Electromyographic assessment of back muscle weakness and muscle composition: reliability and validity issues.

Objective: To assess the reliability and construct validity of various electromyographic indices developed to assess back muscle weakness and muscle fiber composition.

Design: A prospective study with repeated measures performed on 3 days along with comparisons of groups presenting different back strength and/or back muscle fiber composition.

Setting: A biomechanics laboratory within a rehabilitation center.

Participants: Forty male volunteers (20 healthy, 20 with chronic low back pain) were assessed on 3 different days to assess reliability and to make group comparisons. Thirteen healthy women were also assessed once to obtain a third group with known lower strength and different back muscle fiber composition.

Interventions: Not applicable.

Main Outcome Measures: Surface electromyography was recorded for 4 pairs of homologous back muscles while the subjects performed, on a dynamometer, static trunk extension efforts. Electromyographic parameters were computed to assess muscle weakness and muscle fiber composition. The reliability of the data collected across the 3 sessions and comparisons between groups were determined.

Results: Electromyographic parameters generally showed good to excellent reliability, but were insensitive to differences in back muscle strength and did not appear to be related to muscle composition. Some trends were observed in the electromyographic parameters across the force levels, but the large interindividual variability impeded statistical comparisons.

Conclusions: The assessment of muscle weakness and muscle fiber composition through electromyographic analysis does not appear feasible, at least on an individual basis, for the muscles of the back.