When scientists measure changes in muscle cells after strength training using two different methods, the results don't match up well - so they should be looked at separately rather than compared...

When young guys who already work out do a lot of high-rep weight training for a short time, their muscles get bigger but their energy-producing parts actually become less dense, meaning bigger muscles don’t always mean more energy factories inside them.

When young guys who don't exercise start lifting weights for 10 weeks, the energy-producing parts inside their muscles grow faster than the muscles themselves.