Trunk Muscle Weakness as a Risk Factor for Low Back Pain

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Abstract
A 5-year prospective study. To investigate trunk muscle weakness as a risk factor for low back pain in asymptomatic volunteers. Muscle strength has not been sufficiently studied as a risk factor for low back pain. The study participants included 30 male and 37 female volunteers (mean age, 17 ± 2 years), who neither reported nor had ever been treated for low back pain. Trunk muscle strength was measured isokinetically (60 degrees/sec), using the trunk extension and flexion and torso rotation units. The peak torques of the volunteers' extension, flexion, rightward rotation, and leftward rotation were measured, and the agonist/antagonist ratios were calculated as extension/flexion and left rotation/right rotation ratio. The volunteers then were followed prospectively for 5 years to determine the incidence of low back pain and were classified into a non-low back pain group (volunteers with no low back pain during the 5-year follow-up period) and a low back pain group (volunteers who experienced low back pain during this period). The low back pain group consisted of 8 male and 10 female volunteers. There were no significant differences between the non-low back pain group and the low back pain group regarding age, height, weight, the peak torque values, or the left rotation/right rotation ratio. However, the extension/flexion ratio of the low back pain group (men, 0.96 ± 0.27; women, 0.77 ± 0.19) demonstrated significantly lower values than that of the non-low back pain group (1.23 ± 0.28 and 1.00 ± 0.16 for men and women, respectively, P < 0.05). An imbalance in trunk muscle strength, i.e., lower extensor muscle strength than flexor muscle strength, might be one risk factor for low back pain.