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Working Muscles Suffer And Anaerobic Modes Kick In
When blood volume is split among competing interests during exercise in heat, the next victim is active muscle.
Muscles engaged in activity suffer because they aren’t getting as much oxygen from the blood. For endurance athletes oxygen is gold; it’s the fuel that allows us to sustain exercise for longer durations, and without it we’re forced to rely more on pain-inducing anaerobic (without oxygen) modes of producing energy.
Increased anaerobic energy production affects exercise at all intensities and causes a slew of issues including higher total energy expenditure and blood lactate accumulation. Also, carbohydrates are used for energy more than lipids (fat), and since carbohydrate fuel stores are extremely limited in the body, exhaustion is reached much sooner.
In the end, this shift from aerobic to anaerobic modes will generally result in a faster onset of muscular fatigue.