The anterior cruciate ligament (ACL)


ACL injuries–particularly in sports–are an all too-common occurrence. As a matter of fact, ACL researcher, Timothy Hewett estimates that between 75,000 and 250,000 of these types of injuries occur each year the United States. Within these numbers, female athletes suffer a disproportionately higher incidence of ACL injuries–as much as six to eight times higher than their male counterparts.


The ACL, or anterior cruciate ligament, is one of the primary ligaments that keeps your knee joint intact–it aids in the knee’s mobility and stability. Damage your ACL and you’ll more than likely not only experience knee pain, but also instability in the joint.

Open Versus Closed Chain Exercises

Knee joint and ACL rehabilitation exercises fall into one of two categories; open-chain and closed-chain. Open-chain leg exercises can be described as those where the foot on the exercising leg does not remain in contact with the ground or other stationary object throughout the exercise. An example of this type of exercise is the leg extension. Closed-chain exercises, on the other hand, are ones in which the foot on the exercising leg remains in contact with a stationary object, such as the ground, throughout the exercise. Examples of this type of exercise would be a squat or leg press. Generally speaking, closed-chain exercises are thought to be, by many experts in the field, safer for the knee and ACL because they cause less stress on those structures.
While much research has been developed on which of these types of exercises is best for ACL rehabilitation, there appears to be no clear-cut answer. However, one thing that many of the researchers do agree on is that no matter which type of exercise is used for ACL rehabilitation, an important key is to perform the exercise in a way that does not put undue stress on the ACL. Although open-chain exercises have been cited as putting tremendous stress on the knee and ACL, according to researcher G. Kelley Fitzgerald, both types of exercise can be used in ways that do not put undue stress on the ACL. For example, with open-chained exercises, such as the leg extension, Fitzgerald suggests that by limiting the range of motion for that exercise from the normal 90 degrees to approximately 45 degrees, less stress may be placed on the knee joint.

Closed Chain Exercises

Assuming you have been cleared by your physician to exercise after either a surgical or therapeutic intervention, squats are a good overall lower body strength exercise. Performing squats with your body weight or with added external weight such as dumbbells will help you improve your quad and hamstring strength. This exercise can be performed in a variety of ways, depending on your needs. For example, half or “mini” squats where you do not bend your knees to a full 90 degrees, wall squats or slides where you perform the squat with your back against the wall, and single-leg squats are all versions of this exercise.

Open-Chain Exercises

Quadriceps or “quad” muscle weakness is a major concern following an ACL injury, according to Fitzgerald. Isometric leg lifts are a good way to improve the strength in your quads while not creating the stresses on your knee and ACL typically associated with open-chain exercises. Leg extensions with a limited range of motion are another good quadriceps developer for your legs.


With any rehab work there is no one-size-fits-all exercise program. What’s important is that you listen to your physician, listen to your body, and take your time. The goal should be not only to recover from your ACL injury, but to strengthen your body so as to help avoid future injury.