The effects of cold water immersion with different dosages (duration and temperature variations) on heart rate variability post-exercise recovery: A randomized controlled trial

The researchers tested four different "doses" of cold water immersion (CWI) against a passive recovery control group. The intervention was a single session of CWI applied immediately after an intense exercise protocol. The doses varied by:

**Duration:** 5 minutes vs. 15 minutes

**Temperature:** 9°C ± 1°C vs. 14°C ± 1°C

This created four experimental groups:

G1: 5 minutes at 9°C

G2: 5 minutes at 14°C

G3: 15 minutes at 9°C

G4: 15 minutes at 14°C

The control group (CG) sat passively for 15 minutes.

The primary outcome was heart rate variability (HRV) — a measure of how well the autonomic nervous system (ANS) is functioning. HRV reflects the balance between the sympathetic ("fight or flight") and parasympathetic ("rest and digest") branches of the nervous system. Higher HRV generally indicates better recovery and cardiovascular health.

Specific HRV indices measured included:

**MeanRR:** Average time between heartbeats (longer = more relaxed state)

**SDNN:** Overall HRV (higher = better autonomic function)

**RMSSD:** Short-term HRV, reflecting parasympathetic activity (higher = better recovery)

**VLF, LF, HF:** Frequency-domain measures of different aspects of autonomic activity

**SD1 and SD2:** Poincaré plot measures of beat-to-beat variability

**Sample size:** 100 young healthy male participants (20 per group, with some data loss reducing groups to 18–20)

**Age:** 21.73 ± 2.92 years (range approximately 18–27)

**Height:** 1.75 ± 0.06 m

**Weight:** 74.22 ± 10.15 kg

**BMI:** 24.15 ± 2.99 kg/m² (normal to slightly overweight)

**Activity level:** All physically active according to the International Physical Activity Questionnaire (IPAQ)

**Exclusions:** Non-smokers, non-alcoholics, no drugs affecting cardiac autonomic activity, no cardiovascular/metabolic/endocrine diseases

**Setting:** Laboratory at Universidade Estadual Paulista, Brazil, with controlled temperature (22.0 ± 2.34°C) and humidity (40–60%)

**Time of day:** All sessions conducted between 5–10 PM to control for circadian variation

**Important limitation for generalization:** Only young, healthy, physically active males were studied. Results may not apply to women, older adults, sedentary individuals, or those with health conditions.

**Heart rate monitoring:** Polar RS800CX chest strap and wrist receiver (sampling at 1000 Hz)

**HRV analysis software:** Kubios HRV Analysis Software 2.0

**Filtering:** Digital filtering plus manual filtering to remove artifacts, ectopic beats, and premature beats. Only series with >95% sinus beats were included

**Analysis windows:** 256 consecutive RR-intervals were analyzed at each time point:

- Baseline (final 256 intervals of a 20-minute rest period)

- During the recuperative technique (TRec)

- 20, 30, 40, 50, and 60 minutes post-exercise

**Exercise protocol:** A combination of 10 sets of 10 maximal vertical jumps (with 1-minute breaks) immediately followed by a 30-second Wingate cycling test (maximal sprint against 0.075 kp/kg body mass)

**Water temperature control:** Ice added and water stirred, monitored with a thermometer accurate to ±0.3°C

**Immersion depth:** Up to the anterior-superior iliac spine (approximately waist-deep, seated)

**Study design:** This was a randomized controlled trial (RCT) with five parallel groups. Participants were randomly allocated using Microsoft Excel software.

**Randomization:** Yes, computer-generated randomization. This is important because it reduces selection bias and increases the likelihood that groups are comparable at baseline.

**Blinding:** Participants were not informed which group they belonged to regarding intervention duration and temperature, nor which intervention was considered therapeutic. However, this is partial blinding at best — participants in the 5-minute groups knew they were in the water for less time, and those in the 9°C groups could feel the cold more intensely. The researchers administering the intervention were not blinded (they had to set temperatures and timers). There was no sham or placebo control (e.g., room-temperature water immersion).

**Duration:** Single-session study. Total recovery time was 75 minutes including immersion. HRV was measured at baseline, during recovery, and at 20, 30, 40, 50, and 60 minutes post-exercise.

**Washout:** Not applicable (single session, no crossover).

**Statistical approach:**

Normality tested with Kolmogorov-Smirnov test

One-way ANOVA with Tukey's post-hoc for baseline group comparisons

Repeated Measures ANOVA with Bonferroni correction for time and group effects

Sphericity tested with Mauchly's test; Greenhouse-Geisser corrections applied when violated

Sample size calculated a priori based on previous research (18 per group needed for 80% power at 5% significance)

**What this design can prove:**

Causal relationships between CWI dose and HRV recovery (because of randomization and control group)

Which specific dose (temperature × duration combination) is most effective

The time course of HRV recovery over 60 minutes post-exercise

**What this design cannot prove:**

Long-term effects or adaptations (single session only)

Effects on performance, injury prevention, or training adaptation (no performance measures)

Mechanisms (HRV changes could be due to multiple physiological pathways)

Generalizability to women, older adults, or clinical populations

Whether effects persist beyond 60 minutes

Whether the optimal dose changes with repeated use (no habituation assessment)

**Major methodological weaknesses:**

No blinding of researchers or true blinding of participants

No sham control (e.g., thermoneutral water immersion)

Single session only — cannot assess chronic effects

Only males studied

Self-reported compliance with pre-exercise restrictions (no objective monitoring)

Data loss in some groups (though small: 1–2 participants per group)

**Primary outcomes (HRV indices showing anticipated return to baseline):**

**MeanRR (average time between heartbeats):** All four CWI groups showed an anticipated return to baseline values compared to control. This means heart rate recovered faster with any dose of cold water immersion.

**VLF (very low frequency power):** All CWI groups showed faster return to baseline.

**LF (low frequency power):** All CWI groups showed faster return to baseline.

**SDNN (overall HRV):** Only G4 (15 minutes at 14°C) showed anticipated return to baseline and higher values compared to control.

**SD2 (Poincaré plot — long-term variability):** Only G4 showed anticipated return to baseline and higher values compared to control.

**Secondary outcomes (no significant differences):**

**RMSSD (parasympathetic activity):** No significant differences between groups.

**HF (high frequency power):** No significant differences between groups.

**SD1 (short-term variability):** No significant differences between groups.

**Specific statistical results reported:**

G4 (15 min at 14°C) was the only group that showed significantly higher SDNN and SD2 values compared to the control group (CG) during recovery.

For MeanRR, VLF, and LF, all intervention groups showed faster recovery than CG, but G4 was not necessarily superior to other CWI groups for these measures.

The paper reports p-values and post-hoc comparisons but does not provide exact p-values for every comparison in the abstract. Full statistical reporting would require consulting the complete results tables.

**Key pattern:** The 15-minute at 14°C group (G4) was the most consistently effective across multiple HRV indices. Interestingly, the coldest water (9°C) did not outperform 14°C, and shorter durations (5 minutes) were less effective than 15 minutes for some measures.

The study reports that G4 (15 minutes at 14°C) showed "higher values when compared to CG" for SDNN and SD2, and that all CWI groups showed "anticipated return to baseline" for MeanRR, VLF, and LF. However, the abstract does not provide specific effect sizes (e.g., mean differences in milliseconds or percentage changes).

To translate into practical terms:

**MeanRR (heart rate recovery):** If baseline MeanRR was ~900 ms (approximately 67 bpm), and post-exercise it dropped to ~500 ms (120 bpm), an "anticipated return to baseline" means that CWI groups returned to ~900 ms faster than the control group. The time saved was likely on the order of 10–30 minutes, based on the measurement points (20, 30, 40, 50, 60 min).

**SDNN:** G4 showed SDNN values that were statistically significantly higher than control during recovery. A typical SDNN in healthy young adults at rest is ~50–70 ms. Post-exercise, it drops to ~20–30 ms. G4 likely showed SDNN values ~10–20 ms higher than control during recovery, indicating better overall autonomic function.

**Practical significance:** A faster return to baseline HRV means the body's stress response is resolving more quickly. This could translate to feeling less fatigued, better readiness for subsequent training sessions, and potentially reduced cumulative stress over a training cycle.

**Important caveat:** The magnitude of effect was modest and primarily seen in overall HRV measures (SDNN, SD2) rather than pure parasympathetic measures (RMSSD, HF). This suggests CWI affects the overall autonomic balance rather than specifically boosting parasympathetic activity.

**Acknowledged by authors:**

Data loss in some groups (1–2 participants per group)

Single session design limits generalizability to chronic use

Only healthy young males studied

**Critical reader observations:**

**No true blinding:** Participants could feel the cold and knew how long they were immersed. This introduces potential placebo effects and expectancy bias.

**No sham control:** A group immersed in thermoneutral water (e.g., 32–34°C) would have controlled for the effects of immersion itself (hydrostatic pressure, buoyancy) versus temperature.

**Single session only:** Cannot assess whether the effects persist, diminish, or amplify with repeated use over days or weeks.

**Limited generalizability:** Young healthy males only. Women have different thermoregulatory and autonomic responses. Older adults and clinical populations were excluded.

**Exercise protocol:** The combination of jumping and Wingate test is specific and may not generalize to other exercise types (endurance, strength, team sports).

**Short monitoring window:** Only 60 minutes post-exercise. Longer-term recovery (hours, next day) was not assessed.

**No performance measures:** The study did not measure whether faster HRV recovery translated to better subsequent performance, reduced soreness, or improved training adaptation.

**Self-reported compliance:** Participants were asked to avoid exercise, caffeine, and supplements for 24 hours, but this was not objectively verified.

**Industry funding:** Not explicitly stated, but no obvious conflicts declared. However, cryotherapy and recovery products are a commercial industry, so replication by independent groups would strengthen confidence.

**Multiple comparisons:** With many HRV indices and time points, some significant findings could be due to chance. The authors used Bonferroni correction, which helps, but the risk remains.

For someone running their own n=1 experiment:

### What to test

**Primary intervention:** Cold water immersion at 14°C (approximately 57°F) for 15 minutes, waist-deep, immediately after intense exercise.

**Comparison conditions to try:**

- Passive recovery (sitting for 15 minutes)

- Shorter immersion (5 minutes at 14°C)

- Warmer immersion (20°C/68°F for 15 minutes to test if temperature matters)

- Contrast water therapy (alternating hot and cold)

### Minimum meaningful duration

**Single session:** You can detect acute effects within 60 minutes post-exercise

**For chronic effects:** Run for at least 2–4 weeks of consistent post-exercise CWI, measuring HRV daily before and after training

**Measurement protocol:** Record HRV at the same time each day (morning upon waking is standard), plus immediately post-exercise and at 20, 40, and 60 minutes post-CWI

### What to measure (specific metrics)

**Primary metric:** Morning resting HRV (SDNN or RMSSD) — this reflects overall recovery and autonomic balance

**Secondary metrics:**

- Heart rate recovery (HRR): Heart rate drop from peak exercise to 1- and 2-minutes post-exercise

- Perceived recovery: Use a 1–10 scale (1 = completely exhausted, 10 = fully recovered)

- Muscle soreness: Visual analog scale (0–10) for specific muscle groups

- Sleep quality: Subjective rating or wearable sleep tracker

- Training performance: If you track workouts, note if you feel stronger/faster in subsequent sessions

### Key confounds to control for

**Time of day:** Measure HRV at the same time each day (morning is best)

**Pre-exercise state:** Avoid caffeine, alcohol, and heavy meals for 3–4 hours before

**Hydration status:** Dehydration affects HRV and thermoregulation

**Sleep:** Poor sleep the night before will confound results

**Training load:** Keep your training consistent during the experiment (same type, intensity, and duration of exercise each session)

**Water temperature:** Use a thermometer to ensure consistent temperature (±1°C)

**Immersion depth:** Mark the water level on your body (e.g., waist) and replicate exactly

**Ambient temperature:** Room temperature affects how your body responds to cold water

**Menstrual cycle (if applicable):** Cycle phase affects thermoregulation and autonomic function

### What a positive result would look like

**Acute (within 60 minutes post-exercise):** Your HRV (especially SDNN and MeanRR) returns to baseline values 10–30 minutes faster after CWI compared to passive recovery. For example, if passive recovery takes 50 minutes for SDNN to return to baseline, CWI might achieve this in 30 minutes.

**Chronic (over 2–4 weeks):** Your morning resting HRV (RMSSD or SDNN) shows a gradual upward trend (e.g., 5–15% increase) compared to a control period without CWI. You might also notice lower perceived soreness, better sleep quality, and feeling more recovered between training sessions.

**Specific numbers to look for:**

- SDNN increase of 5–15 ms above baseline during recovery

- MeanRR (heart period) returning to within 50 ms of baseline within 30 minutes vs. 50+ minutes

- Morning RMSSD increasing by 10–20% over a 2-week period

**Subjective markers:** You feel "ready to train again" sooner, with less muscle stiffness and fatigue

**Bottom line for self-experimenters:** Try 15 minutes of waist-deep immersion at 14°C (57°F) immediately after your hardest workouts. Track morning HRV and perceived recovery for 2–4 weeks. Compare to a 2-week period of passive recovery. If your HRV trends upward and you feel less beat up, the protocol