Strength Coach: Are Crunches and Back Extensions Breaking your Spine?

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Dan Gabelman BS / PES / USA-W
Mike Boyle Strength and Conditioning
The Sports Club / LA -- Boston
Strength Coach / Private Trainer

Many of my clients and some athletes at MBSC have asked why there are very minimal "crunches" and no back extensions in the training programs they perform. The information presented below is what I simplify in my answers to them and will either be a great reminder or enlightening information on how these exercises affect your spine. In addition I will explain the function of the "core," and why there is still some "crunching " in programs using a straight leg sit-up progression. 

A couple of reasons to avoid these exercises: 

1) "The traditional sit-up imposes approximately 3300 N or about 730 pounds of compression on the spine." (McGill, 88) This means that every time you do a sit-up, knees bent with feet locked under something, sitting up from the ground to vertical, that amount of pressure is crushing your intervertebral discs. Since most people seem to think that 300-500 crunches a day is necessary for six-pack abs that is 990,000 N or 219,00lbs of pressure on your spine and discs at a minimum. Lastly, "The National Institute of Occupational Safety and Health has set the action limit for low back compression at 3300 N. Repetitive loading about this level is linked with higher injury rates in workers, yet this is imposed on the spine with each repetition of the sit-up!" (McGill, 88) 

2) Back extensions are no better. On the roman chair (this is the piece of equipment that is set at 45 degrees and you can lock your feet and rest your thighs on it so that you can drop and lift your upper body, working the back) performing one back extension, imposes over 4000 N or about 890lbs of compression on the spine (McGill 91). In addition, there are many exercises performed on the floor lying on the stomach where some combination of the arms and legs are raised, or the entire torso is lifted off the ground. These types of exercises impose up to 6000 N or over 1300lbs of spinal load and compression (McGill, 91). These exercises are referred to as the "superman", "opposite arm opposite leg raise", "prone cobra", etc. Please avoid these exercises at all costs! Finally, the front squat (my personal preference) and deadlift both work the back and spine in the biomechanically correct fashion. Any variation of these such as the kettlebell deadlift, off-set kettlebell squat, front squat, 1-leg SLDL, trap bar deadlift, plate deadlifts, etc, function the same way. There is no isolation of the muscles and the body works as one functional unit. 

So how does the core function?............Keep reading....