You need a strong core, but most athletes isolate abdominal muscles and ignore everything that surrounds them—the essential hip, trunk, and back muscles that extend off your core and make up your "pillar." "Training your abs while ignoring the other parts of your pillar will throw the whole thing out of balance, leading to problems like lower-back pain," says Paul Goldberg, the strength-and-conditioning coach for the NHL's Colorado Avalanche and author of The Lean Look.
The Fix: Strengthen your lower back by doing reverse sit-ups on a medicine ball. To work your abs, back, and hip and shoulder stabilizers simultaneously, try some overhead squats: Stand with your arms extended straight overhead and a light dumbbell in each hand. Pull your navel toward your spine and then squat until your thighs are roughly parallel to the floor. Keep your core tight as you stand back up.