In only the recent past, adapting for a hot race was a very inexact process. What we now consider old-school coaching advice went something like, ‘Cook yourself, but don’t overcook yourself.’ Needless to say, a lot of athletes overcooked themselves, and would face exhaustion for days on end after a heat adaption workout.

The precise measurements of the CORE sensor allow a much more effective way to heat train – by keeping your core temperature within a narrow range that allows for physiological adaptions without causing excessive heat stress. The trick is finding that range, which can vary amongst individual athletes.

For much of the population, that range centres around 38.5° C/101.3° F. But there are exceptions – some athletes have a lower zone, others a higher zone. Those with a lower zone trying to heat train at 38.5° C/101.3° F will overcook themselves. Those with a higher zone will not get physiological adaptions.

The heat ramp test

To find an individual’s heat training zone, CORE created and validated the heat ramp test. The test measures loss of muscle power due to increased core temperature. It does so by having the athlete hold a constant heart rate while monitoring decrease in power/pace and increase in core temperature. Details on conducting the test are found in the article Heat Training for Sporting Performance.

An ideal heat ramp test looks something like the one below – a smooth, gradual warm up, a constant heart rate, few power surges, and a steadily increasing core temperature. The test ends when power has decreased from the start of the test (which starts when core temp reaches 38.0° C/ 100.4° F).

Real-world testing

Many athletes’ test results look a bit messier – especially self-administered tests being done for the first time. Let’s review one of these ‘messy’ tests and see how we can still use the results. The results were plotted in intervals.icu, a great tool for analysing workout data.

Slow warmup

In this data, kindly provided by an athlete who uses the nickname nfkb, we see that after the 20-minute warmup, core temp was still only at 37.8° C/100.0° F. This is not uncommon, especially in a room that is not super-heated (30° C/86° F). The solution? Dress warmer (or wear a CORE heat training suit). Also, smaller and less powerful athletes (<70 kg/154 lbs, FTP  <250) may need to increase the intensity of the early stages of the warmup to generate enough watts).

In this case, at the end of 20 minutes, the athlete reacted with a surge in power, which quickly raised his core temperature to the threshold for starting the test (38.0° C/ 100.4° F). However, that surge made it difficult to know the starting power of the test. But after 5:00 his power had settled out at around 240w, his heart rate had stabilised at 170, and his core temp still at 38.0° C. Let’s call that the start of the test.

Steady heart rate

With a test start of 240w, the athletes next job was to maintain a heart rate of 170 until power dropped to 192w. The athlete did a great job of that for the next 15 minutes. But then we see a surge in power and heart rate, followed by a big dip in power/heart rate…reflective of the athlete’s indecision if the test was over or not. At that point, his core temperature was 38.8° C/101.8° F.

He decided that maybe it was not over, and then did a great job of stabilising his heart rate for another 7 minutes. During this time, his power slowly decreased to the target of 190w, with his heart rate at 170 and core temperature at 39.0° C/102.2° F, and he ended the test.

Setting the heat training zone

Subtracting 0.5° C from 39.0°C gives the bottom end of the heat training zone. Subtracting 0.3° C gives the top end of the zone. So, his heat training zone is 38.5–38.7° C. (For Fahrenheit, subtract 0.9 and 0.5…giving a range of 101.3–101.7°F).

Validating the heat training zone

How do we know if we have correctly identified this athlete’s heat training zone? Look for a few key indicators in subsequent training sessions:

• A strong sweat response while in the heat training zone
• Moderate discomfort while in the heat training zone (the first session may be very uncomfortable…but for most athletes it become less uncomfortable during subsequent sessions).
• No excessive fatigue in the following days. A heat training session may be exhausting (like a high intensity interval session), but lasting fatigue the next day may be a sign that the zone is set too high. Sometimes that fatigue doesn’t show up until 2–3 days later.

Here we see a heat training session from the same athlete – a warm up followed by high intensity intervals that raised his core temp, followed by z1/z2 efforts to keep his core temp elevated. At 90 minutes into the workout, we see a strong decoupling of power from heart rate…zone 1 watts were causing a heart rate typically found with zone 3 watts. This is a strong indication that core temp was raised high enough to cause physiological adaptions.

And while the athlete found the workout challenging, he reported that by the next day he was fully recovered. This means he hadn’t overcooked himself and had likely identified the correct heat training zone.

Key points for the heat ramp test

• Use a very warm room, dress very heavily, or wear a CORE heat training suit.
• Avoid power surges
• Maintain a steady heart rate
• Be patient!
• Validate the test afterward through heat training sessions.