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Vennila Krishnan and Slobodan Jaric

Coordination of the hand grip (G; acting normally to the grasping surface) and load forces (L; acting in parallel) in bimanual static tasks was studied. L symmetry (either the magnitude or direction) and frequency were manipulated in healthy participants (N = 14). More complex tasks (i.e., the higher frequency and/or asymmetric ones) revealed expected deterioration in both the task performance (accuracy of the prescribed L force profiles) and force coordination (G/L ratio and G-L correlation) suggesting importance of L frequency and symmetry in prehension activities. However, the same tasks revealed a more prominent deterioration of interlimb than the within-limb force coordination. This could be interpreted by two partly different and noncompeting neural control mechanisms where the coordination of interlimb forces may be based on ad-hoc and task-specific muscle coordination (often referred to as muscle synergies) while the within-limb coordination of G and L could be based on more stable and partly reflex mechanisms.

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Leanna Ferrand and Slobodan Jaric

The purpose of this study was to explore the effects of handedness on coordination of grip (G) and load (L) forces in static bimanual manipulation tasks. Participants (N = 10) exerted various L profiles against an externally fixed hand-held device based on presumably open-loop and closed-loop neural control mechanisms, (i.e., mediated and not mediated, respectively, by sensory feedback). Average G/L ratio and the coupling of G and L (i.e., stability of the G/L ratio and correlation between G and L) were separately assessed in each hand. The results revealed a lower average G/L ratio in the non-dominant hand suggesting a more economical grip, while the indices of G and L coupling were similar in two hands. The dominant and non-dominant hand failed to reveal relative advantages in the tasks predominantly based on open- and closed-loop control mechanisms, respectively. We conclude that, due to the static nature of the tested tasks, the particular advantage of the non-dominant hand in G and L coordination could be in line with the recently proposed specialization of the non-dominant limb for control of position. However, the overall results are not in line with classic views of the prevailing open- closed-loop neural mechanisms in the control of the dominant and nondominant limb, respectively.

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Amador García-Ramos and Slobodan Jaric

Purpose: An a posteriori multicenter reliability study was conducted to compare the reliability of the outcomes derived from the linear force–velocity (F–V) relationship (F-intercept [F 0], V-intercept [V 0], F–V slope, and maximum power [Pmax]) using a 2-point method based on 2 distant loads with respect to a multiple-point method based on 4 proximal loads and a multiple-point method that considered all 6 tested loads. Method: Data from 63 healthy men derived from 3 studies were analyzed. The F–V relationship obtained from the bench-press-throw exercise was determined in 2 separate sessions using 3 different combinations of loads: 2-point method (20–70% of 1-repetition maximum [1RM]), 4-load multiple-point method (30–40–50–60% of 1RM), and 6-load multiple-point method (20–30–40–50–60–70% of 1RM). Reliability was assessed through the coefficient of variation (CV), whereas a CVratio of 1.15 was deemed as the smallest important ratio. Results: The 2-point method provided the outcomes of the F–V relationship with greater reliability than the 4-load multiple-point method (F 0, 3.58% vs 4.53%, CVratio = 1.27; V 0, 5.58% vs 7.85%, CVratio = 1.41; F–V slope, 8.57% vs 11.99%, CVratio = 1.40; Pmax, 4.33% vs 4.81%, CVratio = 1.11). The reliability of the 6-load multiple-point method was comparable to the 2-point method (F 0, 3.53%, CVratio = 1.01; V 0, 5.32%, CVratio = 1.05; F–V slope, 8.38%, CVratio = 1.02; P 0, 3.74%, CVratio = 1.16). Conclusion:The distance between experimental points is more important for obtaining a reproducible F–V relationship than the number of experimental points; therefore, the 2-point method could be recommended for a quicker assessment of the F–V relationship.

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Mehmet Uygur, Goran Prebeg and Slobodan Jaric

We compared two standard methods routinely used to assess the grip force (GF; perpendicular force that hand exerts upon the hand-held object) in the studies of coordination of GF and load force (LF; tangential force) in manipulation tasks. A variety of static tasks were tested, and GF-LF coupling (i.e., the maximum cross-correlation between the forces) was assessed. GF was calculated either as the minimum value of the two opposing GF components acting upon the hand-held object (GFmin) or as their average value (GFavg). Although both methods revealed high GF-LF correlation coefficients, most of the data revealed the higher values for GFavg than for GFmin. Therefore, we conclude that the CNS is more likely to take into account GFavg than GFmin when controlling static manipulative actions as well as that GFavg should be the variable of choice in kinetic analyses of static manipulation tasks.

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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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Dragan M. Mirkov, Sladjan Milanovic, Dusko B. Ilie and Slobodan Jaric

The hypothesis that strength of active muscles affects the symmetry of the velocity profiles of voluntary movements was tested. In particular, it was assumed that the duration of acceleration and deceleration phases reflects the ability of the antagonistic muscles to exert torque in such a way that stronger muscle requires less time for action. Twelve subjects performed consecutive 50° flexions and extensions in blocks of either discrete or oscillatory movements. They were tested under high and moderate speed conditions, as well as within different ranges of elbow joint angles. The symmetry ratio (SR; acceleration lime divided by deceleration time) was calculated in order to assess movement symmetry. The results demonstrated SR > 1 under most of the discrete and, particularly, oscillatory movement conditions. A velocity-associated increase in SR was recorded, while different ranges of elbow movements, assumed to provide different torques of the agonist and antagonist muscles, also provided different SR. The findings were generally in line with the predicted effects of movement conditions on muscle strength, particularly those related to elbow angle and elbow angular velocity. Deviations from me ideal movement symmetricity have usually been interpreted as either weakness of various motor control models and hypotheses, or as a sub-optimal control of movements in certain subject populations; the present study suggests an alternative interpretation based upon the ability of active muscles to exert torque.

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Vennila Krishnan, Paulo Barbosa de Freitas and Slobodan Jaric

We investigated hand function in mildly involved multiple sclerosis (MS) patients (N = 16; Expanded Disability Status Scale 1–5, 9-hole peg test 14–32 s) during static and dynamic manipulation tasks using an instrumented device. When compared with healthy controls (N = 16), the patients revealed impaired task performance regarding their ability to exert prescribed patterns of load force (L; force acting tangentially at the digits-object surface). Regarding the coordination of grip force (G; normal component) and L, the data only revealed an elevated G/L ratio, although both the G and L coupling (maximum correlation coefficients and the time lags between them) and the G modulation (gain and offset of G with respect to L) remained comparable in the two groups. Finally, most of the data suggested no MS-specific effects of switching from uni- to bimanual tasks, from available visual feedback to deprived feedback conditions. We conclude that the deterioration in the ability for precise control of external forces and overgripping could precede the decoupling of G and L and decreased G modulation in early phases of the disease. The results also suggest that the applied methodology could be sensitive enough to detect mild levels of impairment of hand function in MS and, possibly, other neurological diseases.

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Daniel Feeney, Igor Jelaska, Mehmet Uygur and Slobodan Jaric

We examined the effects of unilateral muscle fatigue on the performance and coordination of grip (GF; normal component acting between the hand and object) and load force (LF; tangential component) in bimanual manipulation tasks, as well as the associated lateral differences. Eleven participants performed various symmetric bimanual tasks either without fatigue, or after fatiguing the GF producing muscles of either the nondominant or dominant hand. The GF/LF ratio of the fatigued and nonfatigued hand decreased and increased, respectively, while the neither the effects of fatigue on the task performance and GF-LF coordination, nor the lateral differences were revealed. The lack of the fatigue associated effects on most of the tested variables typically observed from unimanual tasks could be explained by bimanual assimilation. The findings also suggest that in daily life switching to bimanual tasks when one hand becomes fatigued could be beneficial regarding preserving the high level of both the manipulation performance and force coordination.

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Dusko B. Ilic, Dragan M. Mirkov and Slobodan Jaric

Nine subjects (experimental group) were tested on rapid elbow flexion and extension movements performed in the same final position, before and after extensive practice of the movements. Nine additional subjects (control group) were also tested, but without any practice between the tests. Comparison of the pretest and posttest results suggested that the experimental group decreased their variable error (i.e., standard deviation of the final movement position) in both practiced (elbow flexion) and nonpracticed (elbow extension) movements. The control group, however, did not improve in either of tested movements. The experimental group demonstrated lower variable error in the nonpracticed elbow extensions than the control group, while the same difference for practiced elbow flexion movements was slightly below the level of significance. The results support the importance of the final position in programming of rapid, self-terminated movements; however, they do not rule out the role of other kinetic and kinematic variables (such as movement distance).

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Amador García-Ramos, Slobodan Jaric, Paulino Padial and Belén Feriche

This study aimed to (1) evaluate the linearity of the force–velocity relationship, as well as the reliability of maximum force (F 0), maximum velocity (V 0), slope (a), and maximum power (P 0); (2) compare these parameters between the traditional and ballistic bench press (BP); and (3) determine the correlation of F 0 with the directly measured BP 1-repetition maximum (1RM). Thirty-two men randomly performed 2 sessions of traditional BP and 2 sessions of ballistic BP during 2 consecutive weeks. Both the maximum and mean values of force and velocity were recorded when loaded by 20–70% of 1RM. All force–velocity relationships were strongly linear (r > .99). While F 0 and P 0 were highly reliable (ICC: 0.91–0.96, CV: 3.8–5.1%), lower reliability was observed for V 0 and a (ICC: 0.49–0.81, CV: 6.6–11.8%). Trivial differences between exercises were found for F 0 (ES: < 0.2), however the a was higher for the traditional BP (ES: 0.68–0.94), and V 0 (ES: 1.04–1.48) and P 0 (ES: 0.65–0.72) for the ballistic BP. The F 0 strongly correlated with BP 1RM (r: 0.915–0.938). The force–velocity relationship is useful to assess the upper body maximal capabilities to generate force, velocity, and power.