ACL injuries are unavoidable in sports. However, many ACL injuries could have been avoided with better preparation prior to competition. Although preventative exercises are often seen as boring and time consuming, after personally having suffered three ACL tears, I will tell you there is nothing more time consuming, boring and frustrating than nine months of rehab.
The Anterior Cruciate Ligament (ACL), located in the middle of the knee, helps stabilize the femur over the tibia. In aggressive contact sports, the ACL may be stretched or torn by a hit to the leg. Many contact ACL injuries are unavoidable and are simply hazards of the game.
However, an ACL tear can occur without contact if an athlete has muscle weakness, poor balance or the inability to decelerate his or her body. Deceleration requires controlling your body weight during rapid direction changes or when landing from a jump. If the muscles around the hip and knee are unable to stabilize the upper leg over the lower leg, an ACL injury can occur.
Although a comprehensive ACL prevention program is beyond the scope of this article (check out STACK's Athlete's Guide to the ACL), I want to highlight three of the best ACL prevention exercises, all of which I have used with my athlete-clients over the years. They benefit athletes of all levels, require no equipment and can be performed in minimal time. For best results, perform them two to three times per week.
This exercise is designed to strengthen the hamstrings and glutes, which help maintain upper- and lower-leg alignment along with the ACL. Its single-leg nature is important for eliminating strength differences between the left and right leg, reducing the chance of injury from an imbalance.
It's important to keep the pelvis in a neutral position during this drill, as energy can "leak" out of the body if there is excessive movement in the pelvis or spine. Tighten your abs, drive your heel into the ground, and lift your hips off the ground without arching your back. (Watch the Cal State baseball team perform the Single-Leg Bridge.)
This is an important balance exercise, aiding in hip stability and alignment of the upper and lower leg. Keeping your hip, knee and ankle in line is essential for preventing your knee from collapsing inward when you land from a jump.
Balance on one leg and keep the knee slightly bent while reaching down to touch your toes with the opposite hand. Return to a full upright position by actively contracting your glute, while again keeping your pelvis and spine in a neutral position. Execute the movement with control and precision, and stop if your technique begins to break down. (Check out Candace Parker's version of the Single-Leg Balance Reach.)
This exercise is designed to address knee alignment during deceleration. An inward (valgus) collapse of the knees is a primary predictor of non-contact ACL injuries.
Stand with feet shoulder-width apart and pre-load by flexing the hips, knees and ankles. With arms back and spine in neutral position, drive arms towards the ceiling and extend through the hips and ankles to jump straight up. Land softly with your knees directly over your feet. Do not allow them to collapse together. Once you can successfully accomplish this drill with proper form, introduce lateral jumps to simulate the game day environment. (Learn perfect Vertical Jump technique.)
This is a plyometric exercise and should only be performed following a thorough warm-up and clearance from your coach.
If you are rehabilitating from an ACL tear, consult your orthopedic surgeon or physical therapist prior to engaging in any form of exercise.
 Hewett TE, Myer GD, Ford KR, Heidt RS Jr, Colosimo AJ, McLean SG, van den Bogert AJ, Paterno MV, Succop P. (2005) "Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study." Am J Sports Med. 2005 Apr;33(4):492-501.