Another metabolic explanation for heat-induced muscular fatigue is that free radical production is increased in skeletal muscles during exercise in the heart(72). Recall that free radicals are molecules with an unpaired electron in their outer orbital(56). This is significant because molecules with unpaired electrons are highly reactive. That is, free radicals bind quickly with other molecules, and this combination results in damage to the molecule combining with the radical. Therefore, accelerated production of free radicals during exercise in the heat could contribute to muscle fatigue because of damage to muscle contractile proteins(56)