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Squats verses Leg Extensions: Open and Closed Kinetic Chain and The rationale for Strength and Function

By Michael P. Keenan, B.S. C.F.T.

A great number of my clients wonder “why am I squatting, and why does my trainer tell me it’s the most important exercise in my routine?” The answer is simple. Out of all functional tasks we perform each day, the “sit to stand” activity predominates them all. I explain to my athletes, the squat prepares you for explosive speed and balance. To my obese clients, the squat uses your hips and thighs, which contain the greatest amount of muscle mass, which will help to burn the greatest number of calories. To my senior exercise clients, I explain that the squat will help them stand up off the toilet, which means they will not have to shout for help from the bathroom! What I will demonstrate more objectively is why the squat is more effective than the leg extension exercise.

First, let’s look at the open verses closed kinetic chain model definition. Open kinetic chain means that the tibia is moving upon a fixed femur, such as in a leg extension or leg curl. There is no force being applied under the foot, therefore, the chain is “open”. Closed kinetic chain means the femur is moving upon a fixed tibia, with the ground or other support (such as a step or leg press platform) under the foot, so the chain is closed. When the chain is closed, the integrity of the joint is more stable. Joint compression forces between the femur and tibia are higher, which keeps the femoral condyles seated properly in relation to the medial and lateral menisci. The quadriceps and hamstrings are co-contracting, decreasing the shearing forces on the meniscus and articulating cartilage, and with proper technique, shearing forces to the posterior patella are minimized. If we were to look at force vectors about the knee, we would see higher compression forces and lower anterior translation forces at the joint during knee extension with a closed kinetic chain activity as opposed to the open kinetic chain. Let’s look at what happens at the knee during an open kinetic chain leg extension.

When the knee is bent to 90 degrees, we know that there are more joint compression forces than anterior translation forces. When the knee begins to extend, there is a sharp change in force vectors about the knee. With each degree of extension, joint compression decreases while anterior translation forces increase. This action repeated over time and with high levels of resistance can have negative effects on the knee joint, namely the menisci, the anterior crusciate ligament and the patella femoral joint. We know that when a muscle contracts slowly, and with high tension, there is a reciprocal inhibition of the opposing muscle group. So now the hamstring muscles relax, allowing for greater anterior translation forces to be applied to the joint. This causes excessive strain on the anterior crusciate ligament and shearing forces to the posterior horn of the menisci. In most cases. the vastus lateralis is allowed to predominate in the extension moment, causing excessive lateral glide of the patella, resulting in a lateral tracking dysfunction. This is especially important for the female client. Females tend to have wider hips, and there fore a larger “Q” angle. “Q” angle is measured from the Anterior Superior Iliac Crest, to the Patella, then to the Tibial Tubercle. A Q angle of greater than 15 degrees will result in a lateral tracking patella with open kinetic chain activities. Those who do not understand this mechanism tend to try to remedy this dysfunction by strengthening the quadriceps in the open kinetic chain position, further exacerbating the condition.

I believe there is a risk to benefit ratio with all exercises. In the body building world, I understand the need for isolation exercise. However, in the general fitness, athletic and medical fitness model, it appears that closed kinetic chain activities offer the highest benefit with the lowest risks. When working with a client with a history of patella femoral syndrome, meniscus tear and anterior crusciate repair, we would say that open kinetic chain knee extension is contraindicated. So what is the proper activity for this individual? Squats, static lunges and step ups. Let’s look at some of the absolutes with relation to this activity. First, just like every activity, there is a progression. The leg press allows the client to work with less than their actual body weight, so we usually begin with this. This is followed by using a physioball placed against a wall, and the client performs squats while leaning back onto the ball. Once the client can perform with activity with proper technique and range of motion, we move away from the ball and perform body weight squats. The next progression is adding weight with a dumbbell, usually held against the chest and placed under the chin. The final progression is placing a bar across the shoulders, which allows for the greatest resistance to be applied to the exercise.

The second absolute is the balance component. Gray Cook has been quick to point out in his research just how reaction forces with cause the body to balance itself without conscious thought. I agree. When squatting, your muscles will work in symmetry with each other, hamstrings, quadriceps, gluteals, back extensors and abdominals all co-contract in concert to perform the activity. Smaller assisting muscles contract to fine tune the movement, muscles that can not be isolated in an open kinetic chain knee extension exercise.

The third component involves energy usage and efficacy of exercise. Obviously, more energy will be required for a squat than a leg extension activity. Weight loss is the number one reason people seek out a personal trainer, so choosing the proper exercise for this purpose is paramount. With concerns to efficacy, the squat is a functional activity, you need to lift, sit, stand and carry in life, whereas in life no one needs to perform high resistance knee extension in the open chain position.

The fourth component is technique and functional range of motion. In general terms, I want my clients to be able to squat to 90 degrees of hip and knee flexion before proceeding to the next level of progression. To allow progression before this is mastered in my opinion is asking for trouble. It gives a false sense of security, balance and proprioception is not properly trained, and more weight will not help this lack of progress. Safety, form, range of motion and balance are the indicators for progression.

I believe that when designing your exercise program, you must look at what are your goals, and then decide which exercises will help you achieve them. My hope was to explain how the squat differs from the leg extension, choose wisely and let your body be your guide.

About the Author:

Michael Keenan is the owner/director of Fitness Together in Manasquan and Lake Como New Jersey. Michael is a Certified Fitness Therapist, and can be reached at www.ftmanasquan.com.

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