Hang from a pull-up bar or stall bars. Without swinging, lift the knees and crunch the trunk to lift the knees up and back toward the elbows—ideally, contact the arms with the knees at the top of each rep. Ensure the pelvis curls up like a crunch rather than just bending at the hips.
 
Control the movement downward and maintain tension in the abs throughout the motion.  
 
Notes
Using stall bars is ideal because the upper body will remain still and swinging will be impossible. If performing from pull-up bars, control the speed to minimize swinging.
 
Purpose
The knees to elbows trains both the hip flexors and abs if done properly—it’s a more accessible variation of the hanging leg raise that will allow athletes to perform a higher number of reps and can be used as a scaled version to work toward hanging leg raises. It’s also a good option if hanging leg raises cause quad cramps for an athlete.
 
Programming
Knees to elbows can be performed in sets of 10-30+ reps.
 
Variations
The speed of the movement can be changed and a pause can be added, usually with the legs horizontal. Moving to straight legs for more difficulty makes the exercise a hanging leg raise. Knees to Elbows can also be done in elbow slings.