Test-Retest Reliability of the Isometric Soleus Strength Test in Elite Male Academy Footballers

BACKGROUND Currently there is no reliability data available for the isometric soleus strength test (ISST), commonly used as a monitoring tool in elite football settings. Isometric strength testing for other muscle groups, most notably the hamstrings, is utilized to identify injury risk and readiness to train/play. To profile athletes efficiently, performance practitioners require optimal measures that are reliable. The aim of this study was to investigate the test-retest reliability of the isometric strength test of the soleus and propose a standardized protocol for its use within an elite male football population. STUDY DESIGN Test-retest reliability single cohort study. METHODS Thirty elite male footballers (age = 22.8 +/- 5.0 years, height = 180.0 +/- 0.08 cm, weight = 70.57 +/- 4.0 kg) performed the ISST, through three maximum 3-second hold efforts with one minute rest between repetitions and 48 hours between tests, in each test. The test was performed mid-competitive season. All data bilaterally were checked for normality using the Shapiro-Wilk test before a Pearson's Correlations and Bland-Altman's analyses were performed. RESULTS Test-retest reliability demonstrated high reliability for ISST bilaterally (Right: r = 0.89; Left: r = 0.79, p<0.05). The standard error of measurement (SEM) (%) was 9.09 - 12.47% and minimal detectable change (MDC) was 25.19 - 34.56 (N) for Peak Force (PF) measures of the ISST. Bilateral levels of agreement were found to be +/- 2 standard deviations (SD) of the interval of agreement bilaterally for ISST (Levels of agreement (LOA): Right: Upper 352.49 - Lower -494.76; Left: Upper 523.82 - Lower -591.30. Bilaterally no significant difference was detected between values (Right: p=0.09, CI: -153.21-10.95; Left: p=0.52, CI: -139.81-72.33). CONCLUSION The results of this study demonstrate high reliability for the ISST. The ISST displays a high test-retest reliability for assessing PF characteristics of the soleus in elite male academy footballers. This test may be beneficial for performance practitioners for profiling soleus function of athletes.