When healthy women carry a heavy backpack—like 20 to 30 pounds—their leg muscles have to work a lot harder, especially the quads and calf muscles, but their walking motion stays pretty much the same.

When healthy women walk faster and carry extra weight at the same time, their hips and knees work harder in a predictable way, but their ankles react in a more complicated, less predictable way — suggesting the brain and spinal cord control these joints differently when both challenges are combined.

When untrained men get stronger during dynamic leg movements like standing up, the biggest factor is how much their thigh muscles fire, especially when combined with changes in the deep thigh muscle’s fiber angle and overall muscle size.

When healthy women walk faster (about 3.4 mph instead of 2.2 mph), their leg muscles work a lot harder—some muscles more than double their activity—but the timing of how they fire stays the same.