Note: This exercise was created by Czech exercise physiologist, Dr. Vladimir Janda and is one of the most challenging sit-up variations as it completely isolates the rectus abdominals by eliminating the hip flexors.
Position your body on the floor in the basic sit-up position; knees to a ninety degree angle with feet flat on the floor and arms either crossed over your chest or to the sides. This will be your starting position.
As you strongly tighten your glutes and hamstrings, fill your lungs with air and in a slow (three to six second count) ascent, slowly exhale. Tip: It is important to tighten the glutes and hamstrings as this will cause the hip flexors to be inactivated in a process called reciprocal inhibition, which basically means that opposite muscles to the contracted ones will relax.
As you inhale, slowly go back in a controlled manner to the starting position.
Repeat for the recommended amount of repetitions.
Variations: The exercise can be done using a dedicated abdominal bench apparatus, having a workout partner exert pulling pressure on your calves (while you maintain your feet on the ground) or wrapping your legs over barbell and pulling back.