I have found over the years of testing in particular with the recovery from an ACL injury that somehow the brain fails to trust the muscles responsible to protect the knees stability and instead distributes the load to the quad, hamstring & calf muscles after an injury. The neurological pathways almost seem disconnected or are never reeducated, almost like the brain doesn’t trust the muscle any longer, so it turns off the power. Lifting weights doesn’t fix the problem, in most cases it just moves the problem somewhere else. So therefore when the athlete goes through physical therapy and starts training & lifting again they create a huge imbalance in the adjoining muscles from the injured leg to the uninjured leg. I have seen muscle imbalances as high as 46% from right quad to left quad and even larger imbalances in the hamstrings after a knee or ankle injury. These imbalances are unknowingly trained into the athletes body when they try to start training again before they have fully healed after almost any type of knee or ankle injury.
The injury does not have to rise to the level of complete ACL failure for the problem to be created. A simple knee strain or rolled ankle can start the wheels in motion within the bodies neurological neuropathway system. It appears that as soon as trust is lost in the muscle, the brain reroutes neurological pathways it feels will better suited to protect the knee, when trust is required. Lifting weights, squatting or leg presses most of the time does not reeducate the neuropathways to the untrusted muscle. Instead heavy weight lifting strengthens the neuropathways to the trusted muscles, which eventually creates such a large imbalance between the muscle groups that appears to happen is the vastly stronger muscles actually injures the weaker muscles leading to a long career of injuries in an athlete. From my experience, muscle imbalances are near the top of reasons for most knee and ankle injuries. The stronger quad or hamstring muscles exert such large force loads on weaker muscle groups during competition that ligaments & tendons start helping or providing support in strength roles. This may help to explain the research done showing that athletes that injure or roll an ankle, have an 80% likelihood of straining their hamstring or groin or tearing a ligament within the same season.
EMG devices should be worn around the knee during recovery training in order for athletes to determine the specific lifting, training or exercises that activate the most amount of neurological connections in order to reeducate the untrusted muscle and grow it back during recovery. In addition some form of muscle trust protocols should be researched and developed to build back the trust in the muscle.
An easy test to see if the brain trusts the muscle again is to surround the knee with EMG sensors and have the athlete do a one leg pistol squat and check the results from the injured knee to the uninjured knee. This will give you a baseline. Then start distracting the brain by having them stand on a vibration plate at around 40hz and have them do the same test. This creates normally enough of a distraction and uncertainty to the brain that if the muscle is not trusted, it will be easy to see, which is typically the case. I did this test on an Olympic long jumper that was 4 years post-surgery and trust was still not regained in the muscles that support the knee. In another instance an athlete was 10 years post-injury. Typically an athlete will have trouble performing the same test on a vibration plate. EMG readings will also be much different and the athlete will become very unstable as the muscles attempt to figure out who is responsible for what. In a few cases I have added strobe glasses to create further instability, which directly impacted neurological output. The results are eye opening. A muscle will appear normal in a muscle composition test and sometimes even give normal EMG readings before trust is required. But as soon as the brain is made uncomfortable and needs to count on these muscles to perform a movement, it reroutes the electrical impulse to the muscles it trusts more, which we could hypotheses is what happens to an athletes body during a stressful competition.
These same tests along with using a stability gyroscope sensor and a muscle mapping protocol I believe can accurately predict the likelihood of suffering a knee or ankle injury within a given playing season. We may not be able to predict ACL tears, but using a scoring system we should be able to determine the level of likeliness for injury.