Hanging Knee Tucks

Start by hanging from a dip bar with your arms slightly wider than shoulder width apart and your legs hanging straight down; if available, use arm straps to eliminate the stress on your forearms.
 
Raise your knees as high as you can into your chest and then lower your knees back down to the starting position; do not allow your body to swing.
 
The entire motion should be done in a smooth, controlled manner that lasts about 4 seconds, with the downward movement no faster than the upward.