Questions Leigh i doing s knee extemsion exercise using a 150 N wcight strapped

Question

Questions Leigh i doing s knee extemsion exercise using a 150 N wcight strapped to her ankdle 40 cm from her knee joint. She holds her leg so that the horizontal distance from her knee joint to the weight ?s 30 cm. 10cm 30 cm 1. 2. 3. What is the moment arm of the weight about the knee joint axis? For this position, what torque is created by the ankle weight about her knee joint axis? If the moment arm of the knee extensor muscles is 3 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

a)
From the diagram, moment arm = 30 cm

b)
Torque = moment arm x Force
= 0.3 x 150
= 45 Nm

c)
Consider that the muscles apply a force F.
Torque applied by muscle about knee joint = 0.03 x F
Torque due to the weight of the leg and the strapped weight is = 0.1 x 10 x 9.81 + 45
= 54.81 N

Since the arrangement is in static equilibrium, net torque is zero.
0.03 F = 54.81
F = 54.81/0.03
= 1827 N

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