The phrase "to strengthen the muscles" means making your muscles stronger and more powerful. It involves exercises or activities that help build up your muscles and make them able to endure more physical activities. Full definition
Very young babies need «tummy time» each day (while they are awake) to help strengthen the muscles in their neck and upper body. (caringforkids.cps.ca)
To encourage your baby to start crawling, you should try to help him in strengthening the muscles of his legs and arms. (newkidscenter.com)
This again points to the importance of strengthening the muscles around the ankle to improve stability. (weckmethod.com)