Superman

Here's an excerpt from Ultimate Back Fitness and Performance:

"Training the back extensors is a challenge if the goal is to minimize spine load. For example, the "superman" is a commonly prescribed spine extensor muscle exercise that involves lying prone while extending the arms and legs. This results in about 6000N (about 1300 pounds) of compression to a hyperextended spine, loading the facet joints and crushing the interspinous ligament. This is not a cleverly designed exercise for anyone." (McGill, p. 227-8)

The National Insitute for Occupational Safety and Health set the action limit for lower back compression at 3300 N. Compression levels past this point has been correlated to a higher risk for injury to the lumbar spine. Incidentally, a straight leg sit up generates about 3500 N, while a bent-leg sit up created about 3350 N. Now, think about all the sit ups you see performed on a daily basis at a gym? Want abs? Strength Coach Mike Boyle often says to perform table push-aways - the best ab exercise he prescribes to his clients and athletes. Obviously this is a joke, but proper nutrition is the key to fat loss - you cannot out-train a poor diet.

So how do we strengthen the back extensors in a safer way? Stuart Mcgill recommends the bird-dog. It's an advanced exercise, and loses its effectiveness dramatically when performed incorrectly. I like to start clients off with the half bird dog:

The key is to be able to extend at the hip, while maintaining a neutral spine. Brace the abdominals, and contract (squeeze) the glute of the raised leg while keeping your chin tucked. Once mastery of this exercise is achieved, progression to the full bird dog can occur:
Here, the stability is reduced to two points of contact on the floor. The opposite arm is raised, and all the same cues apply (neutral spine, abs braced, glute of the raised leg contracted, and no movement occuring anywhere else). Its important that the spine remains neutral throughout. Too often, people overarch the lower back at the top of the movement, while also rounding over at the bottom of the movement. Do not try to touch your elbow and knee together at the bottom, but instead just let them hover slightly off the floor and then raise them back up again. According to McGill, the half bird dog imposes less than 2500 N to the spine, while the full bird dog creates about 3000 N. Both are under the acceptable limit set by the NIOSH, and also with a neutral spine unlike the superman and situp where the low back is taken through full extension and flexion, respectively. Couple that with glute activation, as well as an anti-rotary component, you get a lot of bang for your buck value, with minimal risk of injury to the lumbar spine.

Train Hard (and smart!),

Jason