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 investigate the kinematics of the forelimb during target reaching and food taking in 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 elbow followed a bent movement path with maximal inflection in the middle. The path of the humerus had an ascending parabola-like characteristic. The velocity 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. Several different velocity peaks reflecting specific aspects of the task existed when the bell-shaped velocity profiles were divided in their vectorial components. Angular motion of the elbow consisted of a flexion-extension sequence during the reach and a flexion during the subsequent retraction. After an initial flexion during lift-off the wrist was extended. It kept this extended position during orienting towards the food container. During the retraction phase it was further extended. The angle between the wrist axis and the parasagittal plane changed during the movement. It first increased, then decreased during the last 100 ms before the object was reached. During the retraction it increased again to support the object weight against gravity. The position of the wrist was established by radio-ulnar supination and movements of the whole arm around the shoulder joint. We hypothesize that the position of the wrist axis is the controlled variable during protraction and retraction, regardless of whether it is achieved by radio-ulnar supination or by movements around the shoulder.

Shaping of the cat paw for food taking and object manipulation: an X-ray analysis.

The kinematics of the cat distal forelimb during food-taking were analysed to obtain information on the movement processes within the paw before and during object taking in a species without monosynaptic corticomotoneuronal projections. The behaviour was investigated with two tests: either the table test (TT, food offered on a table located at ground level in a reaching distance of 22 and 28 cm) or the horizontal test (HT, food offered in a small container located at shoulder level, height 18-25 cm, reaching distance 6-12 cm). In five animals, the changes in configuration and the conjoint actions of the wrist, the metacarpophalangeal (MCP) and proximal interphalangeal (PIP) joints were assessed with three-dimensional X-ray cinematography (time resolution 20 ms, spatial resolution 1 mm) and video analysis. While approaching the target, the digits were first extended and subsequently abducted. This 'preshaping' consisted of combined angular changes in the MCP and PIP joints, thereby attaining an adequate grip aperture. Each cat used a stable strategy, but different cats used different strategies. In the TT, preshaping involved an MCP extension and a PIP flexion. In the HT, predominant extension of the MCP, predominant extension of the PIP, or a combination of both were used, followed by MCP flexion and PIP flexion. The grip aperture started to decrease before object contact, earlier in the TT, later in the HT. Grasping was achieved by flexion of first the PIP and later the MCP. The X-ray analysis gave evidence for individuated digit movements. Correlation analysis of the angular position of the joints between the different phalanges showed that digits 3 and 4 acted in concert, as did digits 2 and 5, but with clear independence between the different pairs. Furthermore, the different phalanges served different purposes during the grasp. Digits 3, 4 established object contact, digits 2, 5 were mainly used to stabilize the paw onto the surface. The cat distal forelimb displays a refined movement repertoire during the preshaping and grasping phase of food-taking. During the preshaping phase, the kinematics resembled in some aspects the situation in humans. The results demonstrate the ability of the polysynaptic projections from the cortico-motoneuronal system to organize differentiated distal limb movements, including individuated movements of the digits.