"People do leg lifts to tone the abs, but it's actually one of the worst exercises for the lower back," says Lisa Kinder, certified personal trainer and star of the 10-Minute Solution: High-Intensity Interval Training DVD. "When the legs are lifted, one of the prime movers is the psoas, which attaches to the lumbar spine vertebrae. When this muscle is contracted, it pulls the lower back into hyper-extension and squeezes the discs, which can put a person at risk for a herniated disc."
Instead, Kinder recommends glute bridges (pictured). "This exercise will lift your booty, tone your thighs, strengthen your back, and sculpt your abs," she says.
To do it, lie faceup with knees bent, hip-width apart, and feet flat on the floor. Gently contract abdominal muscles to flatten lower back into the floor (try to maintain this gentle contraction throughout the exercise). Exhale, and keeping abs engaged, lift hips off the floor and lift toes, pressing heels into the floor for added stability. (Avoid pushing hips too high, which can cause hyper-extension in the lower back. Keeping abs strong helps prevent arching.) Inhale as you slowly return to start.