Step Up

Target Muscles: 
Stance: 
One leg is on top of box, knee is at 90˚; other leg is straight
Body Position: 

In erect body position, back is straight, push through the heel of elevated leg until leg is straight (don't lock knee). Slowly return to starting position and repeat.

Positions: 
In erect body position, back is straight, push through the heel of elevated leg
Slowly return to starting position and repeat.
Common Errors: 

Body weight is pushed through toes instead of heels
Box is too high, knee < 90˚

Video: