FitPointz

Here you will find an article on one exercise (new one every month). With a detailed analysis of the muscles used during the exercise and a informative description of how to both perform and assist in the actual performance of the exercise.

A full library of all previous exercises can be found at members online under the sub menu

Exercise: Bent Leg Crunch

Client Instruction:

Starting Position

1. Lie on the floor in a supine position with your knees bent at approximately 90 degrees and your feet flat on the floor and unsupported.

2. Support your head in your hands which are placed just above the back of your neck

3. Keep your elbows out to the side of your head and avoid pulling with your hands

Up Phase

4. From this starting position slowly begin to curl your spine up by contracting your abdominals

5. Raise your head and shoulders up to a point approximately 30-45 degrees from the floor

Down Phase

6. Pause momentarily and then slowly lower your body down to the original starting position

Muscle Analysis Chart - Bent Leg Crunch

*Down Phase
In the down phase of the exercise the body is lowered slowly with gravity. The muscles that concentrically contracted to lift the body are the same that are eccentrically contracting to lower the body.

General Kinesiological Analysis

In a bent leg crunch the spinal joints are flexed by the concentric contraction of the rectus abdominis, external oblique, internal obliques and sternomastoid.

Advanced Kinesiological Analysis

The prime movers in the bent leg crunch are the rectus abdominis, external oblique and the internal oblique. The sternomastoid is recruited as an assistant mover to flex the cervical spine by moving the chin towards the sternum. When the rectus abdominis, external oblique and the internal oblique contract they work as synergists by combining to cause spinal flexion. As they are bi-lateral muscles, it is possible for one side to contract to cause an entirely different spinal action. As such, as they contract to cause spinal flexion, the additional movements of spinal rotation and lateral flexion are neutralised.

The rectus abdominis forms a strong relationship with the hip joint flexors. If the crunch is continued past the 30-45 degrees that is recommended, the hip joint flexors would make a significant contribution to moving the spine towards the knees. By bending the knees the hip joint is placed into flexion, which shortens the hip flexors and places a greater emphasis on the abdominals. It has been suggested that abdominal exercises that emphasize trunk flexion selectively recruit the upper rectus abdominis to a greater degree than the lower rectus abdominis. While this is not entirely conclusive, the body of evidence available suggests that the upper rectus abdominis is used to a similar degree in a bent leg crunch and in the reverse curl. Other abdominal exercises such as the bent leg crunch with twist, V sit and reverse curl can all produce significant involvement of the upper rectus abdominis. It may still be possible in some clients, where the upper rectus abdominis is used more extensively in the bent leg crunch and the lower rectus abdominis is used more is reverse curl movements. It appears more likely that the lower rectus abdominis is used more in reverse curl movements.

Exercise Variations

Bent leg crunches are often varied by incorporating a twist in the movement. The twisting variation has been traditionally viewed as being the main exercise that extensively recruits the external obliques and internal obliques. Some studies have reported that the external obliques are used just as effectively when recruited as synergists in reverse curls, bent leg crunches and V sits. The bent leg crunch can be modified to make the exercise easier to perform by simply elevating the head above the feet by using an inclined bench. Conversely, the exercise can be made more difficult by elevating the feet, without them being held down. In the event that the feet are held down, there is an increased activation of the hip joint flexors. This can be seen in inclined board variations. Previously, straight leg situps were used to strengthen the abdominals. An examination of the full situp demonstrates the increased use of the hip joint flexors for approximately 60 degrees of the movement in comparison of the 30-45 degrees caused by the contraction of the abdominals. To avoid the increased involvement of the hip joint flexors, crunches have been prescribed with the hip joint in acute flexion, with the legs rested on a bench or a stability ball. The increased hip joint flexion forces the pelvic girdle into backward tilt and flattens the lumbar spine. In this position, trunk flexion is isolated over the first 45 degrees reducing the impact of the hip joint flexors.

Weight Machine Variation

A variation to the bent leg sit up is the trunk flexion machine. There are machine manufacturers who have devised many different trunk curl options from a seated position. Generically, the client is seated in the machine with their chest making contact with a breast plate. From this starting position the client pushes their chest against the resistance of the breast plate, at the same time moving a weight. The clients’ hands and arms must be positioned away from the machine so that they do not assist with the movement. An advantage of using these machines is the ability to regulate the resistance to overload the muscles.

Stability Ball Variations

The bent leg crunch can be performed on a stability ball. The client lies in a supine position with the gluteals and the lower back making contact with the ball. The client maintains the knees at 90 degrees of flexion while the spine and hip joint are extended. The stability of the body is maintained through the static contraction of the muscles surrounding the hip, knee and ankle. From this starting position the client attempts to curl up by flexing the spine to between 30-45 degrees. The client’s arms are positioned to the side of their head or across their chest. The trainer kneels to the side of the client and provides additional support when required. It is important to realise that no one single abdominal exercise is sufficiently intense enough to recruit all the abdominal muscles simultaneously. As a consequence, the abdominals must be trained using a variety of exercises with different types of muscle contractions.

© 2007 Fitness Institute Australia

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or any reliance on it. Users should satisfy themselves as to their own or clients medical and physical condition before adopting/using
the information or recommendations made. No responsibility or liability is accepted for any loss or damage suffered
by any person as a result of adopting the above information or recommendations.

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