Partial-body cryostimulation after training improves sleep quality in professional soccer players

This study investigated whether partial-body cryostimulation (PBC) after a standardized training session could improve sleep quality in professional soccer players.

The interventions tested were:

**No cryostimulation (Control):** Players performed their training session but did not undergo cryostimulation.

**90-second PBC:** A single 90-second exposure to -180°C in a partial-body cryochamber.

**Two 90-second PBC exposures:** Two 90-second exposures to -180°C, separated by a 5-minute rest period at room temperature.

**180-second PBC:** A single 180-second exposure to -180°C in a partial-body cryochamber.

All cryostimulation sessions were performed immediately after a 90-minute standardized training session.

The primary outcome measure was **sleep quality**, assessed objectively by the **number of movements during the night** using 3-dimensional accelerometers worn on the wrist. Lower movement counts were interpreted as better sleep quality.

Secondary outcome measures included:

**Sleep efficiency:** Calculated from accelerometer data.

**Subjective sleep quality:** Assessed using a questionnaire.

**Skin temperature:** Measured on the quadriceps after cryostimulation.

**Perceptual thermal sensation:** Self-reported cold sensation after cryostimulation.

**Heart Rate Variability (HRV):** Measured before and after training to ensure similar training load across sessions.

The study included **9 male professional football (soccer) players** from the Niort Football Club (French National Championship Ligue 2).

Their average characteristics were:

**Age:** 24.8 ± 5.5 years

**Weight:** 76.7 ± 7.3 kg

**Height:** 184 ± 0.1 cm

All subjects underwent a medical examination and provided informed consent. The study conformed to the Code of Ethics of the World Medical Association (Declaration of Helsinki) and was accepted by an ethics committee. No specific exclusion criteria beyond being a professional soccer player were mentioned, implying they were healthy and actively training.

**Objective Sleep Quality (Primary Outcome):** Sleep patterns were monitored using a **WGT3X-BT wrist actigraph monitor** (Pensacola, USA). Subjects wore the actigraph every night after a cryotherapy or control session. They manually started the recording when going to bed and stopped it upon waking. The accelerometer measured movement acceleration across three axes (horizontal, vertical, perpendicular). Raw data were recorded with a 60-second epoch length and extracted as the sum of vector magnitude in counts/min (calculated as the square root of the sum of the square of acceleration for each of the three axes) using actiLife software (version 6.11.0).

* **Sleep efficiency** was calculated as (actual sleep time / total sleep time) * 100. "Actual sleep time" was total sleep time minus wake time. "Wake time" was defined as the number of minutes where the number of movements exceeded a threshold of 40 counts/min.

**Subjective Sleep Quality:** Evaluated using the **Spiegel Sleep Quality Perception Questionnaire**.

**Skin Temperature:** The surface temperature of the quadriceps was measured using an **infrared thermometer** immediately after subjects left the cryo device.

**Perceptual Thermal Sensation:** Subjects rated their perceived coldness using a **10-point scale**, from 0 ("neutral") to 10 ("unbearably cold"), in response to the question "How cold do you feel right now?".

**Heart Rate Variability (HRV):** Assessed before and after training sessions using **Polar V800 GPS heart rate monitors** (Finland) to ensure consistent training load.

This was a **randomized controlled crossover trial**.

**Study Design:**

Each of the 9 professional soccer players participated in all four experimental conditions (control, 90-s PBC, 2x90-s PBC, 180-s PBC). The experiment was organized over one month, with each condition taking place once a week on the same weekday and at the same time (12:00 p.m.). This crossover design means each participant served as their own control, which helps to reduce variability between individuals and increases the statistical power for a given sample size.

**Standardized Training Session:**

Before each experimental condition, players undertook a standardized 90-minute training session. This session consisted of:

10 minutes warm-up

30 minutes of technical and tactical work

30 minutes of interval running (15 seconds at 95% of maximal aerobic speed followed by 15 seconds of passive recovery)

20 minutes of plyometric training

This standardization aimed to ensure that the physical exertion and subsequent recovery needs were consistent across all conditions for each player. HRV was also monitored to confirm similar training loads.

**Randomization:**

The order in which each player experienced the different cryostimulation durations (or control) was **randomized**. This helps to minimize the risk of systematic bias, such as the effects of accumulated fatigue or adaptation over the month-long study period, influencing the results of one condition more than another.

**Blinding:**

The study **did not report any blinding** of participants or researchers. Given the nature of cryostimulation (entering a very cold chamber), participant blinding would be impossible. While the researchers analyzing objective actigraphy data *could* have been blinded to the condition, this was not stated. The lack of participant blinding is a significant consideration for subjective measures, as participants' expectations could influence their self-reported sleep quality.

**Intervention Details:**

Partial-body cryostimulation was performed in a Cryotechno® chamber at -180°C. Players wore bathing suits, gloves, socks, and slippers, and were instructed to turn continuously in the cabin. Sessions were medically supervised.

**Washout Periods:**

Each experimental session (training + cryo/control) occurred once a week. This means there was approximately a **6-day washout period** between each condition for a given player. This duration is generally considered sufficient for the acute physiological effects of cryostimulation to subside, minimizing carryover effects from one condition to the next.

**Duration:**

The cryostimulation exposures themselves were very short (90 seconds, 180 seconds, or two 90-second sessions). Sleep quality was assessed for **one night** immediately following each exposure. The overall experiment spanned **one month**.

**Statistical Approach:**

Results were expressed as mean and standard deviation (SD) or standard error.

Gaussian distribution of variables was tested using the Shapiro–Wilk test.

Changes in variables were evaluated using **repeated-measures ANOVA**, followed by **Tukey post hoc test** where appropriate. Repeated-measures ANOVA is suitable for crossover designs where the same subjects are measured under multiple conditions.

**Effect size** was assessed using **Hedges’ g**, categorized as small (0.2 < g ≤ 0.5), moderate (0.5 < g ≤ 0.8), or large (g > 0.8) according to Cohen's scale. Effect sizes provide a measure of the magnitude of an observed effect, independent of sample size.

Statistical significance was set at **p < 0.05**.

A **sample size calculation** (a priori using G*Power version 3.1) indicated that 6 subjects would be sufficient to detect a significant difference in the number of movements during sleep with a desired power of 0.80 and an alpha risk of 0.05. The study used 9 subjects, which was above this calculated minimum.

**What this design can and cannot prove:**

**Can prove:** The randomized crossover design is strong for establishing a **causal relationship** between partial-body cryostimulation and changes in sleep quality *within the individual professional soccer players studied*. By having each player experience all conditions in a random order, the design effectively controls