Wear a knee band, as your knees take the brunt of the force.
Alternating arms improves agility and coordination.
The abdominal crunch is the most basic and one of the most effective exercises you can perform to strengthen and tone your abdominal muscles. Abdominal muscle strength is a key component of flatter, firmer stomach.
While keeping your back straight and head up, dip your legs and jump to a straddled position (one foot in front of the other) while pushing the weight up over your head to a locked out overhead position to complete the movement.
Note that the jump and lift happens in one swift movement with your body "jumping" underneath the dumbbells to receive the weight..
lso note that you need to keep your head up and back straight as you finish the movement.
Don't curve your back, always make sure it is straight at all times.
Avoid this exercise if you have back knee and shoulder problems.