Kinesiology of Exercise

Glute-Ham-Back Machine Sit-Ups

Sit-ups or crunches to develop the abdominal muscles have limited effectiveness as they do not develop strength through the full range of motion possible. Also because of the great intensity and levels of performance needed for almost all sports, athletes and bodybuilders should do exercises commensurate with their levels of ability. 

This means doing exercises which produce greater development of the midsection muscles that are needed for lifting heavy weights and executing exercises and skills that require great force or speed. Doing Yessis Glute-Ham-Back machine (GHB) sit-ups will give you the greater intensity and development needed over a greater range of motion as needed on the highest levels of athletic performance. 

In addition, it is important to understand how you can involve mainly the upper portion of the abdominals with some contraction of the hip flexors or the lower portion of the abdominals with very strong contraction of the hip flexors. These exercise variants are very important in certain sports and for specific development of the different sections of the abdominal musculature.

Major Muscles and Actions Involved

In the Yessis GHB machine sit-up the abdominal muscles are responsible for spinal flexion. In this action your abdominal muscles pull your head and shoulders up and forward from a lower than horizontal position. In some variants the hip flexors contract to rotate the pelvis (together with the trunk) up and forward from a below level position on an axis in the hip joint.

Sports Uses

GHB sit-ups are unique in that they allow for strengthening through the full range of motion of the abdominal muscle action. In the regular sit-up and crunch you begin the action in a straight body position. In the GHB sit-up you begin the action with the spine in a hyperextended position, a position that is often seen in execution of many sports skills, especially in throws.

Exercise Analysis

  • It is important to do the movements in this exercise at a slow to moderate speed in the initial stages while learning to do this exercise effectively. At this time you must keep your body under control at all times. You should not make any quick or jerky movements
    because these can prove dangerous to your spine or hips. When you move slowly into the down position, concentrate on a slight arch in the lower back and a stretching of your abdominal muscles. Feel your abdominal muscles contracting as you rise up. 
  • In addition to the abdominal muscles, the hip flexors are involved in both variants. However, it is important to understand that throughout these exercises, your abdominals and hip flexors are under very strong contraction. The hip flexors, in essence, hold your pelvis in place and provide a strong base for the abdominals to contract and prevent hyperextension of your spine. 
  • Because of this, the GHB machine sit-up is a very safe but advanced exercise. It should also be noted that the hip flexors and abdominals are under constant contraction supporting the trunk in this non-support position (when the trunk is beyond the hips with no support underneath). When you lean back, your muscles undergo a strong eccentric contraction (which is also responsible for muscle development) and then a concentric contraction on the way up. 
  • If you keep your legs straight, it is very difficult (if not impossible) to lower your trunk so that your head is pointing vertically toward the floor with your back excessively hyperextended. If this happens it is usually because the knees are bent, which is potentially a very dangerous position. Hanging down backwards as far as possible creates tremendous pressure on the posterior aspects of the
    discs, which can be a major cause of back problems.
  • The key to successful execution of this exercise is to lower your trunk so that there is approximately 20-30 degrees of hyperextension when doing the exercise. This is more than sufficient to produce a great training effect for the abdominals.
  • When you start using weights for more resistance, you will notice that the greater the amount of weight that you use, the less is the flexion and hyperextension of the spine. The reason for this is that your muscles contract more strongly to hold your trunk in place. As a result it is difficult for them to relax sufficiently to produce the arching of the spine to get in the down position.
  • For further analysis check out our premium Kinesiology of Exercise EBooks.