动力学数学模型下弧圈球运行轨迹的研究     

高颖 《河北体育学院学报》2013,(4):79-82

通过建立乒乓球的动力数学模型,对弧圈球在每一种情况下的运动规律及运行轨迹变化曲线进行分析。结果显示：影响弧圈球运行轨迹的主要因素有乒乓球运动方向、运动速度、旋转速率、加速度及Magnus力,其中乒乓球运动方向和Magnus力的变化是改变乒乓球速度变化的重要因素;在运行过程中,球向的不断变化使弧圈球的速度及加速度也不断改变;而影响乒乓球运行轨迹的直接因素是旋转速率,且弧圈球第一弧线和第二弧线在不同旋转速率下存在着明显的区别。  相似文献   

Strength,power, and postural control in seniors: Considerations for functional adaptations and for fall prevention     

《European Journal of Sport Science》2013,13(6):325-340

Abstract

The ageing neuromuscular system is affected by structural and functional changes that lead to a general slowing down of neuromuscular performance and an increased risk of falling. As a consequence, the process of ageing results in a reduced ability to develop maximal and explosive force, as well as in deficits in static and dynamic postural control. A decrease in the number and size of type II fibres in particular accounts for the age-related decline in muscle mass (sarcopaenia) and strength performance. Multiple denervation and re-innervation processes of muscle fibres seem to be responsible for the reduced number of muscle fibres. Recently, it has been suggested that it is not the decline in motoneurons that accounts for the loss in number of muscle fibres but the disturbed potential of fibre regeneration and re-innervation. Furthermore, an age-related reduction in the number of satellite cells has also been associated with sarcopaenia. The ability to compensate for platform and gait perturbations deteriorates with ageing as reflected in longer onset latencies and inefficient postural responses. All sites within the somatosensory system are affected by ageing and therefore contribute to postural instability. However, morphological changes of muscle spindles appear primarily to be responsible for the impaired ability to compensate for balance threats in old age. Given these neuromuscular limitations in old age, it is important to apply adequate training interventions that delay or even reverse the onset of these constraints. Strength training has the potential to enhance maximal as well as explosive force production capacity. This is accomplished by neural factors, including an improved recruitment pattern, discharge rate, and synchronization of motor units. Furthermore, an increase in number of satellite cells most likely accounts for training-induced muscle hypertrophy. Recent studies have investigated the impact of balance training in old age on the ability to develop maximal and explosive force. In addition, the effects of balance training on reflex activity during gait perturbations were also examined. Increases in maximal and explosive force production capacity and an improved ability to compensate for gait perturbations were observed. It is evident from the literature that researchers are increasingly studying the effects of more specifically designed training programmes on performance in populations of older adults. Thus, in the near future, strength training could be replaced by high-velocity forms of power training and balance training by perturbation-based training programmes. It is hypothesized that this new approach is more efficient in terms of fall prevention than the traditional approach.  相似文献   

In vivo behavior of vastus lateralis muscle during dynamic performances     

《European Journal of Sport Science》2013,13(1):1-13

Length changes in vastus lateralis fascicles were measured in vivo using ultrasonography during one-legged squat jumps (SJ), counter movement jumps (CMJ), and drop jumps (DJ) in the sledge apparatus (n = 9). Patellar tendon forces were recorded simultaneously with an optic fiber technique from 4 subjects. Fascicle length changes were compared with muscletendon unit length changes calculated from kinematic recordings. In general, the tendomuscular and fascicle length changes demonstrated similar patterns. During SJ the fascicles showed shortening throughout the action while during CMJ and DJ they underwent stretch-shortening cycle. In DJ greater muscular activity in braking phase.  相似文献   

Peripheral alterations after two different concentric power protocols     

《European Journal of Sport Science》2013,13(5):327-333

Abstract

The aims of the present study were to assess the ability of explosive concentric contractions to induce high-frequency fatigue, and to determine what effects the number of repetitions and number of sets have upon peripheral fatigue. Nine male students performed two protocols (6 sets, 5 min and 1 week rest between sets and protocols, respectively) with a different number of concentric jumps in a set on the sledge (10 vs. 20 jumps in the first (S10) and the second protocol (S20), respectively). Isometric twitch torque, torque during low-frequency (20 Hz) and high-frequency (100 Hz) electrical stimulation of the vastus lateralis were measured (before protocols and 30 s after sets). Peak twitch torque increased, while contraction and half-relaxation times shortened during the S10 protocol (P<0.05). The increase in peak twitch torque was accompanied by non-significant reductions in contraction time and half-relaxation time in the S20 protocol. High- and low-frequency torques did not change significantly in either protocol. The high- to low-frequency torque ratio increased significantly (P<0.05) after the fourth set in the S10 protocol. Post-activation potentiation was the main reason for enhanced contractile response after both protocols. There were no alterations in high-frequency torque or reductions in the high- to low-frequency torque ratio, even with increased sets and repetitions. Explosive concentric contractions showed no potential to induce high-frequency fatigue, when separated with long periods of rest.  相似文献   

Torque–velocity characteristics and contractile rate of force development in elite badminton players     

《European Journal of Sport Science》2013,13(3):127-134

Abstract

Elite badminton requires muscular endurance combined with appropriate maximal and explosive muscle strength. The musculature of the lower extremities is especially important in this context since rapid and forceful movements with the weight of the body are performed repeatedly throughout a match. In the present study, we examined various leg-strength parameters of 35 male elite badminton players who had been performing resistance exercises as part of their physical training for several years. The badminton players were compared with an age-matched reference group, the members of whom were physically active on a recreational basis, and to the same reference group after they had performed resistance training for 14 weeks. Maximal muscle strength of the knee extensor (quadriceps) and flexor muscles (hamstrings) was determined using isokinetic dynamometry. To measure explosive muscle strength, the contractile rate of force development was determined during maximal isometric muscle contractions. In general, the badminton players showed greater maximal muscle strength and contractile rate of force development than the reference group: mean quadriceps peak torque during slow concentric contraction: 3.69 Nm · kg^?1, s=0.08 vs. 3.26 Nm · kg^?1, s=0.8 (P<0.001); mean hamstring peak torque during slow concentric contraction: 1.86 Nm · kg^?1, s=0.04 vs. 1.63 Nm · kg^?1, s=0.04 (P<0.001); mean quadriceps rate of force development at 100 ms: 24.4 Nm · s^?1·kg^?1, s=0.5 vs. 22.1 Nm·s^?1 · kg^?1, s=0.6 (P<0.05); mean hamstring rate of force development at 100 ms: 11.4 Nm · s^?1·kg^?1, s=0.3 vs. 8.9 Nm · s^?1 · kg^?1, s=0.4 (P<0.05). However, after 14 weeks of resistance training the reference group achieved similar isometric and slow concentric muscle strength as the badminton players, although the badminton players still had a higher isometric rate of force development and muscle strength during fast (240° · s^?1) quadriceps contractions. Large volumes of concurrent endurance training could have attenuated the long-term development of maximal muscle strength in the badminton players. The badminton players had a higher contractile rate of force development than the reference group before and after resistance training. Greater explosive muscle strength in the badminton players might be a physiological adaptation to their badminton training.  相似文献   

The 1985 C. H. McCloy Research Lecture Practical Problems in Exercise Physiology Research     

David L. Costill 《Research quarterly for exercise and sport》2013,84(4):378-384

Abstract

This paper examines the contribution of exercise physiology research to improvements in sports performance. By using illustrations from studies on competitive swimmers, attempts have been made to show the need to develop tests that evaluate the factors essential for success in sports. Such testing should be used to describe the strengths and weaknesses of the athlete, and to offer the athlete and coach valid information to gauge the benefits of training. Aside from the value of monitoring the adaptations to sports training, the primary role of physiological research in sports should be to solve problems that will ultimately aid athletes to achieve their full potential. This paper will conclude with examples of problem solving research and its contribution to performance.  相似文献   

Estimation of Errors in Force Platform Data     

Stelios G. Psycharakis  Stuart Miller 《Research quarterly for exercise and sport》2013,84(4):514-518

Abstract

Young, healthy varsity swimmers were studied over a period of 14 months with regard to the effect of a typical training and competitive collegiate swimming program on plasma cholesterol and phospholipids. Chemical analyses on blood cholesterol and phospholipids were made and compared for competitive and noncompetitive seasons. A dietary analysis was also made during the competitive season. In both the original and subsequent study involving the current (1960–61) varsity swimmers analyses were made of the acute effect of physical exertion on plasma cholesterol and phospholipids.

The exercise, used for conditioning varsity swimming athletes did not significantly lower blood cholesterol and phospholipids. However, this result is not necessarily at variance with the view of some investigators that exercise does exert a hypocholesterilizing effect. Several factors are discussed which are thought to have influenced ultimate results.  相似文献   

String vibration dampers do not reduce racket frame vibration transfer to the forearm     

F-X Li  D Fewtrell  M Jenkins 《Journal of sports sciences》2013,31(11-12):1041-1052

In this study, we examined the effect of string vibration damping devices on reducing racket frame vibration transfer to the forearm. Twenty participants volunteered to hold a tennis racket stationary in a forehand and backhand stroking position while tennis balls were fired at 20 m?·?s^?1 towards two impact locations, the node of vibration and the dead spot. A three-way analysis of variance with repeated measures on damping condition, impact location and stroke condition was performed on the data. The resonant frequency of the hand-held racket was found to be ～120 Hz. No significant differences in amplitude of vibration at the resonant frequency were found for the wrist or the elbow when damped and non-damped impacts were compared. Impacts at the dead spot produced greater amplitudes of vibration (P <?0.01) but no interaction between impact location and string dampers was evident. The string dampers had no effect on the grip force used or the muscle electrical activity in the forearm after impact. In conclusion, we found that string dampers do not reduce the amount of racket frame vibration received at the forearm. We suggest that string dampers remain a popular accessory among tennis players because of their acoustic effects and psychological support rather than any mechanical advantage.  相似文献   

The force–time profile of elite front crawl swimmers     

Danielle P. Formosa  Bruce Mason  Brendan Burkett 《Journal of sports sciences》2013,31(8):811-819

Abstract

In this study, we used recently developed technology to determine the force–time profile of elite swimmers, which enabled coaches to make informed decisions on technique modifications. Eight elite male swimmers with a FINA (Federation Internationale de Natation) rank of 900+ completed five passive (streamline tow) and five net force (arms and leg swimming) trials. Three 50-Hz cameras were used to video each trial and were synchronized to the kinetic data output from a force-platform, upon which a motorized towing device was mounted. Passive and net force trials were completed at the participant's maximal front crawl swimming velocity. For the constant tow velocity, the net force profile was presented as a force–time graph, and the limitation of a constant velocity assumption was acknowledged. This allowed minimum and maximum net forces and arm symmetry to be identified. At a mean velocity of 1.92 ± 0.06 m · s^?1, the mean passive drag for the swimmers was 80.3 ± 4.0 N, and the mean net force was 262.4 ± 33.4 N. The mean location in the stroke cycle for minimum and maximum net force production was at 45% (insweep phase) and 75% (upsweep phase) of the stroke, respectively. This force–time profile also identified any stroke asymmetry.  相似文献   

Effects of stance width on performance and postural stability in national-standard pistol shooters     

Richard N. Hawkins  Joellen M. Sefton 《Journal of sports sciences》2013,31(13):1381-1387

Abstract

The aim of the present study was to determine whether changing stance width would result in a corresponding change in postural and/or pistol stability. Twelve national-standard male air pistol shooters performed 10 shots each at five stance widths (30 cm, 45 cm, 60 cm, 75 cm, and 90 cm). Postural stability was determined by measuring centre-of-pressure changes with a dual force-platform system. Shooting mechanics measures were determined by a NOPTEL ST-2000 optoelectronic training system. Medial-lateral centre-of-pressure excursion (F _4,44 = 7.17, P < 0.001, effect size = 0.99) and speed (F _4,44 = 77.03, P < 0.001, effect size = 3.88) were reduced as stance width decreased. Centre of gravity fine (the percentage of time held within an area the size of the ten-ring) improved during narrower stance widths (F _4,32 = 12.49, P < 0.001, effect size = 0.71). Our findings suggest that stance width affects postural and pistol stability in national-standard air pistol athletes. Moreover, the current method of suggesting a wider stance to improve shooting performance should be reconsidered and perhaps air-pistol shooters should use a 30-cm stance width to improve postural stability and shooting performance.  相似文献