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We examined the effects of time of day on a cycling time trial with and without a prolonged warm-up, among cyclists who tended towards being high in "morningness". Eight male cyclists (mean +/- s: age = 24.9 +/- 3.5 years, peak power output = 319 +/- 34 W, chronotype = 39 +/- 6 units) completed a 16.1-km time trial without a substantial warm-up at both 07:30 and 17:30 h. The time trial was also completed at both times of day after a 25-min warm-up at 60% of peak power. Power output, heart rate, intra-aural temperature and category ratings of perceived exertion (CR-10) were measured throughout the time trial. Post-test blood lactate concentration was also recorded. Warm-up generally improved time trial performance at both times of day (95% CI for improvement = 0 to 30 s), but mean cycling time was still significantly slower at 07:30 h than at 17:30 h after the warm-up (95% CI for difference = 33 to 66 s). Intra-aural temperature increased as the time trial progressed (P0.0005) and was significantly higher throughout the time trials at 17:30 h (P = 0.001), irrespective of whether the cyclists performed a warm-up or not. Blood lactate concentration after the time trial was lowest at 07:30 h without a warm-up (P = 0.02). No effects of time of day or warm-up were found for CR-10 or heart rate responses during the time trial. These results suggest that 16.1-km cycling performance is worse in the morning than in the afternoon, even with athletes who tend towards 'morningness', and who perform a vigorous 25-min warm-up. Diurnal variation in cycling performance is, therefore, relatively robust to some external and behavioural factors.

DeVan et al. [1] undertook a comprehensive cross-sectional study involving the independent variables of IFG (impaired fasting glucose) and exercise training status. In keeping with the effects of many other independent variables, including age [2,3], exercise [4], red wine [5] and cardiovascular disease [6], both FMD% [percentage FMD (flow-mediated dilation)] and D base (baseline artery diameter) were found to be different between the study samples. For example, D base was 0.46 mm larger in non-exercising older adults with IFG than the trained older adults with IFG, whereas FMD% was 3.6% higher in the latter sample. Brachial FMD% is mathematically equivalent to the ratio of D peak (peak diameter) divided by D base, i.e. D peak/ D … DeVan et al. [1] undertook a comprehensive cross-sectional study involving the independent variables of IFG (impaired fasting glucose) and exercise training status. In keeping with the effects of many other independent variables, including age [2,3], exercise [4], red wine [5] and cardiovascular disease [6], both FMD% [percentage FMD (flow-mediated dilation)] and D base (baseline artery diameter) were found to be different between the study samples. For example, D base was 0.46 mm larger in non-exercising older adults with IFG than the trained older adults with IFG, whereas FMD% was 3.6% higher in the latter sample. Brachial FMD% is mathematically equivalent to the ratio of D peak (peak diameter) divided by D base, i.e. D peak/ D …

Regression to the mean is a subject that has received considerable attention in the sports medicine and sport and exercise science literature. Indeed, Shepherd (2003) warns in the title of a recent...

Performances often vary between the heats and finals of breaststroke swimming competitions possibly because the swimmers try to conserve their energy, or for other tactical reasons. Additionally, coaches might advise either a 'positive' or 'even' pace race strategy during the final. The effect of such pacing changes on metabolism (blood lactate, heart rate, ventilation), ratings of perceived exertion, stroke kinematics and turning times have not been investigated. Nine male competitive breaststroke swimmers swam three paced (Aquapacer) 200-m trials, 48 h apart and in random order, at 98%, 100% and at an attempted 102% of their maximal 200-m time-trial speed. Responses in metabolic variables were similar between the 98% and 100% trials, but higher post-exercise blood lactate concentrations and respiratory exchange ratios were observed following the 102% trial. As the pace of trials increased, stroke rate was found to increase proportionately with stroke count. However, during the latter stages of the 100% trial, a disproportionate increase in the stroke count was observed, which led to a significant pacing error. This feature was more obvious in the 102% trial, where participants demonstrated 'positive pacing' and reported higher ratings of perceived exertion than for the 98% trial. During the early stages of the trials, turning times were initially shorter the faster the pace of the trial; however, as the trials progressed, this pattern was found to reverse. We conclude that a slight reduction in pace during near maximal breaststroke swimming altered kinematic but not post-exercise metabolic responses, while an increase in pace led to positive pacing and an increase in both kinematic responses and anaerobic metabolism.

It is important for human health and efficiency to maintain physiological and biochemical processes within narrow bands in spite of external influences. This homeostasis is achieved by means of negative feedback control loops, and superimposed on the “constancy” produced by it, there are rhythmic changes. The main rhythms can have a period—the length of time that elapses until the cycle is repeated—of a day, week, month, or year. Ultradian rhythms refer to cycles with a period of

(2003). Does size matter for sports performance researchers? Journal of Sports Sciences: Vol. 21, No. 2, pp. 73-74.

Eight healthy males were studied for a total of 13 subject-days to assess if gut (from an ingested pill) and axilla (from a thermally insulated skin probe) temperatures would act as a substitute for rectal temperature in field studies of the circadian rhythm of core temperature. Subjects slept and went about their activities, indoors and outdoors, normally. Regular recordings (at 6 min intervals) were made of temperatures from the three sites. In addition, activity was measured (by a sensor on the nondominant wrist) so that the raw temperature data could be "purified," that is, corrected for the direct effects of sleep and activity. Inspection of the raw data indicated that there was a close parallelism between rectal and gut temperatures, but that the parallelism between rectal and insulated axilla temperatures was less reliable. This parallelism was supported by initial calculations of the correlations between rectal and gut temperatures (high and positive) and between rectal and insulated axilla (lower, though still positive) temperatures. Calculation of the limits of agreement between the parameters of the cosine curves fitted to the raw data confirmed that the rectal and gut temperatures were far closer with regard to acrophase and amplitude than were rectal and insulated axilla temperatures (-0.31 +/- 0.89 vs. +0.75 +/- 6.03 h and +0.002 +/- 0.116 vs. +0.083 +/- 0.625 degrees C, respectively). After purification of the temperature data, the limits of agreement for the cosine parameters acrophase and amplitude still indicated that there was a closer agreement between rectal and gut temperatures than between rectal and insulated axilla temperatures (-0.30 +/- 1.12 vs. +0.58 +/- 6.69 h, and +0.007 +/- 0.116 vs. +0.104 +/- 0.620 degrees C, respectively). Part of the explanation of this difference was the unreliable relationships between temperature changes in insulated axilla temperature and bursts of activity and going to bed. It is concluded that, whereas gut temperature is a viable alternativ to rectal temperature (from the viewpoints of both user acceptability and the reliability of data obtained), insulated axilla temperature, though acceptable to subjects, is unreliable from an experimental viewpoint.

Background: Travelling across multiple time zones disrupts normal circadian rhythms and induces “jet lag”. Possible effects of this on training and performance in athletes were concerns before the Sydney Olympic Games.Objective: To identify some determinants of jet lag and its symptoms.Methods: A mixture of athletes, their coaches, and academics attending a conference (n = 85) was studied during their flights from the United Kingdom to Australia (two flights with a one hour stopover in Singapore), and for the first six days in Australia. Subjects differed in age, sex, chronotype, flexibility of sleeping habits, feelings of languor, fitness, time of arrival in Australia, and whether or not they had previous experience of travel to Australia. These variables and whether the body clock adjusted to new local time by phase advance or delay were tested as predictors for jet lag and some of its symptoms by stepwise multiple regression analyses.Results: The amount of sleep in the first flight was significantly greater in those who had left the United Kingdom in the evening than the morning (medians of 5.5 hours and 1.5 hours respectively; p = 0.0002, Mann-Whitney), whereas there was no significant difference on the second flight (2.5 hoursv2.8 hours; p = 0.72). Only the severity of jet lag and assessments of sleep and fatigue were commonly predicted significantly (pConclusions: These results indicate the importance of an appropriate choice of itinerary and lifestyle for reducing the negative effects of jet lag in athletes and others who wish to perform optimally in the new time zone.

Few chronobiologists may be aware of the regression-to-the-mean (RTM) statistical artifact, even though it may have far-reaching influences on chronobiological data. With the aid of simulated measurements of the circadian rhythm phase of body temperature and a completely bogus stimulus (unicorn milk), we explain what RTM is and provide examples relevant to chronobiology. We show how RTM may lead to erroneous conclusions regarding individual differences in phase responses to rhythm disturbances and how it may appear as though unicorn milk has phase-shifting effects and can successfully treat some circadian rhythm disorders. Guidelines are provided to ensure RTM effects are minimized in chronobiological investigations.

The aim of this review is to discuss some issues in the design and statistical analysis of sport performance research, rather than to supply an authoritative 'cookbook' of methods. In general, we try to communicate some possible solutions to the conundrum of how to maintain both internal and external validity, as well as optimize statistical power, in applied sport performance research. We start by arguing that some sport performance research has been overly concerned with physiological predictors of performance, at the expense of not providing a valid and reliable description of the exact nature of the task in question. We show how the influence of certain factors on competitive performances can be described using linear or logistic regression. We discuss the choice of analysis for factorial repeated-measures designs, which is complicated by the assumption of 'sphericity' in a univariate general linear model, and the relatively low statistical power of the multivariate approach when used with small samples. We consider a little-used and simpler technique known as 'analysis of summary statistics'. In multi-group pre- and post-test designs, a useful technique can be to pair-match individuals on their performance scores in a counterbalanced fashion before the intervention or control has been introduced. Finally, we outline how confidence intervals can help in making statements about the probability of the population difference in performance exceeding the value designated as being worthwhile or not.

Rapid travel across multiple time zones disturbs circadian rhythms and induces "jet lag". The aims of this study were 1) to monitor a selection of subjective, physiological and performance variables in elite athletes and sedentary subjects following a westerly flight across five time zones, and 2) to examine whether the promotion of sleep by means of a low-dose benzodiazepine drug influences these responses to transmeridian travel. Subjects comprised eight members of the British men's gymnastics squad, aged 18-30 years, and nine members of the British Olympic Association's support staff, aged 24- 55 years (4 females, 5 males). Subjects were pair-matched for age, sex and athleticism (apart from one person) and assigned to either the treatment (n = 9) or placebo (n = 8) group. All subjects travelled from U.K., arriving at Tallahassee, Florida, at approximately 22:00 hours local time. A test battery was administered to the subjects at 07:00, 12:00, 17:00 and 21:00 hours on the first full day of arrival (this was designated day one) and then on alternate days (day 3, day 5 and day 7). Immediately before retiring to bed on days 1, 2 and 3, subjects were administered, in a double-blind fashion, either 10mg of temazepam or a placebo. Measures in the test battery included sleep quality, sleep length, subjective jet lag (one-to-ten simple analogue scale), tympanic temperature, one-, two-, four- and eight-choice reaction time, grip strength (left and right), leg strength and back strength. Over the 24 h of each test day, subjects also recorded the volume of each urine voided. Data were analysed with a repeated measures general linear model. Alpha was set at 0.01 to control for type I errors with multiple dependent variables. Mean subjective jet lag reduced from 4.6 units to baseline, and mean sleep quality improved by 2.0 units from day 1 to day 5 (P

Humans are diurnal by nature but modern industrialized societies must function throughout the 24 hours of the day. Not only are emergency services required to be able to act at any time, but communications, commerce and industrial processes also never cease their operations. In the military sphere in particular, round-the-clock capability is required. Whereas these processes have been accompanied by an increasing replacement of humans by technology, if only because humans are too slow, expensive, and unreliable, our species still is required to tend such systems and act in relation to the information they give or require. The role of humans has tended to change, therefore, from the that of providing brute strength and even intellectual input to one of vigilance and the minding of machines.

In an informative and balanced review, Mori et al. (2009) explored the perplexing question of why people differ in their individual physiological responses to exercise training. We agree fully with Mori et al. (2009) that gene polymorphisms could account for inter-individual differences in the exercise-modulated response of physiological risk factors of disease, and that a better understanding of associated issues should lead to more effective exercise prescription. In this respect, we wonder whether a statistical artifact, originally identified in the 19th century, should also be considered carefully by researchers on this topic.

Subjective, physiological and physical performance variables are affected following travel across multiple time-zones (jet-lag). The objective of the study was to examine the effects of oral melatonin in alleviating jet-lag by investigating its effects on subjects who had flown from London to Eastern Australia, 10 time-zones to the east. Melatonin (5 mg day(-1)) or placebo capsules were administered to 14 experimental (13 males and 1 female) and 17 control subjects (15 males and 2 females), respectively, in a double-blind study; the time of administration was in accord with the current consensus for maximizing its hypnotic effect. Grip strength and intra-aural temperature were measured on alternate days after arrival at the destination, at four different times of day (between the times 07:00 - 08:00 h, 12:00 - 13:00 h, 16:00 - 17:00 h and 19:00 - 20:00 h local time). In addition, for the first 6 - 7 days after arrival in Australia, subjective ratings of jet-lag on a 0 - 10 visual analogue scale and responses to a Jet-lag Questionnaire (incorporating items for tiredness. sleep, meal satisfaction and ability to concentrate) were recorded at the above times and also on retiring (at about midnight). Subjects continued normally with their work schedules between the data collection times. Subjects with complete data (13 melatonin and 13 placebo subjects), in comparison with published data, showed partial adjustment of the diurnal rhythm in intra-aural temperature after 6 days. A time-of-day effect was evident in both right and left grip strength during adjustment to Australian time; there was no difference between the group taking melatonin and that using the placebo. Right and left grip strength profiles on day 6 were adjusted either by advancing or delaying the profiles, independent of whether subjects were taking melatonin or placebo tablets. Subjects reported disturbances with most measures in the Jet-lag Questionnaire but, whereas poorer concentration and some negative effects upon sleep had disappeared after 3 - 5 days, ratings of jet-lag and tiredness had not returned to 'zero' (or normal values), respectively, by the sixth day of the study. Subjects taking melatonin showed no significant differences from the placebo group in perceived irritability, concentration, meal satisfaction, ease in getting to sleep and staying asleep, frequency of bowel motion and consistency of the faeces. These results suggest that, in subjects who, after arrival, followed a busy schedule which resulted in frequent and erratic exposure to daylight, melatonin had no benefit in alleviating jet-lag or the components of jet-lag, and it did not influence the process of phase adjustment.

Twelve healthy male subjects each undertook two bouts of moderate exercise (70% VO2max for 30 minutes) in the morning (08:00) and late afternoon (18:00) at least 4 days apart. Measurements were made of heart rate, core (rectal) temperature, sternum skin temperature, and forearm skin blood flow during baseline conditions, during the bout of exercise, and throughout a 30-minute recovery period. Comparisons were made of the changes of heart rate, temperature, and skin blood flow produced by the exercise at the two times of day. Student t tests indicated that baseline values for core temperature (37.15 degrees C +/- 0.06 degrees C vs. 36.77 degrees C +/- 0.06 degrees C) and sternum temperature (33.60 degrees C +/- 0.29 degrees C vs. 32.70 degrees C + 0.38 degrees C) were significantly (p < .05) higher in the late afternoon than the early morning. Two-way analysis of variance (ANOVA) indicated that the increases in core and sternum temperatures during exercise were significantly less (p = .0039 and .0421, respectively) during the afternoon bout of exercise compared with the morning, even though the work loads, as determined by changes in heart rate, were not significantly different (p = .798) at the two times of testing. There were also tendencies for resting forearm skin blood flow to be higher in the afternoon than in the morning and for exercise to produce a more rapid rise in this variable in the afternoon. The possible mechanisms producing these responses to exercise are discussed in terms of those that are responsible for the normal circadian rhythm of core temperature. It is concluded that the body's ability to remove a heat load is less in the early morning, when the circadian system is in a "heat gain" mode, than in the late afternoon, when heat gain and "heat loss" modes are balanced more evenly.

Part One: Ergonomics and disabled athletes. Part Two: Environmental factors, ergonomic aids and exercise performance. Part Three: Equipment design and technique analysis. Part Four: Training and fitness assessment. Part Five: Methodology and measurment techniques. Part Six: Health benefits of exercise and workplace-based programmes. Author Index. Subject Index.

In many sports, performance has increased as the result of technological progress. Some developments, such as the fastskin suit in swimming, have had a direct impact on performance (Chatard & Wilso...

Nine healthy female subjects were studied when exposed to the natural light-dark cycle, but living for 17 "days" on a 27h day (9h sleep, 18h wake). Since the circadian endogenous oscillator cannot entrain to this imposed period, forced desynchronization between the sleep/activity cycle and the endogenous circadian temperature rhythm took place. This enabled the effects of activity on core temperature to be assessed at different endogenous circadian phases and at different stages of the sleep/activity cycle. Rectal temperature was measured at 6-minute intervals, and the activity of the nondominant wrist was summed at 1-minute intervals. Each waking span was divided into overlapping 3h sections, and each section was submitted to linear regression analysis between the rectal temperatures and the total activity in the previous 30 minutes. From this analysis were obtained the gradient (of the change in rectal temperature produced by a unit change in activity) and the intercept (the rectal temperature predicted when activity was zero). The gradients were subjected to a two-factor analysis of variance (ANOVA) (circadian phase/ time awake). There was no significant effect of time awake, but circadian phase was highly significant statistically. Post hoc tests (Newman-Keuls) indicated that gradients around the temperature peak were significantly less than those around its trough. The intercepts formed a sinusoid that, for the group, showed a mesor (+/-SE) of 36.97 (+/-0.12) and amplitude (95% confidence interval) of 0.22 degrees C (0.12 degrees C, 0.32 degrees C). We conclude that this is a further method for removing masking effects from circadian temperature rhythm data in order to assess its endogenous component, a method that can be used when subjects are able to live normally. We suggest also that the decreased effect of activity on temperature when the endogenous circadian rhythm and activity are at their peak will reduce the possibility of hyperthermia.

With informed consent, 6 competitive tennis players performed alternate 15 “first” (emphasis-speed) serves and 15 “second” (emphasis-accuracy) serves at 09:00, 14:00 and 18:00 hours. Serve velocity was measured by the digitisation of video footage of each serve. The Hewitt Tennis Achievement Test was employed to measure the accuracy of serve. The amount of spin imparted on the ball was not measured. First serves were at all times of day faster than second serves. First serves were faster but least accurate at 18:00 hours, the time of day that body temperature and grip strength were highest. At 09:00 hours, first serves were just as accurate as second serves, even though velocity of first serves was higher. No effects for time of day were found for the speed and accuracy of second serves. These results indicate that time of day does affect the performance of tennis serves in a way that suggests a nonlinear relationship between velocity and accuracy.

Twelve subjects have been studied in a chamber that isolated them from external noise and lighting. After several control days, one group (n = 6) was subjected to 18 x 27-h 'days' and the other to 11 x 30-h 'days'. Sleep was in the dark, and awake times were spent in normal domestic lighting (150-500 lux). Rectal temperature and wrist actimetry were measured throughout, and the phase of the circadian oscillator was inferred from that of the temperature data, purified to remove direct effects of activity. During the experimental 'days' the rhythms showed a mean period of 24.4 h. A detailed examination of the phase shifts from one day to the next showed that small advances and delays were superimposed upon this drift. Moreover, the mean size and direction of these shifts depended upon the time of exposure to lighting relative to the temperature minimum, as would be predicted from a phase-response curve.

Background Most studies on lifestyle changes in old age have been transverse. We have conducted a longitudinal study. Subjects 112 non-institutionalized subjects were studied in 1984 and again 10 years later (ages in 1984 ranged from 53-82 years). Protocol On each occasion subjects recorded in a diary their times of retiring and rising and of taking meals, during a 'typical week'. They also recorded whether they lived alone or with somebody. Analysis The diaries were scored to establish any effects of age or living alone on the timing and variability of their lifestyle. Results Age was associated with changes in the sleep/wake schedule and mealtimes and a decrease of daily variation in these variables. When these changes were compared in subjects living alone and with somebody, the increase in time spent in bed and the decreases in variability of times of rising and meals were more marked in subjects living with somebody. Conclusions A deteriorating body clock contributes to some of these changes, but an increasingly inflexible lifestyle will offset some of the effects of this decline in circadian rhythmicity.

OBJECTIVE: The consensus of opinion suggests that when assessing measurement agreement, the most appropriate statistic to report is the "95% limits of agreement". The precise form that this interval takes depends on whether a positive relation exists between the differences in measurement methods (errors) and the size of the measurements--that is, heteroscedastic errors. If a positive and significant relation exists, the recommended procedure is to report "the ratio limits of agreement" using log transformed measurements. This study assessed the prevalence of heteroscedastic errors when investigating measurement agreement of variables recorded on a ratio scale in sports medicine and sports science. METHODS: Measurement agreement (or repeatability) was assessed in 13 studies (providing 23 examples) conducted in the Centre for Sport and Exercise Sciences at Liverpool John Moores University over the past five years. RESULTS: The correlation between the absolute differences and the mean was positive in all 23 examples (median r = 0.37), eight being significant (P < 0.05). In 21 of 23 examples analysed, the correlation was greater than the equivalent correlation using log transformed measurements (median r = 0.01). Based on a simple meta-analysis, the assumption that no relation exists between the measurement differences and the size of measurement must be rejected (P < 0.001). CONCLUSIONS: When assessing measurement agreement of variables recorded on a ratio scale in sports medicine and sports science, this study (23 examples) provides strong evidence that heteroscedastic errors are the norm. If the correlation between the absolute measurement differences and the means is positive (but not necessarily significant) and greater than the equivalent correlation using log transformed measurements, the authors recommend reporting the "ratio limits of agreement".

Travel across multiple time zones is a common feature of the lifestyle of contemporary international sport competitors. This entails a disruption of the body's circadian timing mechanisms. Symptoms of ‘jet-lag’ persist until circadian rhythms re-tune into local time. Exercise performance may be impaired during the period of adjustment to the new time zone. Behavioural and pharmacological strategies are available to help accelerate the resynchronization of circadian rhythms. It is recommended that sports administrators should consider the consequences of jet-lag and the time needed to overcome it when planning the international itineraries of travelling athletes.

Although homeostatic mechanisms remain of utmost importance, rhythmic changes are present also. The main ones have a period of 24 h (circadian) or about 2–3 h (ultradian). Circadian rhythms are derived from a body clock, found in the base of the brain, and from the pattern of our sleep wake cycle, including activity and meal times. These rhythms promote the regular changes between an active wake period and a recuperative sleep period. Ultradian rhythms are also widespread and reflect external (lifestyle) and internal factors. The internal factors include biochemical need and some sort of oscillator; but details of how many oscillators, and exactly where they are, remain to be established. Food intake, appetite, digestion and metabolism have been shown to illustrate these principles. Moreover, these principles become important when special circumstances exist as far as meal times are concerned; the particular diffculties of night workers is a good example.

To the Editor One of the most consistent findings in research on percentage flow-mediated dilation (FMD%) is the negative correlation between baseline arterial diameter (Dbase) and FMD% itself. The study by Namtvedt et al 1 is no exception in this respect. FMD% is obviously already an attempt to ‘normalise’ the measured change in arterial diameter (in millimetre) for variability in Dbase, but seems to do this job poorly.2 ,3 In a well-designed study, Namtvedt et al 1 reported that FMD% was 3.7% lower in patients with …

Eight nurses have been studied during rest days and three successive night shifts. Measurements of wrist activity have been made and used to assess the extent to which the pattern of daily activity changes between control (rest) days and days involving night work. One analysis considered wrist activity during time spent in bed; this appears to decrease in parallel with the amount of time in bed that is lost during night work but, when this effect is corrected for, there is greater activity during time spent in bed in the daytime compared with control days (when time in bed is during the night). The dichotomy of activity (between lower values during time spent in bed and higher values when out of bed) also decreases if time in bed is during the daytime while on night shifts. These changes in the amount of wrist activity and the dichotomy between activity in and out of bed are related to the changed quality and quantity of sleep that has been measured by self-report questionnaires and the sleep EEG. It is concluded that results from wrist actimetry can provide valuable information regarding the process of adjustment to night work, and that its convenience (to subject and experimenter), coupled with the new analytical approaches described here, make it a viable method for field studies.

Chronobiology 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 times 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 'morningness', 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.

The following is the abstract of the article discussed in the subsequent letter: This study examined neurohumoral alterations during prolonged exercise with and without hyperthermia. The cerebral oxygen-to-carbohydrate uptake ratio (O2/CHO = arteriovenous oxygen difference divided by arteriovenous

This study was designed to examine the effects of age and time of day on work rates during prolonged, self-paced exercise. Eight young (19-25 years of age) and eight old (48-62 years of age) endurance athletes volunteered for the study. At two times of day (07:00 and 17:00 h), subjects were asked to pedal on a Monark cycle ergometer (Varberg, Sweden) at a self-chosen exercise intensity that they believed they could sustain for exactly 80 min. This self-chosen work rate, rectal temperature, skin temperature (chest, arm, and lower leg), oxygen consumption (VO(2)), expired carbon dioxide (VCO(2)), minute ventilation (VE), heart rate, and perceived exertion (RPE) were recorded every 10 min during the exercise. Preexercise resting measures of rectal temperature, VO(2), and VE were less affected by the time of day in the older group than were those in the young subjects (p0.05). In the morning, rectal temperature was 0.3 degrees C higher in the older subjects than in the young adults. Diurnal variation in mean work rate over the 80-min exercise period was not evident in the old group (p0.10) but amounted to 10 W in the young group (p0.05). Older subjects chose work rates 5.4 W lower than did the young subjects in the morning test session (p0.10). In the afternoon, age differences in work rate amounted to 14.3 W (p0.05). For all subjects, work rates remained relatively constant throughout the exercise period in the morning. In the afternoon, subjects chose high work rates within the first 40 min of exercise, after which work rate decreased sharply to values similar to those recorded in the morning (p0.01). These changes were mirrored closely by changes in (VO(2)) and VCO(2). Perceived exertion increased linearly throughout exercise, irrespective of age or time of day. These results suggest that, in young adults, the mean work rate over 80 min of exercise is higher in the afternoon than in the morning, although the work rate decreased sharply toward the end of the afternoon exercise. In agreement with studies reporting age-related increases in "morningness," age differences in work rate appeared to be least when exercise was performed in the morning.

To evaluate whether circadian rhythms in blood pressure and heart rate are influenced by age, we analyzed 24-h ambulatory blood pressure and heart rate recordings from 31 patients with primary hypertension. Data were collected during hospitalization, after a drug-free run-in period. Set times were administered for lights-on, meals, and lights-off. Daytime napping was prohibited. The patients were divided into sex-matched groups of young (group I: 25-45 years, n = 9), middle-aged (group II: 47-57 years, n = 11), and old (group III: 57-74 years, n = 11) subjects. Hourly data were analysed by fitting a two-component cosine function (24- and 12-h periods). Amplitudes of the circadian rhythms in systolic blood pressure and heart rate were significantly reduced with age. This finding could be partly attributed to the recording of higher nocturnal values in older patients. Elderly hypertensives also evidenced a significantly greater ultradian component (12-h period) in the systolic blood pressure rhythm than did young patients, with the secondary afternoon decline in blood pressure being more pronounced in groups II and III. The 24-h acrophase of heart rate was found to occur approximately 1.6 h earlier than that of systolic blood pressure in the young group (p0.01). This phase advance of heart rate compared with systolic blood pressure was reduced to 1 h in group II (p0.05) and was not evident in group III (p0.1). These results indicate that circadian blood pressure and heart rate profiles of primary hypertensives change with age. Since measures were obtained in a typical clinical setting, these findings have implications for the diagnosis and treatment of hypertension in the elderly. The marked afternoon decline in blood pressure for the elderly patients may also render conventional cosinor analysis inappropriate for accurate description of the circadian rhythms of geriatric hypertensives.

(2002). Sport performance: variable or construct? Journal of Sports Sciences: Vol. 20, No. 4, pp. 291-292.