Correct, these devices actually train opposing muscles, but they do it in a so called iso-kinetic way, meaning the resistance changes depending on the force you are applying. The bigger the force you are using the bigger the resistance becomes. This makes it very difficult to measure improvement as you can only measure repetitions in a given time. With the LDA you get a linear resistance that emulates weight resistance almost perfectly that can be adjusted from aprox. 1 kg to 6 kg thus enabling you to observe the increase in strength and you have also the possibility to check your improvement based on repetitions per given time.