Training indoors is great for a myriad of reasons and workout quality can be kept high, but training in this environment also poses some challenges that are different from riding outdoors. This series will attempt to cover those challenges in detail and provide actionable steps to take to reduce or avoid them completely. The first topic in this series will cover hyperthermia, which is simply a departure from the expected temperature range of the individual from baseline, and more specifically when the core body temperature exceeds 100F with 104F being considered life-threating (i.e. heat stroke). 

Stages of Hyperthermia:

Initial Stage

  • Abnormal sweat rate
  • Labored breathing
  • High pulse rate

Intermediate Stage

  • Nausea/vomiting
  • Headaches
  • Low blood pressure

Advanced Stage (heat stroke)

  • Confusion
  • Cyanosis (bluish/purple coloring)
  • Unconsciousness/death

Causes of Hyperthermia:

Exertional: This is what we’re going to focus on in this post, and ‘exertion’ essentially means exercise. “Muscular exercise increases metabolism by 5 to 15 times the resting rate to provide energy for skeletal muscle contraction. Depending on the type of exercise, 70 to 100 percent of the metabolism is released as heat and needs to be dissipated in order to maintain body heat balance.” (Sawka, et.al). Usually, when you train indoors, the actual exertion and overall intensity tends to be higher relative to leisurely rides outdoors, coupled with the body being quite awful at turning food into mechanical energy, this results in a much higher amount of heat being released and which eventually needs to be dissipated. More on this later…

Environmental: Think of heatwaves, and especially those that are coupled with high humidity. Age also plays a factor here with the elderly not being able to keep cool relative to their younger and more fit counterparts. Don’t be a hero and try to keep the same intensity and exertion compared to riding in cooler temps. When it’s super hot, promise me you’ll bring it down a notch 😘

Drugs: These won’t be covered much for the purposes of this article, but still something to be aware of, especially if you take any psychotropic medications (Xanax, Zoloft, Prozac). These medications may impair the body’s ability to regulate its temperature, and is also another reason why you should always get clearance from your doctor before starting any exercise program (Ohio Dept. of Mental Health).

Effects of Hyperthermia:

Central Fatigue

Central fatigue, related to the central nervous system or essentially the brain, “appears to be primarily related to inhibitory signals from the hypothalamus arising secondary to an increase in brain temperature” (Nybo). The hypothalamus, shown below, is fascinating and is really the primary physiological gatekeeper when it comes to exercising in the heat.

Reference: https://link.springer.com/referenceworkentry/10.1007%2F978-3-540-29678-2_2322

Without getting too tangled in the weeds, the hypothalamus is the body’s thermostat and does a great job keeping the core body temperature in a very tight range. When the body becomes too hot, the hypothalamus will inhibit many things related to heat creation, but for the focus of this article, exercise-induced hyperthermia will reduce voluntary muscular activation. Think of it like the captain of a ship calling for more power from the engine room, but the engine room telling them to go fly a kite! If you want to push up a climb harder, but your body temperature is too high, the hypothalamus will shut you down (hopefully).

Reduced VO2 Max

Ah, VO2 Max, I love and also loathe writing about it since there seems to be an ever growing list of definitions for it. Let’s K.I.S.S and say VO2 Max represents the maximum amount of oxygen that can be utilized by the body.

In another fascinating study, Nybo et. al, took 6 endurance-trained male subjects, and subjected them to a maximal effort at either their baseline core body temperature or hyperthermic (101.5F) after being artificially heated up. They found a decrease in VO2 Max of 16%, and roughly half the time completed between the 2 trials whether they were dehydrated or not:

Reduced Work Rate

If VO2 Max is reduced by that much, it’s safe to assume that overall work rate will be lower under hyperthermic conditions relative to cool, which is exactly what Périard and Racinais found in their study. Similar to Nybo, they took 12 well-trained male cyclists and subjected them to complete 750kJ of work in both a ‘COOL’ environment (64.4F) and ‘HOT’ environment (95F). They also had them complete the trial under hypoxic conditions (HYP below), so please ignore that data set:

They found a 7 minute difference between the trials to complete 750kJ! COOL (48.2 T 5.7 min) compared with HOT (55.4 T 5.0 min).

Long story short, training in the heat – which tends to happen more frequently while riding indoors – can really sap your ability to work at a high level. Fortunately, there are some things we can do so the heat won’t have as much of an effect 🤔

Preventing Hyperthermia:

Pre-cooling

This is something I have been doing recently with my ‘indoor-specialist’ athletes and it has been working well. The theory is that the body has a threshold core temperature, that when surpassed, results in a marked decline in performance (Gonzalez-Alonso, et.al). So, by pre-cooling your body and artificially lowering your core body temperature, you increase the buffer between your exercise starting point and temperature threshold. This is typically accomplished via ice vests, cold-water immersion, or air-conditioned rooms.

Hyper-hydration

The research is a bit limited here, but the theory goes hyper-hydration might improve sweat rates by as much as 33% (Lyons, et. al) which in turn will increase the amount of sweat that evaporates – as long as atmospheric conditions are optimal, i.e. low humidity and circulating air – which lowers and keeps the core body temperature low. If you want to really deep dive on this subject, visit our friends at Skratch Labs and check out their blog post about their hyper-hydration mix.

Clothing

This one should be fairly obvious, but clothing will act as an insulator. When you’re training indoors, or in hot environments, less is more. Now, this doesn’t mean strip down to your birthday suit, but the more skin you can have exposed to circulating air, the better!

Acclimatization

This is the most important thing no matter how you slice it. If you aren’t acclimated to the heat, no amount of pre-cooling, hyper-hydration, or nude cycling will make a difference. Acclimating to the heat happens in different phases and durations, but to give you an overview:

Full adaptation is dependent upon the individual, with most being fully acclimated in the 7-14 day timeframe (Wendt et. al). Interestingly, most endurance athletes are already fairly well acclimated, and will reach full acclimation quicker relative to their unfit counterparts.

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There are also a myriad of other factors, as shown above, that occur when the body is properly acclimated to the heat (Pryor et. al).

Protocols for heat acclimation are also individualized and dependent upon the competition, but 90 minutes of training, for 10 days, with the air temperature above 85F has been shown to be sufficient for the majority of people. This acclimation process should be completed 1-3 weeks before the event date. For a deep dive on this topic, I highly recommend reading: Application of evidence-based recommendations for heat acclimation: Individual and team sport perspectives.

TL:DR

  • Hyperthermia is simply a departure from the expected temperature range of the individual from baseline, and more specifically when the core body temperature exceeds 100F with 104F being considered life-threating.
  • It is caused from exertion (exercise), environmental factors (heat waves), and psychotropic drugs.
  • It results in central fatigue caused by inhibition from the hypothalamus, reduced VO2 Max, and reduced work rate.
  • It can be prevented and reduced by whole-body cooling, hyper-hydration, exposing more skin to circulating air, and most importantly acclimating to the heat.
  • If you are gearing up for a Winter’s worth of indoor training, or moving to a warmer area, remember that your body will need time to acclimate (at least 10 days). So, I’d advise not going ham and keeping the training relatively easy to moderate until acclimation occurs. 

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