The aim of the study was to compare the coordination of hand grip (G) and load force (a force that tends to cause slippage of a grasped object; L) in static bimanual manipulation tasks with the same data obtained from the similar dynamic tasks. Based on the previous findings obtained from dynamic tasks, it was hypothesized that an increase in the rate of L change would be predominantly associated with a decrease in the coordination of the within limb forces (coordination of G and L of each hand as assessed through the correlation coefficients), while a decrease in coordination of interlimb forces (between two G and two L) will be less pronounced. Regarding the pattern of modulation of G, the same increase in L frequency was also expected to be associated with a decrease in G gain and an increase in G offset (as assessed by slope and intercept of the regression lines obtained from G to L diagrams, respectively), as well as with an increase in average G/L ratio. Subjects exerted oscillatory isometric L profiles by simultaneous pulling out two handles of an externally fixed device under an exceptionally wide range of L frequencies (0.67-3.33 Hz). The results demonstrated relatively high correlation coefficients between both the interlimb and within limb forces that were only moderately affected under sub-maximal L frequencies. Furthermore, the hypothesized changes in G gain and offset appeared only under the highest L frequency, while the G/L ratio remained unaffected. We conclude that, when compared with the dynamic tasks based on the unconstrained movements of hand-held objects that produce similar pattern of L change, the static manipulation tasks demonstrate a consistent and highly coordinated pattern of bilateral G and L under a wide range of frequencies. However, the neural mechanisms that play a role in the revealed differences need further elucidation.

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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.

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