The Best Stomach Crunching

 Stomach crunches, also known as abdominal crunches, are an exercise used to isolate and develop the abdominal muscles. The use of a foot holder during a crunch does not play a role in the effectiveness of this exercise. You could wear a blue T-shirt or a green tank top, have someone hold your feet or not while you crunch -- you will end up with the same results. However, the situp, which is sometimes confused with the crunch, is drastically influenced by the introduction of a foot holder.
The Abdominal Crunch
The abdominal crunch isolates the rectus abdominis muscle which is responsible for trunk flexion, or rounding the lower spine. It is performed by lying down with the knees bent at 90 degrees and feet about 12 to 15 inches from the hips. The lower back, or lumbar spine should be pressed into the ground for the duration of the exercise. Crossing your arms over the chest or holding your hands behind the head, the shoulder blades are then lifted off the ground as the lower ribs "crunch" toward the hip points. This is the half-way point of a movement. It is completed by slowly lowering to the start position.
Holding the Feet During Crunches
Anchoring the feet during an abdominal crunch, either by placing them underneath a piece of furniture or having a partner hold them, does not change the effectiveness of the exercise. In terms of isolating the rectus abdominis muscle through trunk flexion, there is no change in intensity or added variety that comes from holding the feet in this exercise.
The Situp
Situps exercise the rectus abdominis and hip flexors, psoas and iliacus. Similar to a crunch, the situp begins with trunk flexion, but instead of returning to the floor, you keep going, lifting the lower spine until the elbows touch the knees. Here, the added component of hip flexion has implications for anchoring the feet.
Holding the Feet During Situps
When you do a situp without someone anchoring your feet, the abdominals work to flex the trunk and stabilize the spine while the hip flexors struggle to contract from an unstable axis -- the hip joint. As soon as you anchor your feet, the hip flexors can be activated with facility as their axis of rotation has just become stable. This will make the situp easier overall, but the rectus abdominis will no longer need to work as hard. To maximize recruitment of the abdominals in the situp, ask your foot holder to press down on your feet as you lift your torso off the ground. However, as you descend, have them let go. This way, you can build strength in the rectus abdominis without a complete takeover by the hip flexors.