Cooling down the use of cryotherapy for post-exercise skeletal muscle recovery

Cryotherapy is a popular method used by professional athletes, which is assumed to improve post-exercise recovery of skeletal muscle function. A common cryotherapy method is cold water immersion, which can involve whole body immersion or only immersion of the exercised limbs. While there is considerable evidence to show that whole body cold water immersion can be beneficial for reducing the risk of hyperthermia when exercising in hot environments [1], mechanistic evidence is lacking as to whether the cooling of skeletal muscle per se can improve the recovery of muscle strength and endurance.

In our recently published manuscript in The Journal of Physiology [2], we addressed the specific question as to whether cooling or heating skeletal muscle affects the acute recovery of muscle contractile function following endurance exercise. To this end, experiments in human subjects involved recreationally-active individuals returning on three separate visits to perform 1 h of moderate-intensity arm cycling exercise. The main difference between visits was whether the upper arms of the subjects were cooled, heated, or not treated (i.e., not heated or cooled) in the 2 h post-exercise recovery period. The upper arm muscles were cooled using arm cuffs continuously water-perfused with ice-chilled water that reduced muscle temperature to ∼15°C compared with the not- treated condition (33°C), which was easily tolerated by the subjects. The heating condition involved continuously perfusing the arm cuffs with warmed water that elevated muscle temperature to 38°C, which pilot testing revealed was the upper limit of what subjects could comfortably tolerate for 2 h. A key finding from these experiments was that there was a positive relationship between muscle temperature during the recovery period and fatigue resistance during subsequent all-out exercise performed after the recovery period. Specifically, mean power output during 3 × 5 min exercise was lowest after 2 h of cooling and highest following 2 h of heating. There were no differences between temperature conditions in cardiorespiratory parameters during the all-out exercise to explain the temperature-dependent differences in mean power output. These results pointed to intramuscular factors being responsible for the worsened fatigue resistance following muscle cooling.

Within the same study, the potential intramuscular mechanisms underlying the temperature-dependent recovery of muscle function were investigated in intact single muscle fibres from mouse flexor digitorum brevis. A major advantage of the intact single fibre technique employed in our study is that Ca²⁺ is a known major regulator of skeletal muscle force generation and this preparation allows us to determine how fatigue-induced changes in intracellular Ca²⁺ handling affect contractile force [3]. In our study, all muscle fibres were fatigued with repeated submaximal contractions (31°C), but thereafter fibres were separated into four different recovery temperatures where they were cooled (16°C, 26°C), not treated (31°C) or heated above the in-vivo physiological temperature of the mouse flexor digitorum brevis muscle (36°C). Recovery of contractile function was assessed by stimulating muscle fibres every 30 min during the 2 h recovery period. The results from these experiments revealed that cooling skeletal muscle fibres depressed the recovery of sarcoplasmic reticulum Ca²⁺ release and submaximal force, whereas heating improved recovery. Some fibres underwent a fatigue test involving repeated contractions following the recovery period, and comparable to the result in the human studies, we were able to show that fatigue resistance was impaired in fibres that underwent cooling versus heating in the recovery period.

A tenet of the temperature-dependent force recovery observed in the single fibres was that the recovery of sarcoplasmic reticulum Ca²⁺ release, contractile force and fatigue resistance only occurred when the fibres were provided with glucose. A major fuel heavily relied upon during endurance exercise are intramuscular glycogen stores, with glucose uptake required to resynthesize glycogen. To test this hypothesis, muscle glycogen content was assessed in whole mouse flexor digitorum brevis muscles recovering at two different temperatures of 26°C and 36°C. Indeed, we confirmed that glycogen resynthesis was temperature-dependent with lower glycogen resynthesis at 26°C than at 36°C. As summarized in Figure 1, we concluded that post-exercise recovery of muscle strength and fatigue resistance following endurance exercise is worsened by cooling, whereas muscle heating accelerates recovery processes.

Figure 1.

Effects of muscle temperature on post-exercise recovery of skeletal muscle function after endurance exercise.

So what are the potential implications of these findings in real world situations? Given that muscle glycogen resynthesis is most rapid within the first few hours immediately post-exercise, using cold water immersion particularly within the immediate post-exercise time frame may worsen skeletal muscle recovery and impair subsequent exercise performance in sporting events where multiple competitions or qualifying rounds are performed within the same day. In addition, competitions performed over consecutive days or weeks can lead to cumulative muscle fatigue (i.e., Tour de France), and it is possible that implementing post-exercise muscle cooling may only compound the problem.

A question that we did not answer but which is one of the most common assumptions of cryotherapy is that healing of muscle damage is improved by reducing inflammation. However, recent evidence has shown that acute 10 min of cold water immersion immediately following one resistance exercise session did not have any anti-inflammatory benefits for skeletal muscle over the following 48 h post-exercise recovery period [4]. In the longer term, another study showed that 10 min of cold water immersion immediately following every resistance exercise session that was performed twice per week for 12 weeks resulted in diminished gains in muscle strength and muscle hypertrophy [5]. Thus, recent evidence is accumulating against the use of skeletal muscle cryotherapy as a post-exercise recovery intervention.

Another compelling conclusion from our recent study is that 2 h of muscle heating is beneficial for post-exercise recovery of muscle strength and endurance [2]. Do these findings suggest that we should take a warm bath or sauna after exercise instead of an ice water bath? We hope that future studies in our lab will provide mechanistic data with translational outcomes that can help establish practical guidelines by which muscle heating can be used to improve post-exercise recovery in various sports.