Reliability and precision of isokinetic strength and muscular endurance for the quadriceps and hamstrings

The importance of accurately reproducing isokinetic strength values is critical for the assessment of training induced changes in muscle function. The purpose of this study was to determine the test re-test reliability of the Biodex System 2 Isokinetic Dynamometer for concentric quadricep and hamstring strength and muscular endurance. Twenty-one healthy volunteers underwent isokinetic testing on 2 occasions separated by 7 days. Each subject performed 5 repetitions at 60 degrees/second followed by a muscular endurance test which consisted of 30 repetitions at 180 degrees/second. The results demonstrated high ICC values ranging from r = 0.88 to r = 0.97 and r = 0.82 to r = 0.96 for variables measured at 60 degrees/second and 180 degrees/second, respectively. SEM values were found to range from low to moderate, 4.8% to 11.6% and 5.6% to 10.8%, at 60 and 180 degrees/second, respectively. ICC coefficients were found to be low, r = 0.52 to r = 0.74, and SEM values were found to be high, 9.8% to 20.8%, for the work fatigue index at 180 degrees/second. The results demonstrate that isokinetic values obtained at 60 and 180 degrees/second are highly reproducible with the Biodex System 2 Dynamometer provided that adequate calibration, gravity correction and patient positioning is recorded and standardized. However, the work fatigue index as a measure of muscular endurance has not been shown to be reliable or precise. Changes in muscular strength over time should be considered valid if these differences lie beyond the SEM values reported in this study.