As a cycling coach, I am often asked about how to prepare for racing in extreme conditions. In hot weather, heat is your biggest enemy. Recently it has become normal to see pro cyclists warming up for time trial stages on trainers, stripped down to their base layers, water bottles in hand and towels draped over the handlebars. What has changed is what they're wearing. When preparing in warm weather, they will now wear ice vests or "cooling vests" from manufacturers like KewlFit. These vests contain removable chemical cool packs designed to help keep the wearer's core body temperature stable during warm-up. The idea behind this is that, while it is important to warm up one's muscles prior to a hard effort, increasing one's core body temperature is detrimental to performance. A cooling vest allows the athlete to warm-up without overheating and compromising their effort.
There is solid science behind the idea as well. Studies have shown marked hot weather performance increases in pre-cooled athletes (Sigurbjörn Á. Arngrïmsson, Darby S. Petitt, Matthew G. Stueck, Dennis K. Jorgensen and Kirk J. Cureton, 2004, Cooling vest worn during active warm-up improves 5-km run performance in the heat, J Appl Physiol 96:1867-1874 and Webster J, Holland EJ, Sleivert G, Laing RM, Niven BE, 2005, A light-weight cooling vest enhances performance of athletes in the heat, Ergonomics 48(7):821-37). The methodologies vary, with some studies using vests, and others using cold baths and/or ice beverages. The most practical pre-cooling technique for most cyclists, however, remains a cooling vest.
So, how do you pre-cool? It's simple – strip down to your base layer (or bare skin) and wear a cooling vest while you do your normal warm-up protocol on the trainer or even on the road. Make sure to also drinking cold fluids and set up your trainer in the shade. In extreme heat, without a cooling vest, it's often better to skip your warm-up entirely. The heat you generate by warming up will only contribute to overheating and become a detriment to your performance. Wait as long as possible before removing your vest, and store it in an insulated bag (often included) so that you can use it to cool down afterwards. Stay cool and you will perform better.