Effect of skill decomposition on racket and ball kinematics of the elite junior tennis serve

Whole body kinematics of the tennis serve have been reported extensively in the literature, yet comparatively less information exists regarding the kinematic characteristics of the swing and toss. In attempting to develop consistency in placement of the toss and racket trajectory, coaches will often decompose the serve and practice it in separate parts. A 22-camera VICON MX motion analysis system, operating at 250 Hz, captured racket, ball and hand kinematics of the serves of five elite junior players under three conditions. The conditions were flat first serves (FS) directed to a 1 × 1 m target bordering the ‘T’ of the deuce service box, a ball toss (BT) in isolation and a free swing (SW) in isolation. Players were instructed to perform BT and SW as in the FS. Paired t-tests assessed within-group differences in hand, racket and ball kinematics between the discrete skill and the two decomposed conditions. Vertical displacement of the ball at its zenith increased significantly during BT compared with the FS and temporal associations between racket and ball motion during the FS (r = 0.861) were affected during task decomposition. This study questions the pervasive use of task decomposition in the development of the tennis serve.     

Physical demand of seven closed agility drills

The present study aimed to quantify the demand of seven generic, closed agility drills. Twenty males with experience in invasion sports volunteered to participate in this study. They performed seven, closed agility drills over a standardised 30-m distance. Physical demand measures of peak velocity, total foot contacts, peak impacts, completion time, and maximum heart rate were obtained via the use of wearable sensor technologies. A subjective rating of perceived exertion (RPE) was also obtained. All measures, with the exception of maximum heart rates and RPE were able to delineate drills in terms of physical and physiological demand. The findings of this study exemplify the differences in demand of agility-type movements. Drill demand was dictated by the type of agility movement initiated with the increase in repetitiveness of a given movement type also contributing to increased demand. Findings from this study suggest agility drills can be manipulated to vary physical and physiological demand. This allows for the optimal application of training principles such as overload, progression, and periodisation.     

网球拍甜区形成的生物力学机制及其实践意义

网球拍有其固有的打击中心，但是在体育运动实践中，人体对网球拍产生了一定的影响，使其打击中心发生了一定的偏移，一系列偏移点形成了所谓的甜区。为了获得更好的打击效果，运动员在使用网球拍时，应该了解人体部分质量的影响，即网球拍的有效质量对打击中心的影响，了解网球拍甜区形成的生物力学机制。     

Non-wear or sleep? Evaluation of five non-wear detection algorithms for raw accelerometer data

ABSTRACT

Detection of non-wear periods is an important step in accelerometer data processing. This study evaluated five non-wear detection algorithms for wrist accelerometer data and two rules for non-wear detection when non-wear and sleep algorithms are implemented in parallel. Non-wear algorithms were based on the standard deviation (SD), the high-pass filtered acceleration, or tilt angle. Rules for differentiating sleep from non-wear consisted of an override rule in which any overlap between non-wear and sleep was deemed non-wear; and a 75% rule in which non-wear periods were deemed sleep if the duration was < 75% of the sleep period. Non-wear algorithms were evaluated in 47 children who wore an ActiGraph GT3X+ accelerometer during school hours for 5 days. Rules for differentiating sleep from non-wear were evaluated in 15 adults who wore a GeneActiv Original accelerometer continuously for 24 hours. Classification accuracy for the non-wear algorithms ranged between 0.86–0.95, with the SD of the vector magnitude providing the best performance. The override rule misclassified 37.1 minutes of sleep as non-wear, while the 75% rule resulted in no misclassification. Non-wear algorithms based on the SD of the acceleration signal can effectively detect non-wear periods, while application of the 75% rule can effectively differentiate sleep from non-wear when examined concurrently.     

运动负荷术语的再思考

运动负荷是运动训练和体育锻炼中的最常用术语 ,对运动负荷的实质进行了分析 ,认为运动练习负荷、生理负荷和心理负荷是运动负荷不可分割的三个方面 ,应防止在运动训练和体育锻炼中的应用的混淆。     

运动员汗液分析技术进展

运动员训练和比赛时出汗导致水分和电解质等成分大量流失,并排出乳酸、尿素、肌酸、肌酐等代谢产物到汗液中。通过汗液分析可快速了解运动时运动员身体状况、机体代谢情况以及运动时机体的适应性,在运动员选材、日常体育训练和体能恢复等方面具有十分重要的实际意义。本文综述了近来运动员汗液分析技术的进展,力求全面反映运动员汗液分析的最新进展。     

预防慢性疾病发生、发展的最佳身体活动负荷研究综述

从身体活动对慢性疾病预防和控制功效的视角,从身体活动对不同年龄群体的生理退化速率、死亡率、疾病发生率、心血管疾病、糖尿病、癌症、骨质疏松症、心理健康的影响等方面对国外相关研究进行梳理,对国外身体活动负荷标准的研究进行归纳,重点分类归纳了预防慢性疾病发生的成年人(18～65岁)、老年人(65岁以上)和已患慢性疾病群体控制疾病发展的不同群体间身体活动的适宜负荷方式和剂量,对身体活动与慢性疾病的研究做出了展望。     

黑龙江省大学生体育课运动负荷现状的研究

通过对黑龙江省10所高校50节体育课的实验和测定,认为体育课负荷强度和练习密度最佳平均率就在130-150/min之间为适度,效果最佳。因此,在高校教学中必须科学地组织教法、合理安排运动负荷,才能有效地增强学生的体质,提高体育教学的质量,活跃学生的身心,培养更多的全面合格人才。     

Anticipation in squash: Differences in advance cue utilization between expert and novice players

The time of occurrence and spatial location of the advance cues used to anticipate the direction and force of an opponent's stroke in squash were examined using a film task. This task, designed to simulate the normal perceptual display of the defensive player, consisted of two discrete parts, each containing 160 individual stroke sequences (trials). In the first part of the film task, the display was occluded at different time intervals throughout the development of the opponent's stroke and the 16 expert and 20 novice subjects had to predict both the direction (down‐wall or cross‐court) and force (drive or drop shot) of the opponent's stroke. In the second part of the film task, visibility to selected advance cues was occluded by placing opaque mats on the film surface. Across all of the film task conditions experts were superior to novices in predicting the event outcome from the information available, highlighting the important contribution anticipatory skills make to expert performance in this sport. Analysis of lateral (direction) error showed that the most critical time periods for extracting information about stroke direction are the periods between 160–80 ms prior to racket‐ball contact and the period of extended ball flight arising at least 80 ms after contact. Whereas both groups were attuned to this ball flight information, only the experts were capable of picking up information from the early part of the opponent's actions. This early information appeared to be provided by the opposing player's arm action. Similar time periods were found to be also important for the prediction of stroke depth, but in this case both experts and novices were similar in their cue dependence.     

对大学体育课学生心理负荷的研究

本文运用心理学多种研究方法,对大学体育课学生的心理负荷进行了定量的描述。探讨了心理负荷与制约因素的关系,研究了意向过程心理负荷曲线的变化特点,论证了心理负荷与学生体质、运动意志、教材喜爱度、课程形式的内在关系,并对典型的心理负荷曲线,进行了全面的分析。     

湘中地区中小城镇社区体育志愿者现状研究

采用文献资料法、调查法、数理统计法、逻辑分析法针对湘中地区中小城镇社区体育志愿者现状进行研究。结果表明：目前湘中地区中小城镇社区体育志愿者数量少,专业水平不高,没有相关的制度,远不能满足社区体育人群的需要。并提出相关建议,以为相关部门提供参考。     

科学体育在全民健身中的作用

通过对科学体育的分析研究,论述了全民健身活动的理论基础,探讨了“运动处方”和“运动负荷价值阈”在全民健身活动中的作用和运用方法。     

The effects of localised fatigue on upper extremity jump shot kinematics and kinetics in team handball

Team handball is a popular sport worldwide that requires numerous throws to be made throughout the course of a game. Because of the upper extremity demands of repetitive throwing, it is possible that fatigue can alter the mechanics of a shot. The purpose of this study was to determine the influence of localised fatigue on jump shot kinematics and kinetics. Eleven male team handball players (23.1 ± 3.1 years; 185.1 ± 8.3 cm; 89.7 ± 12.2 kg) volunteered. An electromagnetic tracking system was used to examine the jump shot prior to and following localised fatigue. The fatiguing protocol consisted of throwing a 2.2 kg medicine ball into a rebounder until volitional fatigue. No significant kinematic or kinetic differences were observed following fatigue. Shoulder external rotation was ?74.8 ± 14.9° prior to and ?79.0 ± 14.7° following fatigue at MER. Scapula, external rotation at ball release (BR) prior to fatigue was ?2.2 ± 7.0° and ?3.2 ± 11.1° following fatigue. Scapular internal rotation, at maximum shoulder internal rotation (MIR), changed from 18.4 ± 11.2° to 20.4 ± 11.8°. Ball velocity decreased from19.8 m · s^–1 to 18.8 m · s^–1 (P = 0.12). Accuracy percentage in the pre-fatigue trials was 60.8 ± 14.1% and 52.8 ± 12.7% following fatigue (P = 0.20). While no significant changes were observed, it is possible that other fatiguing protocols that more closely represent the aerobic and throwing demands of the sport may have a greater effect on the kinematics and kinetics of the jump shot.