Your "core" muscles include some muscles in the abdomen, back, pelvic floor and hips. Their jobs are to support your spine, maintain your balance and move your legs powerfully -- factors that are all crucial in such sports as running, tennis, basketball, skiing, and more.

When we have weak core muscles, it's much harder to maintain good posture; if we get tired, we start leaning forward as we move -- poor form that can lead to muscle strain and decreased lung capacity.

Pilates classes are one of the most favored methods of strengthening the core muscles; some kinds of yoga can also help. If you prefer to strength-train with weights, ask a personal trainer which exercises can work your core best.