ABSTRACT
If the forearm and ball weigh 15 and 20 N, respectively, the biceps must generate B¼ 275 N 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:
B GW R ¼ 0 (14:2)
Therefore, the reaction force acting on the trochlear notch of the ulna is 240 N. 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).
