How does Cold Water Immersion improve recovery?
Despite there being an abundance of research on CWI, the primary mechanisms for its ability to improve recovery are still not fully understood. However, the following theories have been suggested:
- Vasoconstriction (blood vessel constriction).
- Analgesic (pain relieving) effect of the cold water.
- Reducing inflammatory pathways.
- Placebo effect.
- Hydrostatic pressure.
One theory suggests that the immersion into cold water causes vasoconstriction, leading to lower localised blood flow (5). It is thought that the cold temperature activates the thermal nerve cells (nociceptors), leading to a change in sympathetic nerve activity and is therefore responsible for this reduced blood flow. The temperature-induced reduction in blood flow around the damaged tissues, caused by strenuous exercise, reduces oedemas and inflammatory activity (6, 7).
Analgesic effects of the cold water
The second theory postulates that the reduced perception of pain is due to the analgesic effect of the cold water. Whereby the immersion into cold water leads to a decreased nerve conduction speeds and excitability (8), thus reducing nociceptor communication with the sympathetic nervous system (9, 10, 11). Ultimately, this would lead to a reduction in the perception of pain.
Reducing inflammatory pathways
Others have suggested that the decrease in the perception of pain is related to reduced inflammatory pathways – namely: reduced nociceptor sensitisation (12), reduced exercise-induced oedema (13), and reduced white blood cell access (2, 14). This theory is, therefore, a combination of effects, and is often referred to in the research as the primary physiological reason for improved recovery.
Another theory suggests that the effectiveness of CWI on the reduction of post-exercise pain and fatigue is primarily down to psychological perception (i.e. a placebo effect). This suggests that individual simply feels more ‘awake’ during and/or after the immersion into the cold water, causing a decrease in their sensitivity to pain (2, 15). This theory is supported by a recent and extensive review conducted by Hohenauer et al. (3), who reported that CWI reduces the athlete’s perception of DOMS and RPE.
Another theory lies with the effects of hydrostatic pressure on the body during immersion into water. When an individual is immersed into water they are subject to the effects of hydrostatic pressure. For every 1-metre of immersion, the pressure gradient rises by 74mm Hg (mm Hg = millimetres of mercury) – this is almost equal to typical diastolic blood pressure (80mm Hg) (9). As the pressure gradient increases with depth (i.e. the deeper you go, the more pressure imposed), this hydrostatic pressure causes an inward and upward squeezing action on the body. It is this mechanism that causes the effects of buoyancy. The buoyancy reduces the gravitational load on the body, meaning objects such as the human body weigh less when in water. Figure 1 demonstrates the reduction in body weight at various water immersion depths.