The aim of this study was to develop a new method of measuring maximal isometric strength, using an isoinertial-basedweight machine. The aim was also to investigate the reliability of the isometric strength test and the dynamic one-repetitionmaximum (1 RM) test in women and men. Healthy recreational weight trainers (15 women and 15 men) were tested firstly for maximal isometric strength and secondly for 1 RM-strength using an isoinertial-based (gym) knee-extension weightmachine. The isometric test was carried out with the subject performing a muscle action that dislodged a particular testweight (within 5 s) and, as a result, a weighted rope fell out of the weight stack and the trial was regarded as successful. The test was repeated with increments of weight until failure. Intraclass correlation coefficients (ICC) were used to assess test – retest reliability. High ICCs ( 0.90) were found for both the isometric and the 1 RM-strength tests, in both women and men. In conclusion, our new clinical test is a reliable tool for healthy, relatively strong subjects for the establishmentof maximal isometric strength using an isoinertial-based weight machine. The test can be used by physical therapists, athletic trainers and strength and conditioning coaches in clinical practice, working with strength training and rehabilitation.