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Text Box: Summer heat has an impact on your performance very similar to altitude and should be taken equally seriously in how you plan and perform you training.  Like altitudes of 7000+, temperatures in and above 90 degrees Fahrenheit will cause power declines of 10% or more depending on acclimatization.  During Floyd Landisí Tour de France winning ride his Physiologist Allen Lim, riding in the car next to him, noted that every time Floyd poured a bottle of cold water over his head his power increased 30-40 watts, or approximately 10% of his sustainable power.  Floydís previous dayís disaster was simply due to overheating.  The graph below depicts a ride up a 5% 10 mile Cat 1 climb.  Note temperatures at the start of the climb are low to mid 80s climbing to 90-104 degrees in the 2nd half of the climb.






























The athlete has a power threshold of 250w and heart rate threshold of 145bpm and attempted to maintain an average HR of 140bpm, which would be low threshold or zone 4 and normally should not be a problem.  The ride start temperature was 82 degrees, comfortably under normal body temperature and at an average power of 245w zone 4, the avg heart rate during the first 30 minutes was 138bpm,  zone 3/4.  In the second/final hi-lighted 30 minute section the outside temperature rises to equal body temperature, 98.6 and above at a steady HR of 137bpm, but the power drops quickly to an average of 205w as the body is forced into cooling mode.  In this final sector as the heat peaks at 104 degrees,the body is diverting resources to the cooling battle with a power decline of 17% or close to 40watts.  

Some lessons to learn:

To get the most out of any hard training workout you should do them early or late during the coolest part of the day.  (Seven days later this same rider set a personal best of 272w @ HR of  l42bpm in a temperature of 72 degrees)
Expect a 10% or more power loss once temperatures are in the 90ís and if you have a power meter quit trying to achieve a target power and shift over to a target heart rate.
Donít be disappointed if your times up your favorite climb are 10% or so higher than your best, because you are still riding close to a previous best time done in cooler temperatures. 
When your heart rate rises to near threshold, and/or you feel lightheadedness or nausea, slow down and if that doesnít help find the nearest shade and stop and if cool water is available poor some over your head.  The good news is that as long as you respect the heat and respond early to the symptoms, you will recover quickly as soon as your body cools down and regains control of its temperature.  Ignore the symptoms and you could send yourself to the hospital.
If you want to ride in the heat to get acclimatized, do it on easier rides where your body isnít creating as much internal heat, which aggravates the problem.

If the weather has just become hot, be even more careful until your body acclimates to the summer temperature change.  This usually takes a couple weeks, similar again to altitude, but even with acclimation you will still lose significant power.  However if you spend most of the time in air conditioning, donít expect the same level of acclimation.  
Take the heat seriously, ease the pace, listen to your body, watch your heart rate, drink extra, poor water over your head, ride with an ice pack if possible and expect a loss of performance.  If you do this you can still enjoy/survive a ride in the heat.  Above all donít beat yourself up because you seem slower and weaker than normal, and definitely donít decide to push even harder and tough it out as that will just bring about heat exhaustion.

Be Safe, Have Fun and Go Fast
George Chester