Q&A: Leg Strength Science - Enhance Performance and Avoid Injuries

Hey Lyneil,

I have a PT related question for you. When I was going through physical therapy after my knee surgeries, the PT's told me that leg extensions are bad for my knee and I should stay away from them.

I'm getting back into lifting on a regular basis, and I read in muscle & fitness that the only problem with leg extensions is that they don't work the tear drop shaped muscle on the inside of the quad above the knee, which helps stabilize your knee. It said that they're fine to do as long as you're doing exercises that will work that muscle, such as squats in a smith rack. Do you agree with that? And if so, are there any other exercises that you suggest I do for that muscle?

-Bosko

Response-

First, excessive stress to the patellofemoral (knee cap and thigh bone) joint is the primary reason that knee extensions are not recommended by physical therapists or in many rehab protocols for the lower extremity.  The anatomy of the knee is much different than many of the other joint of the body because it has the patella lying over top of it.  The patella is connected to the tibia and the quadriceps muscle by the patellar tendon.  When a force is placed at the end of a lever versus closer to the axis of motion(the joint) the force is considerably increased.  It is like holding a weight in your hand far from your body with your elbow straightened.  It will make your shoulder and arm muscles considerably more fatigued than if you hold it close to your body.   It's physics. The lever is long, the force at the knee is exponential relative to the load applied at the end of the lever.

The patella tendon exerts force on the tibia and is enhanced by the increased distance away from the knee joint secondary to the patella.  This creates a mechanical advantage for the patella tendon allowing less muscle force to tolerate force creation and absorption.  However, when the load is placed further from the body this can cause excessive compression and shear forces at the patella.  This can eventually lead to break down of the soft tissue and stress reactions to the bony structures of the knee especially the patella tendon.

Following a knee surgery the quadriceps muscle is not functioning appropriately as many people have experienced.  This also occurs when the knee joint experiences trauma.  This phenomenon is called reflex inhibition.  The phenomenon occurs because of swelling in the knee joint.  There is a reflex between the joint capsule and ligaments of the knee and the central nervous system that occurs to allow continuous tension to the quadriceps muscle. The swelling that occurs following knee surgery or injury places pressure to the ligaments and joint capsule.  When pressure is place upon the tissue it distends and the small receptors inside the tissue are activated.  Nervous system receptors can only tolerate so much activity before they shut down.  This is called inhibition.  When they shut down so does the reflex loop.  Therefore the quadriceps muscle does not fire appropriately. 

Most times this does not completely shut down all of the muscle fibers.  Some of them are still working which allow the knee to function in a limited capacity.  Many believe this is a survival mechanism that causes the person to have to immediately limit the forces they are able to exert on the injured lower extremity.   The force that is placed upon the knee during knee extension with the weight placed at the end of the shin (a long lever arm) eventually causes stress injury to the soft tissues of the knee that are healthy. This will also cause further detriment to the surgically repaired or damaged tissues that are healing.  Therefore this exercise is not recommended for someone during a rehabilitation of the knee.

From a performance and strengthening standpoint, knee extensions are also inefficient for several reasons.   Less weight is able to be used during knee extensions than squatting or lunging motions which we term closed kinetic chain, CKC (feet are on the floor and multiple joint are moving at once).  During CKC movements greater forces are placed upon the muscle and therefore the muscle will adapt to these forces for increased strength gains.  You may ask if this increase in force can be detrimental?  It can be if poor technique is utilized but that is a topic for another day.  The mechanics are different because the direction of the force is directed close to our center of gravity and not distally as in the knee extension.  This places significantly less compressive and shear forces to the patellofemoral and knee joint.  Therefore we get better improvements in strength and less chance for wear and tear.

The vastus medialis, aka tear drop muscle, is one of the four muscles that comprise the quadriceps muscles that blend into the single patella tendon.  Therefore we have four muscles that exert force on one tendon.  The rectus femoris is a two joint muscle because it attaches at the front of the pelvis just superior to the hip joint and assists with hip flexion.  The other three muscles attach to the femur are one joint muscles.  There have been many studies that show a deficiency of muscle firing in the vastus medialis in people with patellofemoral and knee joint pathology.  There have also been many studies that attempt to preferentially cause the vastus medialis to fire through different types of exercise.  Two of the primary myths that exist are squeezing or pinching the knees together or externally rotating the knee to the outside during squatting exercises.  Neither of these has been shown to be effective.   Most of the muscles which exert force on the knee pull laterally on the patella.  The vastus medialis is the lone, small, muscle that supports the patellofemoral joint medially with dynamic muscle contraction to balance the forces.  If it is weak or not firing it can cause abnormal forces to the patella and potential injury. 

Open chain exercises (feet not on the floor and one joint moving at a time) for the lower extremity is not very efficient for functional development either.  Unless you need to develop strength in a seated position to kick something, the exercise is not functional.  The lower extremities were designed to create and absorb force while in contact with the ground such as in running, jumping, lunging or a double leg take down which I know you know nothing about!(Brendan and I wrestled together in college for those of you who do not know)  Even during most kicking activities, most of the force is created by the trunk and hip muscles.

In conclusion, it is recommended to perform CKC exercises for lower extremity development.  The best exercises are done in single leg positions because the instability and isolation will cause enhanced recruitment of muscle fibers of the lower extremities.  When performed with good technique, this can significantly increase strength and functional ability.  If you are going to utilize machines, hack squats and leg presses are best.  The should also be performed unilaterally to enhance recruitment and functional ability.  If you have enough strength developed it is also wise to incorporate speed into your CKC movements with lighter resistance of course.  The quadriceps muscles are heavily distributed with fast twitch muscle fibers and recruitment can be significantly enhanced by doing plyometrics or adding speed to the movements.  This should only be done if you have first developed considerable strength in CKC exercises.  Also, doing OKC hamstring exercises are not typically thought to be potentially injurious and can significantly increase strength.  I do however recommend strengthening the hamstrings with CKC exercises as swell such as swiss ball hamstring curls and Russian hamstring curls/glute ham machine.  I hope this helps.

Here is an example -

Split Squats 1 set of 3 and 3 sets of 5 with 2 minute rest period between each set followed by 3 sets of 12 Lunge Jumps with a 1 minute rest period between each set.                                        

Reverse lunges off of a 12 inch box 1 set of 3 and 3 sets of 5 with a 2 minute rest period between each set followed by 3 sets of 12 Box Jumps or jump squats with a 1 minute rest period between each set.

Lyneil Mitchell, DPT

A sound mind in a sound body is a short, but full description of a happy state in this world – John Locke