They come in packets of 6

They come in 6 packs

The abdominals or abs can be activated from any form of trunk flexion movement conditions the abdominals. Particularly the Rectus Abdominis.
The movement may consist of spinal flexion or may combine both flexion of the spine and hip.

The question is should they be straight or flexed to reduce the involvement of the hip flexors?

Should we perform trunk curls or are full sit-ups now acceptable, as some of the less well informed sections of the fitness press have suggested recently?

Professor Mc Gill once said that “sit-ups should not be performed at all by most people”. He said this once he finish conducting a study on the spinal loading during various abdominal exercises and discovered that straight leg sit-up impose approximately 3506 N of compression on the spine. A curl-up, however was shown to produce only 1991 N of compression.

The national institute of occupational safety and health set the action limit for the low back compression at 3300 N. repetitive loading above this level is linked with higher injuries rate in workers, yet this load on the spine is exceeded with the high level of rectus adbominis and oblique activities.

Hip Flexor activity during Sit-Up and Trunk Curl
Many people false believe that by flexing the hips, the hip flexors are taken out of the trunk flexion movement.

A study by andersson et al (1995) found that higher levels of activity in the iliopsoas during sit-ups with hips flexed to 45 degrees was more as to compare to a straight leg sit up activates less.

It appears that many of the commonly held beliefs regarding the performance of abdominal exercises are inaccurate or mistaken. What is also apparent is the need for exercise professionals to have an understanding of not the level of muscle activation a specific exercise elicits in a particular muscle group, but also on how great the exercise impart on the spine.

Exercise on the abdominals must be prescribed individually, based on medical history, restriction and even age. As a exercise may be appropriate for a highly trained athlete which are healthy, robust spines may be wholly inappropriate for an individual with history of back pain and poor pine stability, and vice versa.