This exercise develops the soleus muscles. This variation doesn't have overwhelming emphasis on the lower back.
Sit on a flat bench and place a bar near your knee.
Place your feet on something, for example, on severals barbell plates or a step platform. At the bottom, your heel should be as low as possible.
As you exhale, contract your feet and rise up as if your were standing on your toes.
Pause for 2-3 seconds at the top.
As you inhale, return to the starting position, stretching lower legs.
Do not jerk the weight up. Do the exercise slowly.
Lower the heel at the lowest point.
To keep the bar in one position and not let it scratch your skin, put a towel under it and support the bar with your hand.