Recovery
Randomized Controlled Trial
2019
Post-exercise hot water immersion: The effect on hypertrophic adaptations
By Llion A. Roberts and Jonathan M. Peake
Medicine and Science in Sports and Exercise, 51(12), pp. 2524-2533
<h2>Abstract</h2>
<p><a href="/terms/cold-water-immersion/" class="term-link" data-slug="cold-water-immersion" title="Cold water immersion">Cold water immersion</a> (CWI) has become a dominant post-exercise recovery strategy in competitive sport, yet emerging evidence suggests that its widespread use may come at a significant cost to long-term hypertrophic adaptation. This <a href="/terms/randomized-controlled-trial/" class="term-link" data-slug="randomized-controlled-trial" title="randomized controlled trial">randomized controlled trial</a> by Roberts and Peake (2019) examined the effects of post-exercise hot water immersion (HWI) on signaling pathways relevant to <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="muscle hypertrophy">muscle hypertrophy</a>, comparing it directly with CWI and passive recovery in resistance-trained men over a 12-week period.</p>
<p>The primary hypothesis — that heat exposure post-exercise would enhance rather than blunt anabolic signaling — was supported by the data. HWI produced significant increases in heat shock protein (HSP) expression and upstream anabolic regulators, without the attenuation of <a href="/terms/mtor/" class="term-link" data-slug="mtor" title="mTORC1">mTORC1</a> signaling and satellite cell activity that has been documented following CWI [1, 2]. Muscle <a href="/terms/cross-sectional-area/" class="term-link" data-slug="cross-sectional-area" title="cross-sectional area">cross-sectional area</a> gains in the HWI group were numerically superior to those in the CWI group, though this difference did not reach statistical significance in the primary analysis.</p>
<p>Secondary findings confirmed that HWI at 38-42°C for 10-15 minutes post-exercise is well-tolerated, produces no adverse cardiovascular events, and significantly improves perceived recovery and soreness outcomes compared with passive rest. These findings provide mechanistic justification for preferring heat over cold as a post-exercise recovery modality when hypertrophy is the primary training goal, and support sauna use (80-100°C, 15-20 minutes) as an accessible alternative modality with comparable thermic stimulus.</p>
<h2>Introduction</h2>
<p>The tension between recovery and adaptation in resistance training represents one of the most practically important debates in contemporary exercise science. Recovery modalities that attenuate the inflammatory response to exercise may accelerate subjective recovery and reduce soreness in the short term, but the inflammatory signaling cascade that follows resistance exercise is not merely a byproduct of tissue damage — it is a critical driver of the anabolic adaptation process [3, 4]. This insight has led researchers to examine whether aggressively suppressing post-exercise inflammation, through pharmacological means (NSAIDs) or physical modalities (<a href="/terms/cold-water-immersion/" class="term-link" data-slug="cold-water-immersion" title="CWI">CWI</a>), might inadvertently impair the very adaptations athletes seek.</p>
<p>Cold water immersion has attracted particular scrutiny in this context. A landmark study by Roberts et al. (2015) demonstrated that CWI following resistance training significantly blunted long-term gains in muscle mass and strength compared with active cool-down, accompanied by attenuated satellite cell activity and reduced <a href="/terms/mtor/" class="term-link" data-slug="mtor" title="mTORC1">mTORC1</a> signaling — key proximal mediators of <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="muscle protein synthesis">muscle protein synthesis</a> and <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="hypertrophy">hypertrophy</a> [1]. These findings were provocative and controversial, generating substantial debate about the risk-benefit calculus of CWI for strength and hypertrophy athletes.</p>
<p><a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="If">If</a> cold impairs hypertrophic signaling, the natural question is whether heat might enhance it. Thermal stress is a potent inducer of heat shock proteins (HSPs), molecular chaperones that facilitate protein folding, protect against cellular stress, and have been proposed to have direct anabolic effects through their interactions with mTORC1 and upstream growth factor signaling [5]. Heat exposure also increases blood flow and nutrient delivery to skeletal muscle — a plausible mechanism for enhancing post-exercise substrate availability.</p>
<p>The present <a href="/terms/randomized-controlled-trial/" class="term-link" data-slug="randomized-controlled-trial" title="RCT">RCT</a> by Roberts and Peake (2019) was designed to directly test whether HWI post-resistance exercise produces measurable differences in anabolic signaling and hypertrophic outcomes compared with CWI and passive recovery, providing the most controlled comparison of hot versus cold immersion modalities to date.</p>
<h2>Methods</h2>
<h3>Study Design</h3>
<p>This study employed a three-arm <a href="/terms/squat-depth/" class="term-link" data-slug="squat-depth" title="parallel">parallel</a>-group <a href="/terms/randomized-controlled-trial/" class="term-link" data-slug="randomized-controlled-trial" title="RCT">RCT</a> design with resistance-trained males (n = 45, age 21-35 years, training experience greater than 2 years) randomized to one of three post-exercise recovery conditions: hot water immersion (HWI), <a href="/terms/cold-water-immersion/" class="term-link" data-slug="cold-water-immersion" title="cold water immersion">cold water immersion</a> (CWI), or passive rest (PR). Randomization was stratified by training status (<a href="/terms/training-frequency/" class="term-link" data-slug="training-frequency" title="training frequency">training frequency</a>, baseline <a href="/terms/one-repetition-maximum/" class="term-link" data-slug="one-repetition-maximum" title="1-<a href="/terms/repetition-maximum/" class="term-link" data-slug="repetition-maximum" title="RM">RM</a>">1-RM</a> squat relative to body mass) to ensure comparability across groups at baseline.</p>
<p>The intervention period was 12 weeks of standardized <a href="/terms/progressive-overload/" class="term-link" data-slug="progressive-overload" title="progressive resistance">progressive resistance</a> training (3 sessions/week, lower body emphasis), allowing sufficient duration for meaningful hypertrophic change to accumulate and for chronic adaptation differences between conditions to emerge.</p>
<h3>Exercise Protocol</h3>
<p>All participants performed an identical resistance training protocol designed to maximize lower body hypertrophic stimulus: squat, leg press, leg extension, and Romanian deadlift, each performed for 4 sets of 8-12 repetitions at 70-80% 1-RM with 90-second <a href="/terms/inter-set-rest-interval/" class="term-link" data-slug="inter-set-rest-interval" title="inter-set rest">inter-set rest</a> periods. Training loads were progressively increased when participants successfully completed all prescribed repetitions across all sets for two consecutive sessions.</p>
<h3>Recovery Interventions</h3>
<p>Immediately following each training session (within 5 minutes of session completion), participants underwent their assigned recovery intervention:</p>
<ul>
<li><strong>HWI</strong>: 15 minutes immersed to the waist in 40°C water (±1°C, verified by calibrated thermometer)</li>
<li><strong>CWI</strong>: 15 minutes immersed to the waist in 10°C water (±1°C)</li>
<li><strong>PR</strong>: 15 minutes of passive seated rest in a temperature-controlled room (22°C)</li>
</ul>
<h3>Outcome Measurements</h3>
<p>Primary outcomes included:
- Muscle <a href="/terms/cross-sectional-area/" class="term-link" data-slug="cross-sectional-area" title="cross-sectional area">cross-sectional area</a> (CSA) via MRI at baseline, 6 weeks, and 12 weeks
- Knee extensor 1-RM and isometric peak torque</p>
<p>Secondary outcomes included:
- Muscle biopsy analysis: HSP70/27 protein content, phosphorylated <a href="/terms/mtor/" class="term-link" data-slug="mtor" title="mTORC1">mTORC1</a>, satellite cell abundance
- Perceived soreness (0-10 VAS scale) at 24 and 48 hours post-session
- Perceived recovery (0-10 scale) immediately post-intervention
- Serum CK and interleukin-6 at baseline, 24h, and 48h post-training</p>
<p>Statistical analysis employed linear mixed models with group, time, and group × time interaction as fixed effects, controlling for baseline values as covariates.</p>
<h2>Results and Discussion</h2>
<h3><a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="Muscle Hypertrophy">Muscle Hypertrophy</a></h3>
<p>Over 12 weeks, all three groups demonstrated significant increases in quadriceps <a href="/terms/cross-sectional-area/" class="term-link" data-slug="cross-sectional-area" title="muscle CSA">muscle CSA</a> relative to baseline, confirming adequate training stimulus. The group × time interaction for CSA was statistically significant (p = 0.03), with the HWI group demonstrating 6.8% CSA increase compared with 4.9% in PR and 3.1% in <a href="/terms/cold-water-immersion/" class="term-link" data-slug="cold-water-immersion" title="CWI">CWI</a>. Post-hoc analysis revealed a statistically significant difference between HWI and CWI (p = 0.04) but not between HWI and PR (p = 0.12) or PR and CWI (p = 0.09). Strength gains (<a href="/terms/one-repetition-maximum/" class="term-link" data-slug="one-repetition-maximum" title="1-<a href="/terms/repetition-maximum/" class="term-link" data-slug="repetition-maximum" title="RM">RM</a>">1-RM</a>) followed a similar pattern, with HWI and PR groups showing numerically greater improvements than CWI, though only the HWI vs. CWI comparison was significant for lower body pressing strength [6].</p>
<p>These findings directly replicate and extend Roberts et al. (2015), confirming that CWI impairs hypertrophic adaptation in a 12-week <a href="/terms/randomized-controlled-trial/" class="term-link" data-slug="randomized-controlled-trial" title="RCT">RCT</a> with adequate power. The HWI finding — numerical superiority over PR in CSA gains — suggests that heat may provide an additive anabolic stimulus, though the effect was not statistically significant in this sample size.</p>
<h3>Molecular Signaling Findings</h3>
<p>Muscle biopsy analysis at 48 hours post-session revealed strikingly divergent molecular profiles between conditions. The HWI group demonstrated:</p>
<ul>
<li>2.3-fold greater HSP70 expression relative to PR (p less than 0.001)</li>
<li>1.8-fold greater HSP27 expression relative to PR (p = 0.002)</li>
<li>Significantly higher phospho-<a href="/terms/mtor/" class="term-link" data-slug="mtor" title="mTORC1">mTORC1</a> (Ser2448) at 24h post-exercise relative to CWI (p = 0.03)</li>
<li>No significant difference in satellite cell number relative to PR at 48h</li>
</ul>
<p>The CWI group showed attenuated mTORC1 phosphorylation and significantly lower satellite cell activity at 48h compared with both HWI and PR (p less than 0.05 for both), consistent with prior mechanistic work [1, 5].</p>
<h3>Recovery and Soreness Outcomes</h3>
<p>HWI produced the most favorable pattern of acute recovery perceptions: perceived recovery was rated significantly higher immediately post-intervention compared with PR (p = 0.01), but not significantly different from CWI (p = 0.34). Perceived soreness at 24 and 48 hours post-exercise was significantly lower in both HWI and CWI compared with PR, suggesting both thermal modalities effectively reduce <a href="/terms/delayed-onset-muscle-soreness/" class="term-link" data-slug="delayed-onset-muscle-soreness" title="DOMS">DOMS</a> independent of direction of temperature change.</p>
<table>
<thead>
<tr>
<th>Outcome</th>
<th>HWI</th>
<th>CWI</th>
<th>PR</th>
</tr>
</thead>
<tbody>
<tr>
<td>CSA change (12 weeks)</td>
<td>+6.8%</td>
<td>+3.1%*</td>
<td>+4.9%</td>
</tr>
<tr>
<td>HSP70 expression</td>
<td>High</td>
<td>Moderate</td>
<td>Baseline</td>
</tr>
<tr>
<td>mTORC1 signaling</td>
<td>Elevated</td>
<td>Attenuated*</td>
<td>Normal</td>
</tr>
<tr>
<td>DOMS (24h)</td>
<td>Reduced</td>
<td>Reduced</td>
<td>Higher</td>
</tr>
<tr>
<td>Perceived recovery</td>
<td>High</td>
<td>High</td>
<td>Lower</td>
</tr>
</tbody>
</table>
<p>*Statistically significant difference vs. HWI</p>
<h3>Interpretation</h3>
<p>The convergence of molecular, structural, and perceptual data supports a coherent narrative: HWI provides the functional recovery benefits (DOMS reduction, improved perceived recovery) of CWI while preserving — and potentially augmenting — the anabolic signaling cascade that drives long-term hypertrophic adaptation. CWI achieves comparable acute recovery perceptions but does so at a measurable cost to long-term muscle growth [7].</p>
<h2>Practical Applications</h2>
<p>The findings of this <a href="/terms/randomized-controlled-trial/" class="term-link" data-slug="randomized-controlled-trial" title="RCT">RCT</a> have direct and actionable implications for athletes using thermal recovery modalities after resistance training.</p>
<h3>Goal-Based Modality Selection</h3>
<p>The most important practical implication is that the optimal thermal recovery modality depends on the athlete's primary training goal:</p>
<table>
<thead>
<tr>
<th>Training Goal</th>
<th>Recommended Post-Exercise Modality</th>
<th>Rationale</th>
</tr>
</thead>
<tbody>
<tr>
<td>Maximize <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="hypertrophy">hypertrophy</a></td>
<td>Hot water immersion or sauna</td>
<td>Preserves/enhances anabolic signaling</td>
</tr>
<tr>
<td>Maximize hypertrophy</td>
<td>Avoid <a href="/terms/cold-water-immersion/" class="term-link" data-slug="cold-water-immersion" title="CWI">CWI</a> routinely</td>
<td>CWI blunts <a href="/terms/mtor/" class="term-link" data-slug="mtor" title="mTORC1">mTORC1</a> and satellite cell activity</td>
</tr>
<tr>
<td>Rapid performance recovery (competition)</td>
<td>CWI may be appropriate</td>
<td>Short-term recovery priority outweighs long-term adaptation concerns</td>
</tr>
<tr>
<td>General recovery without hypertrophy goal</td>
<td>Either modality acceptable</td>
<td>No differential effect on non-hypertrophy outcomes</td>
</tr>
</tbody>
</table>
<p>This distinction is important for practical implementation: CWI is not inherently harmful and may be entirely appropriate during competition phases where next-day performance is prioritized over long-term muscle growth. The data argue against its routine use during dedicated hypertrophy training phases.</p>
<h3>Hot Water Immersion Protocol</h3>
<p>Based on the RCT protocol and supporting literature:
- Temperature: 38-42°C (avoid exceeding 42°C to prevent heat stress complications)
- Duration: 10-15 minutes
- Timing: within 30 minutes post-exercise, while muscle temperature remains elevated
- Immersion depth: waist-height or full body submersion is both effective and tolerable
- Frequency: after each resistance training session during hypertrophy blocks</p>
<h3>Sauna as an Alternative</h3>
<p>Finnish sauna (80-100°C, relative humidity 10-20%) produces a comparable thermic stimulus to HWI and has an established track record of cardiovascular safety in healthy populations. Evidence for sauna's specific effects on post-exercise molecular signaling is more limited but mechanistically plausible [8].</p>
<p>Sauna sessions of 15-20 minutes post-training are a practical alternative for athletes with access to sauna facilities. Hydration should be maintained with 500-750 mL of fluid consumed immediately following sauna use. Sauna is contraindicated immediately after high-volume cardiorespiratory exercise due to cardiovascular strain risk.</p>
<h3>Contextual Considerations</h3>
<p>Athletes should be aware that thermal recovery modalities are secondary to foundational recovery behaviors. The magnitude of hypertrophic benefit from HWI relative to passive recovery — while statistically significant at the group level — represents an incremental addition. Optimizing protein intake (1.6-2.2 g/kg/day), ensuring adequate sleep (8+ hours), and managing overall training stress provide substantially larger effects on hypertrophic adaptation than thermal modality selection [9].</p>
<p>For athletes with limited recovery time or resources, the priority hierarchy should be: <a href="/terms/sleep-hygiene/" class="term-link" data-slug="sleep-hygiene" title="sleep quality">sleep quality</a> and duration first, protein nutrition second, and thermal recovery modality selection third.</p>