bullshit neithe movement fucks up knees or back..what fucks up knees and back is by doing shitty form too heavy ego lifting... keep it within your range of power and keep the tension ON THE MUSCLE and you wont get those problems.... problems arise when your more worried about the # of lbs on the bar/machine and not as worried about contracting concentric and eccentric and working the muscle.. bodybuilding 101 ... this shit is basic..