosphorylation of FOXO1 and FOXO3a, which is the state in which those breakdown signals are held in check. The single-leg condition produced essentially none of this 3.
That is a meaningful finding for anyone weighing a sauna against a heating pad. It suggests the systemic response — the elevated core temperature, the cardiovascular strain, the circulating signals that come with it — carries more of the adaptive message than local tissue warming does. A sauna is a whole-body stimulus. A heat pack on a quad is not.
Where the evidence stops
Two studies mark the boundary clearly, and both deserve weight. In the first, fourteen healthy older adults averaging 73 years completed eight weeks of infrared sauna, three sessions a week, 45 minutes at roughly 60°C. Capillarisation around both type I and type II fibres rose sharply, by 31 and 33 per cent on the capillary-to-fibre perimeter exchange index. Muscle protein synthesis rates, measured both at rest and after a meal, did not change at all 4.
In the second, fifteen active adults wore heat pads on one calf for eight hours a day, five days a week, for six weeks, raising muscle temperature by 4.6°C. After six weeks there was no change in gastrocnemius cross-sectional area, pennation angle, twitch amplitude, rate of torque development, or maximal voluntary torque — not in the heated leg, not compared with the control leg 5.
Read together with the immobilisation trial, the pattern is coherent rather than contradictory. Heat protects muscle that is being lost. It does not appear to build muscle that is already being maintained. The stimulus is defensive, not constructive, and it matters most precisely when the normal stimulus — loading — has been removed.
What this means in practice
If you are training normally, sauna use is unlikely to add anything to muscle size or strength, and should be chosen for its other documented effects rather than this one. If you are injured, post-operative, immobilised or facing a forced layoff, the case is genuinely different: the available evidence supports heat as a partial countermeasure, and there is little downside beyond the time it costs.
On dose, the honest answer is that no sauna protocol has been tested for this outcome. What the studies share is duration and frequency well beyond casual use — two hours daily in the diathermy work, three to five sessions weekly in the whole-body work — and, in the condition that produced the strongest signalling, a core temperature raised to about 39°C. That last number is the one worth anchoring to: a session that does not meaningfully raise core temperature is not reproducing the stimulus the research describes. Protein intake and any loading a clinician permits remain the primary interventions; heat is an adjunct. And anyone recovering from surgery or managing a cardiovascular condition should clear sauna use with their clinician before starting.
The Contrast Market Perspective
The through-line in this literature is that core temperature, not cabin temperature, is what separated the conditions that worked from the conditions that did nothing. That is a hardware question as much as a behavioural one: a cabin that cannot hold its set point, or that loses heat through poor insulation every time the door opens, delivers a weaker thermal dose than the number on the panel implies — and in the studies above, a weaker dose meant no measurable response. If you are considering heat as part of a recovery or rehabilitation plan and want an honest assessment of what a given cabin would actually deliver, Schedule a consultation.
References
Every claim above draws on peer-reviewed primary sources, listed in full below.
Footnotes
- Hafen PS, Abbott K, Bowden J, Lopiano R, Hancock CR, Hyldahl RD (2019). Daily heat treatment maintains mitochondrial function and attenuates atrophy in human skeletal muscle subjected to immobilization. Journal of Applied Physiology, 127(1), 47–57. PubMed ↩︎
- Hafen PS, Preece CN, Sorensen JR, Hancock CR, Hyldahl RD (2018). Repeated exposure to heat stress induces mitochondrial adaptation in human skeletal muscle. Journal of Applied Physiology, 125(5), 1447–1455. PubMed ↩︎
- Ihsan M, Deldicque L, Molphy J, Britto F, Cherif A, Racinais S (2020). Skeletal muscle signaling following whole-body and localized heat exposure in humans. Frontiers in Physiology, 11, 839. PubMed ↩︎
- Fuchs CJ, Betz MW, Petrick HL, Weber J, Senden JM, Hendriks FK, Bels JLM, van Loon LJC, Snijders T (2025). Repeated passive heat treatment increases muscle tissue capillarization, but does not affect postprandial muscle protein synthesis rates in healthy older adults. The Journal of Physiology, 603(1), 167–186. PubMed ↩︎
- Labidi M, Ihsan M, Behan FP, Alhammoud M, Smith T, Mohamed M, Tourny C, Racinais S (2021). Six weeks of localized heat therapy does not affect muscle mass, strength and contractile properties in healthy active humans. European Journal of Applied Physiology, 121(2), 573–582. PubMed ↩︎