Relationships between the fibromyalgia impact questionnaire, tender point count, and muscle strength in female patients with fibromyalgia: a cohort study.

OBJECTIVE: To test the hypothesis that fibromyalgia (FM) patients with reduced lower extremity strength are more symptomatic and tender than FM patients with normal muscle strength.

METHODS: A total of 840 FM patients and 122 healthy subjects were evaluated between 1998 and 2005. All of the patients completed version 1 of the Fibromyalgia Impact Questionnaire (FIQ) and were assessed for tender points and knee muscle strength. All subjects underwent bilateral isokinetic knee muscle strength testing in flexion and extension. Normative knee muscle strength values were calculated from the healthy subjects, and the FM cohort was divided in 2 groups: 1) patients with normal muscle strength and 2) patients with low muscle strength (2 SDs below normal). The clinical characteristics of these 2 groups were compared.

RESULTS: Significantly reduced knee muscle strength was found in 52% of the patients. There were no clinically significant differences between patients with low versus normal muscle strength. There were no clinically significant correlations between total FIQ score, tender point count, and muscle strength. Only 4.6% of the FIQ scores and 5.1% of the tender point counts were explained by muscle strength.

CONCLUSION: Significantly reduced knee muscle strength was found in more than half of the patients. Patients with subnormal muscle strength were not more symptomatic or tender than patients with normal muscle strength. There were no clinically significant correlations between FIQ, tender point count, and muscle strength; therefore, reduced knee muscle strength appears to be a common objective abnormality in FM that is independent of measurements of disease activity. The implication of this finding in regard to the clinical assessment of FM needs further study.