Explanation
The amount of force a muscle can generate depends not just on the size of the muscle fibers but also on the number of fibers that can be activated and used. Each time you challenge your muscles to do more than they usually do, they learn to use more of your existing muscle fibers. Muscle fibers won't start getting bigger until after you reach the point where you're activating most of them--and you can gain a lot of strength before you get to that point.
Senior Health and Fitness
