Crouch gait is the most common motion abnormality in children with cerebral palsy (CP). This paper presents a new biomechanical model based on a simple rescaling and adjustment to CP patients who develop crouch gait by subject-specific anthropometric data. The model estimates the length of hamstrings, as the distance between the origin and insertion of the muscle, and the velocity of shortening of hamstrings by the first derivative of the length with respect to time. This model has the potential to increase the benefits of three-dimensional biomechanical models as it can discriminate between short, spastic or normal hamstrings. The main advantage of this model in clinical use is that it does not require costly magnetic resonance imaging.