Gail tries to perform an standing arm curl with a 34 lb. dumbbell. The arm is mo

Question

Gail tries to perform an standing arm curl with a 34 lb. dumbbell. The arm is moving slowly that you can assume static equilibrium. The distance between the olecranon and the metacarpals of the hand is 11 inches The biceps muscle inserts 0.04 m from the olecranon. The forearms are light, so we can ignore their mass for now. Using good form, the humerus stays vertical. When the angle between the humerus and the radius is at 145 degrees, how much rotary force is being produced by the muscle? Answer in units of N (Newtons). Review the definition of the rotary force.

Explanation / Answer

'Rotary force' = torque = W*D = (34*0.453)*0.2794 = 4.3 Nm this is for 90o

Similarly for 145o  => 4.3 sin145o = 2.466 Nm

Rotary component: force of a muscle contributing to bones movement around a joint axis; greatest when muscles angle of pull is perpendicular to bone (i.e. 90 degrees). Stabilizing component: degree of parallelforces generated on the lever (bone and joint) when the muscles angle of pull is less than 90 degrees.

Note: (1 lb = 0.453 kg) , ( 11 inch = 0.2794 m)

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