Assessment of extensor and flexor strength in the individual gonarthrotic patient: interpretation of performance changes.

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The intra-session and inter-session reproducibility of knee extensor and flexor strength measurements were examined in 21 gonarthrotic subjects (ten women and eleven men). Using the Cybex 6000 dynamometer, isokinetic peak torque and total work at 30 and 120 degrees/second and isometric peak torque were measured three times on separate days within two weeks by the same examiner. The reproducibility of walking and stair climbing time measurements was also assessed. The concept of critical difference (i.e. the difference between two measurements which would be statistically significant when applied to a reference group in steady state) for the interpretation of muscle strength data obtained by monitoring individual patients is presented. Individual coefficients of variation (CV) were calculated for each muscle strength variable. Depending on the velocity and on whether peak torque or total work were measured, the median CV of intra-session and inter-session extensor strength measurements ranged from 1.5-4.9% and 7.4-10.1%, respectively. CVs for flexor strength measurements were significantly higher. Substantial variability of within subject variances were found, e.g. the 80% central range of CVs for extensor torque at 30 degrees/second was 2.5-29.5% (inter-session). Calculated from CVs, critical differences for inter-session measurements exceeded 30% for all muscle strength variables. Median CVs for walking and stair-climbing time were 7.0% and 4.9%, respectively. In conclusion, the large CVs and corresponding critical differences may be a major limitation in the use of muscle strength measurements in the individual gonarthrotic patient.