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    Default Leg Extensions: Counter Productive?

    To start, let's look at the knee, which is a biaxial joint- meaning it moves in two planes of motion, not a uniaxial joint like the elbow, which is a hinge joint and only moves in one plane of motion. The elbow's only movements are flexion and extension in the sagittal plane. That's all it can do, because any other movements are limited by the olecranon process of upper ulna fitting into the olecranon fossa of the humerus.The articulation of these two bony landmarks limits motion to only flex-ext.


    The knee is a different story. It is a biaxial joint allowing flexion and extension in the sagittal plane, but also some internal and external rotation in the horizontal plane. The tibia and fibula are not directly articulating on the femur, so what prevents hyperextension, anterior & posterior translation of the lower leg, or excessive varus or valgus? A few things- dynamic stabilization of antagonist muscles, and the 4 main ligaments of the knee- the two external (MCL and LCL) and the two internal (ACL & PCL).


    So what causes the knee to extend? Primarily, the four muscles of the quads: the Vastus medialis, lateralis, intermedius- which run from the femur to tibial tuberosity; and the Rectus femoris, which runs fro the anterior-inferior iliac spine to tibial tuberosity.


    Any time that the quads work to extend the knee, they do so by pulling the tibia forward. This is called anterior translation. If the ACL wasn't inside the knee, the tibia would literally move forward away from the patella and rest of the knee joint. That's the primary function of the ACL- limiting anterior tibial translation.


    So, how do we screw that up, training for looks or the "pump" instead of training our bodies the way they're supposed to be trained? We do tons of squats, leg presses, lunges, and (*shudders*) worthless leg extensions (more on those in a bit). Then, as an afterthought, we do a little bit of hamstring work, and call it a day.


    This is a recipe for knee problems of the ultimate degree. Dominant quadriceps activation that is not offset by appropriate hamstrings activation will create anterior tibial translation and increase ACL loading. The ACL can only handle so much stress before it ruptures- partially, fully, or with a tibial avulsion (an avulsion is an injury in which a body structure is forcibly detached from its normal point of insertion- in other words a chuck of the tibia just pops off with the piece of the ACL that didn't tear).


    Because of this, in any program, adequate direct hamstring work is a must. It should be comparable in volume to quad-dominant work.


    When working the hamstrings, deadlifts are not enough. If you look at the three muscles that make up the hams- the semitendinosus and semimembranosus originate on the ischial tuberosity and attach to the tibia, while the Biceps femoris long head originates on the ischial tuberosity and attaches to the fibula and the short head originates on the femur and attaches to the fibula. Got all that? What it means is that while two of the three muscles of the hamstrings are responsible for hip extension (ie, deadlifts), the other one, is more active in knee flexion. Any hamstring work that does not involve some sort of leg curls, is not only incomplete, but not sufficient to offset quad development.


    So- about leg extensions. IMO, they're functionally worthless. For starters, almost nothing that the lower body does in real life is an open kinetic chain movement, but it goes beyond that. Remember the RF originates on the hip? Well there's a principle in kinesiology called Active Insufficiency. Active insufficiency refers to over-shortening of a muscle that crosses two joints, and isn't able to adequately do it's work at either end because of this over-shortening. When hip is flexed (ie, you're sitting there in the leg extension machine), the RF is actively insufficient and unable to properly extend the knee. So, not only are extensions open kinetic chain, they're also not optimal from a hypertrophy perspective of the quads. Lastly, when you're sitting there extending, the knee is under much more stress than it needs to be for two reasons- first, the force on the knee is shear from a weight placed at the most distal portion of the lower extremity, causing a ton of torque to the knee (not compressive force, as in a squat or leg press); second, because the hamstrings are completely removed from the movement, anterior translation of the tibia is occurring with nothing to counter-balance it.


    All of the information here was taken from my collection of physical therapy and PTA textbooks. While I appreciate your skepticism, this is isn't new information or "today's consensus," it's basic anatomy and physiology that has been in the clinical literature for quite some time. You're not often going to find a "study" demonstrating something that is already accepted by the clinical community.


    here's a few studies demonstrating shear forces between squats and leg extensions: http://ajs.sagepub.com/content/24/4/518.short


    "An anterior shear force (anterior cruciate ligament stress) was noted during open kinetic chain knee extension from 40° to full extension; a peak force of 248 ± 259 N was noted at 14° of knee flexion."


    http://www.ncbi.nlm.nih.gov/pubmed/8501090


    "The closed-kinetic-chain exercise produced significantly less posterior shear force at all angles when compared with the open-kinetic-chain extension exercise. In addition, the closed-kinetic-chain exercise produced significantly less anterior shear force at all angles except 30 degrees when compared with the open-kinetic-chain flexion exercise (p < 0.05). Analysis of tibiofemoral compression forces and electromyographic recruitment patterns revealed that the closed-kinetic-chain exercise produced significantly greater compression forces and increased muscular co-contraction at the same angles at which the open-kinetic-chain exercises produced maximum shear forces and minimum muscular co-contraction."


    http://www.azpt.com/Quadriceps%20Act...20Exercise.pdf


    "Exercise in closed kinetic chain promotes more balanced initial quadriceps activation than does exercise in open kinetic chain. This may be of importance in designing training programs aimed toward control of the patellofemoral joint."


    Role of the hamstrings (which are taken out during leg extensions): http://vbschools.net/moodle/pluginfi...%20article.pdf


    "Stress on the ACL during flexion is significantly alleviated by contraction of the hamstrings,
    which exert a posteriorly directed force on the tibia and thus share in the burden of reducing anterior translation."
    Last edited by The Solution; 02-28-2015 at 06:51 AM.
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