The abdomen has four main muscles groups: the transversus abdominis, rectus abdominis and the internal and external obliques. Traditional crunches often target just the rectus abdominis muscles of the core and miss all the rest. Straight-leg situps however, provide a more intense abdominal workout. When you are ready to see results in your abdominal workout, perform a combination of situps including the straight-leg variations.
The standard straight-leg situp not only works the rectus abdominis, but also targets the obliques in your side and adductor muscles of the inner thigh. A popular variation of the straight-leg situp is called the jack-knife. You can feel this exercise from the top of your abdomen to your upper thighs. Start by lying flat on the floor with your legs straight. Lift your upper torso like a full situp and bend your knees, meeting them in the middle of the abdomen. Then, lower your upper body back down to the floor and return your legs to the straight position. Keep the legs straight for an advanced-level isometric burn. To further isolate your core muscles, keep your legs from touching the floor when you extend them.
To work out your abs, obliques and latissimus muscles of the lower back, try straight-leg scissor crunches. The scissor crunch has two main variations, at beginner and advanced levels. The first variation involves crossing your legs straight out on the floor. Place your hands behind the head and cross your left leg over the right. Then, perform 10 to 15 crunches. Switch your right leg over the left and repeat the exercise again. The more advanced variation involves lifting the legs straight out during the crunch. Keep your arms at your side. Raise your abdomen off the floor like a traditional crunch, but kick your legs up toward your chest as you do so. Alternate the left and right legs, scissor kicking them as you perform 10 to 15 repetitions.
Vertical Toe Touches
The vertical toe touch is an intermediate level situp with a straight-leg position. Start by lying on your back with your arms by your sides. Raise the legs straight into the air until your toes are pointed toward the ceiling. If you lack flexibility, bend your legs slightly at the knee. Do not raise your legs beyond 90 degrees; this avoids putting an added strain on your back. Lean into the situp and reach your hands up toward your ankles. You only need to lift your shoulders off the floor to perform this exercise. Hold the situp for five to 10 seconds, then slowly lower your upper body back to the floor.
Straight Leg Reverse Ab Crunch
The straight-leg reverse ab crunch targets the full range of abdominal muscles along with the quadriceps and hip flexors. Technically, the reverse ab crunch is not a true situp; it is a hip raise. Start out lying flat on the floor with your legs straight. Instead of raising your upper body for a situp, roll the legs up toward your abdomen. Continue this motion, lifting your legs straight into the air. Only lift your hips off the floor to keep from injuring your back. Slowly lower your legs back down to the starting position, keeping them as straight as possible. Perform three sets of 10 to 15 repetitions of this exercise for an advanced level workout.
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