Mehmet Uygur

Effects of varying the load force range and frequency on force coordination in static manipulation

A frequency associated deterioration of the hand grip (GF; normal component of force acting at the digits-object contact area) and load force (LF; the tangential component) coordination has been demonstrated in a variety of repetitive manipulation tasks. However, it remains unknown whether the effect originates from the task frequency per se, or from the rate of LF change (a prediction that could be derived from the minimum jerk hypothesis) which increases with both LF frequency and LF range. The aim of the study was to discern between the effects of LF frequency and LF range (which affects only the rate of LF) in static manipulation tasks. Subjects (N=15) exerted a sinusoidal LF pattern against an externally fixed instrumented handle at 5 different LF frequencies (0.67-3.33Hz) and ranges (6-30N). The results revealed weak and mainly non-significant effects of both LF range and frequency on GF scaling (GF/LF ratio). However, both GF-LF coupling (assessed by correlation coefficients) and GF modulation (change in GF relative to LF variation) demonstrated a prominent decrease associated with LF frequency, but not with LF range. The observed findings were interpreted by switching between hypothetical synergies of GF and LF producing muscles separately employed in the phases of the increasing and decreasing forces. From the practical aspect, however, the results suggest that the frequency, but not the LF range should be taken into account when designing rhythmic manipulation tasks, developing standard tests of hand function based on GF-LF coordination, or comparing the data from different studies.

Grip force adaptation in manipulation activities performed under different coating and grasping conditions

The aim of the study was to evaluate grip force (GF; normal component of hand-object interaction) adaptation across different manipulation conditions. We hypothesized (1) that the absolute safety margin (the difference between the exerted GF and the minimum GF that prevents slippage; absolute SM), rather than the relative SM (the same difference relative to the minimum GF required), could be an invariant feature of manipulation, as well as (2) that the SM would be higher in static than in dynamic tasks. Fourteen participants performed the free holding and the static holding tasks that required a same pulling force. Each task was performed using a variety of grasps and two different object coatings that both provided different frictions acting between the hand and the hand-held object. Both tasks revealed an increase in the relative SM associated with an increase in friction, while the absolute SM either remained unchanged (free holding) or suggested a moderate negative relationship (static holding task). Both relative and absolute SM were also higher in the free holding than in the static holding. The later result could be a consequence of the task mechanical conditions (i.e., dynamic vs. static), rather than of the difference in neural control mechanisms (feedback vs. feed-forward, respectively). The obtained findings suggest that the absolute SM (rather than the relative one) should be used in future studies of hand force coordination in healthy and clinical populations, while GF adaptation obtained from static and dynamic manipulation tasks should be separately assessed.

The Effect of Fatigue on the Kinematics of Free Throw Shooting in Basketball

The Effect of Fatigue on the Kinematics of Free Throw Shooting in Basketball Fatigue is an indispensible part of a basketball game which may affect an athletes shooting kinematics. Although kinematic analyses of various sports related movements were extensively done, this study is the first to consider the effects of fatigue on the kinematics of free throw shooting. After measuring the resting heart rate, ten collegiate level, male basketball players (21.8±1.6 y; 192.8±3.6 cm; 84.1±8.5 kg) were asked to perform free throws. Two successful and two unsuccessful free throws were selected. Thereafter, participants were asked to complete the fatigue protocol, which included 30m sprints and 5 vertical jumps at each end, until they reached volitional exhaustion. Additional two successful and two unsuccessful free throws were collected. All shots were recorded by using two digital cameras operating at 60 Hz and placed in a stereoscopic view. The elbow, trunk, knee and ankle joint angles were measured before and after the ball release and at the ball release point. The selected joint angles were compared between successful and unsuccessful free throws, as well as before and after the completion of fatigue protocol. The results demonstrated that fatigue did not effect free throw shooting kinematics (p>0.05) and there was no significant joint angle difference between successful and unsuccessful shots (p>0.05). This study suggested that high level athletes are able to cope with the possible detrimental effects of fatigue while performing coordinated movements such as free throw shooting.

Effects of muscle fatigue on grip and load force coordination and performance of manipulation tasks

Muscle fatigue is known to be associated with a deteriorated muscle coordination and impaired movement performance in variety of voluntary movements. The aim of this study was to investigate the generally underexplored effect of muscle fatigue on both the coordination between grip force (GF; the force component perpendicular to the hand-object contact area that provides friction) and load force (LF; the parallel force component that can move the object or support the body) as well as movement performance in manipulation tasks. Fifteen participants performed a variety of static and dynamic manipulations both with and without a preceding procedure designed to fatigue the arm and hand muscles. The tasks involved exertion of ramp-and-hold and oscillation patterns of LF against an externally fixed instrumented device, and a simple lift of a freely moving device. The results revealed a fatigue-associated decrease in GF scaling (i.e., the magnitude of GF relative to LF) and GF-LF coupling (correlation between GF and LF), while the task performance regarding the accuracy of exertion of the prescribed LF profiles remained unaffected. We conclude that muscle fatigue both partly decouples GF from LF and reduces the overall GF magnitude, which could potentially explain why hand-held objects are more likely to drop when manipulated with fatigued muscles. However, the unaffected task performance could be explained either by the relatively low level of muscle forces required by the tested tasks, the moderate level of the fatigue imposed, or both.

Frictional properties of different hand skin areas and grasping techniques

High friction is crucially important in manipulation activities for reducing the hand grip forces and improving control of manipulative tasks. The aim of this study was to assess the coefficient of friction (COF) of various areas of hand skin. Static COF of nine different grasping techniques applied against two object coatings was assessed by means of the ‘slip point’ method in 16 participants. COF measures proved to be both highly reliable and considerably variable across participants (coefficients of variation ranging from 25 to 75%, depending on the applied grasp). COF was also higher in ‘specialised’ than in ‘non-specialised’ skin areas for grasping, as well as in palms, than in the tips of the fingers. The observed findings are of importance for optimisation of object manipulations and also emphasise the importance of measuring individual COF in ergonomic, biomechanics and motor control studies. Statement of Relevance:The results reveal prominent differences in skin friction not only across various areas of the hand, but also across participants. While the former finding is relevant for optimisation of manipulation activities, the latter emphasises the importance of assessment of individual COF in studies of hand function.

Immediate effects of high-speed cycling intervals on bradykinesia in Parkinson's disease.

Abstract There is growing evidence that speed-based exercise training benefits people with Parkinsons disease (PD). The present study investigates the effects of a single session of volitional, high-speed cycling intervals on a battery of timed functional tests selected for their relevance to the symptom of bradykinesia. Ten subjects with PD (Hoehn-Yahr stage ≤ 3.0) participated in a familiarization session and three test sessions. Functional testing occurred before and after 30 minute sessions in which subjects performed no exercise (NO), pedaled at their preferred cadence (PC), or performed 20, 15-second intervals of high-speed low-resistance cycling (HS-LR). In addition to testing the exercise effects in a within-subjects design, we provide test–retest reliability data, minimal detectable change scores, and correlations among the selected functional tests. Despite the relatively low dose of speed-based exercise, HS-LR elicited significant (p < 0.05) improvements in the four square step test and 10 m walk test. Excepting reaction times, there was high reliability and adequate sensitivity to detect moderate and small differences. Strong correlations among tests of mobility inform the future selection of measures in the experimental design. In addition to what is known about continuous exercise sessions involving high-speed exercise, the present results suggest that brief intervals of HS-LR bicycling are promising and should be examined in a longer duration exercise program.

Two-dimensional static manipulation tasks: does force coordination depend on change of the tangential force direction?

Coordination of the grip force (GF) with a tangential force (TF, often referred to as load force) exerted along a certain line in space (i.e., one-dimensional tasks) during object manipulation has proved both to be high and based on feed-forward neural control mechanisms. However, GF–TF coordination deteriorates when the TF of one-dimensional task consecutively switches its direction (bidirectional task). In the present study, we aimed to explore GF–TF coordination in the generally neglected multi-dimensional manipulations. We hypothesized that the coordination would depend on the number of unidirectional and bidirectional orthogonal components of a two-dimensional TF exertion. Fourteen subjects traced various circular TF patterns and their orthogonal diameters shown on a computer screen by exerting a static TF. As expected, the unidirectional tasks revealed higher GF–TF coordination than the bidirectional ones (e.g., higher GF–TF correlations and GF gains, and lower GF/TF ratio). Regarding the circular tasks, most of the data were in line with the hypothesis revealing higher coordination associated with higher number of unidirectional components. Of particular importance could be that the circular tasks also revealed prominent time lags of GF with respect to TF, suggesting involvement of feedback mechanisms. We conclude that the force coordination in bidirectional static manipulations could be affected by changes in TF direction along either of its orthogonal components. The time lags observed from the circular tasks could be a consequence of the activity of sensory afferents, rather than of the visual feedback provided or the task complexity.

High-Speed Cycling Intervention Improves Rate-Dependent Mobility in Older Adults.

Purpose The aim was to determine the feasibility of a 6-wk speed-based exercise program that could be used to initiate new exercise behaviors and to improve rapid movement in older adults approaching frailty. Methods The intervention group included 14 older adults (3 males and 11 females; mean ± SD, age = 70 ± 7.6 yr, height = 1.6 ± 0.11 m, mass = 76.8 ± 12.0 kg, body mass index = 27.7 ± 4.7 kg·m−2). The control group included 12 older adults (6 males and 6 females; mean ± SD, age = 69.2 ± 6.9 yr, height = 1.7 ± 0.09 m, mass = 78.2 ± 10.9 kg, body mass index = 25.3 ± 2.7 kg·m−2). Subjects included active older adults, including regular exercisers, but none were engaged in sports or exercises with an emphasis on speed (e.g., cycling spin classes or tennis). Stationary recumbent cycling was selected to minimize fall risk, and low pedaling resistance reduced musculoskeletal and cardiovascular load. Two weekly 30-min exercise sessions consisted of interval training in which subjects pedaled at preferred cadence and performed ten 20-s fast cadence intervals separated by 40 s of active recovery at preferred cadence. Results Significant group–time interactions (P < 0.05) supported a 2-s improvement in the timed up and go test and a 34% improvement in rapid isometric knee extension contractions in the exercise group but not in controls. Central neural adaptations are suggested because this lower extremity exercise program also elicited significant improvements in the untrained upper extremities of the exercise group (elbow extension rate of force development scaling factor and Nine-Hole Peg Test, P < 0.05). Conclusion These results demonstrate that a relatively low dose of speed-based exercise can improve neuromuscular function and tests of mobility in older adults. Such a program serves as a sensible precursor to subsequent, more vigorous training or as an adjunct to a program where a velocity emphasis is lacking.

Force Control in Manipulation Tasks: Comparison of Two Common Methods of Grip Force Calculation

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.

Evaluation of novel tests of neuromuscular function based on brief muscle actions.

Abstract Bozic, PR, Celik, O, Uygur, M, Knight, CA, and Jaric, S. Evaluation of novel tests of neuromuscular function based on brief muscle actions. J Strength Cond Res 27(6): 1568–1578, 2013—Although widely used, the standard strength test (SST) is known to provide moderate correlations with functional measures, while being based on sustained maximum forces and a relatively large number of trials. The aim of this study was to compare the concurrent (with respect to SST) and external validity (with respect to the standard balance and maximum power output tests) of 2 alternate tests of neuromuscular function based on brief isometric actions. The first test provides a slope between the rates of torque development (RTD) and peak torques (T) measured from a number of consecutive rapid actions performed across a wide range of T levels (brief force pulses, BFP). The second test (alternating consecutive maximum contractions, ACMC) provides T and RTD from multiple cycles of rapid alternating maximum actions of 2 antagonistic muscle groups. The results obtained from 29 young and healthy subjects revealed moderate-to-high concurrent validity of ACMC (median r = 0.56, p < 0.05) and its similar, if not higher external validity than SST. Conversely, both the concurrent and external validity of BFP seemed to be relatively low (r = 0.23, p > 0.05). Because ACMC could also have advantage over SST by being based on somewhat lower and transitional muscle forces exerted and fewer trials are needed for testing 2 antagonistic muscles, the authors conclude that ACMC could be considered as either an alternative or complementary test to SST for testing the ability for rapid exertion of maximum forces. Conversely, BFP may offer a measure of the neuromuscular system “as a whole” that is complementary to SST by providing outcomes that are relatively independent of muscle size and function.