Question

Consider a situation in which we are carrying a tray with a bottle and glass of wine (total mass = 5.0 kg) with our arm bent by 90º. The length of the forearm (elbow joint to hand) is L = 45 cm and the distance between the elbow joint and radial tuberosity is R = 3.0 cm (you can measure these on your own arm!). Further assume that the mass of the forearm is m = 1.5 kg, and that weight force acts on the forearm at its center. For these values: (a) [3 pt] Determine the force the bicep muscle must apply to hold both the tray and arm in place at a 90º angle. (b) [3 pt] Determine the force acting on the pivot point at the elbow (magnitude and direction), which provides a good estimate of the forces experienced by our joints. (c) [3 pt] Determine the mechanical advantage of this system as the ratio of the weight of the tray to the force exerted by the bicep muscle. You should find a value less than 1; i.e., that the bicep must apply a greater force than the load. Can you explain the evolutionary advantage of having our arms shaped this way if it is a mechanical disadvantage?

Answer 1

Answer of Part(C) :

The evolutionary advantage of this arrangement is that the arrangement of all tissues, ligaments and muscles around the elbow joint is compact.

If we want to improve the mechanical advantage of this arrangement, then we will have to move the connection at radial tuberosity, towards the midpoint of the forearm. This will make the crosssection of the arm much bigger around thew elbow joint.