ACL injury and muscles as stabilizers
Knee injuries are common in athletes and effective strengthening programmes are often required to improve functional strength and stability for further sport career. But which muscles need to get stronger and why? For better understanding let me illustrate the knee joint, the muscles around it and the forces they create.
The ACL is frequently injured in sportsmen and women. The hamstring muscles function to stabilise the knee and support the ACL by preventing the shin bone from shifting forwards as the knee straightens. Therefore, during a rapid extension action such as kicking, the hamstrings are acting eccentrically (muscle lengthening) to help prevent the shin bone from shifting forwards, placing excessive loading onto the ACL (Journal of Human Kinetics, 2012, 33, 63-71).
Exercises that produce a dominant quadriceps activation can increase the shearing-type forces on the knee joint, particularly during movements where the leg is extended rapidly. This is why strong hamstring muscles are so important to balance the knee (Journal Athlete Training, 2012, Aug, 47, 4, 396-405).
Following a knee injury, it is therefore essential to promote exercises that provide simultaneous activity of both the hamstrings and quadriceps - often referred to as a co-contraction. In particular, it's essential to commence co-contraction exercises early in the rehabilitative phase. This encourages effective hamstring and quadriceps muscle control during the latter stages, ready for a return to sport.