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Date Posted: 05:10:57 10/20/04 Wed
Author: J.R.Smith, c.f.t.,s.f.t., p.n.s. - ISSA, USSA, ISFN
Subject: Leg Exttensions...maybe we should drop them?

Most studies on the effectiveness of muscle contraction use electromyography (EMG), which measures the electrical activity or excitability level of the muscles involved. The higher the electrical activity, the more effective the muscle contraction. In EMG studies, researchers can use the peak of the electrical activity (peak force) or, even better, calculate or integrate the full force under the curve. This improved method is called integrated electromyography, iEMG, and was used in the Cornacchia and LaFramboise (1998) study to evaluate muscle efficiency. The study determined which exercise yields the greatest percentage of iEMG for each muscle. The exercises with the highest percentage are considered the best for the particular muscle group.

With regard to the quadriceps the studies found the following.
*Saftey squats (90 degree angle) 88
*Seated leg extensions (toes straight)86
*Half Squats (90 degree angle)78/
*Leg presses (110 degree angle)76/
*Smith machine squats (90-degree angle)60.

Keep in mind that the study does NOT put into account the safety of each exercise. With regard to exercise safety consider the following. if you or your client has had a previous injury or has a pre-exsiting condition, then you may have to arrange the ROM to be commensurate with you or your clients abilities for leg extensions.

Being that the knee is stabilized on the anterior side by the quadriceps, on the medial side by the sartorius and gracilis, on the lateral side by the tensor fascia latae and on the posterior side by the hamstring muscle group from above and the gastrocnemius from below.

Because of the small angle of attachment of the quadriceps to the tibia, a large stabilizing component is always acting on the knee joint. This is particularly important when the hamstrings are contracting strongly and the knee is flexed beyond 90 degrees at which point the hamstrings have a backward dislocating component. To counteract this force there is usually hip flexion, which serves to maintain hamstring length so that tension is maintained.

When the leg is bent 80 to 90 degrees or more and the sartorius, gracilis and gastrocnemius muscles contract, they create a dislocating component at the knee joint. From 180 (straight leg) to 90 degrees of flexion, most of the muscles crossing the knee provide a rotary and stabilizing effect. When there is less than 90 degrees of knee flexion a dislocating component occurs in some of the muscles. Because the knee also has weak bony and ligamentous arrangements, it puts the knee in a weak position. This weak position in a flexed state is what caused the potential damage. Modifying this starting angle can limit any potential problems associated with the leg extension.

However you must also consider that most open chain exercises do not correlate with real-world movements. When was the last time you had to sit down and extend your knee (excepting your last leg workout). The squat, on the other hand, is performed daily anytime you bend your knees to lower your body followed by an extension (i.e. picking something up, assuming you are not using your back). Thus there is greater specificity when using closed chain movements in terms of applicability to the real world.

However, there are other differencs. During a leg extension, only the quadriceps are contracting as opposed to squats where both quads and hamstring are active. This has one very major consequence. During a leg extension, there is not only rotation of the shin about the knee during extension but also some linear translation of the shin. This puts amazing stress on the cruciate ligaments of the knee, especially at the beginning of the movement. However, during a squat, both the quads and hams are firing. Since they are pulling from opposite sides of the shin, there tends to be a lot less demand put on the knee as there is not as much translation.

However leg extensions do have their place, and are still used by mnay physical therapist and occupational therapist usually in rehabilitation and with clients in special populations. HOWEVER,if the leg extension machine is used, it is wise to use a smaller range of motion, perhaps the top third of the movement (from slightly bent to fully straight leg), and light weight to contend with the aforementioned limitations.

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