Gail tries to perform an standing arm curl with a 21 lb. dumbbell. The arm is mo

Question

Gail tries to perform an standing arm curl with a 21 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 0.2 m. The biceps muscle has a moment arm of 0.04 m from the olecranon. The biceps brachii tendon is the size of a nickel (21.21 mm (0.835 in) diameter). (same size cross sectional area as a nickel) 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 108 degrees, how much tensile stress is placed on the biceps tendon? Answer in units of MPa (Megapascals). Correct Answer was 1.25749 MPa. How was this calculated?

Explanation / Answer

'Rotary force' = torque = W*D = (21*0.453)*0.2 = 1.9026 Nm

But with 108o Degrees , torque = 1.9026sin108 = 1.80948 N.m

But with 0.04m moment, F = 45.237 N

Area, A = r2  = 0.35325x10-3 m2

Pressure, P = F/A = 128059.448 = 0.128 Mpa

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