Adaptive force control in grasping as a function of level of developmental disability.

BACKGROUND: The adaptation to the task demands of grasping (grip mode and object mass) was investigated as a function of level of developmental disability. METHODS: Subjects grasped objects of different grip widths and masses that were instrumented to record grip forces. RESULTS: Proportionally, fewer participants from the profound compared with moderate and severe disability groups were able to complete the prehensile tasks. Nevertheless, all participants who completed the task showed adaptive grasping behaviour in terms of level and variability of force produced. There was higher absolute and relative force variability in low mass tasks that was enhanced with greater level of developmental disability. CONCLUSIONS: The findings show task relevant adaptive grasping control with inhibition of force output at very-low-force conditions being the primary performance deficit of the profound disability group as a function of level of developmental disability.

Age differences in noise and variability of isometric force production.

This study examined whether age-related improvements observed in the motor performance of children result from a reduction of noise in the output of the sensori-motor system. Children ages 6, 8, and 10 years and young adults (N = 48, 12 per group) performed continuous, constant isometric force contractions with the index finger at four different force levels with and without visual feedback. The results revealed that: (a) performance improved with increases in age, (b) the force output signal exhibited increased irregularity and a more broadband frequency profile with increases in age under conditions with feedback, and (c) there were no age differences in the irregularity of the force signal and smaller age differences in the frequency profiles under conditions without feedback. It is proposed that the age-related enhancements in performance throughout childhood are primarily due to a more appropriate mapping of the organization of the sensori-motor system to the task constraints rather than to reduction of system noise.

On learning to move randomly.

In 2 experiments, the authors examined whether and to what degree young adults can learn to produce random planar motion of the index finger or fingers. Three different types of information feedback were provided to the participants (N = 8 in each experiment) over up to 5 days of practice across the 2 experiments. The results from both experiments revealed that the participants produced a relatively low level of movement randomness in finger motion and that they did not learn through practice to enhance the stochastic properties of their movement under any feedback conditions. The findings provide further evidence that there are relatively tight constraints on the number of dimensions that are regulating single-limb planar motion and that those constraints are not susceptible to change through typical learning protocols.

Dynamics of self-injurious behaviors.

Self-injurious behavior was examined in a case study of head-banging by an 8-year-old girl with profound mental retardation and an autistic disorder. Trajectories of the arm movements and impact forces of the head blows were determined from a dynamic analysis of videotapes. Results revealed a high degree of cycle-to-cycle consistency in the qualitative dynamics of the limb motions, with one hand motions being faster than those with two hands (inphase and antiphase) and the motions with the helmet about 25% faster than those without the helmet. The impact force of SIBs as a percentage of body weights are near the low end of forces generated in boxing blows and karate hits.