The jackknife sit-up is not dissimilar to the V sit-up, which was a worry when my editor offered it to me without looking me in the eye. Nor is the name designed to ease concerns: ...

Enter: the V-up. Also known as a jackknife, the bodyweight move recruits the entire core and provides a stability challenge that sit-ups and crunches just can't get ya.

Finish off Week 3 strong! Get excited: Today's workout is all about spiking your heart rate with squat thrusts, pop squats, and jackknife sit-ups.

At 97, Elaine LaLanne starts every day with 20 minutes of exercise, including sit-ups and stretches. She and Jack LaLanne, the godfather of modern fitness, helped shape the healthy-living movement.

Jackknife sit-ups. These low-impact sit-ups are great for targeting the lower abs. Lay on a flat surface with your legs out straight, and your arms outstretched overhead.