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Abstract The dynamic properties of six types of tennis balls were measured using a force platform and high-speed digital video images of ball impacts on rigidly clamped tennis rackets. It was found that the coefficient of restitution reduced with velocity for impacts on a rigid surface or with a rigidly clamped tennis racket. Pressurized balls had the highest coefficient of restitution, which decreased by 20% when punctured. Pressureless balls had a coefficient of restitution approaching that of a punctured ball at high speeds. The dynamic stiffness of the ball or the ball-racket system increased with velocity and pressurized balls had the highest stiffness, which decreased by 35% when punctured. The characteristics of pressureless balls were shown to be similar to those of punctured balls at high velocity and it was found that lowering the string tension produced a smaller range of stiffness or coefficient of restitution. It was hypothesized that players might consider high ball stiffness to imply a high coefficient of restitution. Plots of coefficient of restitution versus stiffness confirmed the relationship and it was found that, generally, pressurized balls had a higher coefficient of restitution and stiffness than pressureless balls. The players might perceive these parameters through a combination of sound, vibration and perception of ball speed off the racket. 相似文献
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AbstractImprovements in track and field sports have been attributed to factors such as population increase, drugs and new technologies, but previous research has found it difficult to distinguish the contributions from specific influences. Here it is shown how this is possible by means of a performance improvement index based on useful work done combined with modelling of the annual top 25 performances. The index was set to 100 in 1948 and showed that, by 2012, it had increased in running events to between 110.5 and 146.7 (men’s 100 m and marathon). Underlying global effects accounted for the majority of all improvements (16.2 to 46.7) with smaller influences attributable to an influx of African runners (3.6 to 9.3), and a 4 -year oscillation that arose from staging of the Olympic Games (±0.2 to ±0.6). Performance decreased with the introduction of compulsory random drug testing (?0.9 to ?3.9) the World Anti-Doping Agency (WADA; ?0.5 to ?2.5) and fully automated timing (?0.6 to ?2.5). Changes in elite sporting performance since the 1890s are attributable to societal changes caused by the industrial revolution and globalisation superimposed on millennia of human evolution. 相似文献
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Tertiary Education and Management - How leadership is done in higher education is analysed through a longitudinal interview study among the heads of department at a Swedish university. The focus is... 相似文献
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A dynamic model of the breast during exercise 总被引：1，自引：1，他引：0
The aim of this paper is to develop a method to determine the material characteristics of bras that could limit breast motion
during exercise. A single participant ran on a treadmill at 10 km h−1 wearing either a sports bra, an everyday bra or no bra. The relative motion between the suprasternal notch and the breast
was recorded using a passive marker system at 200 Hz and was modelled as forced damped-harmonic motion with a linear spring
and damper, with the driving force provided by the suprasternal notch. The spring and damper values were found by matching
the model to the experimental data. It was found that both the damping and stiffness values increased with the use of an everyday
bra, and increased further still with the use of a sports bra. The stiffness parameter, however, was shown to be the most
important criterion for minimisation of the breast motion. The model predicted that an increase in breast mass from 100 to
700 g (a 32A-cup to a 32F-cup) increased the vertical motion of the unsupported breast by around 70% when running and 30%
when walking. This was reduced with an everyday bra and further reduced with the high stiffness sports bra. Although predictions
were sensible, the model requires further verification with a cohort of participants. 相似文献
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The purpose of this study was to determine a way of estimating the strain of the breast during exercise and to enable an engineering
analysis of breast motion. Three participants exercised on a treadmill wearing a sports bra, an everyday bra or no bra and
the motion of the body and the nipple were recorded using a 200 Hz passive marker motion capture system. Each participant
also performed a breast drop test to determine the neutral position of the nipple; this allowed strains to be estimated from
the point when nipple acceleration was approximately that due to gravity. Wearing any bra reduced the strain at the nipple
by lifting the breast mass up towards the neutral position where the skin of the breast was neither in tension nor in compression.
Non-linearities in strain occurred during the downward phase of running at strain values exceeding the static value at up
to 70% strain; these were consistent with the previous studies on skin. It was postulated that comfort scores might be related
to elements of the acceleration–strain curve and that nipple motion causing strain beyond static strain might be an important
parameter in understanding breast pain. 相似文献
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One of the tests used to evaluate the performance of sports surfaces measures the peak deceleration of a rigid mass during
contact with the surface, after being dropped from a set height. It is widely used and is included in a number of international
standards.
The test was carried out using two drop heights on four different designs of artificial cricket pitch, with a full set of
acceleration data being collected over the duration of impact. These data were then integrated twice with respect to time
to produce velocity and displacement information. Oblique impacts of cricket balls were also carried out and analysed using
a high-speed video system to evaluate the playing performance of each pitch design.
It was found that, although the pitches gave quite different readings of peak deceleration, they produced similar values for
coefficient of restitution and were therefore considered to ‘play’ in a very similar way. This was due to a high peak deceleration
reading being accompanied by a short contact time and low peak deceleration being accompanied by a long contact time, meaning
that the change in momentum was similar in both cases. It was concluded that for accelerometer tests to be useful, all the
acceleration-time data should be analysed, rather than the peak value alone. 相似文献
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This article presents an analysis of a five-year study on the implementation of a technology-supported distance programme in Swedish firefighter training. Based on activity theory and four interview studies, the attention is directed to firefighter instructors’ perceptions and use of digital technologies with a focus on challenges, contradictions and changes. Two phases are identified, viz an implementation phase and a dissemination phase. During the implementation phase a clear contradiction emerges regarding how the object of the training should be interpreted, which results in the instructors responsible for the implementation reconsidering previous beliefs and, with the support of digital technologies, developing more theoretical and process-oriented approaches and improving the course design. However, during the dissemination phase, when all instructors are involved, a number of conflicts and dilemmas emerge, resulting in the changes achieved in the distance programme to some extent being normalised in the direction of the instructor-centred and exercise-oriented approaches of the campus programme. The article concludes with a discussion about contradictions as well as opportunities that may emerge during the implementation process of a vocational technology-supported distance programme. 相似文献
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The mean of the top 25 performances in 8 men’s and 5 women’s field events since the 1890s showed that performances increased dramatically after the Second World War II and subsequently plateaued during the late twentieth century. A performance improvement index, developed on the basis of work done, was set to 100% in 1948. The underlying rise found in all events was modelled using an exponential function with a superposition of steps and linear changes to account for the introduction of rule changes, drugs testing and the introduction of new technologies. The performance improvement index in throwing events increased to 140.9% compared with 125.8% in jumping events, and women’s performance improvement always exceeded that of men’s. Around half of all events were shown to have reached 99.9% of their predicted limit with a majority reaching it within the next 25 years. It was concluded that performance will only change in the future if an intervention takes place: this could be the emergence of a new technology, a rule change or a new athlete population. 相似文献