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Rainer Blank and Joachim Hermsdörfer

Fast force changes with hand-held objects are an important prerequisite for object manipulation in everyday life. This study examines the development of fastest isometric force changes in a precision grip. One hundred sixty-five children (76 girls, 89 boys), 3–14 years, without neurological abnormalities increased and decreased repetitively isometric grip forces as rapidly as possible by their dominant hand using a small cylindrical pinch grip object (20 g). The frequency of repetitive force changes increased in a linear way from the age of 4 years until about 12 years by 0.23 Hz per year (r 2 = .54) without noticeable gender difference. The ratio of the duration of force increase and decrease slightly declined from 1.05 (4-year-olds) to 0.95 (11- to 14-year-olds). The development of force amplitudes and the mean force were more variable. Temporal parameters become less variable with age, whereas force parameters become more variable. In particular, the temporal parameters of fastest isometric force changes are best predictors for developmental changes. Fastest isometric force changes may be an important basic capacity for fast object manipulation, particularly in young children and in children with movement disorders.

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Dennis A. Nowak, Joachim Hermsdörfer and Helge Topka

Amyotrophic lateral sclerosis is a degenerative motor neuron disorder with progressive and exclusive loss of motor neurons in the spinal cord, brainstem, and motor cortex. Five patients with amyotrophic lateral sclerosis, and 5 age-matched, healthy control subjects performed vertical point-to-point arm movements with an instrumented hand-held object. In between the movements, the object was held stationary. Compared with healthy controls, all patients generated greater grip forces during the phase of stationary holding of the object and greater ratios between grip and load force maximums during the arm movements. We conclude that in amyotrophic lateral sclerosis, the ability to scale the grip force magnitude efficiently according to the actual loading requirements is impaired. When performing upward movements, controls increased grip force in parallel with load force right from the movement onset; during downward movements, controls anticipated an early decrease of load force by constant or decreasing grip forces. In contrast, 3 of 5 patients showed an early increase of grip force during both upward and downward movements, indicating that in amyotrophic lateral sclerosis, the differential regulation of the grip force output according to the direction-dependent load force profile may be impaired. In motor neuron disease, the inaccurate grip force scaling and the impaired temporal coupling between grip and load force profiles may either directly result from deficient motor execution or be secondary to accompanying symptoms, such as dyscoordination of hand and finger muscles due to spasticity.

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Dennis A. Nowak, Joachim Hermsdörfer, Jens Philipp, Christian Marquardt, Stefan Glasauer and Norbert Mai

We investigated the quality of predictive grip force control during gravity changes induced by parabolic flight maneuvers. During these maneuvers gravity varied: There were 2 periods of hypergravity, in which terrestrial gravity nearly doubled, and a 20-s period of microgravity, during which a manipulated object was virtually weightless. We determined grip and load forces during vertical point-to-point movements of an instrumented object. Point-to-point movements were a combination of static (stationary holding) and dynamic (continuous movements) task conditions, which were separately analyzed in our previous studies. Analysis of the produced grip forces revealed that grip adjustments were closely linked to load force fluctuations under each gravity condition. In particular, grip force maxima coincided closely in time with load force peaks, although these occurred at different phases of the movement depending on the gravity level. However, quantitative analysis of the ratio of maximum grip force to the corresponding load force peak revealed an increased force ratio during microgravity when compared to that during normal and hypergravity, We hypothesize that the impaired precision of force coupling with respect to force magnitude during microgravity results from reduced feedback information about the object's mass during the stationary holding of the object in between each movement. The results indicate that the temporal grip force regulation is highly automatized and stable, whereas economical planning of force magnitude is more flexible and might reflect changes of the external loading condition.