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Maria Bellumori, Slobodan Jaric, and Christopher A. Knight

Physical quickness is less in older adults with implications for fall prevention, movement initiation, and activities of daily living. The purpose was to compare control of rapid contractions in young and older adults within two diverse muscle groups: powerful elbow extensors (EE) and dexterous index finger abductors (IFA). Most-rapid force pulses to a variety of levels were recorded and peak force and rate of force development (RFD) were analyzed with linear regression. The resulting slope represents the dependent variable of interest, the RFD-scaling factor (RFD-SF). RFD-SF of EE and IFA strongly correlated both overall (r = .87, p < .01) and separately in young (r = .60, p < .05) and older (r = .77, p < .01) adults. RFD-SF values were different between muscle groups (F 1,28 = 19.1, p < .001) and also less in elderly (F 1,28 = 32.6, p < .001). We conclude that RFD-SF provides a sensitive assessment of muscle quickness that can be used to evaluate neuromuscular function in aging humans.

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Christopher A. Knight

Column-editor : Thomas W. Kaminski

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Christopher A. Knight, Adam R. Marmon, and Dhiraj H. Poojari

Subjects learned to produce brief isometric force pulses that were 10% of their maximal voluntary contraction (MVC) force. Subjects became proficient at performing sets of 10 pulses within boundaries of 8–12% MVC, with visual feedback and without (kinesthetic sense). In both the control (Con, n = 10) and experimental (Exp, n = 20) groups, subjects performed two sets of 10 kinesthetically guided pulses. Subjects then either performed a 10-s MVC (Exp) or remained at rest (Con) between sets. Following the MVC, Exp subjects had force errors of +30%, whereas performance was maintained in Con. There was evidence for both muscular and neural contributions to these errors. Postactivation potentiation resulted in a 40% gain in muscle contractility (p = .003), and there was a 26% increase in the neural stimulation of muscle (p = .014). Multiple regression indicated that the change in neural input had a stronger relationship with force errors than the increased contractility.

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Rahul Marwaha, Susan J. Hall, Christopher A. Knight, and Slobodan Jaric

The aim of the study was to reveal specific aspects of impaired hand function in mildly affected multiple sclerosis (MS) patients. Static manipulation tasks were tested in 13 mildly impaired (EDSS 1.5-4) MS patients and 13 age and gender matched controls. The tasks were based either on presumably visually (i.e., feedback) controlled tracing of depicted patterns of load force (LF; produced by symmetric bimanual tension and/or compression applied against an externally fixed device) or on predominantly feed-forward controlled amplitudes of sinusoidal patterns of LF. The task variables (based on accuracy of exerting the required LF pattern) suggested poor performance of MS subjects in feedback, but not in the feed-forward controlled tasks. The patients also revealed higher GF/LF ratio in all tasks. However, the coordination of GF and LF appeared to be comparable in the two groups. These results continue to support the chosen experimental paradigm and suggest that in mildly affected MS patients, sensorimotor deficits and overgripping precede the decoupling of grip and load forces observed in more severely affected patients.