Every muscle of the body needs to be used frequently, because muscles grow and maintain or gain strength only in response to being challenged. Climbing six flights of stairs rapidly can cause leg pain, especially if you haven’t done it in a long time. That pain is evidence of minor injury to your muscles. In the presence of sufficient amounts of proteins, muscle injury leads to the activation of “muscle satellite cells” and, eventually, to muscle growth. Muscles can be slightly injured and rebuilt by doing simple everyday tasks that are challenging.
