Greg Atkinson

Statistical methods for assessing measurement error (reliability) in variables relevant to sports medicine.

Minimal measurement error (reliability) during the collection of interval- and ratio-type data is critically important to sports medicine research. The main components of measurement error are systematic bias (e.g. general learning or fatigue effects on the tests) and random error due to biological or mechanical variation. Both error components should be meaningfully quantified for the sports physician to relate the described error to judgements regarding ‘analytical goals’ (the requirements of the measurement tool for effective practical use) rather than the statistical significance of any reliability indicators.Methods based on correlation coefficients and regression provide an indication of ‘relative reliability’. Since these methods are highly influenced by the range of measured values, researchers should be cautious in: (i) concluding acceptable relative reliability even if a correlation is above 0.9; (ii) extrapolating the results of a test-retest correlation to a new sample of individuals involved in an experiment; and (iii) comparing test-retest correlations between different reliability studies.Methods used to describe ‘absolute reliability’ include the standard error of measurements (SEM), coefficient of variation (CV) and limits of agreement (LOA). These statistics are more appropriate for comparing reliability between different measurement tools in different studies. They can be used in multiple retest studies from ANOVA procedures, help predict the magnitude of a ‘real’ change in individual athletes and be employed to estimate statistical power for a repeated-measures experiment.These methods vary considerably in the way they are calculated and their use also assumes the presence (CV) or absence (SEM) of heteroscedasticity. Most methods of calculating SEM and CV represent approximately 68% of the error that is actually present in the repeated measurements for the ‘average’ individual in the sample. LOA represent the test-retest differences for 95% of a population. The associated Bland-Altman plot shows the measurement error schematically and helps to identify the presence of heteroscedasticity. If there is evidence of heteroscedasticity or non-normality, one should logarithmically transform the data and quote the bias and random error as ratios. This allows simple comparisons of reliability across different measurement tools.It is recommended that sports clinicians and researchers should cite and interpret a number of statistical methods for assessing reliability. We encourage the inclusion of the LOA method, especially the exploration of heteroscedasticity that is inherent in this analysis. We also stress the importance of relating the results of any reliability statistic to ‘analytical goals’ in sports medicine.

Assessment of flow-mediated dilation in humans: a methodological and physiological guideline

Endothelial dysfunction is now considered an important early event in the development of atherosclerosis, which precedes gross morphological signs and clinical symptoms. The assessment of flow-mediated dilation (FMD) was introduced almost 20 years ago as a noninvasive approach to examine vasodilator function in vivo. FMD is widely believed to reflect endothelium-dependent and largely nitric oxide-mediated arterial function and has been used as a surrogate marker of vascular health. This noninvasive technique has been used to compare groups of subjects and to evaluate the impact of interventions within individuals. Despite its widespread adoption, there is considerable variability between studies with respect to the protocols applied, methods of analysis, and interpretation of results. Moreover, differences in methodological approaches have important impacts on the response magnitude, can result in spurious data interpretation, and limit the comparability of outcomes between studies. This review results from a collegial discussion between physiologists with the purpose of developing considered guidelines. The contributors represent several distinct research groups that have independently worked to advance the evidence base for improvement of the technical approaches to FMD measurement and analysis. The outcome is a series of recommendations on the basis of review and critical appraisal of recent physiological studies, pertaining to the most appropriate methods to assess FMD in humans.

Analysis of High Intensity Activity in Premier League Soccer

The aim of the present investigation was to provide a detailed analysis of the high intensity running activity completed by elite soccer players during match-play. A further aim of the study was to evaluate the importance of high intensity running activity to overall team success. Observations on individual match performance measures were undertaken on 563 outfield players (median of 8 games per player; range=1-57) competing in the English Premier League from 2003/2004 to 2005/2006 using a computerised tracking system (Prozone, Leeds, England). High intensity activities selected for analysis included total high intensity running distance (THIR), total sprint distance (TSD) and the number and type of sprints undertaken. Total high intensity running distance in possession and without possession of the ball was also analysed. The THIR was dependant upon playing position with wide midfield (1,049+/-106 m) and central defenders (681+/-128 m) completing the highest and lowest distance respectively (p<0.001). High intensity activity was also related to team success with teams finishing in the bottom five (919+/-128 m) and middle ten (917+/-143 m) league positions completing significantly more THIR compared with teams in the top five (885+/-113 m) (p=0.003). The THIR and TSD also significantly declined during the 2nd half with the greatest decrements observed in wide midfield and attacking players (p<0.05). Both positional differences in high intensity activity and the observed change in activity throughout the game were also influenced by team success (p<0.05). The results of the present study indicate that high intensity activity in elite soccer match-play is influenced by both playing position and previous activity in the game. These activity patterns are also dependant upon success of the team. This may indicate that overall technical and tactical effectiveness of the team rather than high levels of physical performance per se are more important in determining success in soccer.

Circadian Variation in Sports Performance

SummaryChronobiology is the science concerned with investigations of time-dependent changes in physiological variables. Circadian rhythms refer to variations that recur every 24 hours. Many physiological circadian rhythms at rest are endogenously controlled, and persist when an individual is isolated from environmental fluctuations. Unlike physiological variables, human performance cannot be monitored continuously in order to describe circadian rhythmicity. Experimental studies of the effect of circadian rhythms on performance need to be carefully designed in order to control for serial fatigue effects and to minimise disturbances in sleep. The detection of rhythmicity in performance variables is also highly influenced by the degree of test-retest repeatability of the measuring equipment.The majority of components of sports performance, e.g. flexibility, muscle strength, short term high power output, vary with time of day in a sinusoidal manner and peak in the early evening close to the daily maximum in body temperature. Psychological tests of short term memory, heart rate-based tests of physical fitness, and prolonged submaximal exercise performance carried out in hot conditions show peak times in the morning. Heart rate-based tests of work capacity appear to peak in the morning because the heart rate responses to exercise are minimal at this time of day. Post-lunch declines are evident with performance variables such as muscle strength, especially if measured frequently enough and sequentially within a 24-hour period to cause fatigue in individuals. More research work is needed to ascertain whether performance in tasks demanding fine motor control varies with time of day.Metabolic and respiratory rhythms are flattened when exercise becomes strenuous whilst the body temperature rhythm persists during maximal exercise. Higher work-rates are selected spontaneously in the early evening. At present, it is not known whether time of day influences the responses of a set training regimen (one in which the training stimulus does not vary with time of day) for endurance, strength, or the learning of motor skills.The normal circadian rhythms can be desynchronised following a flight across several time zones or a transfer to nocturnal work shifts. Although athletes show all the symptoms of ‘jet lag’ (increased fatigue, disturbed sleep and circadian rhythms), more research work is needed to identify the effects of transmeridian travel on the actual performances of elite sports competitors. Such investigations would need to be chronobiological, i.e. monitor performance at several dmes on several post-flight days, and take into account direction of travel, time of day of competition and the various performance components involved in a particular sport.Shiftwork interferes with participation in competitive sport, although there may be greater opportunities for shiftworkers to train in the hours of daylight for individual sports such as cycling and swimming. Studies should be conducted to ascertain whether shiftwork-mediated rhythm disturbances affect sports performance.Individual differences in performance rhythms are small but significant. Circadian rhythms are larger in amplitude in physically fit individuals than sedentary individuals. Athletes over 50 years of age tend to be higher in ‘momingness’, habitually scheduling relatively more training in the morning and selecting relatively higher work-rates during exercise compared with young athletes. These differences should be recognised by practitioners concerned with organising the habitual regimens of athletes.

CIRCADIAN RHYTHMS IN SPORTS PERFORMANCE— AN UPDATE

We discuss current knowledge on the description, impact, and underlying causes of circadian rhythmicity in sports performance. We argue that there is a wealth of information from both applied and experimental work, which, when considered together, suggests that sports performance is affected by time of day in normal entrained conditions and that the variation has at least some input from endogenous mechanisms. Nevertheless, precise information on the relative importance of endogenous and exogenous factors is lacking. No single study can answer both the applied and basic research questions that are relevant to this topic, but an appropriate mixture of real‐world research on rhythm disturbances and tightly controlled experiments involving forced desynchronization protocols is needed. Important issues, which should be considered by any chronobiologist interested in sports and exercise, include how representative the study sample and the selected performance tests are, test‐retest reliability, as well as overall design of the experiment.

Jet lag: trends and coping strategies

The number of travellers undertaking long-distance flights has continued to increase. Such flights are associated with travel fatigue and jet lag, the symptoms of which are considered here, along with their similarities, differences, and causes. Difficulties with jet lag because of sleep loss and decreased performance are emphasised. Since jet lag is caused mainly by inappropriate timing of the body clock in the new time zone, the pertinent properties of the body clock are outlined, with a description of how the body clock can be adjusted. The methods, both pharmacological and behavioural, that have been used to alleviate the negative results of time-zone transitions, are reviewed. The results form the rationale for advice to travellers flying in different directions and crossing several time zones. Finally, there is an account of the main problems that remain unresolved.

Match-to-Match Variability of High-Speed Activities in Premier League Soccer

The aim of the present investigation was to determine the between-match variability of high-speed running activities completed by a large sample of elite players over an extended period of time. A further aim of the study was to determine the influence of playing position on the magnitude of this variability. Observations on individual match performance measures were undertaken on 485 outfield players (median of 10 games per player; range=2-57) competing in the English Premier League from 2003/2004 to 2005/2006 using a computerised tracking system (Prozone, Leeds, England). High-speed activities selected for analysis included total high-speed running distance (THSR), high-speed running (HSR), total sprint distance (TSD) and the total number of sprints undertaken. Total high-speed running distance in possession and without possession of the ball was also analysed. Match-to-match variability was generally high across all variables with a mean CV of 16.2+/-6.4% (95% CI=15.6-16.7%) and 30.8+/-11.2% (95% CI=29.9-31.7%) reported for HSR and TSD covered during a game. This variability was generally higher for central players (midfielders and defenders) and lower for wide midfielders and attackers. Greater variability was also noted when the team were in possession of the ball (approximately 30%) than when they did not have possession (approximately 23%). The findings of the present study indicate that match-to-match variability in performance characteristics of elite soccer players is high. This inherent variability means that research requires large sample sizes in order to detect real systematic changes in performance characteristics.

Elevation in cerebral blood flow velocity with aerobic fitness throughout healthy human ageing

It is known that cerebral blood flow declines with age in sedentary adults, although previous studies have involved small sample sizes, making the exact estimate of decline imprecise and the effects of possible moderator variables unknown. Animal studies indicate that aerobic exercise can elevate cerebral blood flow; however, this possibility has not been examined in humans. We examined how regular aerobic exercise affects the age‐related decline in blood flow velocity in the middle cerebral artery (MCAv) in healthy humans. Maximal oxygen consumption, body mass index (BMI), blood pressure and MCAv were measured in healthy sedentary (n= 153) and endurance‐trained (n= 154) men aged between 18 and 79 years. The relationships between age, training status, BMI and MCAv were examined using analysis of covariance methods. Mean ±s.e.m. estimates of regression coefficients and 95% confidence intervals (95% CI) were calculated. The age‐related decline in MCAv was −0.76 ± 0.04 cm s−1 year−1 (95% CI =−0.69 to −0.83, r2= 0.66, P < 0.0005) and was independent of training status (P= 0.65). Nevertheless, MCAv was consistently elevated by 9.1 ± 3.3 cm s−1 (CI = 2.7–15.6, P= 0.006) in endurance‐trained men throughout the age range. This ∼17% difference between trained and sedentary men amounted to an approximate 10 year reduction in MCAv ‘age’ and was robust to between‐group differences in BMI and blood pressure. Regular aerobic‐endurance exercise is associated with higher MCAv in men aged 18–79 years. The persistence of this finding in older endurance‐trained men may therefore help explain why there is a lower risk of cerebrovascular disease in this population.

The Circadian Rhythm of Core Temperature: Origin and some Implications for Exercise Performance

This review first examines reliable and convenient ways of measuring core temperature for studying the circadian rhythm, concluding that measurements of rectal and gut temperature fulfil these requirements, but that insulated axilla temperature does not. The origin of the circadian rhythm of core temperature is mainly due to circadian changes in the rate of loss of heat through the extremities, mediated by vasodilatation of the cutaneous vasculature. Difficulties arise when the rhythm of core temperature is used as a marker of the body clock, since it is also affected by the sleep‐wake cycle. This masking effect can be overcome directly by constant routines and indirectly by “purification” methods, several of which are described. Evidence supports the value of purification methods to act as a substitute when constant routines cannot be performed. Since many of the mechanisms that rise to the circadian rhythm of core temperature are the same as those that occur during thermoregulation in exercise, there is an interaction between the two. This interaction is manifest in the initial response to spontaneous activity and to mild exercise, body temperature rising more quickly and thermoregulatory reflexes being recruited less quickly around the trough and rising phase of the resting temperature rhythm, in comparison with the peak and falling phase. There are also implications for athletes, who need to exercise maximally and with minimal risk of muscle injury or heat exhaustion in a variety of ambient temperatures and at different times of the day. Understanding the circadian rhythm of core temperature may reduce potential hazards due to the time of day when exercise is performed.

Future Perspectives in the Evaluation of the Physiological Demands of Soccer

Soccer (association football) is a team sport that incorporates frequent fluctuations between high and low exercise intensities. These unpredictable changes may be accompanied by unorthodox patterns of movements and the performance of specific skills. The individual activity profiles are highly variable and include elements of self-pacing, since decision making about opportunities to become engaged in play dictates individual activities. Approaches utilised to investigate the demands placed on players during competitive performances include behavioural observations during games, physiological evaluations in matches and assessments of the physical capacity of players. Observations made during games to determine the work-rate patterns of individual players are highly variable and make generalisations based on individual activity patterns conditional, unless the sample sizes are large and data are collected on a number of occasions. The data may also be affected by the diverse methodological approaches to their collection and analysis and a failure to determine the reliability and objectivity of the relevant measuring tools. Techniques that can be used to collect data in matches are limited as the sports rules and regulations restrict some approaches. The validity of applying data from non-competitive matches to the competitive situation must, therefore, be subject to formal scrutiny. There is also a concern as to the degree to which principles of steady-state are applicable to dynamically changing exercise intensities. In the evaluation of the physical capacities of players, the variability in overall soccer performance is reduced to fitness statistics, whereas in reality, soccer performance is a construct based on many different performance components and their interaction at the level of both player and team. Despite these caveats, valuable insights have been acquired into the physiological requirements of the game that have subsequently informed both research projects and impacted upon practice. The challenge for future researchers is to overcome remaining research design hurdles and devise ways to understand more fully the complexities of invasive field games such as soccer. The interactions between individuals within a team require investigation and there is a need to refine and develop methods that employ sophisticated measurement techniques and yet possess both internal and external validity, such as laboratory-based simulations.