Heat is an endurance athlete’s biggest enemy. A human on a bicycle only achieves 20-25% mechanical efficiency, meaning the majority of the energy you produce is lost as heat. And when the air temperature rises that heat leads to a well-documented drop in lactate threshold power output. But the question is: how does this information affect your training and racing strategies?
Choice #1: You could reduce your effort levels in the heat to reflect lower, calculated power ranges. In other words, if your lactate threshold is 275 watts at 73 degrees Fahrenheit, you could reduce it by 6.5% (Tatterson et al, 2000) to 256 watts when it’s 90 degrees.
Choice #2: You could rely more heavily on perceived exertion in hot weather and race as aggressively as you normally would, understanding that when you analyze the data later your power outputs might be lower than expected.
To help you make your choice let’s first look at what happens when you get hot.
Your body directs more blood to the skin
You have a total of about 10 pints of blood in your body and it has to supply oxygen to every cell, including your working muscles. The circulatory system also transports heat, and when core temperature increases your body increases blood flow to the skin for both convective and evaporative cooling (sweat comes from intercellular fluid, which comes from blood plasma). As these two imperatives (oxygen delivery and heat dissipation) compete for resources your cardiovascular system is forced to work harder, which is observed as an elevated heart rate.
Fluid loss accelerates
When you’re working hard you generate a lot of heat and in hot environments convective cooling isn’t as effective because there isn’t a big temperature gradient between skin and air temperatures. As a result you are more reliant on evaporative cooling from sweat, so the floodgates open up. You can lose up to 1.5 liters of fluid per hour in hot conditions. As you acclimate to exercising in heat, your plasma volume will increase, you’ll start sweating sooner, you’ll sweat more profusely, and your sweat will contain fewer electrolytes per unit volume.
Gross efficiency declines
Efficiency can be measured by the ratio between mechanical output (power) and the metabolic output (VO2) to produce it. A 2007 study by Hettinga, et al found that a decline in gross efficiency could account for up to half a cyclist’s heat-induced performance decline. They cited increased metabolic output from increased cardiac output, increased ventilation, and possibly an increase in an athlete’s underlying resting metabolic rate as likely reasons for the reduction in mechanical power production in a hot environment.
So, back to the question. Should you proactively reduce your power ranges for hot-weather workouts and competitions?
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adjust Cycling Power Ranges for Temperature?
No. Why? It is difficult to accurately correct for temperature-related decrements in performance in real time and in the real world. There are other factors in play. Air resistance decreases at higher air temperatures, which can save you watts. You also don’t know your exact hydration status or core temperature in real time. There are some promising software solutions that may soon be able to better correct for these variables and minimize the percent error, but with current technology if you back off based on calculations you’re basically just guessing.
Hot-Weather Performance Strategies
In competition it’s important to remember that everyone is competing in the same heat, and while some athletes will cope with it better than others, all will affected. Your best option is actually the least technical. Train in the heat to acclimatize prior to competition, optimize your cooling strategies, and focus on learning to correlate your perceived exertion to your power data. When it’s go-time in a race, use your perceived exertion (it’s more accurate than you think) and determination to dig deep and do what you have to do; just understand that in the heat your actual power outputs may be 5-7% lower than you expect for that exertion level.
In training it’s the power output that leads to the adaptation. You can achieve “normal temperature” power outputs in high temperature environments, but doing so generates a ton of heat you’ll struggle to dissipate, which means you won’t be able to achieve those necessary power outputs for as long. When air temperature hot (80+ Fahrenheit) and 10-15 or more degrees hotter than the temperature you’re acclimated to, shorten individual lactate threshold intervals and, if you’re able to maintain the appropriate power output for these shorter intervals, add an additional interval so the total time-at-intensity remains the same. For shorter high-intensity interval sessions, like VO2 max workouts featuring 1- to 3-minute efforts, you don’t want to reduce the duration of each interval but can break up your intervals into sets so you can complete the same total time-at-intensity for the workout.
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