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Summer is coming which I am sure will make everyone rejoice. Longer days, warm weather and just a generally good vibe. However some might find these hot days in the transition period a shock to the system.

Most people that ride regularly will know that heat can have an impact on their performance. This can be through a few mechanisms.

  1. Hyperthermia (being too hot) which reduces muscular endurance.

  2. Hyperthermia shifts metabolism (how you burn energy) from more aerobic to more anaerobic. This results in glycogen being consumed at a faster rate.

  3. Reduced cardiac output (reduced amount of blood the heart is pumping). Hyperthermia causes dilation of blood vessels nearer to the skin and pooling of blood in the limbs (in an attempt to cool down) this reduces the volume of blood that returns to the heart increasing cardiac strain.

As well as hyperthermia effecting performance it is well known that dehydration (which often occurs in hot environments during prolonged exercise) can decrease performance. Body water deficits greater than just 2% of body mass can impair aerobic exercise performance by reducing muscle blood flow and altering skeletal muscle metabolism.

Studies looking at dehydration found that in a hot environment, small to moderate levels of dehydration (–2% to –4%) result in a large VO2max decline. Similarly, time to exhaustion is significantly decreased when individuals are dehydrated. This similar to VO2 max is reduced more in a hot environment than in a cool or mild one??. There is a few theories as to why this is but one is that the central nervous system is preventing the person from going as hard or for as long in an attempt to prevent damage to the system due to heat illnesses and dehydration.

However, not everyone is affected to the same degree by the heat, and some people cope better in hot environments than others. It all comes down to the individual.


Although performance is decreased in a hot environment it is not all bad news. Just like with most things the body adapts, heat is no exception here. After a period of acclimatisation the body gets used to the increased heat and performance returns to normal levels. It is recommended that 10-14 days of acclimatisation is needed for a good adaptation period. Adaptions start almost immediately.

How to acclimatise?

According to Dr Phil Chapman of Australian Catholic University to achieve good heat acclimatisation 3 physiological responses are required:

1. High body core temperature (~40°C)

2. Hot skin (>35°C)

3. High cardiovascular strain (HR approaching maximum)

These 3 physiological responses can be achieved during a prolonged submaximal exercise bout in the heat. Heart rate and the temperature of the core and skin will gradually increase until they reach the above limits. Exhaustion will normally then result. In a study Phil was involved in looking at heat acclimatisation protocols cyclist exercised in measured bouts in an environment of 32°C and 65% relative humidity.

They did this for 6 days then had 1 rest day before retesting. The mean power increase from pre-test to post-test was 14%!. The adaptation process was very quick, the first 3 days saw the greatest adaption. The actual exercise protocols you might use in the weeks leading up to an event will depend on the individual and how it fits into your current training program.


A couple of weeks ago everyone saw how Chris struggled leaving the Australian ‘winter’ to head over to the heat of a US just after summer. This resulted in temperature increases from 15-20 averages 20-25 max to temperatures over 30 average and touching 40 as max temp.

Below is a table of his last ride before leaving, his rides over in the US and his first ride upon returning.

I understand the limits to the data as far as the type of ride and efforts etc but this is just intended to provide an overview.

What this table clearly shows is a massive decrease in performance and increase in HR initially as his body tried to increase its cooling effort. We see a 17% decrease in performance alongside an 11% increase in HR initially. This may be partly due to the 7 days he had off the bike but I would put the majority of these effects down to the increase in temperature.

Unfortunately you can see that the UAS was drifting quickly away from Summer and the temperatures decreased quite significantly which will partly skew the analysis. However it is easy to see that by day 5 his power and HR had returned to almost identical numbers to pre-leaving. To cherry pick the data if we look at Day 9 we can see this was still a very hot day. On this day Chris’ performance is on par with or maybe even slightly better than his last ride before leaving. A perfect example of acclimatization.

We can see when he returned home Australia had caught up, beginning to move into summer. His rides on returning have been very good having no issues handling the heat back here.

Chris’ case study is a great example of heat acclimatisation. Chris unintentionally followed a protocol similar to what Dr Chapman had described doing a few longer hard efforts towards the start of his trip which brought his HR up towards max and would have had his skin and core temperature around the mark required. This resulted in the same rapid acclimatisation that Dr Chapman described

Looking at some more specific efforts.

On Day 2 Chris did an approx. 40min effort up a climb during this effort he averaged 269w for an average of 161BPM hitting a max of 174 BPM. This is close to his max of high 170s. This was a hard effort for Chris and he was not soft pedalling it. He did the identical climb on day 10. This time it only took him around 36min. He managed to average 305W for an average HR of 157BPM and maxing out at 169BPM this time.

Over just 8 days Chris had a 13% power improvement with a 3% decrease in HR. Chris also had a large decrease in perceived exertion during the effort i.e. it felt a lot easier for him. He followed this up in the next few days with a few 20min efforts which were within a few percent of his best 20min power ever.

Unfortunately, as mentioned earlier, the temperatures in the US had decreased pretty significantly during the time he was over there so it is not a perfect comparison. However, the temperature was much higher still than before he left. It shows a very good adaptation for Chris to the heat.

Thanks again to Dr Chapman for the information on heat acclimatisation.


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