The lemon squeezer exercise has nothing to do with lemons and everything to do with squeezing the heck out of your abdominals. The move also goes by "V-up" or "jackknife" and sometimes "lemon crushers." Whatever you call it, the exercise is a challenging way to work your rectus abdominis -- the front layer of abdominal muscles that you see in the mirror.
How To Do the Lemon Squeezer
Lie on your back, with your arms and legs extended.
Lift your arms and legs to hover an inch or two off the floor. Engage your belly by pulling your abs in towards your spine.
Crunch upward by bending your knees to your chest, lifting your upper body up off the floor and bringing your arms to reach for your calves. You'll be balanced on your hips.
Release, with control, back to the floor to complete one repetition.
To make the move more difficult, keep the legs straight as you raise the lower body and torso to create the "V" shape at the top of the exercise.
Effectiveness of the Lemon Squeezer
The lemon squeezer feels challenging, and it is. However, it does require a lot of assistance from your hip flexors as you crunch, so it's not just working the front of your abs. In fact, a study published in a 2012 issue of the Journal of Strength and Conditioning Research found that the basic crunch produces more activation in the rectus abdominis, especially the higher region, as compared to the jackknife, or lemon squeezer.
Make It a More Powerful Exercise
Perform a variation of the lemon squeezer using a stability ball to really kick up the intensity and muscle activating power of the move. Research in the Journal of Bodywork and Movement Therapies from 2009 found that performing a stability ball jackknife maximized rectus abdominis activation when compared to a jackknife performed on a stable surface. The stability ball jackknife -- or lemon squeezer -- was also far more effective than a traditional curl-up, or crunch.
Stability Ball Jackknife
Place your shins and tops of the ankles on a stability ball and your hands on the floor. Make a straight line from your head to your toes as you hold your body rigid.
Bend your knees to pull the ball toward your chest and crunch your torso.
Straighten your legs to return to the rigid push-up style position. This completes one repetition.
A Complete Ab Routine
Although the lemon squeezer feels hard, it doesn't comprehensively train your abs -- or your core, which consists of all the muscles of your torso. No single abdominal exercise is comprehensive in getting at all of your abs, though. If you choose to do the lemon squeezer to add variety to your routine, make sure you do other moves along with it.
Your abs flex, stabilize, twist and side bend your spine. Do at least one exercise that takes care of each of these movement patterns. The lemon squeezer covers flexion, as does the crunch. Twisting and rotation may be worked with bicycle crunches or woodchops. Train stabilization with plank variations or the stomach vacuum.