Slobodan Jaric | University of Delaware (original) (raw)

Papers by Slobodan Jaric

Journal of Applied Biomechanics, 2015

This study aimed at (1) evaluating the linearity of the force-velocity relationship, as well as t... more This study aimed at (1) evaluating the linearity of the force-velocity relationship, as well as the reliability of maximum force (F0), maximum velocity (V0), slope (α), and maximum power (P0); (2) comparing these parameters between the traditional and ballistic bench press (BP); and (3) determining the correlation of F0 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 &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt; 0.99). While F0 and P0 were highly reliable (ICC [intraclass correlation coefficient]: 0.91-0.96, CV [coefficient of variation]: 3.8-5.1%), lower reliability was observed for V0 and α (ICC: 0.49-0.81, CV: 6.6-11.8%). Trivial differences between exercises were found for F0 (ES [effect size] &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt; 0.2), however the α was higher for the traditional BP (ES: 0.68-0.94), and V0 (ES: 1.04-1.48) and P0 (ES: 0.65-0.72) for the ballistic BP. The F0 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.

A range of force (F) and velocity (V) data obtained from functional movement tasks (e.g., running... more A range of force (F) and velocity (V) data obtained from functional movement tasks (e.g., running, jumping, throwing, lifting, cycling) performed under variety of external loads have typically revealed strong and approximately linear F-V relationships.

Journal of sports sciences, Jan 4, 2016

The aim of this study was to assess the effect of a unilateral anterior cruciate ligament reconst... more The aim of this study was to assess the effect of a unilateral anterior cruciate ligament reconstruction (ACLR) on maximum voluntary contraction (MVC) and explosive strength of both the involved limb and the uninvolved limb. Nineteen male athletes completed a standard isometric testing protocol 4 months post-ACLR, while 16 healthy participants served as a control group (CG). The explosive strength of the knee extensors and flexors was assessed as RFD obtained from the slope of the force-time curves over various time intervals. Both muscle groups of the involved limb had significantly lower MVC compared to the uninvolved. The involved limb also had significantly lower RFD in the late phase of contraction (140-250 ms) for both knee extensors and flexors (P < 0.05). There was no difference in MVC between the uninvolved limb and the CG. However, RFD of the uninvolved limb was lower compared to CG for both knee extensors (0-180 ms; P < 0.01) and flexors (0-150 ms; P < 0.05). ACL...

Journal of sports sciences, Jan 28, 2016

A mathematical model was developed for the assessment of the starting velocity and initial veloci... more A mathematical model was developed for the assessment of the starting velocity and initial velocity and force of a 100-m sprint, based on a non-homogeneous differential equation with the air resistance proportional to the velocity, and the initial conditions for [Formula: see text], [Formula: see text]The use of this model requires the measurement of reaction time and segmental velocities over the course of the race. The model was validated by comparison with the data obtained from 100-m sprints of men: Carl Lewis (1988), Maurice Green (2001) and Usain Bolt (2009), and women: Florence Griffith-Joyner, Evelyn Ashford and Drechsler Heike (1988) showing a high level of agreement. Combined with the previous work of the authors, the present model allows for the assessment of important physical abilities, such as the exertion of a high starting force, development of high starting velocity and, later on, maximisation of the peak running velocity. These data could be of importance for pract...

Sports Medicine, Feb 1, 2002

Assessment of muscle strength tests has been a popular form of testing muscle function in sports ... more Assessment of muscle strength tests has been a popular form of testing muscle function in sports and exercises, as well as in other movement-related sciences for several decades. Although the relationship between muscle strength and body size has attracted considerable attention from researchers, this relationship has been often either neglected or incorrectly taken into account when presenting the results from muscle strength tests. Two specific problems have been identified. First, most of the studies have presented strength data either non-normalised for body size, or normalised using inappropriate methods, or even several different normalisations have been applied on the same sets of data. Second, the role of body size in various movement performances has been neglected when functional movement performance was assessed by muscle strength. As a consequence, muscle function, athletic profiles, or functional movement performance assessed by tested muscle strength have been often confounded by the effect of body size. Differences in the normalisation methods applied also do not allow for comparison of the data obtained in different studies. Using the following allometric formula for obtaining index of muscle strength, S, independent of body size (assessed by body mass, m) should be recommended in routine strength testing procedures: The allometric parameter should be either b = 0.67 for muscle force (recorded by a dynamometer), or b = 1 for muscle torque (recorded by an isokinetic apparatus). We also recommend using body-size-independent indices of both muscle strength and movement performance when assessing functional performance from recorded muscle strength or vice versa.

Medicine Science in Sports Exercise Official Journal of the American College of Sports Medicine, Oct 1, 2007

Purpose: The aim of this study was to evaluate the effect of external loading on mechanics of ver... more Purpose: The aim of this study was to evaluate the effect of external loading on mechanics of vertical jumping. We hypothesized that the muscular mechanical output could be higher under no-load conditions than in the presence of either positive or negative external loads. Methods: Fifteen physically active men performed maximal countermovement jumps (CMJ) on a force plate while a pulley system provided approximately constant vertical force acting in a way to either reduce or increase the body weight. As a result, the weight of the body approximately corresponded to the gravity acceleration from 0.70 to 1.30g (g = 9.81 mIs j2 ). Results: Regarding the jumping kinematics, we observed a significant (P G 0.001) load-associated decrease in both the peak velocity and lowering of the center of mass during the eccentric jump phase, but not in the duration of the subsequent concentric jump phase. Regarding the muscular mechanical output, both the mean power (P ) and peak momentum (M) revealed significant (P G 0.001) changes associated with loading, and further post hoc analyses revealed significantly higher values (P G 0.05-0.001) of both P and M for 1.00g compared with most of the other loading conditions applied. Conclusion: The results suggest that subject_s own body provides the optimal load for producing maximum mechanical output in vertical jumping. If corroborated by the results of future studies performed on other rapid movement, our findings could support the hypothesis that the muscular system is designed for producing maximum mechanical output in rapid movements when loaded only with the weight and inertia of its own body.

European Journal of Applied Physiology and Occupational Physiology, Feb 1, 1995

The Journal of Strength and Conditioning Research, Dec 1, 2003

Motor Control, Aug 1, 2002

Experimental Brain Research Experimentelle Hirnforschung Experimentation Cerebrale, Mar 1, 2002

Changes in the structure of motor variability during practicing a bimanual pointing task were inv... more Changes in the structure of motor variability during practicing a bimanual pointing task were investigated using the framework of the uncontrolled manifold (UCM) hypothesis. The subjects performed fast and accurate planar movements with both arms, one moving the pointer and the other moving the target. The UCM hypothesis predicts that joint kinematic variability will be structured to selectively stabilize important task variables. This prediction was tested with respect to selective stabilization of the trajectory of the endpoint of each arm (unimanual control hypotheses) and with respect to selective stabilization of the timecourse of the vectorial distance between the target and the pointer tip (bimanual control hypothesis). Components of joint position variance not affecting and affecting a mean value of a selected variable were computed at each 10% of normalized movement time. The ratio of these two components (R V ) served as a quantitative index of selective stabilization. Both unimanual control hypotheses and the bimanual control hypothesis were supported both prior to and after practice. However, the R V values for the bimanual control hypothesis were significantly higher than for either of the unimanual control hypothesis, suggesting that the bimanual synergy was not simply a simultaneous execution of two unimanual synergies. After practice, an improvement in both movement speed and accuracy was accompanied by counterintuitive changes in the structure of kinematic variability. Components of joint position variance affecting and not affecting a mean value of a selected variable decreased, but there was a significantly larger drop in the latter when applied on each of the three selected task variables corresponding to the three control hypotheses. We conclude that the UCM hypothesis allows quantitative assessment of the degree of stabilization of selected performance variables and provides information on changes in the structure of a multijoint synergy that may not be reflected in its overall performance. Fig. 1A-E Possible changes in the components of variance (V COMP and V UN ) between the pre-test (A, B) and post-test (C-E).

Medicine Science in Sports Exercise Official Journal of the American College of Sports Medicine, May 1, 2009

It is well known that both individual muscle and muscle groups produce maximum power against part... more It is well known that both individual muscle and muscle groups produce maximum power against particular external loads. Within the present review, we propose the hypothesis that the lower-limb muscles of physically active individuals are predominantly designed to provide the maximum dynamic output (MDO; assessed as power and momentum) in rapid movements like jumping and sprinting against the load imposed by the weight and the inertia of their own body. The evidence supporting the MDO hypothesis can be found in some general considerations (e.g., certain evolutionary aspects, muscular system design in animals, effects of athletic training) as well as in recent experimental findings. Specifically, here we show that the optimal load for the power and momentum production in vertical jumping in habitually active individuals (but not in strength/power-trained athletes) could be the subject's own body. This also implies that the performance of rapid movements corresponds to body-size-independent MDO of the lower-limb muscles. If supported by future research, MDO hypothesis could 1) provide a theoretical framework for relating both structure and function of the muscular system and for understanding long-term adaptation of the muscular system; 2) suggest that rapid movements, such as vertical jumps, performed without external load could be used for the assessment of MDO (power and momentum) of lower limbs in nonathletic population; and 3) simplify the assessment of physical abilities and neuromuscular function in general through the usage of simple and relatively inexpensive physical performance tests based on natural rapid movements.

Journal of Strength and Conditioning Research the Research Journal of the Nsca, 2005

We review a series of studies that used the framework of the uncontrolled manifold (UCM) hypothes... more We review a series of studies that used the framework of the uncontrolled manifold (UCM) hypothesis to quantify changes in motor synergies with practice. The UCM hypothesis states that control of a multi-element action, at any time, may be associated with creation of a subspace (a UCM) within the state space of the elements. This subspace corresponds to a stable value of an important performance variable or several variables. Strength of a synergy may be estimated quantitatively as proportion of the total variance of its elements, e.g. across several trials at a task, that lies within the UCM. Quantitative analysis of covariation patterns of kinematic and kinetic elemental variables with respect to stabilization of different, task -specific performance variables allowed to monitor changes in motor synergies with practice. The studies have demonstrated two stages in practice-related effects. Early in practice of novel tasks, synergies stabilizing important performance variables emerged and strengthened. Later, in some instances, variability of elemental variables which did not affect the performance variables decreased more rapidly leading to the synergies becoming seemingly weaker. Experiments with transcranial magnetic stimulation applied to the primary motor cortex have suggested that practice led to plastic changes in neural structures mediating motor response to the stimulation.

The Journal of sports medicine and physical fitness