Recovery and training adaptation: the hot-cold equation

If you’re a northern hemisphere dweller where late July is high summer, you might well be struggling to train in the heat and humidity. But take heart because while high temperature and high humidity training can make you sweat copiously and feel sluggish as you try to shed excess heat, there’s a very large silver lining to be had. In a recent SPB article, which was published in our May 2025 issue, we looked at new research on how endurance training in the heat can induce similar improvements in endurance as those generated by training at altitude⁽¹⁾.

The power of heat

In the study we highlighted, athletes who trained for four weeks in high temperature and high humidity conditions [35°C (95F) and high humidity (70% relative humidity)] were compared with those who performed the same training for four weeks in temperate conditions, but where an altitude of 2,500m (8,200ft) was simulated. Although altitude training is known to stimulate extra training adaptations compared to sea level training (due to the lower oxygen content of air, which forces the muscles and circulatory system to become more efficient), the remarkable finding was that it was the heat training that produced the greatest gains.

Both modes resulted in a higher lactate threshold compared to normal training, meaning the athletes were able to sustain higher workloads before the rapid accumulation of fatiguing lactate. But it was only the heat training that resulted in an increase of VO2max (averaging around 0.24litres/minute - significant). In a similar vein, both interventions resulted in increased fat oxidation during exercise, but it was the heat intervention that produced the biggest boost to fat burning while training, increasing the maximum rate of fat burning to 59% of calories burned compared to a maximum rate of 55% of calories burned in the altitude intervention.

It has long been known that when endurance athletes train at moderate altitude (where the concentration of oxygen in the air is lower) for a period of time, a number of physiological responses occur, which over time can lead to positive endurance adaptations and a performance advantage when returning to sea level – most notably an increase in blood volume and haemoglobin^(2,3). But what are the physiological changes produced by heat training that can improve performance?

Because heat training diverts blood away from muscles to the skin (to provide cooling), some physiologists argue that this creates a slight oxygen deficit in working muscles, forcing them to become more aerobically efficient – ie creating a similar stimulus as altitude training⁽⁴⁾. In addition, a period of heat training can boost blood plasma volume, just as in altitude training⁽⁵⁾. Finally, heat training is known to generate a phenomenon in the arteries known as ‘flow-mediated dilation’ effect⁽⁶⁾. In layman’s terms, this dilation effect in arteries ‘stretches’ them, thus improving blood flow to working muscles – an effect that remains even when training or competition is resumed in cooler conditions.

The training adaptation effect

As well as the physiological effects and benefits of heat training (ie heat during exercise), some researchers have speculated that heat applied after exercise might help with post-exercise recovery and training adaptation The rationale for this theory is that recent research has shown that hot water immersion following resistance exercise results in a greater release of growth factor and the hormone testosterone, both of which play a role in muscle recovery and adaptation⁽⁷⁾. Another reason is that while cold-water immersion is known to somewhat improve post-exercise recovery and blood lactate clearance⁽⁸⁾, when it comes to training adaptation over a period of weeks or months, cold-water immersion may actually be harmful.

In a 2014 study, Aussie scientists found that regular 10-minute ice baths at 50°F after training sessions produced less muscle growth in cyclists than traditional low-intensity cool-downs⁽⁹⁾. A similar finding came from a Japanese study, which found that when the forearm muscles and legs were immersed in ice-cold water after training, the strength and muscular endurance gains after six weeks were significantly less that when the limbs were immersed in temperate water at 18C (64F)⁽¹⁰⁾. Scientists now believe that the reason for the poorer adaptation of muscles when they are immersed in cold water after exercise is down to the suppression of inflammation.

During hard exercise, micro-damage occurs within muscles. This damage is repaired during the recovery period but inflammation is an important part of this repair process. When inflammation is suppressed – for example, by making the muscles very cold after exercise - the recovery process is made more difficult. It was way back in 2002 that scientists discovered that anti-inflammatory medication such as Ibuprofen and acetaminophen inhibited protein synthesis in muscles after high-intensity exercise - presumably by blocking the inflammatory damage/repair cycle⁽¹¹⁾. More recently, the cold-water immersion effect of dampening training adaptations has been confirmed in other studies on athletes⁽¹²⁾.