Whey protein supplementation does not significantly improve functional capacity beyond resistance training alone in pre-conditioned older women, as gains in gait speed are similar between supplemented and placebo groups when adjusted for baseline strength and muscle mass.

Whey protein provides leucine that turns on muscle-building signals in muscle cells, which adds muscle mass when combined with strength training. But once a person is already trained and has enough muscle, adding more muscle does not make them walk faster because walking speed depends on how well nerves and muscles work together, not just how big the muscles are.