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Muscle Antagonists & Agonists

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Now I want to add the muscles

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because there are important regional muscles that

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that can be seen about the knee.

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I'm showing you a very stylized picture, a sagittal drawing

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of the knee with a circle for the femur above

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and kind of a box for the tibia below.

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And to it, I've added the anterior cruciate

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ligament and one muscle.

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This is the semimembranosus muscle complex attachments,

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but mainly to the posterior surface of the proximal tibia.

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An antagonist muscle resists a specific movement

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of the joint and agonist muscle

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aids a specific movement of the joint.

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So if we talk about anterior translation of the tibia,

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this muscle, it resists that particular movement.

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It's an antagonist to the movement of anterior translation,

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but it aids the anterior cruciate ligament.

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So we could say it's an agonist to the ligament.

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Let's add a couple different ones here.

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I'm gonna show you now the gastroc anemia attaching

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to the posterior surface of the femur

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and the quadriceps,

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including the patella tendon extension

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attaching to the tibia.

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Now these muscles, again with regard to anterior translation

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of the tibia, all right, are agonists to the movement.

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All right? They're helping the movement,

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but they are antagonists

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to the anterior cruciate ligament, okay?

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They're acting against that ligament.

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So you could imagine with over action, particularly

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of the quadriceps muscle, we might in fact deal with a tear

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of the anterior cruciate ligament.

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So here I just wanted to show you what might occur.

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Let's put tensile force on the quadriceps mechanism,

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and we can see now, okay, that it is an antagonist

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to the anterior cruciate ligament,

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and hence we may get a tear of the anterior cruciate.

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I wanted to illustrate that with a case.

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You're gonna see several examples

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of cases in the two lectures I'm gonna give now.

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Uh, most of the figures

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that you see will come from the internet,

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although the histories are accurate.

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So here we have a female long jumper who landed awkwardly

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with her weight too far back,

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and her knees flexed about 70 degrees.

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Because of that, she activated her quadriceps muscle

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and in an, in an attempt to limit or even stop the fall,

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and we can see here a complete tear

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of the anterior cruciate ligament.

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I.

Report

Faculty

Donald Resnick, MD

Professor Emeritus, Department of Radiology

University of California, San Diego

Mini N. Pathria, MD, FRCP(C)

Division Chief, Musculoskeletal Imaging

University of California San Diego

Eric Y. Chang, MD

Adjunct Professor, Radiology

University of California, San Diego

Brady K. Huang, MD

Clinical Professor of Radiology

UC San Diego Medical Center

Tags

Musculoskeletal (MSK)

MRI

Knee