Q: Why do I always sweat more throughout my workout when I do a warm-up outside in heat, and then come indoors to start my workout, even if it’s a brief warm-up? If I start indoors, I don’t sweat nearly as much. What's going on?
A: Sweating is the primary method of heat dissipation during exercise, regardless of the environmental temperature. High environmental temperature further adds to the metabolic heat stress of exercise. When in the heat, sweating is more important as the body tends to gain rather than lose heat. The more trained you are, or the more acclimatized to the heat, the earlier you will tend to start sweating (Brooks, Fahey, & Baldwin, 2005). This means internal temperature relates more to sweating than skin temperature.
Light exposure can also affect the onset of sweating. Blood vessels dilate in the skin and sweating occurs sooner in athletes who have been exposed to bright light before exercise (Brooks, Fahey, & Baldwin, 2005). The practical significance of this being a possible less than favorable temperature regulation if an individual were to take a nap in a dark room prior to exercising in heat.
Considering the above information, the increased sweat rate after briefly being exposed to a hot environment may be due to both of the above reasons. Purely being outside in bright sunlight, as you were, can cause an increase in sweating. When you combine that with the temperatures we have had lately, you’ve added a sharp increase in skin temperature, with a more gradual increase in core temperature. By the time you’re warmed up, you have already reached and exceeded your sweating threshold. This will then carry over to your workout. Once the response has started, cooling will occur as a result of being indoors, but not to a large enough degree to negate your heat and sun exposure. The longer you remain indoors, the less you will notice that increase in sweating.
Ryne Gioviano, M.S.Ed., NSCA-CPT
Owner of Welligee Personal Training & Lifestyle
Brooks, G.A., Fahey, T.D., & Baldwin, K.M. (2005). Exercise Physiology: Human Bioenergetics and Its Applications (4 ed.). New York: McGraw Hill.