7. The moment arm of the shoulder abductor muscles is approximately 5 cm from th

Question

7. The moment arm of the shoulder abductor muscles is approximately 5 cm from the axis of rotation of the shoulder joint, and the force vector representing shoulder abductor muscle force is oriented at approximately 150 degrees relative to the positive x axis. The weight of the upper extremity of an 80 kg male is approximately 45 N. The center of gravity of the upper-extremity is approximately 0.25 m from the shoulder joint.

a. Draw a free-body diagram of the person described. The person should be facing you and have the left arm abducted parallel to the ground.

b. Calculate the muscle for need to hold the arm in the static position

Explanation / Answer

a) Weight of person is acting downwards .

   The weight of the upper extremity is acting downwards , providing moment in clockwise direction .

Muscle force is acting in upwards direction , providing moment in anticlockwise direction .

b) As, torque = F * r * sin(theta)

=>   45 * 0.25 = F * 0.05 * sin150

=> F = 450 N

=>    the muscle for need to hold the arm in the static position = 450 N

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