Reliability of ankle isometric, isotonic, and isokinetic strength and power testing in older women.

BACKGROUND: Ankle strength (force-generating capacity) and power (work produced per unit of time or product of strength and speed) capabilities influence physical function (eg, walking, balance) in older adults. Although strength and power parameters frequently are measured with dynamometers, few studies have examined the reliability of measurements of different types of contractions.

OBJECTIVE: The purpose of this study was to examine relative and absolute intrarater reliability of isometric, isotonic, and isokinetic ankle measures in older women.

DESIGN: This was a prospective, descriptive methodological study.

METHODS: The following dorsiflexion (DF) and plantar-flexion (PF) measures were assessed twice (7 days apart) by the same examiner in 30 older women (mean age=73.3 years, SD=4.7): isometric peak torque and rate of torque development (RTD), isotonic peak velocity, average acceleration and peak power, and isokinetic peak torque and peak power (30 degrees/s and 90 degrees/s). Several statistical methods were used to examine relative and absolute reliability.

RESULTS: Intraclass correlation coefficients (ICCs) for the DF tests (ICC=.76-.97) were generally higher than ICCs for matched PF tests (ICC=.58-.93). Measures of absolute reliability (eg, coefficient of variation of the typical error [CV(TE)]) also demonstrated more reliable values for DF tests (5%-18%) compared with PF tests (7%-37%). Isotonic peak velocity tests at minimal loads were associated with the lowest CV(TE) and ratio limits of agreement values for both DF (5% and 14%, respectively) and PF (7% and 18%, respectively). Isometric RTD variables were the least reliable (CV(TE)=16%-37%).

LIMITATIONS: This study was limited to a relatively homogeneous sample of older women.

CONCLUSIONS: Test-retest reliability was adequate for determining changes at the group level for all strength and power variables except isometric RTD. Minimal detectable change scores were determined to assist clinicians in assessing meaningful change over time in ankle strength and power measurements within individuals.