A little known secret about ab exercises most fitness experts won't tell you...

Bending your knees during the sit-up is recommended as a way to reduce the activity of the muscles that flex your hip.

This is supposed to reduce the stress to your lower back, and lead to greater isolation of the abdominals.

However, researchers from Stockholm's Karolinska Institute have found that moving the legs from straight to bent actually increases hip flexor activity [1].

They used electromyography (known simply as EMG) to measure muscle recruitment during various exercises. Surface electrodes were placed on the skin over the rectus abdominis, obliquus externus, obliquus internus, rectus femoris, and sartorius muscles. Indwelling fine-wire electrodes were used to measure recruitment of the iliacus muscle.

"There is a misconception that the involvement of the hip flexors will decrease by flexing the legs in sit-ups," says lead author Eva Andersson.

"We found the opposite pattern, i.e. the hip flexor muscles were always engaged to a higher degree with bent than with straight legs."

What's more, there appears to be little difference in lumbar spine compression when sit-ups are done with the legs bent or straight [2]. Both the straight and bent leg sit-up impose large amounts of compression on the spine (3506 and 3350 N respectively).

"The National Institute of Occupational Safety and Health has set the action limit for low back compression at 3300 N; repetitive loading above this level is linked with higher injury rates in workers," notes Professor Stuart McGill in his book Low Back Disorders, "yet this is imposed on the spine with each repetition of the sit-up!"

"Given that the sit-up imposes such a large compression load on the spine, regardless of the leg being bent or straight, the issue is not which type of sit-up should be recommended. Rather, sit-ups should not be performed at all by most people," McGill adds. "Far better ways exist to preserve the abdominal muscle challenge while imposing lower spine loads."

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References
1. Andersson, E.A., Nilsson, J., Ma, Zhijia, M., & Thorstensson, A. (1997). Abdominal and hip flexor muscle activation during various training exercises. European Journal of Applied Physiology, 75, 115-123
2. Axler, C.T., & McGill, S.M. (1997). Low back loads over a variety of abdominal exercises: searching for the safest abdominal challenge. Medicine and Science in Sports and Exercise, 29, 804-811