Background: the validity and reliability of the front plank test (FPT) have been studied in young adults but not in adults aged 34 to 60 years. The aim of this study was to analyze the criterion-related validity and reliability of the front plank test (FPT) for evaluating trunk musculature in adults according to sex and age groups. Methods: a repeated measures design was used to study the reliability of the FPT. A total of 84 adults aged 18–62 years performed trunk muscular force tests and the FPT. Criterion-related validity was assessed using repeated measures ANOVA. Reliability was examined by ICC, error measurements, and Bland–Altman analysis across sex and age groups. Results: significant differences in the activation of the rectus abdominis and external oblique muscles were found compared with the erector spinae and multifidus muscles (p < 0.001). No correlations were found between FPT time and physical activity or rate of perceived exertion at the end (p > 0.05). Low and moderate correlations were found with body fat percentage, trunk fat mass percentage, and external oblique, regardless of sex and age groups (p < 0.05). Extremely high reliability was found regardless of sex and age groups (ICC > 0.98), with low error measurements (RMSE = 11.93–18.73; %CV = 4.91–6.33; SEE = 11.45–16.84). MDC90 values indicated no real change between T1 and T2. Conclusion: FPT is a valid and reliable test for assessing trunk musculature in adults regardless of sex and age groups.

Chronic low back pain (CLBP) is a leading cause of disability worldwide. Trunk muscle weakness is linked to the onset and severity of CLBP. While isokinetic dynamometers are reliable for assessing trunk flexors, extensors, and rotators, the reliability and safety of Functional Electromechanical Dynamometers (FEMD) remain insufficiently studied. Therefore, this study aimed to determine the reliability and safety of trunk strength assessment using FEMD in adults with non-specific CLBP (NSCLBP). Twenty-eight participants with NSCLBP attended three sessions: familiarisation and two assessments (48-72 hours apart). Trunk flexors, extensors, and rotators were assessed using isokinetic and isometric modes with a FEMD. Reliability was analysed using intraclass correlation coefficients (ICC), standard error of measurement (SEM), coefficient of variation (CV), and Bland-Altman plots. Reliability was good to excellent across all trunk muscle groups: flexors (ICC = 0.87-0.92), extensors (ICC = 0.87-0.90), right rotators (ICC = 0.91-0.96), and left rotators (ICC = 0.86-0.96). Mean SEM ranged from 0.88 to 12.63 kg and CV from 6.56 to 15.63%. Three participants reported mild transient discomfort immediately after testing, with no pain reported at 24 hours. These findings indicate that FEMD-based assessment of trunk strength is reliable and safe for patients with NSCLBP, supporting its application in clinical and research settings to monitor rehabilitation.