People who start out less fit tend to see bigger improvements in their stamina and muscle health when they begin exercising, no matter their age or gender — the lower your starting fitness, the more you gain.

For people who aren't super fit to start with, different types of workouts—like steady cardio, intense interval training, and short sprints—boost the energy powerhouses in their muscles by about the same amount, around 23–27%.

Even if your muscles don’t get much bigger from lifting weights, your body still improves how it uses energy, like making your muscles better at burning fuel efficiently.