ARM POSITION CONSTRAINTS WHEN THROWING IN 3 DIMENSIONS

1. Overarm throwing is a skilled multijoint movement with potentially many degrees of freedom. Considering only the arm greater than or equal to 7 degrees of freedom are involved (shoulder 3, elbow 2, wrist 2). For each arm segment 3 degrees of freedom are potentially required to specify its angular position (orientation) at any moment during a throw. Simplification of the control problem for the CNS would occur if there were constraints on these degrees of freedom. The objective was to determine whether such constraints exist at ball release when throwing at targets in different directions using only the arm. 2. The angular positions in three dimensions of the distal phalanx of the middle finger, the hand, the forearm, and the upper arm were simultaneously recorded with search coils as subjects sat with a fixed trunk and threw balls at nine targets in an approximate +/-40 degrees work space. Ball release was signaled by microswitches on the proximal and distal phalanges of the middle finger (proximal and distal triggers). 3. On throwing at any one target the hand at ball release adopted a similar orientation for each throw, i.e., for a particular vertical and horizontal angular position the hand adopted a similar torsional position. On throwing at targets throughout the work space, angular position (rotation) vectors describing hand positions in space at ball release were confined to a two-dimensional surface rather than a three-dimensional volume. This constraint in hand torsion occurred near and at ball release but not throughout the entire throw. It was not due to mechanical factors because such a surface was not obtained when subjects deliberately twisted their arms when throwing. Thus at ball release during a ''natural'' throw the hand was constrained to 2 of its possible 3 angular degrees of freedom. 4. The same constraint was also found for finger, forearm, and upper arm angular positions in space at ball release as determined at both the proximal and distal triggers. A consequence is that at ball release the entire arm was constrained to 2 of its possible 7 degrees of freedom. 5. The two-dimensional position vector surface for each arm segment was similar to that obtained when pointing with a straight arm at the same targets. In both cases they showed torsion and were twisted like the surface obtained by rotations around the horizontal and vertical axes of a Fick gimbal. However, in some subjects the throwing surfaces were tilted from the vertical. This tilt correlated with aspects of the backswing rather than with the speed of the throw. 6. It is concluded that when throwing naturally at different targets using only the arm a major constraint exists in arm position at ball release, i.e., the arm adopts only one angular position for every throwing direction and thus obeys Donders' law. The nature of this constraint is similar to that found during straightarm pointing. In both cases arm segments are restricted to various extents to positions similar to those adopted by rotations of a Fick gimbal. For throwing this may be the consequence of an attempt to keep hand orientation at ball release fixed with respect to gravity. The nature of these constraints may provide information about the properties of the underlying coordinate system used by the CNS when pointing and throwing.