Did you know that your lower leg, calf and foot muscles have to withstand 6-8x your bodyweight?
Got achilles tendinitis, achilles problems, shin splints or plantar fascitis? The feet are the 1st to hit the ground when you run, so weakness or mobility deficits below the knee can even contribute to runner's knee, IT band syndrom, knee pain, hip pain, or even low back pain!
Depending on your age, you should be able to do a certain # in order to run reasonably well and without pain. How many of these can you do? Do both sides feel the same amount of effort and get to the same # of reps? Can you get your heels all the way up?