Active therapy for chronic low back pain Part 2.: Effects on paraspinal muscle cross-sectional area, fiber type size, and distribution

Design. Randomized prospective study to compare the effects of three types of active therapy on the back muscle structure of chronic low back pain patients. Objectives. To analyze the effects of 3 months active therapy on gross back muscle size and muscle fiber type characteristics and their relationship to changes in muscle function. Summary of Background Data. Many studies have documented a diminished muscular performance capacity in cLBP patients, but few have supported this with evidence of alterations in either the macro- or microscopic structure of the paraspinal muscles. Investigations of the changes in muscle structure following active rehabilitation are even rarer. Methods. Assessments of trunk muscle cross-sectional area (using MRI), erector spinae fiber size/type distribution and pathology (percutaneous biopsy), and muscle function (see Part 1) were made in a group of 59 individuals with cLBP, who were participating in a randomized trial of active therapies for cLBP (physiotherapy, muscle training on devices, aerobics). Results. Fifty-three out of 59 patients (90%) completed the therapy. At baseline, significant correlations were observed between the size of the paraspinal muscles and isometric hack extension strength (P=0.0001), and be tween the proportional area of the muscle occupied by each fiber type and the fatigability of the muscle (P=0.012). following therapy, there were small (few percent) increases in trunk muscle size in the aerobics and physiotherapy groups and a similarly slight decrease in the devices group. Changes in erector spine size correlated only weakly and nonsignificantly with changes in back extension strength. There were no major changes in fiber type proportion or fiber size in any group following therapy. Conclusion. Three months active therapy is not sufficient to reverse the typical "glycolytic" profile of the muscles of cLBP patients or to effect major changes in back-muscle size. The alterations in muscle performance observed (increased strength and endurance; Part 1) were not explainable on the basis of structural changes within the muscle.