When people lift weights, their arm and leg muscles show similar changes in muscle fiber numbers — and in both, we don’t see clear signs of new muscle fibers forming.

Lifting weights for three months didn’t increase the number of muscle fibers in young guys’ biceps, so their muscles got bigger without adding new fibers.

Lifting weights for up to 6 months probably doesn’t increase the number of muscle fibers in your arms or legs — instead, your muscles grow because the existing fibers get bigger, not because you grow new ones.

Whether you're new to lifting weights or already experienced, your muscles seem to add new fibers at about the same rate when you start resistance training — at least in the first 6 months.

When you lift weights, your muscles get bigger not because you're growing new muscle fibers, but because the ones you already have are getting thicker. Scientists don't think you actually create new muscle fibers from training.

Lifting weights for three months makes the muscle fibers in young men's biceps get bigger — both the endurance-type and power-type fibers.