Physical activity increases until around 82F, and then plateaus till around 95F. At that point, we start to become less active again, especially if we're over the age of 65 or clinically obese. At this point, hot becomes too hot. This also explains some regional differences Obradovich uncovered: residents of the cooler, northern states are likely to become more active as the planet warms, while residents of warmer, southern states will likely become slightly less active. In the north, the increase in pleasantly warm days will balance out the number of days that are too hot to trot, while the south will experience many more of those too hot days.