Vibration and Rebound Velocity Characteristics of Conventional and Oversized Tennis Rackets

Abstract

Stroboscope photography and accelerometry techniques were used to measure rebound velocities of tennis balls from impact locations on the strings and vibration levels at the rotation point on the racket handle for both conventional and oversized tennis rackets. The oversized rackets demonstrated lower vibration levels and higher rebound velocities than their conventional counterparts when balls struck by the racket were compared along a transverse axis drawn perpendicular to the racket shaft and through the geometric center of the strings. These differences were, however, only significantly different (p < .01) at the impact location 6 cm along this axis toward the top edge of the racket. Higher rebound velocities were recorded at all impact points from the oversized rackets along the axis in line with the racket shaft. Significantly lower vibration levels were apparent at locations 4 cm, 6 cm, and 8 cm from the string center away from the racket handle. The lower vibration levels, particularly at the extremes of the racket face, in conjunction with higher rebound velocities, support the concept that this new racket design is of practical benefit to users.     

The effect of a fast bowling harness in cricket: an intervention study

The influence of a bowling harness, as a training aid, was assessed as a means of modifying bowling technique. Thirty-three 13-year-old bowlers received a standardized 15 min of bowling coaching twice a week for 8 weeks. They were randomly assigned to one of two groups. The 13 participants in the intervention group used the bowling harness throughout the coaching, while also receiving verbal and visual feedback. The 20 participants in the non-harness group received the same visual and verbal feedback. Three-dimensional videography (200 Hz) of each player's bowling action enabled the calculation of transverse plane shoulder alignment counter-rotation, separation angle, lateral flexion and hyperextension of the trunk before and after the intervention. The restriction applied by the harness produced a significant reduction ( P = 0.006) in separation angle and forced the bowler to adopt a position at back-foot impact that reduced the 'twist' in the spine. However, it had no effect on restricting other aspects of trunk movement during the critical phases of the bowling action. No significant long-term modifications to technique were found after the coaching intervention when players were assessed without the harness.     

The Influence of Tennis Racket Flexibility and String Tension on Rebound Velocity Following a Dynamic Impact

Abstract

The effects of string tension and longitudinal racket flexibility on post-impact ball velocity were investigated in tennis. Six wooden rackets, two with flexible shafts, two with medium and two with stiff shafts were strung with synthetic gut at tensions of 245N (55 lb), 289N (65 lb) and 334N (75 lb).

A pneumatically driven racket-arm was triggered by a stimulus from a photo-electric cell positioned at the exit nozzle of a ball machine so that impact occurred with the racket perpendicular to the path of the ball. New tennis balls were fired to impact each racket at the geometric center of the strings and 5 cm above the geometric center. The average horizontal velocity of the ball, both before and after impact, was determined using stroboscope photography.

A significant interaction between racket stiffness and string tension was recorded for an inward ball velocity of 22.7 m/s and a racket velocity of approximately 6.8 m/s. String tension had no significant influence on rebound velocity for a stiff racket following impact with a moving racket. Medium and flexible rackets produced the highest coefficients of restitution when strung at 245N (55 lb) compared to 289N (65 lb) and 334N (75 lb).     

Variance and Invariance in Expert and Novice Triple Junipers

Abstract

The purpose of this study was to examine relative timing, relative distance, and effector independence characteristics of the triple jump. We videotaped expert and novice triple jumpers using standing and running starts. On some trials, the triple jumpers were required to take off from their nondominant leg. Gentner's (1987) interaction test was used to assess the proportional duration model in describing the timing of the component submovements. For both experts and novices, the proportional durations varied with the type of start. For experts, the relative timing of the jump components was independent of the take-off leg. The findings were mixed for the novices. Similar results were obtained when relative distance was examined. These results are discussed with respect to constructs related to variability in skilled performance.     

Is the Study Process Questionnaire (SPQ) a good predictor of academic achievement? Examining the mediating role of achievement-related classroom behaviours

Studies have shown that the Study Process Questionnaire (SPQ)—which provides a measure of student approaches to learning—is a relatively weak predictor of academic achievement. The present study sought to explore whether students’ achievement-related classroom behaviours, as observed by teachers, can be used as a mediator between student approaches to learning and academic achievement. The SPQ was administered to 1,608 students enrolled in six different diploma programmes offered by a polytechnic in Singapore. Data were analysed by means of correlation and path analysis. In line with existing studies, the results revealed that student approaches to learning was a weak predictor of academic achievement. However, achievement-related classroom behaviours turned out to be a significant mediator between student approaches to learning and academic achievement, effectively doubling the explained variance in academic achievement. Implications of these findings for using the SPQ are discussed.     

Self-Esteem, Locus of Control, College Adjustment, and GPA Among First- and Continuing-Generation Students: A Moderator Model of Generational Status

The role of generational status (first-generation vs. continuing-generation college students) as a moderator of the relationship between psychological factors and college outcomes was tested to determine whether generational status acts as a risk factor or as a sensitizing factor. The sample consisted of 322 undergraduate students who completed online measures of self-esteem, locus of control, and academic adjustment and provided self-reports of GPA. Generational status significantly moderated the relationship between psychological factors and academic outcomes. Generally, it was found that the relationship between psychological factors and academic outcomes were strongest among first-generation students. Further, it was found that for the majority of the interactions with locus of control, first-generation status acted as a sensitizing factor that amplified both the positive and negative effects of locus of control. In contrast, for self-esteem, first generation status acted as a risk factor that only exacerbated the negative effects of low self-esteem. These results are interpreted as reflecting motivational differences between first- and continuing-generation students and are discussed with respect to the social/cultural capital hypothesis that is most frequently presented in the existing literature.     

Consensus or dissensus? Stakeholders’ views on the role of play in learning

Across cultures and eras, children have engaged in play as part of the process of growing and maturing into adulthood. Play has been recognized as an effective form of pedagogy to promote learning in the early years. However, beliefs about what play is and how it should be practiced vary across Hong Kong and in other countries. Because Chinese culture places a heavy emphasis on academic achievement, a play-based curriculum has not been widely implemented in the region. Through classroom observations and interviews with key stakeholders in early childhood education, namely parents, teachers, and principals, this study reveals the complexity of views on this matter and uncovers the root cause of the difficulties in supporting play in the Hong Kong kindergartens. These findings may help promote the play-based curriculum in Hong Kong and other countries where play is in crisis.