BACKGROUND: Strength assessments are used to monitor physical progression and evaluate the impact of training interventions, which is extremely valuable for both athletic performance and clinical populations. For results to be useful, measurements must be relevant, reliable and show sensitivity to change. OBJECTIVES: The aim was to establish the practicality of isometric force assessment at two different knee-joint angles; 90° (ISO90) and 120° (ISO120). More specifically, to address the familiarisation effects, reproducibility and sensitivity of measurements associated with each method of assessment, and the discrepancy in force output when altering the angle at the knee joint. METHODS: Thirty-five strength trained males attended three sessions; familiarisation (T1), test (T2) and retest (T3), separated by 7 days. During each session, ISO90 and ISO120 was assessed using an incline leg press device. RESULTS: Force output was similar during T1, T2 and T3 for ISO90 and ISO120, separately (p > 0.05). Measurements taken from both assessment methods demonstrated good reproducibility (ICC = 0.96, CV < 5%) and showed sufficient sensitivity to detect changes in performance. Force output was greater during ISO120 (5153 ± 1446 N) versus ISO90 (2660 ± 597 N, p < 0.001) but the magnitude of the difference in force output showed great intrasubject variability. CONCLUSION: Isometric assessment performed on a leg press device requires minimal habituation to demonstrate a good degree of reproducibility and sensitivity to detect small changes in performance. It is a simple and practical method to evaluate strength at different joint angles, which may prove useful in strength diagnosis in performance and clinical contexts.