Temporal constraints in reaching and grasping behavior

Under specific task conditions, many types of human movement behavior such as speech, locomotion and handwriting exhibit a temporal constraint on the many potential degrees of freedom in the form of relative timing among movement components. In the present experiments, we show evidence of temporal constraints in the control of adult prehensile movement independent of transport duration (experiment 1) and initial conditions (experiment 2). In both experiments, subjects were required to reach and grasp a dowel, mounted vertically on a joystick over a distance of 23 cm. Using light emitting diodes (LEDs) placed on the subject's finger, thumb and forearm, individual LED trajectories were captured on film and later kinematically analyzed. In experiment 1, transport duration varied from 300 to 800 ms and in experiment 2, initial conditions were manipulated by requiring subjects to adopt different initial grip postures. Regardless of these manipulations, little change was observed in the temporal occurrence of maximum aperture and onset of grip closure relative to overall movement duration. Functional coupling between fingers and forearm was indicated by high, within-trial co-variation between aperture size and forearm velocity. These results provide further evidence for a temporal constraint on the individual components of prehensile action.