Eccentric muscle function is thought to play an important role in human movement. In particular, eccentric muscle function may be especially relevant to the execution of high force fast stretch-shortening cycle (SSC) tasks within athletic performance (e.g. the ability to tolerate large external forces and regulate leg spring stiffness during jumping and sprinting). Although chronic eccentric training has been demonstrated to induce greater increases in SSC performance and leg spring stiffness compared with other training modalities, there have been no studies demonstrating a link between eccentric muscle function per se (i.e. maximum eccentric strength and eccentric muscle function under fast SSC conditions) and the performance of high force locomotive tasks such as jumping and sprinting in trained athletes. Furthermore, few studies investigating the effects of chronic eccentric training on athletic performance have recruited resistance trained athletes undertaking an ecologically valid physical preparation program. The overall purpose of this thesis was to elucidate the role of eccentric muscle function and training in athletic performance. In addition to addressing the gaps in the literature with scientific rigour, this research was intended to directly influence the practice of strength and conditioning coaches working within athletic performance. It remains difficult to assess eccentric muscle function during functional multi-articular movement in a practical environment with trained athletes. Therefore, two novel assessment protocols were investigated. Firstly, an assessment of maximum lower body isoinertial eccentric strength was demonstrated to successfully identify an eccentric back squat one repetition maximum (1RM). Furthermore, it was found that the eccentric back squat 1RM was 28 ± 8 % higher than the concentric back squat 1RM in resistance trained participants (n = 10). Secondly, eccentric muscle-tendon unit (MTU) function under fast SSC conditions was inferred from braking phase kinetic variables during a drop jump (DJ) in sprint trained participants (n = 13). Both novel assessment protocols exhibited acceptable reliability.

The role of eccentric muscle function in athletic performance was then investigated in two cross-sectional studies. The first cross-sectional study investigated how eccentric muscle function contributed to reactive strength in highly trained sprinters (n = 12) in comparison to a non-sprint trained control group (n = 12). Trained sprinters exhibited a higher DJ reactive strength index (RSI; Effect Size [ES] ±90% confidence limits [CL]: 3.11 ±0.86) attained primarily by a briefer contact time (ES: -1.49 ±0.53). Very large differences in mean braking force (ES: 2.57 ±0.73) were observed between groups which was closely associated with contact time (r = -0.93). Higher levels of reactive strength exhibited by trained sprint athletes may therefore be underpinned by a shorter and more forceful eccentric muscle action. The second cross sectional study investigated the role of isoinertial eccentric strength and eccentric muscle function under fast SSC conditions (i.e. during a DJ) in modelled stiffness regulation during maximum velocity sprinting in highly trained sprinters (n = 11) in comparison to trained team sport athletes (n = 13). Trained sprinters attained a higher maximum sprinting velocity (ES: 1.54 ±0.85), briefer ground contact time (ES: -1.39 ±0.80) and higher modelled vertical stiffness (ES: 1.74 ±0.96) in comparison with team sport athletes. Trained sprinters also exhibited a moderately higher RSI (ES: 0.71 ±0.74) via the attainment of a briefer and more forceful ground contact phase, while only a possible small difference in isoinertial eccentric force (ES: 0.38 ±0.56) was found between the two groups. RSI demonstrated large to very large associations with maximum velocity (r = 0.72) and vertical stiffness (r = 0.67), whereas isoinertial eccentric force exhibited weaker correlations with maximum velocity (r = 0.56) and vertical stiffness (r = 0.41). The stronger association between modelled stiffness regulation at maximum velocity and eccentric muscle function under fast SSC conditions (i.e. DJ mean braking force) compared with maximum isoinertial eccentric strength indicates that the regulation of lower body stiffness may be a somewhat task-specific motor strategy. Therefore, the cross-sectional investigations identified that eccentric muscle function contributes to reactive strength and maximum velocity sprinting capabilities in highly trained athletes.

The final investigation determined the effects of a lower body resistance training program incorporating accentuated eccentric loading (AEL) in comparison with a traditional (TRAD) resistance training program in resistance trained team sport (i.e. Rugby Union) athletes (n = 14) undertaking a broader physical preparation program. Two four-week phases of distinct eccentric phase tempos were completed (i.e. slow and fast tempo). Strength, power, speed and muscle properties were assessed at baseline and following each training phase. The slow AEL protocol elicited superior improvements in back squat strength (ES: 0.48 ±0.34), 40m sprint performance (ES: -0.28 ±0.27), maximum sprinting velocity (ES: 0.52 ±0.34) and vertical stiffness at maximum velocity (ES: 1.12 ±0.72) versus slow TRAD training. In contrast, the second four-week block of fast AEL training elicited a small increase in reactive strength (i.e. RSI via a moderate reduction in contact time), but impaired 40m speed and maximum sprinting velocity. In addition, fast AEL was less effective in improving lower body power (ES: -0.40 ±0.39) versus fast TRAD. This study demonstrated that four weeks of AEL training with a slow eccentric tempo can induce superior improvements in lower body strength, maximum velocity sprinting speed and stiffness regulation in resistance trained athletes in an ecologically valid setting. However, a subsequent four-week phase of AEL training emphasizing a fast eccentric tempo did not lead to additional improvements in strength and may have impaired maximum velocity sprinting capabilities. It was proposed that the second phase of eccentric training could have exceeded the recovery capabilities of the athletes undertaking a concurrent program.

In summary, this thesis identified that eccentric muscle function contributes to high force fast SSC function and therefore athletic performance. However, the regulation of eccentric force under task specific conditions may be more important than maximum eccentric strength. Eccentric training can induce superior improvements (i.e. in comparison to TRAD training) in strength and speed in trained team sport athletes undertaking a concurrent training program, however, it should be incorporated judiciously.