E4 Joints Levers

Topic

In general, this joint acts as a lever that modifies the relationship between the muscular force—specifically the force defined in another model of this system—and the resulting force exerted on an object external to the body.

Within biomechanics, this joint is classified as a first-, second-, or third-class lever, depending on the relative positions of the joint fulcrum, the muscular input force, and the resistance of the external load. Consequently, most human joints function as third-class levers, with the muscle inserted close to the joint fulcrum; this sacrifices mechanical advantage in exchange for greater speed and a wider range of motion at the distal end.

The joint's mechanical advantage is the ratio of the muscular force arm to the external load resistance arm—specifically, the lever arm of the muscular force defined in another model of this system acting on a particular bone. If the muscular force arm is shorter relative to the external resistance arm, the joint's mechanical advantage is correspondingly lower.

The torque generated within the joint determines the mechanical demand placed on both the articular cartilage and the surrounding periarticular tissue. In a joint like the knee, for instance, the compressive force can exceed the body's total weight many times over during a landing—an extraordinary demand on the articular cartilage that surpasses even the extreme tension placed on the heel tendon (as described in another model of this system). Consequently, in this model, the joint acts as a lever that transmits muscular force to the cartilage itself; this causes the hyaline cartilage to behave as a biphasic material—comprising a solid phase and an internal interstitial fluid—identical to the articular cartilage involved in wear-related degradation models. The effective stiffness of the hyaline cartilage increases with the rate of the applied load, driven by the hydrostatic pressure of the interstitial fluid within the cartilage. A higher loading rate results in even greater effective stiffness, thereby providing the joint with enhanced protection against sudden impacts.

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