One of the most devastating injuries you can sustain as an athlete is a tear or rupture of an anterior cruciate ligament (ACL), the key strap of connective tissue which stabilizes the knee joint and connects the back of the femur (thigh bone) with the front of the tibia (shin bone). The injury is a particularly painful one - and requires lots of recovery time and some fairly sophisticated rehabilitation techniques in order to restore normal knee function.
It's true that it is possible - at least in some endurance sports - to train for a while with a ruptured ACL, at least after the pain and swelling associated with the initial injury subside. The writer of this article, who is a runner, actually trained consistently and without major problems from 1978 to 1994 with a completely severed ACL in his right knee - until the lack of stability associated with the missing ACL finally caused his knee to turn into a red-hot mass of angry tissue which made normal workouts impossible. However, it's far better to surgically repair a breached ACL before serious endurance training is undertaken, in order to minimize the long-term risk of developing osteoarthritis and/or other disorders (the writer wasn't aware that the ACL was missing until an MRI revealed its absence in 1994).
The word 'cruciate' in this key ligament's name means cross-shaped or marked with a cross, a seemingly odd designation for a straight strap of connective tissue which is roughly the size of one's little finger. However, within the knee joint, especially when the tibia is rotated in an internal direction (counterclockwise for the right knee, clockwise for the left knee), the anterior cruciate ligament (ACL) runs over the front of - and is roughly perpendicular to - another key piece of connective tissue called the posterior cruciate ligament (PCL), creating a 'cross' of connective-tissue cords within the knee. An easy way to picture this is to cross your index finger under your middle finger; the middle finger represents the ACL and the index finger is the posterior cruciate.
The cruciates provide support for the knee and also guide rotational movements at the knee joint. Basically, the ACL prevents hyperextension of the knee, limits excessive forward movement of the tibia during knee flexion, and controls internal rotation of the tibia. It's possible that the ACL also controls external rotation of the tibia, especially since it tends to be wrapped around the inside of the lateral femoral condyle (the bony projection at the outside bottom of your femur).
Rupturing an ACL
The possible scenarios for injury are varied. For example, someone might collide with you as you are running along, slamming into your leg near the knee while the foot of that leg is planted on the ground. The sudden wrenching movement of the tibia which results can tear the ACL surprisingly easily. Alternatively, you might step on someone else's foot while running (or while landing after jumping during a basketball or volleyball game), causing your knee to hyperextend and rip the ACL out of its moorings. Or, you might simply attempt to come to a sudden stop, often while twisting your leg at the knee, to get out of the way of another athlete. The extreme knee flexion and torquing which result can easily damage the ACL.
If an ACL is seriously harmed during activity, you'll often hear a 'pop' when the injury occurs, and swelling will take place almost immediately. In addition, the knee itself will tend to be quite unstable, usually giving way during weight-bearing.
Although the following exercises aren't a foolproof guarantee against ACL problems, they mimic some of the above injury-producing movements (in a controlled and strengthening - but not threatening - manner) and represent a great way to minimize the risk of ACL injury. Each exercise is accompanied by a description of why it is helpful:
Exercise 1: The Six-Way Lunge with Arm Drop
To carry out this exercise, begin by standing with your feet parallel and hip-width apart. Your arms should be bent at the elbows so that your hands are directly in front of your shoulders. Then, take a long step forward with your right foot, as if you were doing a lunge, and lean your upper body forward approximately 45 degrees at the waist as you do so. Drop your hands on either side of your right knee as your right foot makes contact with the ground. Quickly extend your right knee (eg, straighten your right leg) and return your body to the initial, full-standing position. Repeat with your left leg.
Then, from the starting position (feet shoulder-width apart and pointing straight ahead, hands up in front of shoulders), step directly to your right with your right foot into a lateral-lunge position. Your upper body should also face to the right, and it should lean forward over your right leg at approximately 45 degrees from the vertical position. Again, drop your hands on either side of your right knee as your right foot makes contact with the ground (your left foot should remain pointing straight ahead). Quickly extend your right knee and return your body to the starting position. Repeat the same motion with your left leg moving to the left.
Finally, from the starting position, twist your body around at the hips and step diagonally and to the rear with your right foot into a backward-lateral-lunge position. Your upper body should face to the right-rear at about 'four o'clock' position, and it should be inclined over your right leg at approximately 45 degrees from the vertical. Again, drop your hands on either side of your right knee as your right foot makes contact with the ground (your left foot should remain pointing straight ahead). Quickly extend your right knee and return your body to the starting position. Then, repeat the overall motion with your left leg, moving it to the left-rear ('eight o'clock') position.
Repeat this entire sequence (forward right leg, forward left leg, right-side right leg, left-side left leg, back-and-right right leg, back-and-left left leg) three times for a total of 18 repetitions of stepping. Rest for 30 to 60 seconds, carry out the 18 reps again, rest for 30 to 60 seconds, and perform the 18 reps one last time, for a total of three sets. Progressively toughen the exercise over a period of four to six weeks by increasing the speed of the exercise and also the resistance held in your hands (use small dumbbells weighing up to 10 pounds).
Why does the six-way lunge help prevent ACL problems? This key exercise stretches and strengthens the hamstring muscles on the back of the thigh in all three planes of motion (sagittal, frontal, and transverse). Strong and flexible hamstring muscles give the ACL an assist in its job of controlling the knee joint and preventing the tibia from moving excessively during knee flexion.
Exercise 2: The One-Leg Squat with Lateral Hop
To carry out this exercise, stand with your left foot forward and your right foot back, with your feet about one shin-length apart (your feet should be hip-width apart from side to side). Place the toes of your right foot on a step or block which is about six to eight inches high. Most of your body weight should be directed through the heel of your left foot.
Bend your left leg and lower your body until the left knee reaches an angle of about 90 degrees between the thigh and lower part of the leg. Then, hop upward and laterally, so that your left foot lands about six to eight inches to the left of your take-off point. Upon landing, immediately descend into another squat and again hop upward, but this time hop back to your original take-off point (six to eight inches back to the right). Finally, hop to the right a distance of about six to eight inches, descend into another squat, and then hop back to the starting, straight-ahead position. Be sure to maintain upright posture with your upper body as you do this, and hold your hands at your sides throughout the squatting and hopping movement. Complete a total of 12 lateral hops (to the left and to the right) with your left leg before switching over and doing the same thing with your right leg. Perform a total of three sets with each leg, with 30 to 60 seconds of rest in between.
Make certain that you perform these one-leg squats with lateral hops only on soft dirt, an aerobics floor, a wooden gym floor, a grassy surface, soft dirt, a rubberized track, or some other resilient surface which provides some 'give.' Hopping repeatedly on concrete or asphalt may increase the risk of overuse injuries to the lower part of your leg.
Why are the one-leg squats with hops important? Sudden changes in direction while running and jumping can cause injury to the ACL due to the increased stress placed on the knee. Lateral hopping movements help prepare the ACL and muscles around the knee for these sudden (and often unpredictable) movements in the frontal (side-to-side) plane. Even if you're not very worried about your ACLs, this is a great exercise!
Exercise 3: Zig-Zag Runs
Start by running at half-speed straight ahead for about five metres. At the five-metre mark, cut quickly to your left for several strides by pushing off your right foot and moving in a left-forward (diagonal) direction. Then cut back toward the right for several strides by pushing off your left foot and moving in a right-forward (diagonal) direction. Repeat this sequence for a total of eight to 10 cuts (four to five to the right and four to five to the left). Perform three to five sets of this exercise with a break of 30 to 45 seconds between each run.
Gradually increase the intensity of your zig-zag runs over a period of four to six weeks by running faster and also by changing the number of strides between direction changes (vary the number of strides between one and five). If you participate in sports which require running backwards (eg, rugby, soccer, and basketball), complete some of the sets in a backward direction.
Zig-zag runs help develop the balance and body control required to move in multiple directions at various speeds. These runs require the knee joints to move through a number of different angles and directions, thus mimicking movements which can lead to ACL injury in unprepared athletes.
Exercise 4: The High-Bench Step-Up
Yes, this old 'stand-by' for runners is great for warding off ACL difficulties. Begin from a standing position on top of a bench which is approximately knee high, with your body weight on your left foot and your weight shifted toward your left heel. The right foot should be free and held slightly behind the body. Then, lower your body in a controlled manner until the heel of the right foot touches the ground, but support all of your weight on your left foot. Return to the starting position by driving down with the left heel (the one that's on the bench, not the one which hits the floor) and straightening your left leg. Repeat for a total of of 10 to 12 repetitions, and then switch over to the right leg. Maintain absolutely upright body posture with your trunk throughout the entire movement, with your hands held at your sides (with or without dumbbells).
Make this exercise progressively more difficult over time by increasing the resistance (up to 20-pound dumbells may be used) and by increasing the height of the step (up to mid-thigh height). Increasing the height of the step increases the involvement of the hamstring muscles on the back of the thigh; these are the muscles which function in tandem with the ACL to stabilize the knee joint. Strong yet flexible and coordinated hamstrings help minimize the risk of ACL injury.
Why female ACLs are more vulnerable
One of the curious aspects of ACL problems is that they tend to occur more frequently in female athletes, compared to males. For example, research carried out by Elizabeth Arendt, MD, and Randall Dick, the National Collegiate Athletic Association's Assistant Director of Sports Sciences, has shown that female collegiate soccer players have twice as many ACL injuries as males, while female basketball participants have three times as many serious ACL maladies as their male peers.
The reason for this is unclear, but some sports-medicine experts theorize that it is the result of anatomical differences. For one thing, the 'intercondylar notch' - a small chasm at the bottom of the femur through which the ACL passes - tends to be smaller in females. Theoretically, during cutting and jumping movements, the narrow female notch may fray and weaken the ACL.
Another female anatomical characteristic - the wider pelvis - may also be a contributing factor. The wider female pelvis tends to aggrandize the angle made at the knee between the femur and tibia when the foot is planted on the ground during running, increasing both inward pressure on the knee and external rotation of the tibia. These two factors seem to put the female ACL under greater stress, compared to the more injury-free male structure.
In addition, it's possible that the ACL is considerably more lax in females than males. As it turns out, there are both oestrogen and progesterone receptors on the ACL; increases in one or both of these hormones may slacken the ACL, heightening the risk of damage (if this is really true, female athletes would tend to be at higher risk during certain stages of their menstrual cycles). It's known that a female's ligaments tend to loosen up as a result of the hormonal changes associated with pregnancy, so this theory is not too far-fetched.
Studies carried out at the University of Michigan have found that female athletes have more knee-joint laxity, less strength in their leg muscles, and slower muscle-reaction times, compared to males, all factors which would increase the risk of ACL trauma. Of particular interest is the reaction-time disparity: it's clear that to optimize your chances of keeping your ACL intact, you need to do more than simply boost the strength of your hamstrings; you also should increase the speed with which they react to cuts, landings from vertical jumps, and sudden stops. If they're very strong but react slowly, they may still be unable to support the ACL in time to avoid injury (that's why gradually increasing the speed of movement of our protective exercises is so important).
Overall, the exercises presented above should help keep your ACLs from blowing up. If you're a female athlete, it particularly behoves you to make the exercises part of your regular strength training.
Text: Owen Anderson
Exercises: Walt Reynolds