Most leg press machines provide movement in a straight line. With straight line movement there is more shearing force at the end point of the movement, and if one locks out at the end point there is greater potential for hyper-extending the knee. As an example see here (warning graphic). At Austin Fitness Trainers and at New Orleans Fitness Trainers we use MedX equipment. The MedX leg press provides an arcing range of motion similar to the rotation of the knee, and it can be set so that there is no locking out at the end point.