Sit-ups is one of a group of |exercises| called calisthenics that build muscular endurance and improve |stomach| muscle strength and firmness. Sit-ups can also prevent or lessen back problems as strong abdominal muscles provide better support for the back. There are three sets of abdominal muscles that sit-ups can strengthen which are all otherwise hard to |exercise|. If done correctly, sit-ups will provide strength and power for activities that involve the torso, such as running and tennis. Before adding sit-ups to your |exercise| routine, learn the variations to avoid (the old-fashioned straight-leg sit-up is potentially dangerous) and the variations that will provide the benefit you're seeking (the diagonal crunch tones the muscles that shape the sides of your waist).