Pathology
A strain is an injury sustained by a muscle that has been exposed to excessive tensile stress, often referred to as a pulled muscle. However, it is not just an excess of stretch alone on a muscle that creates the strain. Muscle strains occur most often from tensile force on a muscle while it is in a state of contraction.

Most muscle strains occur when a muscle is engaged in an eccentric contraction. This is because the forces are greater on a muscle in an eccentric contraction than during isometric or concentric contractions. A common example of how this happens to the back is reaching forward and to the side to pick something up (see Figure 1). A significant load is not necessary in order for the strain to occur. Simply the weight of your body in this awkward position is enough to cause a muscle strain in the lumbar region.

Eccentric contractions occur in the erector spinae muscles on each side, as well as in the quadratus lumborum muscle on the side opposite the one you are bending toward (see Figures 1 and 2). Attempting to resist gravity's pull on your torso puts these muscles at a significant mechanical disadvantage. This is the type of situation that often leads to a strain. Keep in mind the short intrinsic muscles of the spine (see Figure 4) are also frequently overwhelmed and may be strained, especially if the pain is close to the spine.

It is important to identify the forces acting on the spine to evaluate when eccentric muscle actions are occurring and thereby identify the potential mechanics of a strain. For example, a primary action of the iliopsoas muscle is forward flexion of the torso. This muscle is frequently described as a problem in many back-pain problems. Bending over to pick something up involves forward flexion of the torso; however, it is not the iliopsoas that is commonly injured in this scenario. Similar to the example described above, it is the erector spinae muscle group working eccentrically that has sustained the strain.

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