Journal of motor behavior
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The exceedingly large grip forces that many older adults employ when lifting objects with a precision pinch grip (Cole, 1991) may compensate for a reduced capability to produce a stable isometric force. That is, their grip force may fluctuate enough from moment to moment to yield grip forces that approach the force at which the object would slip from grasp. We examined the within-trial variability of isometric force in old (68-85 years, n = 13) and young (n = 11) human subjects (a) when they were asked to produce a constant pinch force at three target levels (0.49, 2.25, and 10.5 N) with external support of the arm, hand, and force transducer and (b) when they were asked to grasp, lift, and hold a small test object with a precision grip. ⋯ Thumb and finger forces for grip are produced through coactivation of many muscles and thus promote smooth force output through temporal summation of twitches. We conclude that peripheral reorganization of muscle in older adults does not yield increased instability of precision grip force and therefore does not contribute directly to increased grip forces in this population. However, force instability may affect other grip configurations (e.g., lateral pinch) or manipulation involving digit abduction or adduction forces.
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Diminished tactile sensibility and impaired hand dexterity have been reported for elderly individuals. Reports that younger adults with severely impaired tactile sensibility use excessive grasp force during routine grasp and manipulation tasks raise the possibility that elderly persons likewise produce large grasp forces that may contribute to impaired dexterity. Impaired pseudomotor functioning also occurs in elderly subjects and may yield a slipperier skin surface that enhances the possibility for excessive grasp force. ⋯ Twopoint discrimination limina in the older subjects averaged about four times greater than in the younger subjects. Increased grasp forces in elderly persons may result from other factors, such as increased variability in grip force production. The contributions of excessive grasp forces to impaired dexterity in older persons still need to be addressed experimentally.
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Journal of motor behavior · Sep 1984
Motor programming: does the choice of the limb which is to carry out the response imply a delay?
In many activities, the human being must quickly decide on the response to be produced following a change in the environment. In some of these situations, the limb that the individual chooses to carry out a response seems to be a significant element in performance. Thus, if the individual carries out the response with the limb closest to the target, the performance can improve because it will take less time to achieve the goal. ⋯ In order to do so, the subjects performed a two choice reaction-time task. For this task, some subjects knew beforehand which hand they had to use to carry out the response while other subjects were unaware of this fact. The results of two experiments indicated that the choice of the limb which is to carry out the response requires no particular delay when the movement to be produced is externally guided.
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Articular proprioception is normally considered to provide accurate information about limb position, particularly in ball skills in which the eyes are be occupied with tracking the ball. If this is so, then preventing sight of the catching hand without interfering with visual tracking of the ball should affect the accuracy of catching. The experiment reported here indicates that is not the case. ⋯ The errors made are in positioning of the catching hand, which frequently does not contact the ball. In addition, subjects showed larger changes in the felt length arms after catching without sight of the hand than did those who could hand while catching. Visual information about the position of the hand for catching, and this may be because the proprioceptive system is by vision.
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Choice reaction time prior to a motor response has been shown to depend on the nature of the response to be made. This effect is assumed to represent variations in programming time. However, as the length of a response sequence increases this effect becomes smaller, suggesting that some response programming is postponed until after the response sequence is initiated. ⋯ For sequences comprised of two Morse Code responses (e.g. dit-dah) the initial reaction time was independent of the terminal response. However, programming of this terminal response was apparent as a lengthening of the duration of intervals within the response when the terminal response was dah rather than dit. When programming of parts of the sequence is postponed beyond the reaction time interval, the programming occurs later and influences the timing of the sequence of responses.