Muscles are like work animals that are quick on the uptake. If you carefully increase the load, step by step, they learn to take it. As long as you explain your expectations to them by actually showing them examples of the amount of work they have to endure, your muscles will comply and gradually get stronger. It doesn’t happen overnight, of course. But as long as you take your time and do it in stages, they won’t complain—aside from the occasional long face—and they’ll very patiently and obediently grow stronger.