Muscle action potentials and efficiency were measured during shot putting and ball throwing in right and left handed normals. The dominant and the nondominant side were compared in trained and untrained persons in order to investigate motor learning effects. 2. Trained shot putters show a coordinated sequence of activation of trunk, leg and arm muscles of both sides which precedes the final arm extension. After turning the body from an initially inclined and twisted position the final arm extension transfers the force of the accelerated body mass to the shot. 3. When shot putting or throwing is performed by the nondominant arm in subjects trained for the dominant arm the coordination of the contralateral dominant arm is lacking, in contrast to the performance by the trained arm. 4. In untrained persons the shot putting is effected mainly by arm extension on either side. The preceding trunk and leg action is very incomplete and without coordination of the contralateral arm, whereas the shot putting arm shows stronger triceps brachii innervation. The distances achieved by untrained shot putters reach only one-fifth or one-third of those of highly trained persons. 5. In ball throwing the throwing arm shows final coactivation of the biceps and triceps muscles coordinated with trunk and contralateral arm movements. The distances reached by throwing with the untrained arm are about half of those of the trained dominant arm. 6. Trained sportsmen put the shot with the untrained nondominant arm to 73% of the distance achieved by the trained arm. Untrained persons, however, show an approximately equal, smaller range of shot with the dominant and nondominant arm (8% side difference). 7. A biomechanical factor causing different performances of trained and untrained persons in shot putting is the different force of the energy transferring mass: the untrained person thrusts mainly with the arm which has barely 1/20 of the mass of the whole body, used by the trained shot putter. 8. That bilateral training and not hemispheric dominance is the decisive factor producing the improved efficiency is demonstrated by three observations: a) the maximal efficiency and bilateral coordination of shot putting in trained persons, b) the lack of contralateral activation of the dominant arm in shot putting and throwing by the nondominant arm, and c) the minimal left and right side differences in performance of untrained persons.
