Trunk extensor and flexor strength measured by the Cybex 6000 dynamometer. Assessment of short-term and long-term reproducibility of several strength variables.

STUDY DESIGN

Isokinetic trunk extensor and flexor strength (peak torque, total work, power acceleration time, and torque acceleration energy) at 30 degrees/sec, 120 degrees/sec, and 180 degrees/sec and isometric extensor and flexor peak torque were measured in 24 healthy women. Measurements were performed in three separate sessions at the same time of day and by the same experienced examiner. The number of days between Session 1 and Session 2 and between Session 1 and Session 3 ranged from 1 to 7 (median, 2 days) and from 23 to 48 (median, 32 days), respectively. The test results were not revealed before all sessions were completed.

OBJECTIVES

To determine the intrasession, short-term, and long-term reproducibility of several trunk strength variables measured by the Cybex 6000 isokinetic dynamometer (Lumex, Inc., Ronkonkoma, NY).

SUMMARY OF BACKGROUND DATA

Only few and insufficient data regarding the reproducibility of trunk strength measurements exist. Previous studies have focused on coefficients of correlation to express reproducibility. Correlations, however, are misleading and inappropriate for this purpose. The critical difference is the difference between two results that would be statistically significant when applied to a reference group in steady state. The critical difference in percentage is calculated as 2.77 X coefficient of variation (%).

METHODS

Isokinetic dynamometry (Cybex 6000). Reproducibility was expressed as pooled coefficients of variation, individual coefficients of variation, and critical differences.

RESULTS

Substantial between-subject variation of coefficients of variation was found for all strength variables; for example, the 80% central range for individual coefficients of variation for long-term reproducibility of extensor strength measurements at 30 degrees/sec ranged from 2.5% to 19.5%. Critical differences calculated using the pooled variances for long-term reproducibility were higher than 20% for all measures, mounting to 80% for acceleration time.

CONCLUSIONS

Strength testing is of limited value in the individual subject because of considerable day-to-day variability. This finding has important clinical implications.