Muscles trained: Abdominals, hip flexors
Lift the top end of an adjustable flat bench up so that it's in a slight incline position. Lie o a bench with your hips lower than your head and grab the end of it for support. Bend your hips and knees 90 degrees. Pull your hips upward and crunch them inward. Keep your hips and knees at 90-degree angles. Pause lower, and repeat.