Hanging Leg Raises
Grab a chinup bar with an overhand, shoulder-width grip (or use elbow supports, if available), and hang from the bar with your knees slightly bent and feet together. Simultaneously bend your knees, raise your hips, and curl your lower back underneath you as you lift your thighs toward your chest. Pause when the fronts of your thighs reach your chest, then slowly lower your legs back to the starting position. Do three to four sets of 8 to 12 reps.
Source: Men's Health