Raised blood pressure (BP) remains the leading modifiable risk factor for cardiovascular disease morbidity and mortality globally. As such, primary prevention strategies are required to improve risk factors to prevent the development of hypertension (HTN). Isometric exercise training (IET) is becoming an established intervention for reducing resting BP. However, few studies have investigated the effects of IET in a population at increased risk of developing HTN. Therefore, this thesis examined the effects of IET, using a novel home-based wall squat intervention, in a pre-hypertensive male population. Specifically, the thesis aimed to explore the potential mechanism/s responsible for improved BP control using an acute isometric exercise (IE) stimuli and a four-week IET intervention. Firstly, acute IE was shown to elicit a step-wise increase in BP, heart rate and cardiac output and associated increase in sympathetic activity. In the immediate recovery period, there was a hypotensiveresponse, which was associated with parasympathetic activation, increased baroreceptor reflex control and reduced peripheral vascular resistance. The hypotensive response was also associated with improved indices of cardiac function, including a reduced estimated filling pressure. Four weeks of IET was shown to significantly reduce resting and ambulatory BP. Improved autonomic cardiovascular control, with increased parasympathetic over sympathetic activity, greater baroreceptor reflex sensitivity and reduced peripheral vascular resistance potentially mediated the decreased BP. A reduction in plasma interleukin-6 and asymmetric dimethylarginine suggests an anti-inflammatory response and improved vascular function, respectively, following IET. Finally, improved myocardial diastolic function suggests positive cardiac adaptations in response to BP reductions. Collectively, the findings of this thesis highlight potential mechanistic pathways for improved BP control in a prehypertensive population and demonstrates wider cardiovascular benefits of IET beyond BP reductions, which are important observations for risk reduction in this population.

In Men's Artistic Gymnastics the accelerated backward giant circle on high bar is used to generate the rotation required for the subsequent skill. When used prior to a dismount at the end of a high bar routine the gymnast performs a number of backward giant circles in order to generate sufficient rotation to perform the dismount. The most common dismounts from high bar require the gymnast to perform two backward somersaults in the layout position. Of all the dismounts performed by elite male gymnasts it is the double layout somersault dismount which requires the most rotation. Observations of elite gymnasts have shown that two different techniques may be adopted in the accelerated giant circle performed before release. Since gymnasts are able to perform the dismount from both types the question arises: What is the best technique for increasing rotation using accelerated backward giant circles?

Increasing the amount of energy derived from fat is likely to have important health and performance applications. This thesis applied contemporary knowledge in order to investigate strategies for optimising whole-body fat oxidation, in male subjects. Study 1 investigated the efficacy of Xenadrine EFXTM (Gx), a weight-loss supplement containing bitter orange, green tea, and guarana, in promoting fat metabolism during 6 h rest. The ingredients in Gx are purported to increase lipolysis and fat oxidation. Indeed, a metabolic effect of Gx was evident. However, carbohydrate oxidation was increased by 15.5 ± 5.6 g over the 6 h period with a concomitant decrease in fat oxidation of -5.9 ± 3.1 g, while the metabolic rate remained unchanged. Study 2 extended the work from the first experimental study, by examining the acute effects of GX during 60 min sub-maximal treadmill walking. Although the onset of the walking exercise resulted in an approximately five-fold increase in the rate of fat oxidation, a reduction in the energy derived from fat, and increase in the energy derived from carbohydrate, was again evident with Gx. In Study 3 the intensity of exercise was manipulated, during treadmill running, in order to determine the exercise intensity eliciting the highest rate of fat oxidation (Fatmax). There was a large variation in the rate of fat oxidation across the range of exercise intensities investigated, with Fatmax occurring at 65.9 ± 1.0 % V02max, while individual peak rates of fat oxidation were 0.54 ± 0.04 g·min-1. This study highlighted the need for careful consideration of the intensity of exercise in order to optimise fat oxidation. Study 4 identified a relationship between Fatmax and the lactate threshold, during treadmill exercise. Analysis of plasma glycerol and NEF A concentrations indicated that the reduced rates of fat oxidation at high exercise intensities were not mediated by reduced lipolysis or plasma FA availability. These finding's were interpreted as supporting either depletion of the free carnitine pool, or inhibition of CPTI by a reduction in intra-cellular pH, as mediating the reduction in fat oxidation at exercise intensities in excess of Fatmax. Based upon the hypothesis that a reduction in intra-cellular pH might contribute to the reduced rates of fat oxidation at high exercise intensities, Study 5 sought to increase intra-cellular H+ buffering capacity. Four weeks supplementation with ~-alanine resulted in a significant increase in the muscle concentration of carnosine, an imidazole dipeptide contributing to intra-cellular physico-chemical H+ buffering. Despite the augmented carnosine concentration there were no changes in substrate utilisation across the range of exercise intensities studied, whilst Fatmax remained unchanged. This data suggests that a reduction in intra-cellular pH does not mediate reduction in fat oxidation during intense exercise. Taken together, the findings of this thesis demonstrate the importance of exercise as an intervention for facilitating fat metabolism, and highlight the primacy of carbohydrate metabolism within the fat-carbohydrate interaction.

In equestrian science, anecdotal evidence suggests that certain fitness parameters are beneficial for riding position. Studies have investigated the demands of riding; however, there is no objective data linking the general fitness of riders to its effects on riding position. This research set out to investigate the perceived importance of off-horse exercise on riding position, the impact that certain fitness parameters have and the effectiveness of a specifically tailored exercise programme. Nearly 90% of respondents stated they believed off-horse exercise to impact on their riding position, with dressage, leisure and older riders to be most likely to participate. The average fitness of riders including aerobic capacity, core stability, balance, flexibility and posture was then researched regarding the effect these might have on riding position. In a cross sectional study the relationship with riding position and pressure distribution, looking at the laterality of the seat, were explored. With the exception of cardiovascular fitness these were then tested for transferability of skill in an intervention trial. A mechanical horse and video analysis were used for analysis and the TekScan pressure mat recorded pressure distribution. Aerobic capacity was found to have no impact on riding position in this study and hip flexibility was assumed to have the greatest impact on riding position. After the intervention, the range of motion of the angles was decreased suggesting a more stable and quiet seat due to increased core stability. In line with other research asymmetries in the rider were found in pressure distribution, which were significantly decreased post intervention (trot and canter). No improvements in medio-lateral symmetry were found. Transferability of skill in riding position can be explained with the high similarity of standing posture and riding position. Overall horse rider specific training regimes can be recommended for improvement of riding position, but combination with riding instruction is suggested.

Olympic Rowing is a ‘power endurance’ sport with a range of anthropometric, physiological and technical requirements. Literature examining the physiological determinants of elite rowing performance has rarely included the analysis of different groups or their longitudinal development. Elite rowing traditionally adopts a ‘squad based’ approach to training which often fails to recognise the potential benefits of individualised training. To date, limited data exist examining the individualised profiling of elite rowers leading to the inclusion of bespoke training prescription in order to maximise performance. Study 1 investigated the relationship between 2,000m ergometer performance and regularly monitored physiological variables, which contribute to selection, in male and female elite senior and development rowers. Analysed individually, there were large differences in the relationships observed across gender and competitive level, with submaximal aerobic capacity (power at 4 mmol・l-1 lactate; W4mmol・l -1) being the only variable to significantly correlate with 2,000m performance in all squads. Results were further analysed using bivariate regression to examine the degree of shared variance between physiological status and performance. W4mmol・l -1 was able to explain 25-59% of the variation in performance. Other variables were able to explain the variance in performance to differing degrees, depending on the squad. This suggests that coaches and practitioners should examine performance determinants of homogenous groups, as the determinants of performance may be different depending on gender and competitive level. Study 2 investigated the importance of W4mmol・l -1 by tracking its longitudinal development in a large group of elite male rowers completing the same training programme. Changes in W4mmol・l -1 were analysed in order to investigate progression rates and differences between Olympians (OLY) and non-Olympians (NON). OLY improved significantly following each of the first 3 years of elite level training. The results of a case series analysis of individual athletes, including a double Olympic gold medallist with >12 years of international experience, suggested a clear upward trend in W4mmol・l -1 throughout a career, despite fluctuations within individual seasons and Olympiads. Improvements were attributed to the physiological adaptations associated with a consistent and well executed high volume/low intensity training model. Differences in the development of W4mmol・l -1 between OLY and NON were not significant until the 3rd year of elite level training. The stagnation in W4mmol・l -1 observed in NON athletes at this time was ascribed to a ceiling of aerobic development or an inability to effectively polarise training in order to maximise adaptation. At this point alternative training methods could be introduced in order to avoid stagnation in development and subsequent performance. Physiological profiling during the early stages of an athlete’s career could also identify those more likely to thrive in a high volume/low intensity training programme. Study 3 involved the implementation of a physiological ‘Spider Profile’ for club rowing coaches. Using key performance determinants, development athlete’s relative strengths were identified in order to inform the training process. Results were compared to senior athletes and ‘Olympian Standards’. U23 international athletes possessed significantly greater maximal and sub-maximal ‘rowing specific’ endurance capacities than noninternational rowers, and were significantly weaker than senior athletes in measures of maximal strength. It was therefore suggested that in order to improve their chances of U23 and senior team selection, development athletes should prioritise the improvement of technical and aerobic indices of performance rather than strength and power. Also, the identification of new athletes should be weighted more towards endurance factors than maximal strength and power production. Study 4 refined the physiological profiling system developed in the previous studies and used it to implement training interventions that improved individual weakness in a group of six elite male rowers. Athletes were assigned to either an endurance (END, N=4) or maximal power (MAX, N=2) group depending on the results of a complete physiological profile. All rowers completed a generic rowing training programme (mean volume = 131 km per week) with 2 of the 14 sessions per week comprising either high intensity aerobic interval training or additional weight lifting. Results were analysed as a case series with individual responses discussed as a lack of control group made the relative impact of training interventions difficult to assess. Three out of four END athletes improved aerobic indices, in particular V&O2peak, but made no improvements in markers of power production. MAX athletes improved their maximum power and aerobic performance. This was attributed to increased mechanical efficiency, muscle coordination and recruitment, strength related technical improvements and/or the reduced relative intensity of sub-maximal work leading to conservation of energy. In conclusion, the minor adaptation of a generic rowing training programme can have a marked effect on the physiological adaptation of athletes struggling to make progress in a traditional high-volume/low-intensity system. In summary, this thesis has highlighted that the analysis of heterogeneous groups of rowers does not provide the level of detail necessary to describe elite performance. Instead, due to individual differences in determinants of performance, a case series approach is a more appropriate means of identifying strengths and weaknesses and implementing interventions to make improvements. Aerobic indices of performance are highlighted as the most important descriptors at both a development and international level. In particular sub-maximal capacity, which is superior in elite development athletes, can be used to differentiate between those that achieve senior team selection, Olympic success, and those that fail to reach the upper echelons of the sport. Spider Profiles are an effective tool which highlight individual strengths and weaknesses in development athletes. Such profiles can be used to provide bespoke interventions to individuals failing to make an impact in elite rowing teams, and the subsequent improvements made can have a global effect on performance if they can be applied to the rowing stroke effectively.

Sleep is a recurring state that is considered a critical process in the optimal attainment of daily functions and recovery in athletes. However individuals from elite sports, such as soccer, may be exposed to a number of situations that may impact sleep within the competitive season (such as inconsistent schedules and travel), which may result in sub-optimal sleeping patterns. However, research documenting the sleep of soccer players is at present limited. Therefore it would seem important to investigate how soccer players sleep to further the understanding of how sleep may be impacted. On this basis, the aim of the current thesis was to examine the typical sleeping patterns of youth soccer players and the factors effecting sleep. This was completed through a series of investigations conducted during the competitive youth soccer season. The aim of the first study (Chapter 3) was to validate a commercially available wireless sleep-monitoring device (WS). This was done in an attempt to provide a viable methodology to measure sleep within the habitual environment of soccer players. Nine randomly selected male participants were monitored over 3 nights and comparisons were made between the WS and other established field measures of sleep (Wristwatch actigraphy, sleep diary and Firstbeat bodyguard heart rate system). The relationships between the WS and the other sleep devices, indicated strong to very strong correlations (r > 0.6) and no significant differences between a range of outputs; total sleep time (Actigraphy assumed sleep time [0.97] & Sleep Diary [0.87] p > 0.05), sleep onset latency (Actigraphy [0.69] p > 0.05) and number of awakenings (Sleep Diary [0.69], p > 0.05). There were also low bias and narrow limits of agreement (LOA) within the comparison of mean differences with the WS for assumed sleep time (2 ± 17 min 95% LOA: -30 to 34 min [Actigraphy]), sleep onset latency (7 ± 17 min, 95% LOA -28 to 40 min [Actigraphy]), and number of awakenings (0.05 ± 1, 95% LOA -3 to 3 [Sleep Diary]). These results suggested that the WS is a viable device for the detection of these selected sleep variables. Chapter 4 looked to provide a comparison of sleep measures between a sample of youth soccer players (N=8) and non-athletes (N=8). Both groups were monitored over a period of 6 days within the habitual setting using the WS. The findings showed the soccer player group attained greater amounts of sleep quantity in comparison to the non-athlete group (504 ± 22 vs. 433 ± 46 min [+71 min] total sleep time, ES: 2.0, Large, p < 0.01). This may have been as a result of a later time of final awakening (08:54 ± 00:14 vs. 07:34 ± 00:46 [+77 min], ES: 1.7, Large, p < 0.01). Such an observation suggested that the soccer players were afforded greater time in bed as a result of the imposed soccer schedule. The soccer players also displayed a statistically greater time spent in wake (13(13) vs. 3(5) min [+10 min], PS: 0.86 ES: 1.5, Large, p < 0.05) on average each night. This data suggested that the sleep of the youth soccer players might be less efficient (as a consequence of greater levels of disturbance), despite desirable quantities of sleep being attained than non-athlete controls. Chapter 5 provided a daily comparison of sleep measures conducted over a 14-day assessment period. It is apparent that youth soccer players attained more sleep quantity in the nights preceding the match day (M-2: 488 ± 53 min [ES: 0.91, Moderate; p = 0.06] & M-1: 486 ± 64 min [ES: 0.84, Moderate; p = 0.02] respectively) in comparison to the night of the day after the match day (M+1: 422 ± 61 min). Such a finding suggested that youth soccer players adopt behaviours that reduce their sleep quantity on the designated recovery day (M+1) by >60 min, which may impact the recovery processes associated to this day. Relationships between sleep parameters and training and match load indicated a 100 au rise in RPELOAD (RPE * Duration) increased the time spent in wake (42 s [90% CI: 0 to 84 s]; ES: 0.36, Small; p = 0.098). It was also observed that an increase of 1000 m total distance increased the time spent in wake (40 s [90% CI: 5 to 75 s]; ES: 0.33, small; p = 0.06) A 100 m rise in high-speed running distance increased the number of awakenings observed (0.14 [90% CI: 0.03 to 0.25]; ES: 0.28, p =0.04) and the time spent in wake on average each night (1.5 min [90% CI: 0.78 to 2.3 min]; ES: 0.57, Small; p = 0.04). A similar outcome was observed in Chapter 6 were a 100 m rise in average high-speed running distance across three different 14-day training periods during the youth soccer season showed a 5 min increase in the time spent in wake on average (ES: 0.88, moderate; p = 0.04). Such outcomes provided a potential link between increases in training intensity (i.e. high-speed running distance) and sleep disturbances within youth soccer players. Increases in high-speed running distance also related to an increase of 24 min (90% CI: 12 to 36 min) on average for total sleep time (ES: 1.3, large; p < 0.01). Similarly increased high intensity heart rate (>85% max HR) was shown to effect total sleep time +20 min (90% CI: 6 to 32 min; ES: 0.87, moderate; p = 0.035). This may suggest that increases in intensity also may impact the amount of sleep quantity within youth soccer players. At present the mechanism for this response largely remains unknown. Within Chapter 7, a practical sleep hygiene strategy (10 min showering at ~40 °C, 20 min before time of lights out) was investigated. A group of ten youth soccer players were evaluated under normal sleeping conditions (control) and a shower intervention period, each consisting of three days within a randomized cross over trial design. Sleep information was collected using the WS. In addition to skin temperature, which was evaluated using iButton skin thermistors. The iButtons were used to establish both distal and proximal skin temperatures. This data was also used to create the distal to proximal gradient (average of distal measures – average of proximal measures = DPG). The data demonstrated that the shower intervention elevated distal skin temperature by (+1.1 °C [95% CI: 0.1 to 2.1 °C]; ES: 0.44, Small; p = 0.04) on average during a 10-minute period prior to lights out in comparison to the control condition. This elevation was also present during the first 30 minutes following lights out (1.0 °C [95% CI: 0.4 to 1.6 °C]; ES: 0.65, Moderate; p < 0.01), which was also accompanied by an increased DPG between conditions (0.7 °C [95% CI: 0.3 to 1.2 °C]; ES: 0.45, Small; p < 0.01). Additionally it was observed that on average the sleep onset latency of the youth soccer players was lower in the shower intervention condition (-7min [95% CI: -13 to -2 min]; ES -0.55, Moderate; p = 0.007). However no other sleep variable was affected as a result of the intervention. These findings demonstrate that a warm shower performed before lights out may offer a practical strategy to alter the thermoregulatory properties of distal skin temperature, which may advance sleep onset latency within youth soccer players.

American football is one of the most popular sports in the United States. However, unlike other football codes, little is known about its physical demands. Aside from a limited amount of research conducted on college players during training and matches, no research exists on players at the elite level, in the National Football League (NFL). Therefore, the primary aim of this thesis was to evaluate the physical demands of training in the NFL. This aim was achieved by establishing a systematic approach to training evaluation using three main phases of study: (1) Evaluation of monitoring strategies in American football; (2) Description of American football training demands with an emphasis on periodization; and, (3) Examination of the consequences of training with an emphasis on injury risk. The first study of this thesis (Chapter 3) showed that three commercially available inertial sensors were able to differentiate between fundamental American football actions (e.g., sprinting, change of direction, and collisions) during movement tasks in a controlled setting and may be useful for quantifying the physical demands of training. During training sessions, Session Rating of Perceived Exertion exhibited a variety of individual responses making sRPE challenging to use when exclusively evaluating the physical demands of training (Chapter 4). Therefore, more objective measures (e.g., GPS and inertial sensors) were evaluated during training (Chapter 5) and indicate that commonly used measures of distance and velocity may not adequately describe the physical demands for some position groups. As such, inertial sensors offer more flexibility to classify a broad range of activities within the sport. A number of inertial sensor metrics are available to the practitioner in commercially used systems. Chapter 6 utilized a principal components analysis to reduce eleven variables to 3 principal components, explaining 79% of the variance within the data. These findings suggest that a small number of variables (e.g., Player Load and IMA) may be adequate when describing the training demands of the sport. Given the reduction in measures to report, Chapter 7 used Player Load and IMA to describe the periodization strategies across a season and within the training week employed by the coaches of one NFL team. Training load was observed to decreases across the season with no clear periodization structure. Conversely, within the weekly micro-cycle, coaches appear to employ some pattern of periodization whereby training load is seen to systematically decrease as the game nears. The final phase of this thesis (Chapter 8) investigated the consequences of American football training by exploring the relationship between training load measures (Player Load, IMA, and Impacts) and non-contact soft tissue injury. Several logistic regression models were compared using Bayesian Information Criterion (BIC). The best model suggested that sessions with greater volume (PLTotal) and intensity (ImpactsHigh) were associated with non-contact soft tissue injury in American football players and may have implications for practitioners when designing training programs within the sport. Collectively, this thesis has the potential to not only offer practitioners within American football a way forward in terms of evaluating training demands but also may be influential to the broader scope of sports science given some of the novel statistical approaches taken to understanding training load monitoring.

The aim of the present thesis was to examine the way in which biological motion is coded and imitated during imitation learning by improving upon methodologies currently used in the literature to examine imitation of underlying movement kinematics. Across four experiments, imitation of the kinematic structures of biological and non-biological motion models was examined to investigate the processes involved in imitation learning. The purpose of the first experimental chapter, Chapter Two, was to examine the way in which biological motion kinematics were coded during imitation learning by establishing whether imitation of biological motion kinematics was a function of lower-level visuomotor processing or top-down attentional modulation. Results showed that not only were imitations of typical and atypical biological motion different, but both models were imitated as accurately during spatially incompatible trials as compatible. Accurate imitation of spatially incompatible atypical biological motion confirmed biological motion coding is a function of lower-level visuomotor processing. Following results from Chapter Two, Chapters Three, Four and Five assumed lower-level visuomotor processing of biological motion and were designed to further examine whether this lower-level visuomotor processing of biological motion was modulated by top-down attentional factors (e.g. end-state-targets, visual attention, social primes). The first of these top-down modulations was included in Chapter Three, which examined the influence of end-state-targets on biological motion coding during imitation learning. Although kinematics was not modulated by end-state-targets, movement time was less accurate when end-state-targets were present, which suggests that lower-level and top-down processes operate together during the processing of visual information during imitation learning. In addition to end-state target modulation, imitation data further confirmed the coding of atypical biological motion by demonstrating differences in imitation of two relatively similar atypical biological motion models (atypical17 and atypical26). The top-down attentional factor examined In Chapter Four was visual attention, which was measured by recording eye movements during observation of the model stimuli. Analysis of eye movements demonstrated that visual attention was directed towards the model throughout the entirety of the observation phase during trials where end-state-targets were both present and absent. As goal-directed eye movements were not made during observation of the models, results suggest that the kinematic data contained within each of the models was observed and consequently featured in the representation formed for motor execution. Chapters Two, Three and Four provide a fundamental understanding of how biological motion is coded during imitation learning by using robust protocol that improves upon the validity of those used in the current literature and specific modulations that discredit significant top-down modulatory explanations for biological motion coding. The way in which biological motion coding occurs in neurotypicals (no neurologically atypical patterns of thought or behaviour) is important when trying to understand where deficiencies in those with intellectual disabilities occur. The intellectual disability most closely associated with the current thesis is autism, where deficiencies in imitation are suggested to be linked to social components. Therefore, to establish a foundational understanding of how social context influences neurotypical imitation, Chapter Five examined the influence of social primes on the coding of biological motion. Results showed that social primes modulated the accuracy of imitation, where peak velocity was more like those of the models following observation of an anti-social prime. In addition, observation of both the pro- and anti-social primes was shown to reduce the variability of imitation relative to observing no social prime at all. These findings demonstrate that social primes are being coded and incorporated into the motor output such that both the accuracy and consistency of imitation of biological motion are modulated. Together, the results presented in the current thesis demonstrate imitation of novel, atypical biological motion is a function of complimentary lower-level and top-down processes that facilitate the coding of both underlying kinematics and environmental context.

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.