Q: What does it mean to “lock” your knees, and why do exercise trainers advise against it?
A: Locking your knees means putting them in full extension so that your legs are completely straight. When doing calisthenics, lifting weights or using exercise machines, locking your knees puts maximum stress on the joint, increasing your risk for injury. Similarly, fully flexing your knees, as in deep squats, increases stress and can cause pain or injury. Even sitting for long periods—especially with your knees bent at 90° or more—can stress the knee joint. Knees need to move.
Try to avoid the locked-knee position when lifting weights, cycling (for instance, if your seat is too high) or doing any activity that puts extra weight on your knees. When working out at the gym or in an exercise class, ask a trainer to evaluate your technique.