The forces encountered at the knee during resistance training can be classified as either shear or compressive. Consider the femur (thigh bone) and its relation to the tibial plateau (the top of the lower-leg bone) upon which it sits. When the femur presses directly down on the tibia (as it does when you are standing upright), the force is compressive: Its directed so that bone-to-bone contact provides integrity. A shear force is applied in a direction that would cause the femur to slide perpendicular to the tibia were it not held in place. Fortunately, strap-like connective tissue segments called ligaments prevent this dislocation from occurring, but this type of force places the joint under considerable stress. It should definitely be avoided, especially if your knees are susceptible. Preventing a dislocating shear force at the knee requires paying attention to the position of your lower leg as you perform compound leg exercises (movements that involve both hip and knee motion). Isolation exercises like leg extensions, where motion occurs at the knee exclusively, inherently involve shear force at the knee, since the resistance must be applied perpendicular to the long axis of the bone.
If youe squatting, for example, your lower leg should stay close to perpendicular to the floor throughout. If it proceeds forward, the force becomes shearing and injury potential is increased. A good rule of thumb when performing squats and leg presses is to prevent your knee from passing in front of your toes as you descend. If your knees pass over your toes on a leg press, simply place your feet higher on the platform of the carriage. If they are positioned properly, your lower leg will stay perpendicular to the platform throughout. Moving your feet forward also reduces shear force on squats performed on a hack squat, horizontal squat or Smith machine. If your compound movement of choice for legs is free-weight squats, repositioning your feet in this manner is impossible (you must maintain your balance and if you move them forward, youll fall rearward). Instead, shear force is avoided in this case by coordinating motion at the knee and hip. Descending with the bar positioned directly above the middle of your femur should guarantee correct alignment. If you try to keep it further back (maintaining a more upright torso posture, which will shift the emphasis to your quadriceps), it will be impossible to keep your lower leg from moving past 90 degrees in relation to the floor.
Another important consideration for injury prevention is maintaining proper frontal plane alignment at the knee as motion occurs. This simply means that whichever way your toes are pointing, your knees should follow suit. This might seem like an easy posture to ensure, and it usually is as you descend. As you come back up, however, its a much greater challenge. When the ascent is difficult and you hit a sticking point, its typical to allow the knees to cave inward. This creates a favorable line of pull for the stronger part of your quadriceps to help you through the rep. Unfortunately, this twisting motion is a surefire way to injure your knee. Specifically, a cartilage tear is a distinct possibility. In addition, any time an abnormal position is assumed (the knee is a uniplanar joint not meant to articulate in the frontal plane), ligaments are stretched and they do not possess elastic tendencies. As a result, stretching them on a repeated basis will cause them to remain lengthened and compromise joint integrity.
Ligaments are also over stretched when excessive depth is achieved on squats and presses. Simply squatting shallow, on the other hand, is less effective for enacting neuromuscular gain. A thorough evaluation of your specific benefit-risk ratio will help you decide how deep you should go. A competitive athlete, for example, might be better served to squat deeper to maximize adaptation, especially if their competitive event involves assuming a similar posture. A weekend warrior with knee problems, on the other hand, would be better off playing it safe and not exceeding a depth where their thigh is parallel to the floor.
Another common leg training execution error involves the heel picking up during the descent on free weight squats. The culprit in this case is usually tight hamstrings and calves. Due to their shortened status, these muscles do not have sufficient length to accommodate the movement range being covered, so the heel is pulled upward through the Achilles. Stretching exercises designed to enhance movement range might help, but I have found that some people are simply not built to squat in this manner. For these individuals, machine squats and leg presses can be substituted. These allow the foot to be placed further in front of the body, thereby decreasing the degree of dorsiflexion at the ankle during the descent. As a result, the foot can be kept flat.
Fred DiMenna, a Certified Strength and Conditioning Specialist and Lifestyle and Weight Management Consultant, is a two-time Natural Mr. United States and a WNBF drug-free professional bodybuilder. Visit him on the web at www.freddimenna.com or email him at: firstname.lastname@example.org.