Effect of unilateral knee extensor fatigue on force and balance of the contralateral limb
OBJECTIVES To determine the activation of the gluteus medius in persons with chronic, nonspecific low back pain compared with that in control subjects, and to determine the association of the clinical rating of the single leg stance (SLS) with chronic low back pain (CLBP) and gluteus medius weakness. DESIGN Cohort-control comparison. SETTING Academic research laboratory. PARTICIPANTS Convenience sample of people (n=21) with CLBP (>12wk) recruited by local physiotherapists, and age- and sex-matched controls (n=22). Subjects who received specific pain diagnoses were excluded. INTERVENTIONS Not applicable. MAIN OUTCOME MEASURES Back pain using the visual analog scale (mm); back-related disability using the Oswestry Back Disability Index (%); strength of gluteus medius measured using a hand dynamometer (N/kg); SLS test; gluteus medius onset and activation using electromyography during unipedal stance on a forceplate. RESULTS Individuals in the CLBP group exhibited significant weakness in the gluteus medius compared with controls (right, P=.04; left, P=.002). They also had more pain (CLBP: mean, 20.50mm; 95% confidence interval [CI], 13.11-27.9mm; control subjects: mean, 1.77mm; 95% CI, -.21 to 3.75mm) and back-related disability (CLBP: mean, 18.52%; 95% CI, 14.46%-22.59%; control subjects: mean, .68%; 95% CI, -.41% to 1.77%), and reported being less physically active. Weakness was accompanied by increased gluteus medius activation during unipedal stance (R=.50, P=.001) but by no difference in muscle onset times. Although greater gluteus medius weakness was associated with greater pain and disability, there was no difference in muscle strength between those scoring positive and negative on the SLS test (right: F=.002, P=.96; left: F=.1.75, P=.19). CONCLUSIONS Individuals with CLBP had weaker gluteus medius muscles than control subjects without back pain. Even though there was no significant difference in onset time of the gluteus medius when moving to unipedal stance between the groups, the CLBP group had greater gluteus medius activation. A key finding was that a positive SLS test did not distinguish the CLBP group from the control group, nor was it a sign of gluteus medius weakness.