Iraklis Kollias

Perceptual-motor contributions to static and dynamic balance control in children.

Abstract The authors addressed balance control in children from the perspective of skill development and examined the relationship between specific perceptual and motor skills and static and dynamic balance performance. Fifty 11- to 13-year-old children performed a series of 1-legged balance tasks while standing on a force platform. Postural control was reflected in the maximum displacement of the center of mass in anterior-posterior and mediolateral directions. Simple visual, discrimination, and choice reaction times; sustained attention; visuomotor coordination; kinesthesis; and depth perception were also assessed in a series of perceptual and motor tests. The correlation analysis revealed that balancing under static conditions was strongly associated with the ability to perceive and process visual information, which is important for feedback-based control of balance. On the other hand, when greater task demands were imposed on the system under dynamic balancing conditions, the ability to respond to the destabilizing hip abductions-adductions in order to maintain equilibrium was associated with motor response speed, suggesting the use of a descending, feedforward control strategy. Therefore, like adults, 11- to 13-year-old children have the ability to select varying balance strategies (feedback, feedforward, or both), depending on the constraints of a particular task.

Using principal components analysis to identify individual differences in vertical jump performance.

e standing vertical jump has been used by physical T” education teachers and coaches as one of the most common tests to evaluate muscular strength of the lower limbs, power, and the so called “jumping ability” in many different sports. Several investigators have used the vertical jump as an experimental paradigm to evaluate the role of different strength training protocols in basketball and volleyball (Bobbert &Van Soest, 1994; Brown, Mayhew, & Boleach, 1986). However, the validity of the vertical jump as a measure of muscular strength of the lower limbs has been questioned lately, mainly due to the moderate association reported between muscular strength and vertical jumping performance (Genuario & Dolgener, 1980). It has been shown that strength training results in just small improvements (8-12%) of jumping performance (Blamer & Noble, 1979; Brown et al., 1986). Subsequent studies have shown that other factors such as the sequence of muscle activation orjoint reversals, individual joint contributions, optimal position of the body center of mass at the instant of take off, or the ability to transfer mechanical energy from the proximal to the more distal segments can also be important for determining successful performance in the vertical jump (Bobbert &Van Ingen Scheneau, 1988; Dowling & Vamos, 1993; Fukashiro, 8c Komi, 1987; Hudson, 1986). These studies allowed the identification of several predictor variables related to both timing and

Biomechanical Differences in Elite Beach-Volleyball Players in Vertical Squat Jump on Rigid and Sand Surface

The purpose of this investigation was to detect whether differences exist concerning the dynamic and kinematic parameters of vertical squat jump (SJ) on rigid (RS) and sand (SS) surface. Fifteen elite male beach volleyball players (age: 25.6 +/- 6.2 years; height: 188.0 +/- 3.5 cm; body mass: 83.2 +/- 6.0 kg; mean +/- SD, respectively) performed SJ. Force platform and kinematic analyses were used with paired sample T-tests to evaluate the differences. Vertical jump height was significantly smaller (p < .001) on SS than RS. Maximal force and maximal power were significantly higher on RS than SS (p < .05 and p < .01 respectively). Impulse time was larger in SS but with no significant difference (p = .286). Kinematic analysis revealed significant differences between the values of ankle joint during starting posture (p < .01) and of hip joint at the moment of take-off (p < .05). Ankle joint range of motion and angular velocity was larger in SS (p < .05). In conclusion, SJ height on SS was smaller than on RS because of the compliance and the instability of the sand. This resulted in a reduction in maximum force and take-off velocity. Furthermore, the compliance of SS made it hard for the ankle to push along the vertical axis of the movement of the body and as a result it slipped behind in an attempt to maximize propulsion. As a result, the body tries to balance and equalise this movement and move the hip to larger extension.Abstract The purpose of this investigation was to detect whether differences exist concerning the dynamic and kinematic parameters of vertical squat jump (SJ) on rigid (RS) and sand (SS) surface. Fifteen elite male beach volleyball players (age: 25.6±6.2 years; height: 188.0±3.5cm; body mass: 83.2±6.0 kg; mean ±SD, respectively) performed SJ. Force platform and kinematic analyses were used with paired sample T‐tests to evaluate the differences. Vertical jump height was significantly smaller (p < .001) on SS than RS. Maximal force and maximal power were significantly higher on RS than SS (p < .05 and p < .01 respectively). Impulse time was larger in SS but with no significant difference (p = .286). Kinematic analysis revealed significant differences between the values of ankle joint during starting posture (p < .01) and of hip joint at the moment of take‐off (p < .05). Ankle joint range of motion and angular velocity was larger in SS (p < .05). In conclusion, SJ height on SS was smaller than on RS because of the compliance and the instability of the sand. This resulted in a reduction in maximum force and take‐off velocity. Furthermore, the compliance of SS made it hard for the ankle to push along the vertical axis of the movement of the body and as a result it slipped behind in an attempt to maximise propulsion. As a result, the body tries to balance and equalise this movement and move the hip to larger extension.

Comparing jumping ability among athletes of various sports: vertical drop jumping from 60 centimeters.

&NA; Kollias, I., V. Panoutsakopoulos, and G. Papaiakovou. Comparing jumping ability among athletes of various sports: Vertical drop jumping from 60 centimeters. J. Strength Cond. Res. 18(3):546–550. 2004.—Drop jumping performance (DJP) is of high importance in order to achieve sporting performance in both team and individual sports. The purpose of the present study was to compare DJP among athletes from various sports. One hundred thirty‐eight male athletes (age: 22.3 ± 3.6 years, body height: 1.87 ± 0.08 m, body mass: 81.8 ± 10.8 kg) from 6 different sports performed drop jumps from 60 cm (DJ60) on a force plate. Results revealed that volleyball players jumped higher (p < 0.001) than other athletes. However, track and field athletes produced higher peak force and higher power output using a shorter upward phase (p < 0.001). Further examination using principal components analysis (PCA) revealed that team sport athletes and single scull rowers exhibited DJP utilizing force and time parameters differently than track and field athletes. Conclusively, DJP was different among athletes of various sports. Furthermore, PCA can be a useful method for evaluating the above mentioned differences and for monitoring drop jumping training programs.

Bilateral inter-arm coordination in freestyle swimming: effect of skill level and swimming speed.

The aim of this study was to examine the influence of level of skill and swimming speed on inter-limb coordination of freestyle swimming movements. Five elite (2 males, 3 females; age 18.9 ± 1.0 years, height 1.71 ± 0.04 m, body mass 62.1 ± 7.0 kg) and seven novice (age 22.0 ± 2.0 years, height 1.77 ± 0.04 m, body mass 74.8 ± 9.0 kg) swimmers swam a sprint and a self-paced 25 m freestyle trial. The swimming trials were recorded by four digital cameras operating at 50 Hz. The digitized frames underwent a three-dimensional direct linear transformation to yield the three-dimensional endpoint kinematic trajectories. The spatio-temporal relationship between the upper limbs was quantified by means of the peak amplitude and time lag of the cross-correlation function between the right and left arms endpoint trajectories. A strong anti-phase coupling between the two arms, as confirmed by peak amplitudes greater than 0.8, was noted for both groups and swimming speeds. Significantly higher (P  < 0.05) peak amplitudes were observed for the sprint compared with self-paced swimming. No significant differences in the strength of inter-limb coupling were noted between the elite and novice swimmers (P  > 0.05). Time lags were very close to 0 ms and did not differ between groups or swimming speeds. We conclude that in freestyle swimming, the intrinsic anti-phase (180° phase difference) inter-limb relationship is strongly preserved despite the physically powerful environmental influence of the water and this “preferred” pattern is not affected by level of skill. In contrast, increasing movement speed results in stronger inter-limb coupling that is closer to the anti-phase inter-limb relationship.

Angular momentum and landing efficiency in the long jump

Abstract The purpose of this study was to compare the amount of angular momentum and the efficiency of landing for the hang and 2½ hitch kick long jump techniques. Twelve male long jumpers participated in this investigation and were divided into two groups based on their flight technique (hang group, n=6; hitch kick group, n=6). The participants performed three jumps with full run-up, the best of which was selected for analysis. A two-dimensional DLT analysis of the take-off, flight, and landing was conducted for the two groups. Three cameras (JVC GR-DVL 9800 GL, Victor Company, Japan) with a sampling frequency 50 Hz were used. Two of the cameras were placed perpendicular to the run-up, on the right and left side of the take-off board, to record the take-off and the flight of the athletes (from both sides of the movement); the third was placed perpendicular to the pit, on the right side, to record the landing. The two groups differed in their angular momentum at the instant of take-off (hang: −11.92 kg·m2·s−1; hitch kick: −22.69 kg·m2·s−1; P<0.001) in the front direction, and in linear and angular velocity, mainly of the arms, at the instant of take-off. According to the results, the 2½ hitch kick group had greater angular and linear velocities (P<0.05). The 2½ hitch kick group had better landing efficiency than the hang style group (P<0.05), as the athletes in the former group managed to land their pelvis in front of their heels during landing. On the other hand, the hang style group landed their pelvis almost 5.81 cm behind the heels during landing and shortened their final distance. It seems that the 2½ hitch kick group had better landing efficiency because of their greater amount of angular momentum. The hang style group could have achieved better landing through an increase in angular momentum, which depends on an increase of the linear and angular velocities of the two arms during take-off.

Pelvis-Upper Trunk Coordination at Butterfly Stroke and Underwater Dolphin Kick: Application on an Elite Female Butterfly Swimmer

Pelvis-Upper Trunk Coordination at Butterfly Stroke and Underwater Dolphin Kick: Application on an Elite Female Butterfly Swimmer Swimming strokes kinematics are traditionally monitored, analyzed and extracted for a small part of a race. This practice may facilitate methodological limitations due to restricting issues arising from video analysis, though it is not scientifically solid, despite the cyclic nature of swimming movements, which may justify it. Skill level, swimming speed and fatigue are just some of the factors or constraints that may interfere with the cyclic nature of swimming and therefore affect inter-segmental coordination from stroke to stroke. There are studies that describe inter-segmental coordination in swimming using up to four consecutive stroke cycles. Nevertheless, studyinG coordination especially under the perspective of dynamic systems theory demands a large number of strokes to be analyzed in order to bring out the underlying coordination pattern, accounting also for its stability over time.

Biomechanical differences of arm swing countermovement jumps on sand and rigid surface performed by elite beach volleyball players

ABSTRACT The purpose of this study was to investigate the possible arm swing effect on the biomechanical parameters of vertical counter movement jump due to differences of the compliance of the take-off surface. Fifteen elite male beach-volleyball players (26.2 ± 5.9 years; 1.87 ± 0.05 m; 83.4 ± 6.0 kg; mean ± standard deviation, respectively) performed counter movement jumps on sand and on a rigid surface with and without an arm swing. Results showed significant (p < .05) surface effects on the jump height, the ankle joint angle at the lowest height of the body center of mass and the ankle angular velocity. Also, significant arm swing effects were found on jump height, maximum power output, temporal parameters, range of motion and angular velocity of the hip. These findings could be attributed to the instability of the sand, which resulted in reduced peak power output due to the differences of body configuration at the lowest body position and lower limb joints’ range of motion. The combined effect of the backward arm swing and the recoil of the sand that resulted in decreased resistance at ankle plantar flexion should be controlled at the preparation of selected jumping tasks in beach-volleyball.