Exercise training is known to increase endothelial function and provoke arterial remodelling both locally and systemically. This thesis was designed to further examine these relationships by investigating the acute response to different exercise intensities, with and without shear rate modification. Shear rate modification was also used to examine the impact of systemic exercise training on brachial haemodynamics. Finally, the effect of inactivity on vascular function and arterial remodelling were studied using novel models of inactivity. The aim of Study 1 was to examine the effect of shear stress on upper limb brachial artery dilation during acute cycle exercise of different intensities. The impact of three randomised bouts of 30 mins leg cycling (50, 70 and 85% HRmax) on brachial artery blood flow, shear rate (SR) and brachial diameter, was measured bilaterally and simultaneously. SR was further manipulated in one arm via forearm heating (40±1°C) in a water bath (+0C) throughout the exercise bouts. Exercise induced stepwise increases in SR in the unheated arm (~0C) (P<0.05). In the +o( arm, SR was significantly greater than in the ~o( limb. Brachial artery diameter increased post- exercise in ~o( by 3% (50%HRmax; P>0.05 vs. baseline), 7% (70%HRmax; P<0.05) and 9% (85%HRmax; P<0.05). In the+°C arm, post-exercise brachial diameter increased at all exercise intensities (P<0.05) and was significantly greater (P<0.05) than in the ~°C limb at 50% (12%), 70%HRmax (14%) and 85%HRmax (15%). In conclusion, increases in shear rate during incremental lower limb exercise are associated with increases in brachial artery diameter. This response is exaggerated with larger SR induced by localised heating, indicating that leg exercise has systemic effects on arterial diameter and that SR is an important stimulus to vasodilation during exercise in humans. The second study examined brachial artery flow mediated dilation (FMD) (using high resolution echo-Doppler) pre, and post (0, 1, 2, 24hr) 3 bouts of acute 30 min exercise at different (50, 70 and 85% HRmaxl intensities. Shear rate area-under-the- curve (from cuff deflation to peak dilatation; SRAUcl was calculated as the eliciting stimulus for FMD. Both baseline diameter and SRAUC were elevated by exercise. With covariate-control of these variables, the change in brachial artery FMD was negligible after exercise (~5 minutes post exercise) at 50% HRmax (6.3±2.6 vs. 5.9±2.5%; 95%(1 for difference: -0.59 to 1.34%) whilst a larger changes in FMD were noted after the exercise bouts at 70% (6.1±1.8 vs. 4.7±1.9%; 95%(1 for difference: 0.08 to 2.58%) and at 85% HRmax (6.6±1.6 vs. 3.6±2.2%; 95%(1: 0.41 to 5.42%). A further 2-way ANOVA revealed there were no changes in FMD at any other time- point post exercise (1, 2, 24hrs) and FMD normalised by Ihr post. These data indicate, for the first time, a 'dose-response' relationship between exercise intensity and the reduction in FMD, even when exercise-mediated changes in shear and baseline diameter are accounted for. The purpose of Study 3 was to examine the contribution of shear stress to changes in vascular function in the non-exercising upper limbs in response to lower limb (systemic) cycling exercise training. Subjects participated in an 8-week cycle training study undertaken at 80% HRmax, with unilateral cuff inflation around the forearm during each exercise bout. FMD, partly NO-mediated endothelial function (i.e. ischaemic handgrip exercise (iEX)), and endothelium-independent dilation to a NO donor (i.e. glyceryl trinitrate (GTN)) were measured at 2, 4 and 8 weeks. Cycle training increased FMD in the non-cuffed limb at week 2 after which, responses returned towards baseline levels (5.8±4.1, 8.6±3.8, 7.4±3.5, 6.0±2.3 at 0, 2, 4 and 8 weeks, respectively; ANOVA: P=0.04). No changes in FMD were observed in the cuffed arm. In addition, no changes were evident in response to iEX or GTN in either the cuffed or non-cuffed arms (P>O.05) across the 8 week intervention period. These data suggest that lower limb cycle training induces a transient increase in upper limb vascular function in healthy young humans which is, at least partly, mediated via shear stress. Exercise training is associated with rapid changes in endothelial function, which occur within days of starting training. Whilst long-term physical inactivity has a strong effect on vascular structure, little is known about the immediate impact of inactivity on vascular function. Therefore, Study 4 measured changes in vascular function before, during (day 4) and after 8 days of unilateral forearm inactivity induced by wearing a sling on the non-dominant arm. Maximal handgrip strength of the inactive forearm decreased after 8 days, confirming physical deconditioning. There were no significant changes in brachial artery baseline diameter, FMD, iEX or GTN across the 8 days in either arm (P>0.05). A significant decrease in peak blood flow was found in the intervention arm (2-way interaction: P=0.03) that is suggestive of remodelling of forearm resistance vessels. However, measures of (largely and partly) NO-mediated endothelial conduit artery function were not altered across an 8 day period of inactivity. Whilst increases in mean arterial shear stress are known to induce improvements in arterial function and remodelling in humans, animal data have demonstrated that retrograde shear is associated with pro-atherogenic effects. However, relatively little is known regarding the effect of retrograde shear rate on vascular function in humans in vivo. In order to provoke retrograde shear, subjects wore a compression sleeve on one forearm for 8 clays. Measurements were taken before and during acute (lhr) exposure to a compression sleeve on baseline day O. Measurements were taken after 4 and 8 days exposure to the compression sleeve. There were no significant changes in mean or antegrade shear rate during exposure to the compression sleeve. However, the compression sleeve resulted in an immediate increase in retrograde shear rate in 6 subjects (P<O.05, intervention-group), but remained near resting levels in the other 6 subjects (P>O.05, control-group)i.e. subjects in whom the compression sleeve did not increase retrograde shear were the control group. The intervention group demonstrated a significant decrease in FMD after 1 h compression sleeve (P<O.05), but not in the control group (P>O.05). After 8-days using the compression sleeve, no significant changes in FMD, iEX, or GTN-response in the intervention and control group (all P>O.05) were observed. In conclusion, short-term increases in retrograde shear rate decrease FMD, but not chronically. Data in this thesis provide evidence for the role of blood flow and shear stress, as a result of exercise and inactivity, and its immediate effects upon the vasculature.

The periodic alternation of light and darkness over the solar day has a fundamental role in synchronising human circadian rhythms. Artificial light is known to alter circadian timing depending on time of administration; early morning light can advance circadian timing while late evening light can delay circadian rhythms. Nevertheless, there is a lack of research on how manipulations of the body clock, via bright light, can enhance human performance and/or alleviate the effects of jet-lag after transmeridian travel. The doctoral programme communicated in the present thesis was designed to fill some of these gaps in knowledge. In the first experiment, the acute effects of light exposure on thermoregulation, pineal function and autonomic nervous system function (indicated by the rate pressure product) were examined. At 20:00 h, eight healthy men were exposed to a baseline period of dim light « 12 lux) for 45 min followed by 0 lux for another 60 min. Thereafter, participants were exposed to either polychromatic bright light with blue photons (2500 lux), blue photons removed (2,500 lux) or 0 lux for 30 min. Baseline data was altered for all variables in the two light conditions, suggesting an "expectancy effect". There was evidence that light attenuated the nocturnal fall in core body temperature and rise in melatonin. These data highlight that the circadian system and other areas of the brain which are stimulated by light are still sensitive to short duration exposure and these effects are amplified by the inclusion of blue spectrum light. In a follow-up study, the effects of light exposure (blue photons included polychromatic bright light vs. no light) on subsequent early morning performance were examined under conditions of thermal stress. Participants were woken at 06:30 h. At 07:00, participants entered an environmental chamber set at 35°C and 60% relative humidity. Three 10-min bouts of exercise (55% V02max) were completed on a cycle ergometer with each bout separated with 10 min of passive recovery. Participants then completed a 10-km cycling time-trial. Core body temperature was measured every 30 s throughout the experiment using intestinal thermistors. The time of the sleep-trough in core temperature occurred approximately 1.75 h later following bright light (L) vs. no light (NL) (p = 0.07). Just prior to the time-trial, Tc was 0.27 ± OA2°C lower in L than NL (p = 0.07). The time trial was completed 1.43 ± 0.63 min quicker in L vs. NL (p = 0.001). Immediately after the time trial, intestinal temperature was 38.21 ± 0.56°C in BL compared to 38.64 ± 0.42"C in NL (p = 0.10). These data provide the first evidence that a 3D-min exposure to bright light prior to sleep can delay circadian timing to the extent that exercise performance is improved in hot conditions during the subsequent early morning. In the third study, the effects of a simulated dawn during the last 30 minutes of sleep on the subsequent dissipation of sleep inertia were examined and it was questioned whether subsequent improvements in simulated work and physical performance were mediated. Eight participants, who reported difficulty with morning waking, were administered in a random order to a control (C) and a dawn Simulation (OS) trial (starting 30 minutes prior to waking). Subjective ratings of sleep quality and alertness were obtained alongside measures of cognitive performance (addition and a reaction time tasks measured at 5, 30 and 75 minutes after waking at habitual workday times). Physical performance was also measured 35 minutes after waking using a self-paced cycling protocol. After waking in the OS condition, perceived sleep quality was 1.16 ± 0.89 (p = 0.01) points higher compared with C. Ratings of alertness were significantly higher in OS than C throughout.

No description available.

Shift-work is increasingly common in society and is associated with a number of health inequalities. The health effects of shift-work can include a redudion in quality and quantity of sleep, insomnia, chronic fatigue, anxiety and depreSSion,decreased vigilance, metabolic syndrome, adverse cardiovascular (especially blood pressure), gastrOintestinal effects and reproductive effects in women. Shift-work is also associated with disturbances to a workers domestic and social life. Physical activity is known to either prevent or alleviate these health inequalities in those with 'normal diurnal' lifestyles such as day-workers. However, shift-work generally decreases opportunities for physical adivity. Moreover, the favourable affects of physical activity on such health inequalities have not been confirmed in shift-workers. The studies in the present thesis are designed to examine the associations between shift-work and physical activity covering a wide range of physiological and psycho-social variables in shift-workers. The first descriptive study was designed to provide the first detailed and multi-researchstrategy examination of LTPA and its correlates in shift-workers. One hundred and sixty one shift-workers partiCipated in the initial cross-sectional study. A cohort of ten of these partiCipants also volunteered for further diary- and inteNiew-based studies. PartiCipants completed the SSI together with a LTPA questionnaire. Gender, job type, age and shift-work experience were explored as correlates of LTPA. The cohort also completed a 7-day diary and wore an accelerometer for assessment of activity counts during work, leisure, and bedtime when working each shift-type (days, nights and rest). Participants also completed a semi-strudured email-administered interview. The total energy expenditure per week in LTPA of male shift-workers was found to be twice that of women. Midwives recorded the lowest LTPA. Firefighters reported the most LTPA. The time spent out of bed during night shift days was 4-h higher compared with rest days. Data from this study indicated that LTPA is generally low amongst shift-workers, the majority of whom are overweight or obese. Jobtype and gender are much more influential on LTPA than age or experience. Shift-workers spend more of their time on rest days in bed. The second descriptive study was designed to explore the relationships between coping strategies adopted by shift-workers and their leisure-time energy expenditure. The importance of coping strategies has been highlighted in previous research. Ninety-five participants completed an adapted version of the completed the SSI together with a LTPA qUestionnaire. Predictors of age, time spent in shift-work, gender, marital status and the various shift-work coping indices were explored with step-wise multiple regression. Leisuretime energy expenditure over a 14-day period was entered as the outcome variable. Gender (p < 0.023) and time spent in shift-work (p < 0.051) were found to be predictors of energy expenditure, with the most experienced, male shift-workers expending the most energy during leisure-time. Overall 'disengagement' coping scores from the SSI were positively related to leisure-time energy expenditure (p < 0.054). In males, disengagement of sleep problems (p > 0.086) was found to be negatively correlated to energy expenditure, whereas disengagement of domestic-related problems was found to be positively related to energy expenditure (p < 0.001). These relations were not found in female shift-workers (p > 0.762). These data indicated that experienced male shift-workers participate in the most leisure-time physical activity. These people 'disengage' more from their domestic-related problems, but less from their sleep-related problems. The next laboratory-based study was designed to examine the acute effects of evening exercise and meal frequency on psychophysiological and performance-related variables during a subsequent period of simulated night-work. Nine healthy participants, completed at least two crossover trials beginning at 18:00 h. Between 19:00-20:00 h, participants either rested or exercised at 50%V02peak and then remained awake throughout the night, completing various tasks until 05:15 h. Six participants completed a total of four trials in which they exercised or rested while either one standardized (SOkJlkg) meal at 22:00 h or two smaller (30 kJ/kg) meals at 22:00 and 02:00 h were eaten. Core Body Temperature (Tc), wrist activity, mood, sleepiness, arousal, self-chosen work-rate, and reaction time were all measured throughout the simulated night-shift. Following exercise, Tcwas Significantly lower throughout the night-shift compared with no prior exercise (95% Cl = 0.00 to 1.01°C), even though wrist activity was higher and sleepiness was lower after exercise. Self-chosen workrate was significantly higher (95% Cl = 20 to 43 W) and reaction time faster during the nightshift that followed exercise. Reaction time and alertness were worst when only 1 meal was ingested during the night-shift (p <0.04). These data indicate that a single bout of evening exercise can improve sleepiness as well as mental and physical performance during a subsequent simulated night-shift ... The findings from the studies in this thesis indicate that many of the short-term health benefits of exercise are apparent in contexts of shift- and night -work. The relatively small final intervention study demonstrates the potential utility of an individualised lifestyle intervention based on motivational interviewing for shift-workers.

The detrimental nature of movement variability has recently been reconsidered with suggestions that it has a functional role to play in performance. Movements in golf can be attributed to the organismic, task and environmental constraints from which they emerge with these swing movements affecting shot outcomes. A three-dimensional analysis of address position variability revealed that higher skilled golfers present reduced alignment variability in angular relationships between the shoulders and stance compared to less skilled counterparts. Whilst there were no group differences in impact variability, both points in the swing displayed reducing variability from proximal to distal aspects of the kinetic chain. With the popularity of strength and conditioning growing within the golfing world it has become important for coaches to be able to assess golfers’ physical constraints. Two-dimensional analysis, representative of that used in coaching environments, assessed the relationship between the overhead squat and deterioration of posture in the golf swing. Results showed small but significant relationships between this test and golf swing postural kinematics. An 8-week intervention to address overhead squat physical constraints resulted in no change in 3D swing kinematics. Strength and conditioning as a stand-alone intervention provides no benefits to postural kinematics suggesting the need for coaching.

Taking part in regular physical activity leads to adaptive response that enables the body’s antioxidant defence to be better equipped to fight against oxidative stress. Exercise intensity seems to be one of key factors that determines the effectiveness of exercise. The work presented in this thesis used novel approaches, through the application of emerging technologies, to study physical activity and its effects. This thesis contributes to the existing literature by being the first to investigate the effect of exercise on a marker of oxidative stress in the brain, an organ that becomes impaired (including oxidative damage) with ageing and diseases associated with ageing. The finding from this thesis suggests that brain glutathione (GSH) of young sedentary men as measured by magnetic resonance spectroscopy (MRS) was altered in response to acute exercise, in an exercise intensity dependent manner. Observed changes in peripheral markers of oxidative stress were also exercise intensity dependent. The brain seems to be protected against hyperperfusion injury during high intensity phase of high intensity interval exercise. Objectively measured physical activity levels were not significantly increased by an unsupervised home-based exercise intervention in older adults, potentially due to a lack of progressive goals based on adherence to physical activity.

There is no information available in the scientific literature that documents a specific assessment protocol for analysing a soccer player’s maximal power capabilities. As soccer-associated muscular power has not previously been investigated, it is not known how important power is in elite soccer and, if it is, the physiological and genetic determinants of soccer-associated power remain unknown. Such information could be used to optimise soccer-specific talent identification and development strategies. With this in mind, the overriding aim of our thesis was to investigate the physiological and genetic factors underpinning powerful actions in elite youth soccer. When devising an assessment of soccer-associated muscular power, there needs be a detailed analysis of the specific actions performed during elite competitive match-play that can be described as powerful. The aims of our first experimental study (Chapter Three) were to compare the frequency and durations of powerful actions during competitive English Premier League [under 18 (U18) and under 21 (U21)] elite youth soccer matches using a novel soccer specific powerful action (SSPA) notational analysis coding system. We found that while elite soccer match-play requires players to perform powerful actions in multiple directions [68 horizontal accelerations (in the horizontal-forward or mediolateral directions), eight sprints, and six vertical jumps (three bilateral and three unilateral)], horizontal accelerations of short duration (< 1.5 s) from different starting speeds were the most dominant type of explosive action. This activity profile provides a strong rationale for devising a muscular power assessment protocol that evaluates the ability to produce maximal power in multiple directions, from a unilateral stance. Our data also suggests that such a protocol could provide a specific lower body power profile in elite soccer players (ESP). The aim of our second study (Chapter Four) was therefore, to determine whether countermovement jumps (CMJs) in different directions [CMJs: bilateral vertical (BV), unilateral vertical (UV), unilateral horizontal-forward (UH) and unilateral medial (UM)] assessed independent lower-limb power qualities, and if CMJ performance differed between ESP (representing an English Premier League Academy regularly at U18 and under U21 levels) and non-elite soccer players (NSP). We found that unilateral CMJs in different directions assessed independent peak vertical power (V-power) and resultant take-off velocity capabilities, and the UH CMJ required significantly greater bicep femoris electromyographic (EMG) activation in comparison all other CMJs. Moreover, in comparison to NSP, ESP achieved greater V-power during all CMJs (p≤0.032) except for BV (p=0.197), and also achieved greater UH CMJ projectile range (51.6 ± 15.4 vs. 40.4 ± 10.4 cm, p=0.009). Our results suggest that unilateral CMJs in different directions, but not the commonly used BV CMJ, are determinants of U18 and U21 elite soccer playing status and can be used by applied practitioners as independent assessments of soccer-associated muscular power. As the physiological determinants of performance are of use to the applied practitioner for informing talent identification criteria, and prescribing detailed training intervention strategies, the primary aims of the third and fourth studies were to investigate the neuromuscular (Chapter Five) and tendon (Chapter Six) determinants of unilateral CMJs oriented in different directions. Our data suggests that unilateral CMJ performance is associated with direction-specific neuromuscular and tendon properties in U18 and U21 ESP. While UV CMJ performance was related to the size (quadriceps femoris muscle volume and physiological cross sectional area), architecture (vastus lateralis pennation angle) and ability to activate (vastus lateralis EMG activation level) the knee extensor muscles, UH CMJ performance was related to the elongation and compliance properties of the patellar tendon, and was inversely correlated with vastus lateralis fascicle pennation angle. Our findings highlight the importance of targeting specific neuromuscular and tendon properties when assessing and developing muscular power performance in U18 and U21 ESP. Many physiological changes occur during puberty (Viru et al., 1999) and our findings in Chapters Four, Five and Six may only be applicable in U18 and U21 ESP. Therefore, in our fifth study (Chapter Seven) we aimed to investigate the importance of acceleration, sprint, horizontal-forward CMJ and vertical CMJ capabilities at different stages of maturation in elite youth soccer. Elite soccer players and CON were grouped using years from/to predicted peak height velocity (PHV, a measure of growth velocity and an indirect measure of pubertal phase) to determine maturation status (ESP: pre-PHV, n=100; mid-PHV, n=25; post-PHV, n=88; CON: pre-PHV, n=44; mid-PHV, n=15; post-PHV, n=54). By comparing performance of ESP and control participants (CON) matched for maturation status, we found that acceleration and sprint performance were associated with elite youth soccer at all stages of maturation, but maximal power (horizontal-forward and vertical jumping) capabilities may only be important for elite youth soccer at mid- and post-peak height velocity. Our data could imply that assessments of acceleration and sprint capabilities should be included in soccer talent identification protocols at all stages of maturation, but maximal power should only be included at mid- and post-PHV. The purpose of our sixth experimental study (Chapter 8) was to investigate if specific gene single nucleotide polymorphisms [SNPs: ACTN3 R577X (rs1815739), BDNF G > A (rs6265), COL5A1 C > T (rs12722), and COL2A1 C > T (rs2070739)] played a role in determining elite youth soccer player status, and speed and power capabilities, in ESP and CON at different stages of maturation. We found that ACTN3 R- and BDNF G-allele frequencies were more frequent in post-PHV compared to pre-PHV ESP. Moreover, while the COL2A1 CC genotype was associated with greater horizontal power and faster 20 m sprint performance, BDNF GG genotype appears to positively influence 20 m sprint performance during the pre-PHV period only. Overall, our findings illustrate that elite soccer may require different genetic profiles before and after maturation, and genetic screening could be included in talent identification criteria to help predict maximal power and sprint potential in ESP. In summary, we devised a muscular power assessment battery that measured independent power qualities and could discriminate between U18 and U21 ESP and NSP. Our subsequent analysis showed that the physiological factors underpinning unilateral CMJ performance were direction-specific, and UV and UH CMJ capabilities were underpinned by separate neuromuscular and tendon properties, and should be assessed and developed, independently in U18 and U21 ESP. We then recruited a larger cohort of ESP and CON, at different stages of maturation, and demonstrated that muscular power was important for elite soccer performance at mid and post-PHV, but not pre-PHV. Finally, we showed that genetic profiles of ESP differed between pre- and post-PHV, and that certain gene variants [COL2A1 C > T (rs2070739), BDNF G > A (rs6265)] were associated with specific power and speed capabilities in ESP. Overall, our studies provide novel information that could have significant implications on soccer-associated power related talent identification and training intervention strategies in elite youth soccer academies.

Player's performance in competitive matches is partly determined by the systematic training programmes that they complete. The differences in the organisation of training may result in differences in the physiological stress placed on players. This study aimed to investigate the influence of training organisation on the physiological, metabolic and molecular responses to soccer-specific intermittent exercise in skeletal muscles. In Chapter 3, the physical demands of professional soccer training were examined by quantifying the physiological loads and work-rate profiles of elite players throughout the programmed pre-season and in-season training for a one week period. The physiological loads in pre-season were significantly higher than those in the in-season period (p < 0.05). Similar activity profiles were, however, observed during each training period irrespective of the time of the year. These findings demonstrate that pre- season training was more intensive than in-season training though these differences were not linked to changes in the activity patterns during sessions. Technical/tactical training seems to be an important component in increasing the physiological strain observed in pre-season training. This study also indicates that soccer training seems to elicit different demands to those associated with match-play. In Chapter 4, a laboratory-based soccer-specific training simulation (LSSTS) was devised on a motorized treadmill. Attempts were made to re-create both similar overall exercise intensities and patterns of discrete activity observed in training. The validity of this protocol was evaluated by comparing the physiological responses of professional players with those of healthy subjects who completed the LSSTS. Physiological measurements such as mean HR and % of HRmax associated with the simulation were similar to those obtained in the actual training session. These data suggest that the protocol is suitable in re-creating a soccer-specific training session in the laboratory. This protocol is, therefore, sufficient to use in investigations to study the physiological responses and the molecular adaptations of skeletal muscle to soccer-specific intermittent exercise. In Chapter 5, the effect of a single bout of soccer-specific intermittent exercise on metabolic stress and acute molecular responses associated with mitochondrial biogenesis was investigated in human skeletal muscle. The LSSTS was utilised as the sports-specific exercise protocol. The levels of blood metabolites and muscle glycogen were significantly altered during and after exercise (p < 0.05). Simulated soccer-specific training also acutely activated the expression of PGG-1a mRNA in human skeletal muscle (p < 0.05). There was, however, no significant change in the phosphorylation of AMPK and p38MAPK. This would suggest that the global effect of soccer-specific intermittent exercise on aerobic performance may be partly mediated by adaptations associated with mitochondrial biogenesis in skeletal muscle. ' In Chapter 6, the effect of prior soccer-specific training on the physiological, metabolic and molecular responses to a subsequent bout of soccer-specific intermittent exercise was evaluated. Two experimental trials (BETWEEN DA Y trial, one bout of soccer-specific intermittent exercise in a day Vs WITHIN DA Y trial, two consecutive bouts of soccer-specific intermittent exercise performed in a day) were completed on two separate occasions. There were significant increases in physiological responses during the second bout of exercise in the WITHIN DAY trial, compared to those obtained in the, BETWEEN DAY trial (p < 0.05). A more pronounced increase in NEFA and glycerol was observed in the WITHIN DAY trial compared with the BETWEEN DAY trial post-exercise following the second bout of exercise (p < 0.05). The expression of PGC-1a mRNA significantly increased following exercise compared to pre- and post-exercise values. There was, however, no difference in phosphorylation of AMPK and p38MAPK and the expression of PGC-1a mRNA between either trial. Based on these findings, it would seem that different approaches to training organisation may be more important for the acute physiotoqlca: responses to soccer-specific intermittent exercise than the molecular changes underpinning chronic adaptations.

No description available.

This thesis aims to identify physiological and biochemical variables, comparing sex, training status, age and maturity in sub-elite, endurance trained adolescents. Maximal lactate steady state was investigated and the effects of endurance training programmes measured.T he first study assessedth e reliability of absolute running speed, V02, and HR that correspond to the fixed blood lactate reference values of 2.0 and 2.5 mmo1. L"1 and the lactate threshold (LT) and found these measures to be reliable after endurance-trained adolescent runners completed two identical incremental treadmill tests within a 7-10 d period The second study was designed to determine the relationship between physiological variables and endurance running performance in this age group. Track-based, running performance times were available for 18 boys and 14 girls for the 800 m, and 16 boys and 13 girls for the 1500 m. The participants were tested using a step-wise incremental treadmill test and a Wingate anaerobic power test (WAnT) on separate occasions. The results from this study found that for the 1500m, running speeds corresponding to the fixed [BLa ] were a useful measure for assessing performance in endurance trained boys and girls. Unlike previous studies, peak V02 was not a significant physiological predictor of 1500m performance in either boys or girls. For the 1500 m performance in girls the anaerobic measure was no longer significant once variations in size or age had been taken into consideration. Whereas V VO2 peak and running economy may prove to be of some value when considering the 800m for boys, the running speed corresponding to a [BLa ] of 2.5 mmol-L-1 was the only meaningful physiological predictor variable for girls once differences in age and body size had been accounted for. The third study had three main objectives: (1) to identify the exercise intensity that corresponds to the (MLaSS) in adolescent, endurance trained runners, (2) to examine possible between sex differences, and (3) to compare the MLaSS with commonly cited fixed blood lactate reference variables. The participants were first tested using a step-wise incremental treadmill test to establish the blood lactate profile and peak VO 2. The running speed and % peak VO 2 at the MLaSS were not significantly different to those corresponding to the fixed [BL& ] of 2.0 and 2.5 mmol-L-1 (P>0.05). The % HR max at 2.5 mmol-L-1 was also not different to that at the MLaSS, whereas at 2.0 mmol-L-1 it was slightly lower (P<0.05). The running speed, % peak VO 2, and % HR max at the fixed [BLa] of 4.0 mmol-L-1 were significantly higher than those at the MLaSS (P<0.05). In conclusion, it is clear that the MLaSS corresponded to the relatively high exercise intensity in this sample of athletes. It would appear that the running speed, % peak VO 2, and % HR max at the MLaSS lies somewhere between the fixed [BLa ] of 2.0 and 2.5 mmol"L-1. These results confirm earlier work that has suggested a fixed [BLa ] of 2.5 mmol-L-1 may be used with young people' to assess and monitor endurance running performance in place of the more commonly used 4.0 mmol-L-1 that has received so much attention in adult-based studies. The fourth study examined the effect of exercise training on endurance performance, blood lactate profile in relation to running speed (RV) and cardio respiratory function (peak V02) in adolescent runners. This study demonstrated that resting HR, LT and 1 Use of the expression young people is increasingly common since the publication of the text, Young People and Physical Activity by Armstrong and Weisman in 1997. It is used within this document to generically represent the 6 to 18 year age group. 11 RV, HR, V02 and peak V02 at LT were significantly influenced by endurance training. When running time, running velocity and run performance time pre and postintervention were included in the analysis, the intervention did not have a significant effect on peak VO2. When percentage body fat was included as a covariate, there was a positive association with pre and post-training for all groups. The conclusion from these data is that maturity and training both have an effect, especially at supra suggested training levels. The results of the four inter-linked studies support an age-related increase in endurance in aerobic and anaerobic performance and indicated significant differences between boys and girls. From a coaching viewpoint the results reveal that, from the age of 14 to 18 years, runners should be introduced to high intensity training and that changes to the format of middle distance running performance in adolescent competition are recommended.