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An ACL patient is doing leg extensions to strengthen his quadriceps muscles. With no additional weight…
An ACL patient is doing leg extensions to strengthen his quadriceps muscles. With no additional weight added, the patient’s lower leg (shank+foot) has a mass of 6.25 kg. The center of mass of the lower leg segment is located 32 cm from the axis of rotation of the knee joint. The leg is flexed 25 degree down from horizontal. The quadriceps muscle acts through the patellar tendon, which inserts 5 cm from the axis of rotation of the knee joint and is oriented at an angle of 10 degree with respect to the tibia, as shown. The motion is performed slowly enough that we can assume (quasi) static equilibrium. How much force (F_q) must the quadriceps exert in order to support the weight of the limb? What are the reaction forces at the knee (R_x and R_y)? How do the magnitudes of these force compare with the load imposed by the weight of the limb? Do your values seem reasonable? Why or why not?
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