This exercise is an excellent one for improving running technique. It requires good balance, but works all of the quadriceps.

1 Stand tall with your planted leg slightly flexed at the knee and your hands on the support bar. Pull in your abdominal muscles slowly, exhale and start to raise your knee. 

2 As your knee nears 90 degrees at the hip, slowly extend your lower leg as far as is comfortable without compromising your body position.

3 Inhale, slowly flex your lower leg and return your knee to the starting position - do not allow your foot to touch the ground.

4 Repeat for the desired number of repetitions, then repeat the exercise using your other leg.