Should you Lock your Knees when you Squat and Deadlift?

One of the common pieces of bad advice we hear is that athletes should not "lock" or fully extend the knee joint because it is unsafe. Coaches and practitioners who do not follow the latest research believe this is safer for the knees because it keeps the quadriceps and hamstrings more engaged, which in their mind means the joint is more supported. Unfortunately, this is just plain false. Your cruciate ligaments, (ACL, PCL, MCL, LCL) work together to lock the knees in place and stabilize the knee joint. They are in their most shortened position ONLY when the knee is locked and the joints are stacked, which creates optimal stability between the tibia (shin bone) and femur (thigh bone). This is a large reason why powerlifting federations require athletes to start and finish a squat with locked knees and to fully "lock out" a deadlift with extended knees and hips. If the knees remain unlocked, the femur is creating a moment arm (oh hey, physics!) that places shearing force on the knee joint, adding stress to an already unstable joint position. Think of it this way -- if you never train a specific skill or position that your body CAN perform, it is unlikely you will be strong in that skill or position. So when you accidentally find yourself required to support that skill or position, whether through an accident or a fall or a stupid alcohol-influenced decision, you are more likely to land yourself an injury because you have not practiced and strengthened that skill/position.