Bend your knees to perform exercise-ball crunches.
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When it's time for your abdominal workout, it's a good bet that you'll be getting down on the floor and performing crunches. You'll find an almost endless variety of crunches, some of which require machines or free weights, although most are body-weight exercises. Among those many crunches, you'll also find some in which you bend your knees and others you perform with your legs fairly straight.
Try the Bent-Knee Crunch
You typically keep your knees bent to perform crunches. You can do dozens of bent-leg crunch variations, including crunches on your back, with your feet in the air, on your side, with your back on an exercise ball or with your legs in motion. The foundational exercise is the standard crunch performed face up, with your feet flat on the floor and your knees bent. Place your hands behind your head, contract your abs and then lift your head and shoulders off the floor. Return under control to the starting position.
Include the Straight-Leg Crunch
Straight-leg crunches form a smaller part of the crunch family. You can perform straight-leg crunches by lying face up on the floor with your legs straight and then moving your upper body as you would for a standard bent-knee crunch. Alternatively, lie on your back, raise your legs so they're perpendicular to the floor and then perform the standard crunching motion with your torso. Your knees will typically be slightly flexed in the latter exercise, but not bent nearly as much as in a standard crunch.
Identify the Muscles
Linear crunches - any crunch in which your upper body moves directly toward your legs - target the rectus abdominis muscle and also work the obliques. Twisting crunches work in reverse. When you lift your head and shoulders and then twist your torso to the right or left. you target your obliques, while your rectus abdominis assists in your movements. These principles apply whether you're doing bent-knee or straight-leg crunches.
Do It Daily
The general rule that you should not train muscle groups on consecutive days doesn't apply to your abdominal muscles because these muscles work constantly to stabilize your torso as you walk, run and even sit. As a result, you can perform crunches every day if you wish. Exercise scientist Len Kravitz recommends performing eight repetitions each of five to 10 exercises during an abdominal workout. You should also vary your workout over time, performing different exercises to avoid reaching a fitness plateau. In this respect, alternating bent-knee and straight-leg crunches can offer the variety you need.