Gouvêa, AL, Martinez, CG, and Kurtenbach, E. Determining maximal muscle strength in mice: validity and reliability of an adapted swimming incremental overload test. J Strength Cond Res 34(8): 2360–2368, 2020—At present, there are no reliable methods to determine maximal muscle strength in small rodents. Here, we established an adapted swimming incremental overload test (SIOT) as an instrument for this purpose. First, to validate the test, BALB/c mice received 20 mg·kg−1·d−1 of dexamethasone (DEXA group) or water (control group). After 14 days, with a cumulative dose of 120 mg·kg−1 of dexamethasone, the SIOT could detect a decrease of approximately 7% in muscle strength. In addition, this decrease was consistent with a significant reduction in body (above 13.5%) and muscle (approximately 15%) weight in DEXA atrophic animals. To establish the SIOT reliability, another group of animals was evaluated for 5 consecutive days. In this second protocol, the SIOT was executed with an initial load corresponding to 12% of the mouse body weight (BW) fixed to the tail. Increments between 1 and 5% of the BW were added during each attempt to obtain the highest load that was tolerated for a time interval of 5–7 seconds. On the last day, the SIOT reliability test was performed by 2 different raters to obtain the inter-rater reproducibility. The adapted SIOT was shown to be reliable when measured by the same rater (intraclass correlation coefficient [ICC] = 0.939) and by 2 different raters (ICC = 0.830). The Bland-Altman graphical representation did not demonstrate heteroscedastic errors. Therefore, the SIOT proved to be a sensitive and reliable method to measure muscle strength, and it can be applied to small animals in different models of muscle atrophy.