Why is it easier to run in the cold?

Overheating is one of the main factors limiting physical activity. During running, thermoregulatory metabolism, which keeps core temperature steady at rest, is adjusted through a body temperature independent mechanism to compensate for the exertional heat generation. The colder the environment, the higher the metabolism at rest, and the more complete the compensation is. As a marathoner, one of us logged many miles of training which serve as an interesting dataset spanning several summer and winter seasons. Strikingly, the average pace during the winter months appears to be about 1 minute per mile faster than during the summer months, displaying a huge difference in performance. Besides, every long distance runner knows that it may take a noticeably longer time to start sweating when it is cold outside in spite of an often faster pace and warmer clothing. This raises the question why colder environment possibly leads to a slower temperature growth and to a potentially better performance. High body temperature is a major regulatory signal to limit the physical effort and, thus, to prevent the temperature from growing further. So, for the effort to remain at high level, the temperature should stay away from this threshold for as long as possible. The rate of temperature change is defined by a balance between 2 processes: heat produced vs. heat dissipated per unit of time. Importantly, both heat production and skin thermal conductance have lower limits: a certain level of metabolism is required to maintain basic functions, and the skin even with fully constricted vessels does not completely insulate the body. To maintain constant temperature, the production of heat must exactly compensate for the dissipation of heat. When possible, mammals keep their heat dissipation at minimum not to spend energy for regulatory thermogenesis. Physical activity is actuated by muscle contractions, which are not extremely efficient processes. In fact, more than 80% of the energy generated in the muscles is wasted in the form of additional heat. This heat depends on the exercise intensity only. It may look like the best way to limit the temperature growth is to increase heat dissipation, which in both humans and rodents occurs in major part through an increase in blood flow in the skin. However, greater cutaneous blood flow competes with blood supply to other organs including muscles. That may be a reason why during exercise cutaneous vasodilation does not kick in until the temperature reaches really high levels close to the fatigue threshold. Heat dissipation can be represented as the product of the difference of temperatures inside and outside of the body and the thermal conductance of skin. In this context, one may think that at colder ambient conditions heat dissipation naturally increases due to a greater difference between the body temperature and the temperature of the environment. However, this higher dissipation occurs before the exercise even starts and, hence, is compensated by the thermoregulatory metabolic heat production. Summarizing the above, cold environment cannot provide an ergogenic advantage through the lowering core body temperature unless metabolism unrelated to exercise is reduced. Thermoregulatory system may turn off or