Research shows that while the standard assumption is that knees are locked in standing, actually in reality standing is possible with low leg stiffness without locking the knee. Medically, a knee is locked when a torn part of the meniscus has displaced into a part of the knee or due to a tight Popliteus muscle.
The view that 'knee locking act alone by itself causes syncope' is not backed up by scientific evidence. However, locking one's knees and maintaining a rigid, completely unmoving standing position for long periods of time might interfere with proper blood flow from the legs and cause vasovagal syncope.
It turns out the medical science behind “don’t lock your knees” doesn’t exactly hold up. While the advice to avoid locking knees is widespread – even outside of military communities – the actual cause of fainting is usually loss of blood flow to the brain which can be brought on by any number of things, but the act of locking ones knees has nothing to do with passing out. While the act of locking ones knees and maintaining a rigid, completely unmoving position for a long period of time may interrupt proper blood flow, the act of locking the knees alone does not by itself cause fainting spells.
Syncope also commonly called fainting or “passing out” is a medical term used to describe a temporary loss of consciousness due to the sudden decline of blood flow to the brain. If one is about to faint, there are a lot of accompanied symptoms described here. There are also several types of syncope which have specific trigger events or they seem to occur through other causes such as dehydration, BP medication and other medical conditions. If it occurs very often without a triggering event, it might be a sign of an underlying heart disease requiring medical attention.
Prolonged standing associated with heat or emotional stress can cause gravitational blood pooling in the lower extremities. This might act as a trigger for the fainting spell and may be amplified by the knee locking.
“If dehydration is present the situation is worsened,” explains Dr. Richard Nicholas, chairman of the UAMS Department of Orthopedic Surgery. “The low rate of blood return to the heart in these situations results in hypo-perfusion of the brain and at times the fainting spell. Standing at attention, or with one’s knees locked, may amplify the process.”
Medical advise to counter this knee locking mechanism is to move about or contract the leg muscles which might help in stimulating the blood flow to the heart and improve circulation to the brain.