THE ANATOMY

The anterior cruciate ligament (ACL) connects the femur to the tibia at the knee.  It runs from the front of the tibial plateau (top of the tibia) towards the back and inside surface of the end of the femur.  The primary role of the ACL is to prevent anterior translation of the tibia on the femur.  In other words, it prevents the lower leg from sliding too far forward compared to the upper leg.  It is also responsible for resisting hyperextension of the knee and medial (towards the inside) and lateral (towards the outside) rotation of the lower leg on the upper leg.

THE PROBLEM

According to emedicine, about 200,000 people sprain their ACL every year, and about 100,000 go on to have surgical reconstruction.  The mechanism of injury can be contact or non-contact.  Non-contact injuries are usually associated with rapid deceleration of the lower limb, knee hyperextension, or forced rotation of the lower limb.  Women are 4-6 times more likely to suffer a non-contact injury to their ACL.  A few possible explanations for this are that:

• women tend to have a wider pelvis, and therefore an increased angle at the knee (more “knock-kneed”); this places more of a demand on the hip musculature to control the leg and increases the demand on the ACL to stabilize the knee
• women tend to have strong quadriceps and weak hamstrings; this is referred to as an increased quad:hamstring ratio
• women tend to have a narrower femoral notch, which is the space at the end of the femur through which the ACL courses; this puts more physical pressure on the ACL and can wear it down
• women tend to have decreased neuromuscular control compared to men; this refers to the timing, strength, and endurance of the musculature surrounding the hip and knee

Gender aside, what can be done to prevent an ACL injury? Let’s use this information to formulate a training program that will help prevent an ACL injury in men and women.

THE SOLUTION

Emphasize the Gluts:

• The gluteus maximus is the primary muscle responsible for external rotation of the leg and the gluteus medius is the primary muscle responsible for abduction of the leg.  Simply put, these two muscles prevent the knees from coming together (knock-kneed) and rotating inward during functional activities like walking, running, jumping, etc.
• According to research, the best exercise to target the gluteus medius is side-lying leg lifts.  This is a great starting point, and can later be applied to more functional, upright positions.  TIPS! When performing side-lying leg lifts make sure that your leg is in line with your upper body or slightly behind you (extended); don’t let your hips roll backwards; keep your toes pointing up towards the ceiling
• To target the gluteus maximus, squats are a good starting point.  You can modify squats by using an exercise ball, a barbell, dumbells, wall sits, etc… BUT the best exercise to target the gluteus maximus is the single-leg squat.  That doesn’t mean you should jump right into single-leg squats though; progress slowly.  TIPS! When squatting bend at the hips, not through your low back.  Keep your knees in line with your 2nd or 3rd toe on each foot.  Don’t let your knees move forward in front of your toes (you should be able to see your toes if you look down) And don’t squat past 90°; it’ll do more damage than it’s worth.

Don’t forget the Hamstrings:

• This goes back to the quad:hamstring ratio.  When the quads contract they pull on the front of the tibia (the motion the ACL is trying to limit).  When the hamstrings contract they pull on the back of the tibia, assisting the ACL.  So it makes sense that you want to strengthen this synergistic muscle group.  The easiest way to target the hamstrings is to use a weight-machine, either in sitting or prone (on your stomach).  It is true that the hamstrings are co-contracting during squats, but they are not the primary muscle group involved; the quads are working to extend the knee, and the glut max is the primary muscle extending the hips.  TIPS! Using a fitness ball can also be a great way to workout the hamstrings.  Lie on your back and rest your heels on an appropriately sized fitness ball.  Bridge up, bringing your pelvis off of the floor.  Once stable, roll the ball towards you by bending at the knees.

Eccentric Control:

• An eccentric contraction occurs when the muscle is contracting and lengthening at the same time.  This happens all the time.  The ability to safely and efficiently perform eccentric contractions is crucial for overall control of the lower extremity (deceleration injuries!).  The best part about this is that you are already doing exercises that have an eccentric component to them (it’s everywhere).
• Try this.  Focus on the downward phase of the squat and count to 5 before reaching the bottom.  Or when performing hamstring curls on the machine, count to 5 while straightening your leg.  Or when performing side-lying leg lifts, count to 5 as you lower your leg.  This is simply emphasizing the eccentric component of the lift, which is crucial for overall stability.

First Things First:

• One of the biggest mistakes people make is progressing themselves too soon, or skipping the starting point altogether.  You have to crawl before you can walk, and you have to be able to safely perform exercises like the ones I’ve mentioned (side-lying leg lifts, squats progressing to single-leg squats, hamstring curls) before beginning a more intense program such as a plyometric program.  If your technique is faulty, you will actually be placing yourself at an increased risk for injury.
• For resistance training and aerobic training guidelines, read this article!
• If you need more help getting started, go see a physical therapist or a qualified personal trainer!