Figure 4.9 Mechanical advantage. (A) The quadriceps has less of a mechanical advantage than the force of the weight because of its shorter moment arm. The muscle group will have to generate more force than the resistance if the muscles are going to extend the knee through its range of motion into extension. (B) The gastrocnemius force has a greater mechanical advantage because of its longer moment arm compared with the body weight’s moment arm. This second-class lever system allows the muscle to generate less force and be more efficient in the amount of force needed to lift the heel. However, the amount of displacement (the distance the heel is lifted off the surface while toes remain in contact) is less compared with the amount of displacement seen in (A).
QLf
AB
Figure 4.10 Sagittal T1-weighted MRIs of the knee. (A) The line of pull and moment arm of the quadriceps muscle without the patella. (B) The patella functions as a pulley by deflecting the line of pull of the muscles away from the joint axis, effectively increasing the moment arm of the muscle’s force. (From Levangie, P, Norkin C. Joint Structure and Function: A Comprehensive Analysis, 5th ed. Philadelphia, PA: F.A. Davis Company, 2011: p. 41, with permission.)
