If you're short on time, include this core conditioning move to target your midsection quickly and safely. Begin with the bent-knees variation and progress to the more intense straight-leg version as you get stronger.
Lie on your back with your arms at your sides and your knees together and bent directly above your hips, shins parallel to the ceiling. Inhale with your head and your hands resting on the floor. On the exhalation, contract your abdominals and bring your head, chest and shoulders off the floor while reaching your hands past the outsides of your knees. Pause for two seconds, then slowly move your knees a few inches farther away from your chest. Pause in this position for six seconds. Think of pulling your pubic bone up to your breastbone to avoid arching your lower back. Then bend your knees over your hips and return to the start position. Repeat 12 times.