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Athletes commonly consume caffeine as an ergogenic aid. The effect of caffeine ingestion on the immune response in humans, and in particular following exercise, has received little scientific attention. Caffeine may affect immune responses via adenosine receptor antagonism or adrenaline-mediated mechanisms. Following the relatively recent removal of caffeine from the WADA list of prohibited substances, its use is expected to increase further, therefore any influence of caffeine on immune function is of particular relevance. The aim of this thesis was to investigate the influence of caffeine ingestion on immunoendocrine responses following prolonged and intensive cycling in humans. Initially, caffeine (6 mg. kg4 body mass) did not affect neutrophil functional responses when participants rested for 3.5 h following ingestion (Chapter 4). Caffeine ingestion attenuated the post-exercise decline in neutrophil oxidative burst response when stimulated by f-MLP (Chapter 6) but not PMA (Chapter 5). An in vitro study suggested that this most likely occurs as a result of adenosine-receptor antagonism by caffeine (Chapter 8). Although caffeine ingestion improved pre-loaded TT performance by 4%, there was no attenuating effect of caffeine on post-exercise f- MLP-stimulated neutrophil oxidative burst responses (Chapter 7) nor was there any benefit of co-ingesting caffeine with CHO on oxidative responses, compared with either supplement alone (Chapter 9). Caffeine supplementation did not affect total circulating leukocyte or neutrophil count at rest or following exercise, but increased circulating lymphocyte count both at rest and during exercise following ingestion. Caffeine ingestion was consistently associated with an increased plasma adrenaline concentration but not noradrenaline concentration at rest and following exercise. Caffeine generally had no effect on plasma cortisol concentration, though it was consistently associated with an increased plasma IL-6 concentration following exercise of various duration and intensity. In conclusion, the attenuating/positive effects of caffeine ingestion on post-exercise neutrophil oxidative burst responses appear to be stimulant-dependent. None of the experimentals tudies-,h, owever, indicated that caffeine ingestion was more detrimental than placebo on the f-MLP-stimulated neutrophil oxidative burst response to exercise.

The benefits of carbohydrate ingestion before and after prolonged exercise are well known to scientists, coaches and athletes alike. However, the type of carbohydrate consumed can have a significant effect on substrate metabolism both at rest and during exercise. The glycaemic index (GI) is a method of classifying carbohydrate-containing foods according to their postprandial glycaemic responses. This is a useful tool to aid the selection of appropriate carbohydrates for both pre- and post-exercise meals. This thesis examined the influence of ingesting mixed meals with different glycaemic indices both before exercise and during the post-exercise recovery period in male and female subjects.

Evidence supports that the ingestion of carbohydrate solutions in the post-exercise period can facilitate the restoration of exercise capacity both through providing the fluid necessary for rehydration and through stimulating carbohydrate storage. The accrual of this evidence has now established many components of the optimal carbohydrate feeding strategy during recovery and further progress has been sought through investigating the potential influence of other macronutrients. Specifically, combined ingestion of protein and carbohydrate may promote a more rapid re-synthesis of endogenous glycogen stores than when either nutrient is ingested in isolation. This possibility has led to speculation that ingestion of a mixed carbohydrate-protein solution (CHO–PRO) might restore the capacity for physical exercise more completely during a short-term recovery than when a matched quantity of carbohydrate alone is ingested. However, evidence in support of this hypothesis is not yet available and the present series of studies will therefore attempt to directly examine the effects of CHO–PRO ingestion on recovery of exercise capacity.

The debilitating effects of heat stress that accelerate the rise in body core temperature are well known. Although fluid replacement has been shown to be effective to attenuate the rise m body core temperature during exercise, the physiological responses to ingestion of fixed volume of drinks at different temperatures are unclear.

Although oxygen transport (i.e. maximal cardiac output) is the usual determinant of maximal exercise capacity (i.e. VO2UAX ) in healthy humans, it is not the sole determinant of exercise performance. Whilst the potential capacity of the respiratory system has been described as being 'over-built' for exercise, a number of respiratory system functions have been linked with exercise limitation, in both trained and untrained individuals. The purpose of this research was to examine the effect exercise has on the functions of the respiratory system. For reasons which will be outlined function of the respiratory system can be described, in part, by airflow profile and breathing pattern. Initially, the tidal airflow profile and breathing pattern, at rest and during exercise of various intensities was examined; specifically looking at how tidal breathing variables are altered in response to increased metabolic demands. Initial findings were that there is vast diversity in the resting tidal breathing profile (« = 148), with significant differences (p < 0.05) being observed in a number of variables between males and females. Onset of exercise alters the majority of tidal airflow characteristics but the pattern of change is similar in both sexes. Over a range of exercise intensities the termination of exercise and increased ventilation rates vary, as does the magnitude of some of the respiratory profile changes, but many of the timing changes are the same, particularly those of breathing frequency (fe), the ratio between inspiration and expiration (ti/tToi) and the late occurrence of peak inspiratory flow (tpir). This consistency of characteristic changes with exercise termination strongly suggests that they may play an important role in exercise limitation. Following maximal incremental cycling the majority of tidal breathing characteristics returned to pre-exercise values within 10 minutes of the end of exercise, including the rapid shallow breathing observed in a number of subjects, which has been associated with respiratory muscle fatigue. Subsequent trails showed that both the inspiratory and expiratory muscle's ability to produce maximum respiratory pressures was significantly impeded post exercise, probably due to respiratory muscle fatigue. Flow and volume characteristics of maximum exercise ventilation differed significantly from those obtained by the maximum voluntary ventilation manoeuvre, highlighting the inappropriateness of using the MVV as a measure of ventilatory capacity during exercise. Changes in respiratory system functions are reflected in changes in airflow profile. This thesis explores the use of profile measurements to detect and measure factors that limit exercise.

Research into endurance running performance (economy, optimal running biomechanics, and injury mechanisms) lacks a universal running technique; however, landing heel-toe identified eighty percent of runners. A new Gravitational hierarchical model of running was developed based upon a novel technique---Pose running---owing to deficiencies in the current hierarchical model (Hay and Reid, 1988) on the interaction of forces involved in running. A major deficiency in the current hierarchical model is viewing gravity as only active during flight. In contrast, the Gravitational hierarchical model defines a gravitational torque as the motive force in running. Ground reaction force is not a motive force but operates according to Newton's third law. The ground can only propel a runner forward via muscle activity, but leg and hip extensor muscles have consistently proven to be silent during leg extension. High Achilles tendon forces at terminal stance suggest the elastic recoil creates a re-bound effect in the vertical direction only, thus reducing work against gravity. Two experienced Pose runners were compared (study 1) with two experienced heel-toe runners using primary and secondary research variables derived from the Gravitational hierarchical model, which effectively distinguished the two techniques while establishing gravity as the motive force. For example, maximum horizontal acceleration of the centre of mass occurred before maximum horizontal ground reaction force, supporting the Gravitational hierarchical model that only a gravitational torque could create acceleration. Finally, sixteen male recreational endurance heel-toe runners (study 2 using the same primary variables as study 1) were randomly assigned into two groups where one group received a 7-hour Pose intervention over 7-days while the other group remained as heel-toe runners. A 2 x 2 mixed factorial ANOVA where group (control vs. treatment) and trial (pre to post changes) assessed the primary research variables. Significant interactions were explored using Tukey post hoc tests, which found significance (Pose runners pre-post test) for stance time (P= 0.001), centre of mass to support limb at 25 ms (P= 0.042), centre of mass displacement during stance (P = 0.001), knee flexion angular velocity during stance (P= 0.005) plus swing (P= 0.043) and stride frequency (P= 0.002). After 12-days the Pose group's posttest time-trial (2400 m) improved by a mean of 24.7 s compared with a 3 s decrease in the heel-toe group. No significant changes pre-post test, were found for an economy run (2400 m) at 3.35 m-s -1. A preliminary prospective 3-month injury report found no injury incidence in the Pose runners compared to six injuries in the heel-toe group. It was concluded, that the Pose technique is a valid biomechanical technique, which improved performance, while reducing injuries. Future work should further quantify gravity's role in accelerated running by ascertaining whether the horizontal acceleration of the centre of mass also occurs before maximal horizontal ground reaction force.

When performing team sports such as soccer, rugby and hockey in hot climates, the ingestion of water is often favoured over sports drinks containing carbohydrate. This is because the onset of fatigue under these environmental conditions is typically the result of hyperthermia and dehydration rather than carbohydrate depletion. However, during other modes of exercise conducted in hot environmental temperatures drinking a dilute carbohydrate–electrolyte solution (CES) has been shown to increase exercise performance when compared with plain water. There is currently a dearth of knowledge regarding the benefits that may be derived from consuming these solutions during prolonged, intermittent high-intensity exercise in the heat. One reason for this is the impracticalities associated with measuring core temperature during unconstrained exercise. Therefore, the aim of this thesis was to determine a reliable means of continuously measuring core temperature wirelessly during unconstrained exercise. Thereafter this methodology was used to investigate the influence of ingesting water and a 6% CES during a protocol that closely simulates the demands of intermittent field sports in 30°C.

Intense, unaccustomed exercise is documented to result in muscle damage and transient immune perturbation. Strenuous exercise is considered to generate free radicals and confers an oxidative stress burden on the body, supplementary to that of the normal in vivo metabolism. Excessive amounts of free radicals can cause cellular and tissue damage and are implicated in the manifestations of muscle damage in the post-exercise period. A further consequence of intense exercise is the initiation of an immune and inflammatory response during and post-exercise. Whilst these processes are vital for normal immune function, an exaggerated response can exacerbate cell damage and be self-propagating. One route to modulating these responses is by intervention with antioxidants. Boosting of the existing antioxidant defences is hypothesised to provide additional protection against free radical damage. The series of investigations presented within this thesis attempt to provide further elucidation of the proposed role of antioxidants on post-exercise responses.

This thesis examined the physiological characteristics of elite adolescent male and female sports perfonners compared to non-elite adolescents in relation to age and maturity. The determinants of endurance perfonnance in adolescent males was also investigated. In Chapter 3a the reliability of the laboratory tests employed in the thesis were examined. Seven male and 11 female adolescents (age: 13.6 ± 0.7 and 13.3 ± 0.5 years, respectively) perfonned a submaximal oxygen uptake CV02)-blood lactate test and a peak V 02 test on a treadmill, and a maximal 30 s cycle ergometer sprint, on 2 days, 6 weeks apart. Pearson correlation coefficients for male and female data combined were: peak V 02, r = 0.93 (p < 0.01); submaximal V02 at 8.8 km.h-l,r = 0.91 (p < 0.01); and peak power output, r = 0.90 (p < 0.01). T-tests revealed a difference between submaximal V02 of males (t = 8.51; p < 0.01). Mean difference and limits of agreement for peak V 02, submaxima1 V O2 and peak power output, were -0_03 ± 6.4 rnl.kg-l_min-l, -0.8 ± 2.7 rnl.kg-l.min-l, and -11 ± 91 W, respectively_ The laboratory tests employed within this thesis were deemed suitably reliable for use in detecting any physiological differences between elite and non-elite adolescents. In Chapter 4 the physical characteristics and sexual maturity of 109 elite (67 male, 42 female) and 123 non-elite (65 male, 58 female) adolescents (aged 12 to 16 years) were examined. Height, body mass, sum of 4 skinfolds and sexual maturity were compared. Sexual maturity was more advanced in the elite males (main effect group: p < 0.05; main effect group: p = 0.08, n.s.; genital and pubic hair developmerIt, respectively)_ Elite females were older at stages of pubic hair (main effect group:p < 0_01) and breast development (main effect group: p < 0.05) and age at menarche (13.2 ± 0.9 vs. 12.3 ± 0.9 yrs; p < 0.01). The results suggest that elite males were more advanced in sexual maturity with elite females characteristic of late maturers. Chapter 5 examined peak V Oz in 101 elite (65 male, 36 female) and 114 non-elite (62 male, 52 female) adolescents, aged 12 to 16 years. Absolute (A) and relative (R) peak V O2 were higher in elite males and females when compared by age (main effect group: p < 0.01). Also, when compared by sexual maturity absolute and relative peakV02 were higher in elite males (A - main effect group: p < 0.05; genital development; R - main effect group: p < 0.01; pubic hair / genital development) and elite females (A - main effect group: p < 0.01; pubic hair / breast development; R - main effect group: p < 0.01; pubic hair / breast development). Differences in peak V02 between elite and non-elite adolescent males and females cannot be wholly attributed to differences in maturity. Chapter 6 examined submaximal V 02 and blood lactate concentration in 108 elite (66 male, 42 female) and 120 non-elite (64 males, 56 females) adolescents, aged 12 to 16 years. Running at 7, 8.8 and 10.6 km_h-l, non-elite males were more economical (lower submaximal V02; rnl.kg-l.min-l) than elite males when compared by age (main effect group: p < 0.05) and maturity (main effect group: p < 0.01; pubic hair development). In contrast non-elite females were less economical than elite females at equivalent ages (main effect group: p < 0.01) and stages of sexual maturity (main effect group: p < 0.05; p < 0.01; pubic hair and breast development, respectively). Submaxima1 blood lactate concentration was lower in elite males and females when compared by age (main effect group: p < 0.01) and by stage of pubic hair (main effect group: p < 0.01) and genital (main effect group: p < 0.05) and breast development (main effect group: p < 0.01). Superior running economy in the non-elite males may be due to a greater body fatness improving economy, or be due to an uneconomical running gait in the elite males. Superior running economy in the elite girls may have been related to biomechanical factors or the relative contribution from aerobic and anaerobic energy sources. Lower submaximal blood lactate concentration in both elite males and females may be related to the lower relative exercise intensities at each work rate. Chapter 7 examined the determinants of 3 km run performance in 13 active adolescent males aged 15.3 ± 0.4 yrs. Peak V02 (59.8 ± 4.1 m1.kg-1.min-1; range: 52.9 - 67.1) was most highly associated with running performance (13:02 ± 01:27 min:s; range: 10:35- 16:07; r = - 0.90; P < 0.01). Running velocity at peak V02 (16.9 ± 2.2 km.h-1; range: 13.4 - 21.0) also demonstrated a high correlation (r = -0.77; P < 0.01) with performance. Endurance capacity run time (running at a treadmill speed which elicited 83 ± 2 % (range: 81 - 87 %) of peak V~) was the only other variable examined to relate to 3 km performance time (r = -0.77; P < 0.01). Stepwise multiple regression analysis revealed the best predictor of 3 km running time (s) to be a combination of peak V02 (ml.kg-1.min-1) and body mass (kg). Chapter 8 investigated the short-term maximal power output in 63 elite and 39 non-elite males and 39 elite and 57 non-elite females aged 12 to 16 years. Absolute (W) and ratio scaled (W.kg-1) peak power were higher in elite males (p = 0.063, n.s.; p < 0.05; absolute and ratio scaled, respectively) when compared by age. Absolute peak power was lower at age 13 years in elite females, yet higher at subsequent ages compared to non-elite (age x group interaction: p < 0.01). Elite females demonstrated a lower fatigue index (%) when compared by sexual maturity (main effect group: p < 0.05). Superior short-term power output of elite males may be related to advanced sexual maturity and possibly differences in body composition. The [mal chapter examined the longitudinal development of peak V O2, submaximal V O2 (ml.kg-1.min-1), and power output in 11 elite and 5 non-elite males over 2 or 3 consecutive years. Linear additive multilevel regression modeling revealed that sum of skinfolds remained the same in the elite group yet increased in the non-elite. Peak V O2 was greater in the elite males by 9.1 ml.kg-1.min-1, with no change in age. Oxygen uptake at 10.6 km.h-1 declined by 1.2 ml.kg-1.min-1 per yr in each group. Percentage peak V O2 at the same running speed was 12 % lower in the elite and declined by 1.3 % each year in both groups. Also at the same treadmill speed, blood lactate concentration was 1.5 mmoU1 lower in the elite males compared to the non-elite, however there was no decline with age. Optimum running economy and that at 65 and 85 % peak V O2 declined by 8.0, 9.2 and 6.6 ml.kg-1.km-1, respectively, each year, yet was not different between groups. Both peak and mean power output increased disproportionately in the elite group, with percentage fatigue increasing similarly in both groups. Blood lactate 2 min post-sprint increased by 0.94 mmol.r1 per year in both groups, yet increased by - 3.0 mmor at Tanner (1962) pubic hair stage 3. The athletic superiority of young elite male and female performers may be related to advanced maturity in males but later maturity in females. A high peak V 02 is an important characteristic of young elite athletes, and also an important determinant of endurance performance. The influence of running economy upon performance remains to be elucidated, however, young elite athletes are characterised by lower blood lactate concentrations during submaximal exercise. The role of power output to performance appears to be influenced by a maturity-related factor, in addition to training.