The etiology of epicondylalgia lateralis humeri (tennis elbow) is not fully understood. A biomechanical model is introduced for those types of epicondylalgia where damage at the origin of the wrist and finger extensor muscles is caused by overloading. It shows that grasping and pinching always cause a flexing moment at the wrist joint. To avoid flexion of the joint, there must be equilibrium of moments, which is attained by activity of the extensor muscles. Simultaneous measurements of force and electromyograms support the biomechanical model.