I thought this was the most interesting part, even if it was from a different study:
"Oh, and machine squats were found to have 30-40% higher shear forces than free weight squats"
Andrews JG, Hay JG and Vaughan CL. Knee shear forces during a squat exercise using a barbell and a weight machine. Biomechanics VIII B.H. Matsui and K. Kobayashi, eds. Champaign: Human Kinetics, 1983: 923-927