Start in high plank position with core tight and hands directly under shoulders. Keep core engaged to stabilize hips as you drive right knee toward chest. Return right leg to starting position, then repeat with left leg. Continue alternating legs to pump knees as fast as possible. Add This Daily Ab Workout to Your Training Routine. These six exercises make a great warmup before a ride or any cross-training session. 20 Lying Bicycle Crunches With Overhead Dumbbell (hold the weight just off the ground, but don't let it touch).