Normal Low Back Biomechanics

About 30 years ago, low back pain affected 50% of the population. Throughout this time, emphasis was placed upon strengthening the abdominal musculature in order to offset this high incidence. Today, the incidence of low back pain is 80%. Obviously, something was wrong!

Although increased intra-abdominal pressure (IAP) offers low back support, studies reveal that abdominal muscle contraction has no correlation to iap. IAP is actually accomplished by the contraction of the latisimus dorsi muscles, along with the compression of the thorax down upon the abdomen.

When a person is standing upright, the abdominal muscles are only responsible for the first 10 to 20 degrees of forward bending. That is all!

After that, gravity takes over while the erector spinae (back extensor) muscles and the latisimus dorsi muscles contract eccentrically, offering low back support down to 60 degrees. Beyond 60 degrees, an eccentric contraction of the hamstring muscles assist forward bending. All these eccentric contractions basically help you resist gravity and keep you from falling forward.

When you lift something from the floor, it is concentric contractions of the quadriceps, buttock and erector spinae muscles, which return you back to standing.

In order to strengthen the low back, it is essential to exercise the supportive musculature both concentrically and eccentrically. When performed properly, every exercise has a concentric and eccentric component .

A concentric muscle contraction occurs when a load (weight) is placed upon a muscle and as the muscle fibers contract, they shorten. An example of this type of contraction would be the performance of the first half of an arm curl exercise; whereby you contract the bicep muscle and bring the weight toward your chest, thus shortening the bicep's length. An eccentric muscle contraction occurs when a load is placed upon a muscle and as the muscle fibers contract, they lengthen. An example of this type of contraction would be the performance of the second half of an arm curl exercise; whereby you contract the bicep muscle in order to slowly return the weight toward your original starting position, thus having the bicep muscle fibers lengthen.

As you are now aware, during forward flexion, it is eccentric contractions of the low back musculature that provides the necessary support and concentric contraction during lifting. Hence, it is vital to increase the strength of the back extensor muscles with both eccentric and concentric types of exercise. This is accomplished on a back extension machine; concentric contractions as you extend backwards and eccentric contractions as you return the weight slowly to your original starting position.

Although the abdominals have been found not to play as significant a role in low back rehabilitation as once thought, they are still important.

Remember, in normal spines the strength ratios between extension muscles to flexion muscles are 1.3:1; meaning the erector spinae (extensor) muscles should be 30% stronger than the stomach (flexor) muscles. In people with chronic low back pain, this ratio is less than 1:1. It is essential that you maintain this ratio during your rehabilitation.

The latisimus dorsi muscles should also be strengthened since they play such an active role in supporting the erector spinae muscles, as well as increasing abdominal pressure. The exercises of preference are lat pulldowns and lat rows.

Since the quadriceps (leg extensors) are instrumental during lifting, they too are a vital muscle group to work. The exercise of choice is the leg extension machine. Once again, it is vital to have the leg extensors 30% stronger than the leg flexors (hamstrings).

Copyright © 1999 Dr. Ronald A. Weinstein. All rights reserved.

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