[aap1966]

UV radiation breaks down folate, so dark skin in equatorial regions protects against low serum folate levels, as well as the larger melanocytes (melanin containing cells) acting as a UV block and minimising skin damage ("sun burn").
Pale skin is an adaptation to allow more UV radiation into the skin to facilitate Vitamin D production as distance from the equator increases and UV exposure drops. There is probably biological no advantage to blond hair, it's just a genetic variation that poses no disadvantage in higher latitudes, thus the variation can persist. However, blond hair is finer and (on paler skin) less obvious that darker hair, so blondes appear "less hairy". There are some sex selection arguments regarding consequent attractiveness, but I find most of those dubious.
Red hair may be a remnant feature from hybridisation with Neanderthals that again offered no survival disadvantage at higher latitudes and (like blondness) could persist.

Different genetic groupings ("races") also have physiological variations regarding renal and carbohydrate metabolism thought to be related to starvation and low hydration environments, so it's not just "skin deep".

As for muscle metabolism: there is very little in the literature but just ponder this. How many white guys do you see in the Olympic 100m sprint finals, and how many black guys in the 50m Freestyle finals? That contrast is more than just societal.