Ascientific discovery made decades ago (known as "Wolff's Law") established that every human has a natural adaptive response that tells the body to grow bone and muscle strength on its own... Just like when your skin sweats when it gets overheated, and your pupils dialate in response to darkness.
This response is called "bone remodeling" and can be intentionally triggered through a series of movements that precisely compress the musculoskeletal structure to at least 4.2 times it's body weight for a specific amount of time... once a week. (view research)
Once activated, this response begins to actually transform the internal cellular structure of your bones. Our bone cells consist of Osteoblasts (builders), Osteoclasts (chewers), and Osteocytes (bone).
Until the age of about 20, a combination of hormones and "musculoskeletal compression" activities like walking, lifting, jumping, playing, etc... keep the "builders" dominating the "chewers".
Then as adults, they start to even out and eventually the "builders" can't keep up anymore and the roles start to reverse. So we get weaker bones... and in effect weaker muscles. (Note: Muscles will only grow as strong as the bones can handle because of neural inhibition)