Forces in Flexing the Elbow


"You need your elbow flexor muscles to initiate elbow flexion against gravity, but NOT to keep the elbow flexing"

True or False? Explain your answer with refrence to one of Newtons laws.


Newton's second law says that the net force on an object must be the product of its mass and its acceleration. F=ma. Suppose for example that you are standing with your arm hanging straight down, not moving. In this state the elbow flexor muscles would be relaxed. To raise your forearm to the horizontal requires force from the flexor muscles to overcome three different kinds of resistance to motion. First it must provide the F=ma force to get the forearm moving. Once the forearm is swinging with constant speed, no further acceleration is required so a=0 and therefore F=0. That is not the whole story with regard to the flexor muscles however. As long as the forearm is moving there will be a frictional force opposing the motion. This is quite small in a healthy elbow joint but it is not zero. More significant is the other force resisting the upswing of the forearm. As the center of mass of the forearm plus any load carried in the hand, moves out from under the elbow joint, there will be a torque tending to pull the forearm back down to its vertical alignment. This comes from gravity acting on the center of mass. The flexor muscles must overcome this weight component as well.

My answer then would be "false". Be careful though. The person asking the question may be looking only for the F=ma part of the answer. It is true that the component of force required to overcome inertia goes to zero once the angular velocity of the forearm achieves its constant value.