For injury screening to effectively identify individuals with at-risk behaviours, risk factors should be identified and validated carefully through appropriate prospective study designs. In the context of injury prevention in sport, the main aim of screening is to draw a line between those who are at risk of getting injured and those who are not. In order to effectively screen for anterior cruciate ligament (ACL) injury risk, injury screening should not be based on a singular observation in a single task as it is unlikely to effectively identify those who are at risk with acceptable sensitivity and specificity. Observations of ACL injury could be evaluated through a more mechanism-informed risk factors as this may provide a better justification of an individual’s movement pattern. If an individual who is at risk would demonstrate a particular behaviour across different tasks, this collection of variables characterising an individuals’ at-risk behaviours across tasks could form an individual’s “movement signature”. This thesis therefore aimed to critically evaluate the biomechanical risk factors for non-contact ACL injury during dynamic sporting activities and to explore some novel approaches to characterising movement characteristics for screening. Through a systematic review, the first study in this thesis critically evaluated the current research trends on the in vivo biomechanical risk factors of the ACL injury in dynamic activities and identified a lack of high quality (level 1), prospective evidence. Only one prospective cohort study was identified; therefore, more prospective cohort studies are required as research since the time of this systematic review did not provide further prospective evidence. Study two sought to develop more prospective evidence but unfortunately no ACL injuries were observed therefore, no new biomechanical risk factors for ACL injury could be identified. Utilizing the data collected from the prospective cohort, study three led to the development of a novel approach of injury screening by verifying the existence of individual movement signatures. The task-invariant movement signatures were also able to identify at-risk movement behaviour. Further exploration of mechanism informed multi-planar variables in study four showed that task-invariant movement signatures also exist in multi-planar variables, and may better inform at-risk behaviours. This thesis has furthered the understanding of biomechanical risk factors and moved towards the development of more effective injury screening tools.

Hamstring muscle injuries constitute a major concern in football and a major challenge for physiotherapists working in this sport, being an injury with long absence from playing and training. Although clinical strategies to rehabilitate these injuries and clinical injury risk assessments have been explored over the years, a broader comprehension of how variables regarding running performance may in some form relate to hamstring injury risk has been missing for clinical professionals. Together with this, its incidence has been increasing despite many preventive efforts, which reveals a necessity for developing risk assessment methods to better inform preventive strategies. The key involvement of hamstring muscles during accelerations and decelerations during football running actions justifies research into acceleration related observations. Therefore, the aims of the current programme of research were to develop new laboratorial and load monitoring strategies related to acceleration actions, by exploring biomechanical factors from a physiotherapist perspective. Additionally, implementing assessments and exposing some key limitations of these assessments in professional clubs is also described throughout the experimental studies of this thesis (chapters 3 and 5). For the purposes of this thesis, force development variables were analysed in chapter 3, during maximal accelerations on a non-motorised treadmill, and comparisons between professional players with and without previous injuries were performed. Results from this study revealed no differences between groups during both maximal acceleration and steady state of a maximal sprint effort. A second approach regarding risk analysis and acceleration variables considered the mechanical load based on trunk-mounted accelerometry used in outfield training, as detailed in chapter 5 of the present thesis. In this study mechanical load expressed by PlayerLoadTM, an accelerometer-derived variable aimed to express the rate of change in acceleration, was collected for the training sessions during three weeks previous to a hamstring injury event, in English Premier League clubs, using matched healthy controls. Although the results did not show significant differences between experimental and control group, this exploratory method may constitute a promising method to assess hamstring injury risk. Reliability and validity of the acceleration related variables were addressed first for each of the two experimental studies detailed in chapter 3 and 5. For this purpose, a pilot study on reliability of force collection using a non-motorised treadmill was performed to test the experimental protocol with results showing good overall reliability. For the PlayerLoadTM, a laboratorial study detailed in chapter 4 using a laboratorial overground soccer simulation protocol was adopted and convergent validity with subjects´ anthropometrics together with reliability analysis of four isolated football actions (jogging, side cut, stride and sprint) was performed. Results of this study revealed no association between PlayerLoadTM and the subjects height or body mass and also an overall good reliability for the four actions analysed. In summary, the research presented in this thesis helped better understand the current value and limitations of screening and monitoring acceleration related variables in the context of hamstring (re-)injury prevention in professional football, introducing to the clinical field a different perspective of addressing hamstring behaviour during acceleration actions, and its hypothetical relation with hamstring injury.

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The aim of this thesis was to examine how experts learn using the theory of deliberate practice. Expert and intermediate Gaelic football players practiced kicking, with their learning being assessed between a pre-test and retention test. A novel method to measure the tenets of deliberate practice during the activity, as opposed to retrospectively, was used throughout the thesis. Findings support previous research on the mechanisms and strategies engaged in by experts as they aim to improve performance and how they differ to lesser-skilled individuals. In line with the theory of deliberate practice, in Chapter 2 and 3 the experts rated practice higher for effort and lower for enjoyment, as well as practicing a more relevant skill in Chapter 2, when compared to intermediates. Moreover, they improved kicking accuracy between pre-test and retention test, whereas the intermediate group did not. In addition, the thesis identified differences between the cognitive mechanisms of experts and intermediates that underpin their respective performance. Expert groups engaged in greater cognitive processing during (Chapter 2 and 3) and between (Chapter 3) practice sessions when compared to intermediates. Chapter 4 examined the impact of applying these expert cognitive processes to the deliberate practice and performance of a youth intermediate group. A training group practiced kicking with an intervention designed to increase cognitive processing, whereas a control group practiced kicking without intervention. Findings support previous research by providing evidence of the outcome of such an intervention on deliberate practice. The training group demonstrated greater cognitive effort and less enjoyment during practice and greater improvements in accuracy after practice compared to the control group. Overall, findings in this thesis support the theory of deliberate practice and extend the research on the role of cognitive processing in effective skill acquisition.

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Team sports such as soccer follow an intermittent pattern of exercise, which is known to place greater demands on thermoregulation than continuous exercise of a similar intensity. Time to exhaustion has been shown to be dependent upon environmental temperature, while clothing is known to create a microenvironment at the surface of the skin. The aim of this thesis was to determine the thermoregulatory response to soccer-specific intermittent exercise during different conditions of clothing and environment. The thermal and physiological responses of the feet to continuous and soccer-specific intermittent exercise were evaluated. Intermittent exercise was found to induce an increase in foot skin temperature of a greater magnitude than during continuous exercise of the same overall intensity- The findings indicate that the foot maintains an altered thermoregulatory response not evident elsewhere on the human body. The localised and whole-body physiological and thermal responses to soccer footwear were examined during soccer-specific intermittent exercise. Soccer footwear does not have a significantly detrimental effect on physiological responses compared to training shoes. Nevertheless, there was evidence of increased thermal strain when wearing the soccer boot, which may become significant in a hot environment. Similarly, the localised and whole-body physiological and thermal responses of the hands were evaluated when wearing goal keeping gloves during simulated goalkeeper activity- Goalkeepers' gloves restrict heat loss from the hand and in order to alleviate this problem, phase control materials (PCM's) have been developed to reduce heat load and maintain a comfortable skin temperature. All sites of skin measurement, except mean body skin temperature, showed uniformly that a PCM glove caused a greater increase in skin temperature than a glove with normal foam material. Therefore, the particular specification of PCM used in this study promoted heat gain rather than the intended heat loss and was therefore inappropriate to enhance thermal comfort when used in a goalkeeper's glove. The effects of three different environmental conditions (10°C, 20°C and 30°C) on soccer-specific intermittent exercise were examined, Results showed that the physiological strain-associated with soccer-specific intermittent exercise is greatest in the heat (30°C) with parameters such as heart rate, mean skin temperature, rating of perceived exertion, thermal perception, change in body mass and skin blood flow all lowest during exercise in the cool. Exercise in the cool condition (10°C) may be the optimal environment for performance of soccer-specific intermittent exercise, A significant relation was found between core temperature and prolactin (marker of brain serotonin activity) suggesting that central serotinergic mechanisms of fatigue may play a role in exercise performance during soccer-specific intermittent activity performed in the heat. The effects of traditional soccer fabrics and technical fabrics on the physiological and thermoregulatory responses to soccer-specific intermittent exercise were evaluated. Analysis revealed that slight differences between traditional and technical clothing ensembles in physiological parameters, such as heart rate, mean skin temperature, body mass loss and rating of perceived exertion, were not significant. Therefore, wearing technical fabric clothing gives no particular benefit over a traditional fabric ensemble. In The lack of differences between clothing materials lead to the conclusion that an elite soccer team competing under extremes of temperature in international climates would be best advised to concentrate on proper acclimatisations nutrition and fluid replacement strategies prior to competition than on the specifics of clothing design.

This thesis aimed to provide an examination of the kinematic and metabolic responses of breaststroke swimming, including the effects of changing pace. Study 1 provided a description of competition swimming including rarely reported temporal elements (start time CST) to 15 m, turning times (IT s) for the 7.5 m ingress and 7.5 m egress, end time (ET) - fmal5 m) as well as repeated measurements of turning times, mid-pool swimming velocity, stroke rate (SR) and stroke length (SL) as races progressed. Mid-pool swimming velocity, ST and IT were found to be significantly related with finishing time (FT) and each other suggesting that coaches should adopt an holistic approach to the training of breaststroke swimmers. Stroke rate and IT were found to increase as races evolved while mid-pool SV and SL decreased. A detailed comparison of the 100 m and 200 m events suggested that event specific preparation might be needed and that there was a potential for swimmers of the 200 m event to reduce their ST. In Study 2 a multiple regression analysis utilising kinematic and temporal variables demonstrated that SV was the primary determinant of FT. Turning time was the secondary determinant of IT in all events except the men's 100 m where ST had greater relative importance. The analysis produced precise predictive equations of FT which could be used by coaches to predict race performance and to prescribe race pace training. Studies 3, 4 and 5 established that breaststroke swimmers could be paced precisely and reliably using the Aquapacer during moderate to high intensity 200 m breaststroke trials, subject to the onset of fatigue. Stroke kinematic (SR, stroke count SC) and metabolic responses (blood lactate, gas exchange and heart rate) elicited during the trials of Study 3 were found to be reproducible. During 200 m trials paced at 98 %, 100 % and 102 % of a subject's mean maximal 200 m speed (Study 4) SR was elevated to increase swimming velocity, and to compensate for a deterioration in SL caused by lactacidosis. Lactacidosis occurred because swimmers were operating beyond their maximal aerobic power even during the 98 % trial and hence the additional speed in the faster trials required an increased anaerobic contribution. In support of this, post exercise blood lactate concentrations were significantly negatively related to FT in the 98 % and 100 % trials. Turning times were initially shorter at the start of the faster trials but a marked deterioration followed demonstrating their sensitivity to lactacidosis. An anomaly across studies was that changes in kinematic variables were less predictable during the final 100 In of positively split men's 200 In races (Study 1) WId trials (Study 4). A subsequent comparison of positively split, evenly split and negatively split 175 In trials (Study 5) demonstrated that stroke kinematics remained significantly related over the whole distance of the evenly split trial compared with the positively split trial. Subjects also demonstrated reduced blood lactate, RER and RPE values following the evenly split trial compared with the positively split trial. It was suggested that coaches should experiment with an evenly split race strategy to determine if it produces shorter FT s compared with the positively split patterns currently adopted in competition. A common finding was that breaststroke swimmers exhibited unique SR : SL ratios so that both SR and SL were poorly related to FT in all the studies. It was suggested that coaches, using the Aquapacer" , could entrain a swimmer's ideal SR to elicit a more evenly paced and consistent competition performance. Pacing the ideal SR could also be used for a race specific test, because if a swimmer becomes better able to maintain IT s and SL over a given distance the FT would be improved which might indicate a potential for an improved competition performance. Finally, a model of a maximal even paced 200 m breaststroke swim was outlined and the effect of a change of pace discussed with respect to the model.

The intermittent exercise pattern associated with soccer makes analysis of the demands of the sport more complex than in many individual sports. The aim in this thesis was to determine the physiological and metabolic responses to soccer-specific exercise. The demands associated with elite level match-play were evaluated by techniques of motion-analysis. Laboratory based soccer-specific intermittent exercise protocols were then devised to determine the physiological strain associated with soccer and investigate the effects of increased ambient temperature and whole body pre-cooling on performance. The work-rate profiles of elite South American soccer players and English Premier League players, performing in international and club level respectively, were determined. English Premier League players covered a greater total distance during a game than the South American players (P< 0.05). Differences were found for the total distance covered for playing positions with midfield players covering larger distances than forwards. Defenders covered a greater distance jogging backwards than forward players with forwards sprinting a greater distance than defenders. Work-rate was reduced in the second half of the game for all player. The total distance covered by the international players was done mainly at submaximal intensities. High intensity exercise was infrequent and bouts were of short duration. No significant correlations were observed between the work-rate profile and anthropometric charactersitics of individuals. The use of the doubly-labelled water technique to indicate the energy expenditure during soccer match-play was investigated. The doubly-labelled water technique cannot determine energy expenditure during a soccer match as the rate of turnover of the isotopes is too small to allow the accurate estimation of energy expended. Laboratory based soccer-specific intermittent protocols elicited physiological responses that were similar in magnitude and pattern to soccer match-play. Physiological demands fluctuated with exercise intensity during intermittent exercise. Oxygen consumption and heart rate were not significantly different during soccer-specific intermittent exercise and steady-rate exercise at the same average intensity. Rectal temperature did not differ significantly between the two protocols, although intermittent exercise performance resulted in a greater rise in rectal temperature as the protocol progressed (P< 0.05). Sweat production did not differ significantly between the two exercise sessions, though the rating of perceived exertion was significantly higher (P< 0.05), for the session as a whole, during intermittent exercise. Intermittent exercise performance at 26 ° C did not result in significant increases in the physiological, metabolic or thermoregulatory responses when compared to intermittent exercise at 20 ° C. The physiological and metabolic responses were also similar when intermittent exercise was performed after a whole body pre-cooling manoeuvre. Rectal temperature was lowered by the pre-cooling strategy prior to exercise (- 0.6 ± 0.6 ° C, P< 0.05). Rectal temperature during exercise was only significantly lower after pre-cooling than during exercise at 26 ° C. No significant differences were observed in rectal temperature during exercise between the normal and pre-cooled condition. The increase in rectal temperature during the second half of the protocol was significantly greater than the increase observed at 26 ° C or under normal conditions. This may be a consequence of an altered thermoregulatory response due to the pre-cooling manouvre. In conclusion, the work-rate demands of soccer seem to be predominantly aerobic in nature with anaerobic bouts and the performance of specific match activities increasing the demands placed on players. The demands of intermittent exercise are not significantly different from continuous work performed at the same average intensity though there is tentative evidence for a decrease in the efficiency of the thermoregulatory system during intermittent work. No adverse effects upon intermittent exercise performance were noted under conditions of moderate heat stress, while any thermoregulatory benefits of whole body pre-cooling during intermittent work are probably only transient.