X-ray kinematic analysis of forelimb movements during target reaching and food taking in the cat

We have used a three-dimensional X-ray cinematographic approach to investig ate the kinematics of the forelimb during target reaching and food taking i n five cats. Measurements of the trajectory of the limb during the reaching movement showed that the movement paths of the metacarpophalangeal joint ( MCP) and the wrist were sigmoidal with a long nearly linear segment. The el bow followed a bent movement path with maximal inflection in the middle. Th e path of the humerus had an ascending parabola-like characteristic. The ve locity profiles of the MCP and wrist were nearly bell-shaped and skewed to the left, whereas the profiles of the elbow joint were more or less double peaked with the second peak occurring 60-40 ms before object contact. Sever al different velocity peaks reflecting specific aspects of the task existed when the bell-shaped velocity profiles were divided in their vectorial com ponents. Angular motion of the elbow consisted of a flexion-extension seque nce during the reach and a flexion during the subsequent retraction. After an initial flexion during lift-off the wrist was extended. It kept this ext ended position during orienting towards the food container. During the retr action phase it was further extended. The angle between the wrist axis and the parasagittal plane changed during the movement. It first increased, the n decreased during the last 100 ms before the object was reached. During th e retraction it increased again to support the object weight against gravit y. The position of the wrist was established by radio-ulnar supination and movements of the whole arm around the shoulder joint. We hypothesize that t he position of the wrist axis is the controlled variable during protraction and retraction, regardless of whether it is achieved by radio-ulnar supina tion or by movements around the shoulder.