Leigh is doing a knee extension exercise using a 100 N weight strapped to her an

Question

Leigh is doing a knee extension exercise using a 100 N weight strapped to her ankle 40 cm from her knee joint. She holds her leg so that the horizontal distance from her knee joint to the weight is 30 cm.

1. What is the moment arm of the weight about the knee joint axis?

2. For this position, what torque is created by the ankle weight about her knee joint axis?

3. If the moment arm of the knee extensor muscles is 4 cm about the knee joint axis, what amount of force must these muscles produce to hold the leg in the position described? Consider that in addition to the added weight at the ankle, the leg itself weighs 10kg, and the center of mass of the leg (shown by the crosshair circle in the figure) is located at a distance of 10 cm from the knee joint axis.

Explanation / Answer

1)the moment arm is along perpendicular to the bottom part f the leg

2)Since gravity works vertically, the 30 cm distance should be used since it will be
perpendicular to the force of gravity working through the weight.
T = F * d(perpendicular) =100N(0.30m) = 30 Nm

3)The torque due to the weight must be equal in magnitude, but opposite in direction.
30Nm + 10Nm= Fmuscle *0.04

Fmuscle= 40/.04= 1000 N

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