ABSTRACT

If the forearm and ball weigh 15N and 20N, respectively, the biceps must generate B = 275N to maintain a static posture. Force equilibrium conditions require that the sum of all forces acting on the forearm in the vertical direction be zero to maintain static equilibrium:

Therefore, the reaction force acting on the trochlear notch of the ulna is 240N. This type of example is widely used in ergonomics training and teaching, because it illustrates how mechanical forces acting on biological tissues (biceps force B and joint reaction force R) can be much greater than the magnitude of externally applied forces. A similar free body

analysis can be conducted with forearm rotation to develop a dynamic biomechanical model (Chaffin and Andersson, 1984).