Most females are quad dominant meaning they heavily utilize the quadriceps muscles during actions that require quads, hamstrings, and glutes. This is important to note because if the muscles are not firing in the proper sequencing there could be a delay in the firing sequence causing the athlete to be more prone to injury. The different biomechanics of most female athletes causes a different activation pattern at heel strike which makes the quads contract first causing a sling shot effect on the ACL. In most men the hamstrings fire more efficiently eliminating this issue.
For example, the hamstrings control eccentric forces (or lengthening of the muscle) and help an athlete to safely slow down after accelerating. If there is a delay in the firing of the hamstring complex (where the quads fire first) then that muscle group is not able to fire quickly enough to help the athlete safely decelerate causing the ACL to take over and ultimately sprain or tear. It is ideal is for the hamstring complex to be at least 60% of quadriceps strength.
In addition to messing up the proper firing sequencing, quad dominance also creates increased wear on the knee joint over time particularly at the Patella (knee cap) because it is attached to the quadriceps complex via the quadriceps tendon. If the athlete has strong hamstrings and glutes those muscles will work together to load the forces of the movements of the body more to the posterior chain or muscles on the back side of the body. This is important because it takes pressure off the knees and the lower back.
By strengthening the hamstrings and muscles of the posterior chain or the backside of your body, symptoms of quad dominance will decrease. This is why it is so essential in ACL rehab to focus on contracting your hamstrings before the quads. It will teach your body the proper firing sequences as it relearns muscle contraction and muscle activation patterns. Yes, it is critical to fire the quads soon after surgery but for the female athlete hamstring activation before quads is the best route. After a week or two when pain levels decrease and hamstring contractions are becoming easy then incorporate the quad activation into your rehabilitation routine.