The Minimum Number of Attempts for a Reliable Isometric Strength Test Score

The maximal isometric strength test is widely used in general, athletic, and clinical populations. However, this test has often been used without considering how many trials are needed or appropriate to obtain reliable test scores. Thus, the aim of this study was to examine the maximal isometric strength test in upper and lower body for the purpose of determining the reliability of best isometric strength score and the ideal number of trials for a reliable test score.

One hundred and twelve healthy adults were assigned to four different groups (elbow flexion with the vertical and horizontal forearm positions, knee flexion, and knee extension). After familiarization, all participants performed three maximal isometric voluntary contractions with the designated muscle group. The reliability of best isometric strength test score was calculated using the intraclass correlation coefficients (ICC) from a one-way ANOVA model. The Spearman–Brown prophecy formula was used to identify the minimum number of trials needed.

The reliability of the best maximal isometric strength test scores out of three trials were high (ICC = 0.92 to 0.98). At least two trials of maximal isometric strength test for knee flexion and one trial for elbow flexion and knee extension would be necessary to achieve an acceptable reliability coefficient of 0.80. In addition, one trial of elbow flexion would be enough for women, but men would need two trials.

Our results suggest that overall, at least two trials would be necessary to test isometric strength in upper and lower body muscles in both sexes.