An accurate trunk muscle strength assessment seems very important to design and individualize training and rehabilitation programs in clinical and sport settings. Hand-held dynamometers (HHDs) are interesting alternatives to isokinetic dynamometers for assessing trunk isometric muscle strength because they are inexpensive instruments and easy to use. This cross-sectional observational study aimed to examine the reliability of two novel sitting tests for assessing trunk flexion and extension isometric strength using an HHD and their relationship with two other novel isometric tests that use an isokinetic dynamometer. Twenty-four female amateur athletes (age: 24.5 ± 2.64 years; body height: 164.45 ± 6.33 cm; body mass: 63.17 ± 10.35 kg) participated in this study. A test-retest design was carried out one-week apart to examine the reliability. The relationship and the degree of agreement between the HHD and the isokinetic dynamometer measurements were analysed using Pearson correlation and Bland-Altman analysis, respectively. In general, the reliability of all isometric strength tests was good, with ICCs ranging from 0.65 to 0.87 and typical error < 15%. Pearson correlations were moderate, with values of r = 0.47 (R2 = 0.22) and r = 0.42 (R2 = 0.18) for flexion and extension strength, respectively. Bland-Altman plots showed no agreement between HHDs and isokinetic measurements. All trunk isometric tests using both, an isokinetic dynamometer and HHDs, provide reliable measurements for assessing trunk flexion and extension strength. According to the comparative analysis, both measurement types are different and cannot be used interchangeably. Health and sport professionals should choose the test that best suits the biomechanical characteristics required for functional goals or success in a given sport.
