A practicable EMG-force model is evaluated for muscles about the knee. The model included envelope signal processing and a gain-dependency of knee angle and angular velocity. Six healthy subjects participated in the experiments. For calibration, maximal isokinetic contractions about the knee were performed on a dynamometer with recordings of knee joint movement, net moment and EMG of thigh muscles. The model parameters were fitted on these calibration experiments. For validation, estimated moments from the EMG levels were compared to the actual exerted moments of two independent isokinetic contractions. Averaged RMS values of the difference ranged from 11 to 20% of the actual exerted moment. For isokinetic concentric contractions, the present model is suitable as a method to estimate muscle moments.