Good question and one that is under investigation. Researchers in Sweden, for example, tried to measure the strain on the anterior cruciate ligament (ACL) during four exercises commonly used in the early phases of rehab.

They found that the position of the knee joint was the key factor. The straighter the knee, the greater the strain. At this point in the research of this topic, we don't have actual strain measurements on the ligament for different positions and movements.

Researchers are also looking at differences in the kind of strain. Strain from compression through the joint may be very different from a shearing force across the knee. So twisting the knee when the foot is planted on the ground with the knee straight and the body weight over it may be more dangerous than other positions with the knee bent.