Every day (sometimes every other day), I do heel raise exercises. I stand on one leg and use my calf muscle to go up onto my toes. I repeat this exercise as many times as I can, which is about 40 repetitions per foot right now. I bend that knee a bit (so as not to lock and strain my knee), and repeat for each leg.
This exercise helps strengthen the calf muscle (which is the muscle on the back of your lower leg). When I first started, I was only able to do about 30 repetitions, but now I can do about 40.
This exercise is important for runners and walkers (and people who are on their feet a lot), because a strong calf muscle helps prevent over-use injuries like shin splints, stress fractures, and broken bones. Strong calf muscles also help stabilize the back and pelvis, helping them move correctly and protect your gait and form.