Trim your middle section and strengthen your back with this modified plank.
Face seat of a sturdy bench or chair, hold outside edges, and step back into plank position, legs extended, holding weight between arms and toes. Slowly lower one knee toward the ground, without touching, then bring back to plank position and lower other knee. Keep abs contracted and back straight. Continue alternating knees. Perform up to 24 repetitions, 12 on each leg, per set.