"Leg muscles" refers to the group of muscles found in our legs that help us move, walk, run, and do activities using our legs. These muscles are responsible for giving strength, stability, and flexibility to our legs. Full definition
Although parents tend to have varying opinions about this, it is a fact that the dancing, jumping and swinging that the babies do on a jumper require movement of leg muscles. (momdiscuss.com)
This movement strengthens leg muscles, and your infant will need that strength to roll over, which will probably happen around 4 to 6 months of age. (kidshealth.org)