These will work perfectly with your gym pull up bar or any other pull up bar . These ab straps are very well made and will help round out your workout routine, allowing you to perform such exercises as abdominal crunches and the like. To perform hanging knee raises, adjust the Ab Straps so the loop is about as long as your forearm, then hang the Ab Straps from the parallel grips of your pull up bar. Slide your arms through the loops then bend your elbows and grasp the handles of the pull up bar. Lift your feet off the ground and engage your stomach muscles by holding your stomach in. Raise your knees up to your chest and lower them to the start position. You can also perform crossover knee raises by bringing your knees across your body or single knee raises alternating each leg. Try straight leg raises for more resistance