Validation of a videogrammetry technique for analysing American football helmet kinematics     

Ann Bailey  James Funk  David Lessley  Chris Sherwood  Jeff Crandall  William Neale 《Sports biomechanics / International Society of Biomechanics in Sports》2020,19(5):678-700

ABSTRACT

Professional American football games are recorded in digital video with multiple cameras, often at high resolution and high frame rates. The purpose of this study was to evaluate the accuracy of a videogrammetry technique to calculate translational and rotational helmet velocity before, during and after a helmet impact. In total, 10 football impacts were staged in a National Football League (NFL) stadium by propelling helmeted 50th percentile male crash test dummies into each other or the ground at speeds and orientations representative of concussive impacts for NFL players. The tests were recorded by experienced sports film crews to obtain video coverage and quality typically available for NFL games. A videogrammetry procedure was used to track the position and rotation of the helmet throughout the relevant time interval of the head impact. Compared with rigidly mounted retroreflective marker three dimensional (3-D) motion tracking that was concurrently collected in the experiments, videogrammetry accurately calculated changes in translational and rotational velocity of the helmet using high frame rate (two cameras at 240 Hz) video (7% and 15% error, respectively). Low frame rate (2 cameras at 60 Hz) video was adequate for calculating pre-impact translational velocity but not for calculating the translational or rotational velocity change of the helmet during impact.  相似文献   

False-performance feedback does not affect punching forces and pacing of elite boxers     

Israel Halperin  Dale W. Chapman  Kevin G. Thompson  Chris Abbiss 《Journal of sports sciences》2019,37(1):59-66

Prior research indicates that providing participants with positive augmented feedback tends to enhance motor learning and performance, whereas the opposite occurs with negative feedback. However, the majority of studies were conducted with untrained participants performing unfamiliar motor tasks and so it remains unclear if elite athletes completing familiar tasks respond in a similar fashion. Thus, this study investigated the effects of three different versions of false-performance feedback on punching force (N), pacing (force over time) and ratings of perceived exertion (RPE) in 15 elite amateur male boxers. Athletes completed a simulated boxing bout consisting of three rounds with 84 maximal effort punches delivered to a punching integrator on four separate days. Day one was a familiarisation session in which no feedback was provided. In the following three days athletes randomly received false-positive, false-negative and false-neutral feedback on their punching performance between each round. No statistical or meaningful differences were observed in punching forces, pacing or RPE between conditions (P > 0.05; ≤ 2%). These null results could stem from the elite status of the athletes involved, the focus on performance rather than learning, or they may indicate that false feedback has a less potent effect on performance than previously thought.  相似文献   

Early Years Parenting Mediates Early Adversity Effects on Problem Behaviors in Intellectual Disability     

Vasiliki Totsika  Richard P. Hastings  Eric Emerson  Chris Hatton 《Child development》2020,91(3):e649-e664

A family developmental framework was applied to data from families of children with intellectual disabilities to understand the role of parenting in the path from early adversity to problem behaviors in mid-childhood. Data from 9 months to 11 years tested the Family Stress Model in families of 555 children. Adversarial parenting between 3 and 5 years mediated the path from early adversity (family poverty and maternal psychological distress at nine months) to problem behaviors at 7 and 11 years. Positive parent–child relationship only mediated the path to conduct problems. Multiple mediation was not present. Early adversity impacts both positive parent–child relationship and adversarial parenting between three and five, but the latter is crucial for problem behaviors in mid-childhood.  相似文献   

Comparing the magnitude and direction of asymmetry during the squat,countermovement and drop jump tests in elite youth female soccer players     

Chris Bishop  Lucas A. Pereira  Valter P. Reis  Paul Read  Anthony N. Turner  Irineu Loturco 《Journal of sports sciences》2020,38(11-12):1296-1303

ABSTRACT

The aims of the present study were to provide an in-depth comparison of inter-limb asymmetry and determine how consistently asymmetry favours the same limb during different vertical jump tests. Eighteen elite female under-17 soccer players conducted unilateral squat jumps (SJ), countermovement jumps (CMJ) and drop jumps (DJ) on a portable force platform, with jump height, peak force, concentric impulse and peak power as common metrics across tests. For the magnitude of asymmetry, concentric impulse was significantly greater during the SJ test compared to CMJ (p = 0.019) and DJ (p = 0.003). No other significant differences in magnitude were present. For the direction of asymmetry, Kappa coefficients revealed fair to substantial levels of agreement between the SJ and CMJ (Kappa = 0.35 to 0.61) tests, but only slight to fair levels of agreement between the SJ and DJ (Kappa = ?0.26 to 0.18) and CMJ and DJ (Kappa = ?0.13 to 0.26) tests. These results highlight that the mean asymmetry value may be a poor indicator of true variability of between-limb differences in healthy athletes. The direction of asymmetry may provide a useful monitoring tool for practitioners in healthy athletes, when no obvious between-limb deficit exists.  相似文献   

Power training in elite young soccer players: Effects of using loads above or below the optimum power zone     

Irineu Loturco  Lucas A. Pereira  Valter P. Reis  Chris Bishop  Vinicius Zanetti  Pedro E. Alcaraz 《Journal of sports sciences》2020,38(11-12):1416-1422

ABSTRACT

This study aimed to examine the effects of two jump squat (JS) training programs involving different loading ranges in under-20 soccer players during a preseason period. Twenty-three elite young soccer players performed sprint speed (at 5-, 10-, and 20-m), change-of-direction (COD) speed, JS peak-power (PP), and countermovement jump (CMJ) tests pre and post four weeks of training. Athletes were pair-matched in two groups according to their optimum power loads (OPL) as follows: lower than OPL (LOPL; athletes who trained at a load 20% lower than the OPL) and higher than OPL (HOPL; athletes who trained at a load 20% higher than the OPL). Magnitude-based inferences were used to compare pre- and post-training measures. Meaningful increases in the PP JS were observed for both groups. Likely and possible improvements were observed in the 5- and 10-m sprint velocity in the LOPL group. Meanwhile, possible and likely improvements were observed in the CMJ, 5- and 10-m sprint velocity, and COD speed in the HOPL group. Overall, both training schemes induced positive changes in athletic performance. Soccer coaches and sport scientists can implement the JS OPL-based training schemes presented here, either separately or combined, to improve the physical performance of youth soccer players.  相似文献   

Bodies in flux: scientific methods for negotiating medical uncertainty,C. Teston. Chicago,IL: University of Chicago Press, 2017, 243 pp., Paperback – $35/Hardcover – $105/E-Book – $35     

Chris McCracken 《Technical Communication Quarterly》2018,27(2):191-194

  相似文献   

Experimental and Numerical Analysis of a Novel Display Case Design: Case Study of the Renovated Anne Frank House     

Karin Kompatscher  Bart Ankersmit  Edgar Neuhaus  Marcel A.P. van Aarle  Jos W.M. van Schijndel  Henk L. Schellen 《文物保护研究》2020,65(5):262-284

ABSTRACT

Many museums are housed in historic buildings, sometimes the building itself is part of the museum collection. Creating a stable environment by providing a nearly constant temperature and relative humidity at correct levels decreases the risk of object degradation. Maintaining this steady indoor environment, however, increases energy consumption and risks to the historic building. Museum display cases offer a solution to the mitigation of risks to which valuable objects may be subjected by providing an extra layer of protection to indoor climate fluctuations. The Anne Frank House is a historic house museum located in Amsterdam. The museum has undergone several renovations in the last years to deal with an increase in the number of visitors to over 1.2 million a year. The original diaries and other documents of Anne Frank are permanently on display in the Anne Frank House. With the recent refurbishment the possibility arose to design a new state-of-the-art display case. This study presents the results of the experimental research related to the design, performed in-situ. The temperature and relative humidity in the new exhibition space and inside the new display cases were monitored to gain insight into the hygrothermal behavior of these controlled environments. A complementary numerical study was performed to investigate effects of dynamic climate control of the exhibition gallery and climate conditions in the display case under various circumstances. Four main conclusions are presented in this paper. The investigated display case design is able to provide a stable relative humidity environment by means of silica gel, while using an active box-in-box climate control system to create stable temperature conditions. The inner case temperature depends on the temperature supplied by the display case air handling unit. Protocols must be in place in case of malfunction or failure of the climate control system of the display case. The air handling unit of the case needs to be shut off to create a passive environment for the objects on display until necessary actions are taken. Exhibition gallery set points can be less stringent when susceptible museum objects are on display in the display case. The environments are separated and provide an opportunity for energy saving set point strategies. The last conclusion drawn is that the numerical study provides valuable insight into imposing dynamic control of set points for temperature and relative humidity in the exhibition gallery and the effect on the display case environment.  相似文献   

A rich study of James’ pragmatism     

Chris Haufe 《Metascience》2018,27(2):255-257

  相似文献   

A study of results overlap and uniqueness among major Web search engines     

Amanda Spink  Bernard J. Jansen  Chris Blakely  Sherry Koshman 《Information processing & management》2006

The performance and capabilities of Web search engines is an important and significant area of research. Millions of people world wide use Web search engines very day. This paper reports the results of a major study examining the overlap among results retrieved by multiple Web search engines for a large set of more than 10,000 queries. Previous smaller studies have discussed a lack of overlap in results returned by Web search engines for the same queries. The goal of the current study was to conduct a large-scale study to measure the overlap of search results on the first result page (both non-sponsored and sponsored) across the four most popular Web search engines, at specific points in time using a large number of queries. The Web search engines included in the study were MSN Search, Google, Yahoo! and Ask Jeeves. Our study then compares these results with the first page results retrieved for the same queries by the metasearch engine Dogpile.com. Two sets of randomly selected user-entered queries, one set was 10,316 queries and the other 12,570 queries, from Infospace’s Dogpile.com search engine (the first set was from Dogpile, the second was from across the Infospace Network of search properties were submitted to the four single Web search engines). Findings show that the percent of total results unique to only one of the four Web search engines was 84.9%, shared by two of the three Web search engines was 11.4%, shared by three of the Web search engines was 2.6%, and shared by all four Web search engines was 1.1%. This small degree of overlap shows the significant difference in the way major Web search engines retrieve and rank results in response to given queries. Results point to the value of metasearch engines in Web retrieval to overcome the biases of individual search engines.  相似文献   

Perceptions in Role Change from Teacher to Counselor: Intra-Role Conflict and Motivation for Change1     

Chris D. Kehas  Jane L. Morse 《Counselor Education & Supervision》1971,10(3):200-208

Twelve former teachers who had been counseling for no more than two years were interviewed on how they experienced the processes of role change from teacher to counselor. This article extends the findings reported in an earlier paper and deals with intra-role conflict and with the motivation for changing roles. The following hypotheses emerged from the study: (a) if teaching experience heightens the conflict in perceived incompatibility of role expectations, then it is dysfunctional to becoming a counselor; and (b) teachers enter counseling primarily because they seek to fulfill an inner desire for growth toward what they perceive as their ideal role in education. The implications of the testimony given by the counselors for counselor preparation programs and for the structure of roles in schools were examined.  相似文献