Determine the internal muscle force (N) to maintain equilibrium for the knee fle

Question

Determine the internal muscle force (N) to maintain equilibrium for the knee flexion exercise in the image using the following information (using Image J software):

This woman weighs approximately 125 pounds

The distance from the sagittal plane knee axis of rotation to the insertion point of the hamstrings is 3.7 cm.

The distance from the sagittal plane knee axis of rotation to the shank-foot segment COM is 29.2 cm.

A published research paper reported that (on average) in females, the shank accounts for approximately 6.68% of body weight.

Explanation / Answer

Given: The shank accounts for 6.68% of total body weight.

Shank weight = 0.0668 x 125 pounds = 8.35 pounds = 3.79 Kg

COM of Shank lies 29.2 cm away from the knee joint.

The gravitational force acting on the Shank is given as shank mass x g = 3.79 x 9.8 = 37.14 N

The torque produced by gravitational force acting upon the shank is equal to

= r x F= 29.2 cm x 37.14 N = 10.84 N-m

The insertion point of hamstring is 3.7 cm away from knee joint. To maintain the equilibrium, the muscle force of hamstring have to produce an equal amount of torque as gravitational force acting on the shank.

Therefore

N x 3.7 cm = 29.2 cm x 37.14 N

N= 293.1 N

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