Resistance training has long been utilised as a strategy for improving sport performance in competitive athletes dating back to early civilizations. In modern times, much emphasis has been placed on better understanding the application of specific training methods to optimise performance at the individual level. One such application is the concept of eccentric-based training (ECC), which has been known to researchers and practitioners for decades; however, only recently have these notions been investigated in the context of applied human performance. Direct comparison of eccentric- and concentric-only training tends to favour the former with respect to hypertrophy, strength and muscle architecture. Furthermore, the existing literature for ECC tends to emphasise heavy- or tempo-controlled exercises. Current research has suggested that relatively fast ECC may provide a superior stimulus for eliciting adaptation in measures of strength and explosive ability compared to slow ECC. The purpose of this thesis was to examine the theoretical and applied underpinnings for ECC in practical training environments, develop an assessment tool for prescribing ECC, and to investigate the effects of ECC on practical measures of performance in competitive athletes. Evidence suggests that neuromuscular adaptations resulting from eccentric-based training methods could benefit athletic populations competing in team sports. The purpose of Chapter 2 is to review existing literature on the effects of eccentric-based training on performance qualities in team sport athletes. Variables related to strength, speed, power and change of direction ability were extracted and effect sizes were calculated. Eccentric-based resistance training appears to be an effective stimulus for developing trivial to large effect size differences in strength, speed, power, and change of direction in team sport athletes. However, the range of effect sizes, testing protocols, and training interventions suggest more research is needed to better implement this type of training in athletic populations. According to the findings of our literature review, eccentric-based training (ECC) is an effective training strategy in athletes; however, despite the theoretical benefits the uptake by practitioners is currently unknown. The purpose of Chapter 3 was to survey strength and conditioning practitioners about their use of ECC with athletes. Our survey adds to the existing body of literature showing implementation of ECC by practitioners in the field to improve physical performance through strength, hypertrophy, and power. Sport performance (64%) was the top ranked reasons to include ECC, and specifically targeted the improvement of strength (35%), hypertrophy (19%), and power (18%). ECC intensity was prescribed as percentage of concentric 1RM (34%), RPE (20%) or velocity (16%). A majority of respondents did not monitor ECC load (58%) or use eccentric-specific testing (75%). The methodology between practitioners is generally non-uniform and tends to follow existing guidelines for traditional or concentric-based training despite major physiological differences between the two modes of muscle action. The efficacy of ECC is well supported, yet there appears to be a lack of defined protocol for integrating ECC research into practice. Therefore, the prescription of ECC may be improved through the development of a framework for strength and conditioning practitioners to use for specific programming outcomes. A greater understanding of eccentric contribution to sport performance may help define testing and monitoring procedures for the prescription of ECC interventions. Several gaps in practice were identified in Chapter 2 and Chapter 3, namely the lack of protocol for including ECC in a team sport environment. The purpose of Chapter 4 was to investigate the reliability of an incremental eccentric back squat protocol with trained rugby union athletes. Force plates and a linear position transducer captured force-time-displacement data across six loading conditions, separated by at least seven days. Eccentric peak force demonstrated good intraclass correlation coefficient (≥ 0.82) and typical error values (≤ 7.3%) for each load. Variables based on mean data were generally less reliable (e.g., mean rate of force development, mean force, mean velocity). These findings indicate this lower body, multi-joint protocol meets common standards reliability for longitudinal monitoring of eccentric force and velocity characteristics in trained athletes. In Chapter 5, the eccentric force-velocity relationship was investigated in an applied setting with a multi-joint movement. The purpose of this study was to investigate the force-velocity-load relationship in an incremental eccentric back squat test with professional male rugby union athletes. Each increase in barbell load tended to result in a linear change in relative eccentric mean force (REMF), eccentric mean velocity (EMV), and eccentric peak velocity (EPV). The direction of this change was dependent on the variable being measured with linear positive changes noted in REMF, and linear negative changes in EMV and EPV .We observed a plateauing effect for relative eccentric peak force (REPF) as load increased. These results show that for “peak” variables lighter loads produced similar magnitudes of force, but generally moved at higher velocities than heavier loads. These observations suggest that the eccentric force-velocity-load relationship may vary depending on individual characteristics and the parameters used. Individual characteristics explain large proportions of variance in eccentric capacity, thus the individualisation of ECC appears justified. Further research may investigate the responsiveness of these qualities to different types of eccentric training and athletic performance. Fast eccentric contraction velocities have been suggested to alter muscle architecture and improve fast twitch muscle fibre composition to a greater extent than slow eccentric contractions. These findings may have implications for improving physical performance in athletes, although most research involving ECC has been conducted with either specialized equipment or with untrained participants. The purpose of Chapter 6 was to compare the effects of fast-tempo and controlled-tempo ECC using a traditional exercise on strength, speed, and jumping ability in trained athletes. Nineteen semi-professional rugby union athletes completed 6-weeks of an off-season ECC resistance training program. Within-group differences from pre- to post-test showed trivial to small improvements (g = 0.01 to 0.45) in both groups across all performance variables. The observed differences in physical performance between- and within- groups are largely similar following 6-weeks of ECC in trained athletes. Differences in the execution and prescribed intensity (%1RM) of the ECC exercises suggest that performance may change to a similar extent through different mechanisms. These findings indicate that the individualised prescription of ECC is warranted and such an approach may help to optimise performance outcomes in trained athletes. This thesis deepens the understanding of ECC and its effect on physical performance in trained athletes, and adds new knowledge to the field allowing practitioners to prescribe ECC relative to individual capabilities. Future research should examine how to prospectively optimise ECC on the basis of individual eccentric force and velocity characteristics.