Exercise Analysis - Leg Extensions

Gym lore is a funny thing and this is apparent when you consider an age-old tenet bandied about the weight room. When it comes to training legs, squats have been ordained king of all exercises with leg extensions relegated to pauper status. In my opinion, this caste system is somewhat unfounded.

Squatting is classified as a compound (multi-joint) exercise because it involves motion at both the hip and knee. Consequently, the quadriceps (muscles of the front thigh), hamstrings (muscles of the back thigh) and glutes (muscles of the butt) are all active. So, if your criterion for gauging exercise superiority is the ability to use a single one to work the vast majority of the lower body, it’s justified to hold squats in such high esteem. But if you want to target any of these groups specifically, squats take a backseat and a single-joint isolation movement like leg extensions reigns supreme.

A leg extension is any exercise where the leg is extended at the knee against opposition provided by a load applied to the lower leg. This movement is typically done on a leg extension machine, which is a staple of any well-equipped gym. Leg extensions provide the ability to isolate three of the four front-thigh muscles collectively known as the quadriceps. These three muscles work together to extend (straighten) the leg when the hip is flexed (bent). The vastus lateralis is located on the outer portion of the thigh bone (femur), while the vastus medialis is situated on the inside. The vastus intermedius lies between the two. The fourth muscle of the quadriceps (the rectus femoris) also extends the knee, but its contribution during conventional leg extensions is minimal because it crosses the hip, so it is shortened and rendered inactive when you are seated.  

Old school lifters often eschew leg extensions or consign them to secondary status by performing them as an afterthought following compound movements like squats and leg presses. However, putting emphasis on leg extensions is warranted in many cases. A major limitation associated with all compound exercises is that the weakest link in the chain of muscles being used is the one that will break first and be the only one to receive optimal stimulation. In addition, subtle technique modifications can be unknowingly employed to switch emphasis to stronger areas when motion at multiple joints is present. With this in mind, it’s easy to understand why the degree of stimulation the aforementioned extensors of the knee receive from compound movements like squats is highly dependent on who is performing the exercise and the specific technique they employ.

Training the three vastus muscles of the quadriceps is important for ensuring optimal knee function. For example, the vastus medialis is typically implicated as a weak link when the malady known as patellofemoral syndrome is present. In this case, the kneecap deviates to the outside of the thigh because insufficient stabilization is provided by the muscle opposing this shift. When the kneecap tracks laterally in this manner, its underside becomes irritated. Strengthening the vastus medialis and improving range of motion in the outer thigh muscles that pull the kneecap away from its normal position is the age-old remedy for addressing this painful affliction.

Leg extensions are an effective isolation exercise that can be used specifically to strengthen important muscles that help to keep your knees healthy. However, a Catch-22 exists because the loading pattern associated with this exercise can be problematic when injury is already present. With leg extensions, the load is applied at a right angle with respect to the lower leg bone (tibia). Consequently, a shear force is present at the knee. Conversely, if the feet are positioned appropriately with respect to the hip, compound movements involve a compressive force because the opposition is directly in line with the tibia. This is less dangerous because it ensures that the opposing load will serve to compress the joint.  

A shear force at the knee is particularly problematic when ligament injury is present. Ligaments attach bone to bone, so a load that separates the tibia from the femur will stress these critical soft tissue elements. Exercisers suffering from patellofemoral syndrome might also experience discomfort when performing leg extensions. If excessive pain is present, a modified version where only the end portion of the movement range is traversed (terminal knee extensions) can be implemented. In addition, shear force can be reduced by shortening the opposing load’s moment arm (perpendicular distance between the line of application of the load and the joint). This can be done by applying the opposing load higher on the tibia (with ankle weights, for example) or by changing the angle of the thigh (angling it upward).

If you can perform leg extensions without pain or discomfort, there’s no reason to be conservative when using this movement. For example, even though it’s an isolation exercise typically thought of as a “shaping” movement, resistance that provides sufficient opposition should always be chosen. Just like every other resistance training exercise, the opposing load should cause exhaustion and an inability to continue to perform strict, full-range repetitions within two minutes. If this criterion is not met, minimal adaptive response will occur.

This article was originally published in New Living Magazine, which can be accessed on-line at www.newliving.com.




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