Biomedical subjects

S Jaric

Publications and source records attributed to S Jaric.

18 recordsLinked to original sources

PubMed2006

Evaluation of tests of maximum kicking performance.

AIM: Despite the important role of kicking in various athletic activities, the reliability of tests of maximum kicking performance has not been evaluated. The aim of the present study was to assess the reproducibility of performance of standing kick, instep kick and drop kick. METHODS: Male physical education students (n=77) were tested on maximum kicking performance by means of a standard Doppler radar gun. RESULTS: The maximal ball speed in the standing kick, instep kick and drop kick (averaged across the subjects and trials) were 19.8+/-1.9 m s(-1), 26.7+/-2.7 m s(-1) and 25.3+/-2.2 m s(-1), respectively. There were no significant differences in the tested performances among the consecutive kicking trials of each test. The intraclass correlation coefficients ranged between 0.94 and 0.96 (95% confidence intervals 0.93-0.97). The limits of agreement for maximum ball speed in all three tests ranged from 0.2+/-1.4 m(-1) to 0.3+/-1.3 m s(-1), suggesting that in 95% of repeated trials the ball speed might be from 1.2 m s(-1) less to 1.6 m s(-1) greater than the original estimate. The coefficients of variation for all kicking tests were between 2.6% and 3.3% (95% confidence intervals; 2.2-3.9%) suggesting a low intra-subject variability. CONCLUSIONS: Due to a high reliability, relative simplicity, and a small number of participants needed to detect worthwhile changes, the evaluated kicking tests could be highly recommended for sport specific profiling and early selection of young athletes, as well as for the assessment of training procedures and other interventions applied on individual teams of elite soccer, rugby or American football players.

Adult↗

PubMed2003

Vertical posture and head stability in patients with chronic neck pain.

OBJECTIVE: To evaluate postural performance and head stabilization of patients with chronic neck pain. DESIGN: A single-blind comparative group study. SUBJECTS: Patients with work-related chronic neck pain (n = 9), with chronic whiplash associated disorders (n = 9) and healthy subjects (n = 16). METHODS: During quiet standing in different conditions (e.g. 1 and 2 feet standing, tandem standing, and open and closed eyes) the sway areas and the ability to maintain the postures were measured. The maximal peak-to-peak displacement of the centre of pressure and the head translation were analysed during predictable and unpredictable postural perturbations. RESULTS: Patients with chronic neck pain, in particular those with whiplash-associated disorders, showed larger sway areas and reduced ability to successfully execute more challenging balance tasks. They also displayed larger sway areas and reduced head stability during perturbations. CONCLUSION: The results show that disturbances of postural control in chronic neck pain are dependent on the aetiology, and that it is possible to quantify characteristic postural disturbances in different neck pain conditions. It is suggested that the dissimilarities in postural performance are a reflection of different degrees of disturbances of the proprioceptive input to the central nervous system and/or of the central processing of such input.

Adult↗

PubMed2002

Evaluation of methods for normalizing muscle strength in elite and young athletes.

BACKGROUND: The following equation has been most often used in order to present recorded muscle strength (F) normalised for various measures of body size (such as body mass m) Fnor = F x m(-b). Since previous studies have provided inconsistent results, the purpose of the present one was to assess the value of the exponential parameter b (b-value) that (a) provides both the normalised strength independent of body mass and (b) demonstrates the highest possible correlation between the normalised strength and standard tests of movement performance. METHODS: Thirteen groups of male and female athletes of both different age and different sport specialisation were tested for maximal isometric force of various leg muscle groups, as well as for vertical and standing long jump. RESULTS: The data obtained on most of the experimental groups suggested that the b-value close to the theoretically predicted b=0.67, but also different from b=0 and b=1, should be used for calculation of the normalised strength independent of body mass. Although the strength normalised using b=0.67 and b=1 demonstrated higher correlation with jumping performance than the non-normalised strength (i.e., b=0), the data also suggested that b>1 could provide the highest prediction of movement performance. CONCLUSIONS: Muscle strength should be calculated as force per kg(2/3) rather than as absolute force or force calculated per kg of body mass in order to assess strength independent of body size in routine testing of muscle function. However, different and probably task specific normalisation methods could be required in order to predict performance of various movement tasks by assessed muscle strength.

Adolescent↗

PubMed2001

Effects of plyometric training on jumping performance in junior basketball players.

BACKGROUND: Although the plyometric training has proved its efficiency, it remains generally unknown whether a limited amount of plyometric training could improve movements in subjects who already demonstrate high level of performance. METHODS: Three different training regimens were performed in order to study effects of plyometric training on elite junior basketball players. While control group (CG) participated only in the regular midseason training activity, another two groups performed a limited amount of plyometric training employing drop jumps from the height of either 50 cm (EG-50) or 100 cm (EG-100). The height of the maximal vertical jump (CMJ), as well as the maximal voluntary force (F) and the rate of force development (RFD) of hip and knee extensors were tested prior to and after the training. RESULTS: An increase in CMJ (4.8 and 5.6 cm in EG-50 and EG-100, respectively), as well as in F of hip extensors and RFD of knee extensors was observed in both experimental groups, while no significant changes were recorded in CG. When the pretest scores were used as a covariate, both experimental groups demonstrated higher increase in CMJ and RFD of knee extensors then CG. However, no differences were observed between EG-50 and EG-100. The multiple correlation between four isometric parameters and CMJ revealed R2=0.29. CONCLUSIONS: A limited amount of plyometric training could improve jumping performance in elite junior basketball players and this improvement could be partly related with an increase in F of hip extensors and RFD of knee extensors. However, neither of the two initial heights of the applied drop jumps proved to be more effective.

Adolescent↗

PubMed2000

Changes in movement symmetry associated with strengthening and fatigue of agonist and antagonist muscles.

The hypothesis that strengthening or fatiguing procedures applied on active muscles can affect the symmetry of rapid, discrete movements was tested. Subjects (N = 12) performed rapid, consecutive elbow flexions and extensions between 2 targets before and after (a) applying a strength training program, (b) fatiguing elbow flexors, and (c) fatiguing elbow extensors. The results demonstrated that an increase in strength of elbow extensors caused by applied strength training is associated with an increase in the symmetry ratio (i.e., acceleration time divided by deceleration time) of elbow flexion movements. The symmetry ratio also increased and decreased in movements when agonists and antagonists were fatigued, respectively. Because the strength training and fatiguing procedures are both known to affect muscle force, the data are interpreted as changes in muscles' ability to exert the force while acting as agonists or antagonists. Namely, muscles need equal impulses of force (torque multiplied by time) to accelerate and, thereafter, to decelerate the limb while performing a rapid, discrete movement. The symmetry ratio may therefore be changed so that more time will be provided for muscles that become relatively weaker (compared with their antagonists) because a strengthening or fatiguing procedure has been applied, whereas a shorter time period should be sufficient for action of their stronger antagonists. Although, in the literature, the studied phenomenon has been discussed as a predominantly motor control phenomenon, the present data suggest that the movement symmetry could also be related to agonists' and antagonists' ability to exert force, particularly while performing rapid, discrete movements.

Adult↗

PubMed2000

Changes in movement variables associated with transient overshoot of the final position.

Transient overshoot (TO), which is assessed as the distance between the movement amplitude and the final position, was measured in a series of rapid, discrete elbow flexion movements performed under different distance and loading conditions by 7 participants. A positive relationship was found between kinematic variables (peak velocity, peak acceleration and deceleration, and the symmetry ratio) and the magnitude of TO, particularly in short movements performed against a light load. The relationships between TO and electromyographic (EMG) variables were low and mainly insignificant. Thus, TO contributes to the variability of rapid, discrete movements and therefore should be taken into account as an additional parameter in studies of the scaling of movement variables with movement mechanical conditions. TO could also represent a consequence of mechanical properties of the single-joint system rather than an independently programmed primary submovement.

Adult↗

PubMed1999

Changes in movement final position associated with agonist and antagonist muscle fatigue.

We have tested the hypothesis that agonist and antagonist muscle fatigue could affect the final position of rapid, discrete movements. Six subjects performed consecutive elbow flexion and extension movements between two targets, with their eyes closed prior to, and after fatiguing the elbow extensor muscles. The results demonstrate that elbow extension movements performed in the post-test period systematically undershot the final position as compared to pre-test movements. However, attainment of the aimed final position in elbow flexion movements was unaffected by fatiguing of the extensor muscles. Undershoot of the final position obtained in extension movements was associated with agonist muscle fatigue, a result that was expected from the point of view of current motor control theories, and that could be explained by a reduced ability of the shortening muscle to exert force. On the other hand, the absence of the expected overshoot of the final position when the antagonist is fatigued, indicates the involvement of various reflex and/or central mechanisms operating around the stretched muscle that could contribute to returning the limb to the standard final position after a brief prominent overshoot.

Adult↗

PubMed1999

Effects of direction and curvature on variable error pattern of reaching movements.

A number of studies have analyzed various indices of the final position variability in order to provide insight into different levels of neuromotor processing during reaching movements. Yet the possible effects of movement kinematics on variability have often been neglected. The present study was designed to test the effects of movement direction and curvature on the pattern of movement variable errors. Subjects performed series of reaching movements over the same distance and into the same target. However, due either to changes in starting position or to applied obstacles, the movements were performed in different directions or along the trajectories of different curvatures. The pattern of movement variable errors was assessed by means of the principal component analysis applied on the 2-D scatter of movement final positions. The orientation of these ellipses demonstrated changes associated with changes in both movement direction and curvature. However, neither movement direction nor movement curvature affected movement variable errors assessed by area of the ellipses. Therefore it was concluded that the end-point variability depends partly, but not exclusively, on movement kinematics.

Adult↗

PubMed1999

Anthropometric, strength, and power predictors of sprinting performance.

BACKGROUND: The purpose of this study was to examine relations between sprinting performance (i.e. average velocity within both the initial acceleration and maximum speed phases of sprint running) and some standard anthropometric, strength, and power tests. METHODS: Twenty-four male students of physical education were timed over the distances of 0.5-15 m and 15-30 m from the sprint start. Several measures of muscle isometric strength (knee extensors, hip extensors and flexors) and power (height of the counter movement jump and the average power of leg extensors during continuous jumping) were also collected, in addition to the lean body mass and the percentage of both muscle and fat tissue. RESULTS: The results obtained demonstrated that, except for the height of the counter movement jump, all correlation coefficients between the selected variables and sprinting performance were low and, therefore, insignificant. As a consequence, multiple correlation coefficients were also low (0.43 and 0.56 for the initial acceleration and maximal speed phase, respectively). CONCLUSIONS: Most of the standard anthropometric, strength and power tests could be poor predictors of sprinting performance. A better assessment of sprinting performance could be based on more specific tests that, unfortunately, require more complex measurements.

Adult↗

PubMed1998

Changes in the symmetry of rapid movements. Effects of velocity and viscosity.

Five subjects made rapid, discrete elbow flexion movements over different distances, against different inertial loads, as well as under distance and load combinations that kept movement time constant. The results demonstrated that an increase in peak movement velocity was associated with an increase in the temporal symmetry ratio of the movement (acceleration time divided by deceleration time), as well as with an increase in both agonist electromyographic (EMG) burst duration and antagonist EMG latency. Since an increase in peak movement velocity is associated with faster agonist muscle shortening, as well as with faster stretching of the antagonist muscle, we hypothesize that the velocity-related changes in movement symmetry can be viewed as, at least partially, a consequence of muscle viscosity. Viscosity increasingly resists the shortening agonist and assists the lengthening antagonist when movement velocity increases. Therefore, the agonist muscles require more time to produce the required impulse, while the antagonist muscle can brake the movement in a shorter period of time. In order to test the hypothesis that viscosity is responsible for the velocity-associated changes in the symmetry ratio, we performed a second experiment with distance and load combinations identical to those of the first experiment, but with different external viscous loads, which resisted the slower and assisted the faster movements. The results demonstrated that the movements became more symmetrical in the presence of the viscous load. There were also changes in agonist duration and antagonist latency. We conclude that changes in the symmetry associated with changes in movement velocity may be due to the effects of either muscle viscosity or changes in how muscles are activated to account for differences in viscous force.

Adult↗

PubMed1998

Effects of arm and leg loading on sprint performance.

The effects of loading on sprint kinematics were examined in 24 male students. The moment of inertia of either the arms or legs was increased by up to 50% of their unloaded values and the time for distances of 0.5-15 m and 15-30 m from a sprint start was measured. An increase in leg loading was associated with a gradual decrease in velocity of both sprint phases, while the change associated with arm loading was modest and significant only in the second phase. The decrease in sprint velocity was predominantly due to a reduction in stride rate, while the stride length remained almost unchanged. It was concluded that leg loading affected sprint velocity more than arm loading, and also that the velocity was reduced due to a decrease in the stride rate rather than in the stride length.

Adult↗

PubMed1998

Learning a motor task involving obstacles by a multi-joint, redundant limb: two synergies within one movement.

In this study, we investigated the hypothesis that the problem of motor redundancy could be solved using synergies representing rules for relative joint involvement to ensure a desired endpoint trajectory which may be context-dependent and may change with practice. Subjects practised a planar movement 'as fast as possible' from an initial to a final position, avoiding three round obstacles. The improvement in motor performance included a decrease in movement time and in the error scores. It was accompanied by the emergence of two distinct synergies. The first one involved elbow and shoulder movements and was used to move from the initial position to the first obstacle, and from obstacle to obstacle, whereas the second, involving the wrist, was used while going around the obstacles. The first synergy was seen before practice and showed an increase in joint coupling with practice. The second synergy was not seen prior to practice; it could not be explained by the pseudo-inverse transformation. We concluded that the central nervous system (CNS) has options for solving the redundancy problem, and that the solutions may be chosen based on such considerations as accuracy requirements, inertial properties of segments, and efficacy of particular joints to move the endpoint along a desired trajectory.

Acceleration↗

PubMed1998

Learning transfer from flexion to extension movements: importance of the final position.

Nine subjects (experimental group) were tested on rapid elbow flexion and extension movements performed in the same final position, prior to 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) movement. 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 significant level. 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).

Adult↗

PubMed1994

The effects of practice on movement distance and final position reproduction: implications for the equilibrium-point control of movements.

Predictions of two views on single-joint motor control, namely programming of muscle force patterns and equilibrium-point control, were compared with the results of experiments with reproduction of movement distance and final location during fast unidirectional elbow flexions. Two groups of subjects were tested. The first group practiced movements over a fixed distance (36 degrees), starting from seven different initial positions (distance group, DG). The second group practiced movements from the same seven initial positions to a fixed final location (location group, LG). Later, all the subjects were tested at the practiced task with their eyes closed, and then, unexpectedly for the subjects, they were tested at the other, unpracticed task. In both groups, the task to reproduce final position had lower indices of final position variability than the task to reproduce movement distance. Analysis of the linear regression lines between initial position and final position (or movement distance) also demonstrated a better (more accurate) performance during final position reproduction than during distance reproduction. The data are in a good correspondence with the predictions of the equilibrium-point hypothesis, but not with the predictions of the force-pattern control approach.

Adult↗

PubMed1993

Principles for learning single-joint movements. I. Enhanced performance by practice.

This study investigated changes in myoelectric and mechanical variables for movements made "as fast as possible" as a function of practice in the context of the dual-strategy hypothesis of motor control (Gottlieb et al. 1989b). Five male subjects made 1400 rapid elbow flexion movements in ten blocks of 20 trials over seven experimental sessions. Improved performance was defined as increased peak movement velocity, decreased peak velocity variability, increased acceleration and deceleration, a proportionately greater increase in peak deceleration than peak acceleration, and greater consistency in terminal location. The changes observed over experimental sessions were very similar to (but larger and more consistent than) those seen for the first experimental session, with the partial exception of the timing of the antagonist electromyogram (EMG). In general, the increases in the values of the measured mechanical variables covary with myoelectric measures in the same way as when subjects are asked to intentionally change speed in accordance with the rules of the speed-sensitive strategy (Corcos et al. 1989). However, there are differences between subjects in the extent to which speed changes can be attributable to the agonist muscle, the antagonist muscle, or in the timing between the muscles. In one of the five subjects, the latency of the antagonist EMG decreased over blocks on the 1st day but increased over experimental sessions and was consequently activated proportionately later in the movement. This suggests that extended practice can give at least some subjects flexibility in modifying the motor programs that underlie movement.

Adult↗

PubMed1993

Principles for learning single-joint movements. II. Generalizing a learned behavior.

The previous paper in this series showed that changes both within and between experimental sessions can be understood in the framework of the dual-strategy hypothesis of motor control, with a modification sometimes required for the timing of the antagonist muscle. The present paper extends these findings by determining how practicing movements at one distance generalizes to changes in performance at other distances. Five subjects made elbow flexion movements over five different distances (pretest). They then performed 1400 movements (seven sessions of ten blocks of 20 trials) at only one of those distances. The subjects then repeated the flexion movements over the five different distances (posttest). On the posttest, subjects decreased their average movement time by 20 ms. In addition, their movements became less variable. The electromyographic pattern of the faster movements was characterized by a more rapidly rising electromyogram, for three of the subjects, and an antagonist latency that decreased.

Adult↗

PubMed1992

Effects of practice on final position reproduction.

Three subjects practiced fast, accurate 36 degrees elbow flexion movements to a 2.5 degrees target for 14 sessions of 100 trials (total, 1400 trials). Subjects then returned for a 15th experimental session in which they were asked to perform 15 movements under identical conditions to the practice condition. They were then tested under three experimental conditions without visual feedback: (1) identical to the practice conditions, (2) with small shifts in starting position (+/- 3 degrees of the practiced starting position), that were insufficient for subjective discrimination and, therefore, subjects were instructed to repeat the practiced movements; and (3) with a large shift in starting position (range, +/- 15 degrees of the practiced starting position), under the instruction to move to the same target. Experimental conditions 2 and 3 demonstrated that shifts in starting position were partially correlated with shifts in final position. These results are interpreted from the point of view of the equilibrium-point hypothesis of motor control.

Adult↗

PubMed1988

Practice improves even the simplest movements.

Three subjects practiced accurate, fast elbow flexions of 54 degrees to a 3 degrees wide target. Movements of 36 degrees, 54 degrees and 72 degrees were then tested. Comparison over the three distances showed that the normally monotonic relationship between movement distance and movement time is alterable by specific training. Subjects learn to go faster over the practiced distance by refining their neural commands to the muscles. The benefits of practice only partially transfer to other distances. We conclude that many of the relationships seen among movement variables in simple tasks are plastic in nature and affected by prior experience.

Adult↗