Work by Shefer et al. suggests why exercisers have better muscle function than nonexercisers as they age. It turns out that endurance exercise doesn't just tone muscles, it also increases the number of the muscle stem cells that regenerate muscles after injury or illness. Enhanced stem cell numbers might also delay sarcopenia, the decline in muscle mass that occurs with aging. The experiments on rats showed that the number of muscle stem cells (called satellite cells) increased after rats spent 13 weeks running on a treadmill for 20 minutes a day. Younger rats showed a 20% to 35% increase in the mean number of stem cells per muscle fiber, while older rats showed a 33% to 47% increase.