Answer: The large quadriceps muscle in the upper leg

Chapter , Problem 29

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The large quadriceps muscle in the upper leg terminates at its lower end in a tendon attached to the upper end of the tibia (Fig. P8.29a). The forces on the lower leg when the leg is extended are modeled as in Figure P8.29b, where T S is the force of tension in the tendon, wS is the force of gravity acting on the lower leg, and F S is the force of gravity acting on the foot. Find T S when the tendon is at an angle of 25.0 with the tibia, assuming that w 5 30.0 N, F 5 12.5 N, and the leg is extended at an angle u of 40.0 with the vertical. Assume that the center of gravity of the lower leg is at its center and that the tendon attaches to the lower leg at a point one-fifth of the way down the leg.