Suppose you stand on one foot while raising your other leg up behind you. Your m

Question

Suppose you stand on one foot while raising your other leg up behind you. Your muscles will have to apply a force to hold your leg in this raised position. We can model this situation as in the figure. The leg pivots at the knee joint, and the force to hold the leg up is provided by a tendon attached to the lower leg (tibia) as shown. The tendon provides the torque to raise the lower leg. 5.0 cm 50 cm Assume that the lower leg and the foot together have a combined mass of 4.4 kg, and that their combined center of gravity is at the center of the lower leg. (a) How much upward force must the tendon exert on the lower leg (tibia) to keep it in this position? Ftendon on tibia (b) As you hold your leg in this position, the upper leg (femur) exerts a force on the lower leg (tibia) at the knee joint. What is the direction of this force? left right up down (c) What is the magnitude of the force of the upper leg (femur) on the lower leg (tibia) at the knee joint? Fr femur on tibia

Explanation / Answer

a)

F = force applied by tendon

Fg = force of gravity = mg = 4.4 x 9.8

r = location of center of mass relative to joint = 50./2 = 25 cm

d = distance of point where tendon force act from joint = 5 cm

using equilibrium of torque

Fg r = F d

4.4 x 9.8 x 25 = F x 5

F = 215.6 N

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