The second lactate threshold (LT2) has long been associated with endurance performance in trained endurance athletes. In Australia, the state institutes and academies of sport utilise the modified maximum deviation (D-max) method to determine LT2 in endurance athletes for the purpose of establishing training intensities and indicating whether athletes are appropriately adapting to training loads. For LT2 methods to be considered valid, it had been suggested that strong associations with simulated endurance performance and the maximal lactate steady-state (MLSS) are required. Although research has established a strong relationship between the modified D-max and endurance performance, there are gaps in the literature which this thesis aims to investigate. iii appropriate measurement of ovarian hormones. Therefore, the second study extended study one, to explore the same experimental aims, in endurance-trained women (n = 12) whilst controlling for ovarian hormone concentrations through the use of hormonal contraceptives. When comparing to the men's results from study one, power output and V O2 at LT2 were also found to be significantly correlated with 40 km cycling performance in endurance-trained women. Additionally, combining maximal (peak power output and V O2max) and fractional utilisation (V O2 and power output at LT2) variables produced the strongest prediction of performance in women (r 2 = 0.87) and men (r 2 = 0.95). In women, VT2 was significantly higher than LT2 for all variables measured, despite no differences observed for the men. More women (73%) than men (50%) completed 30 min of cycling and elicited a steady-state [La -] at their LT2 power output, despite LT2 being significantly higher than the self-selected power output during a 40 km cycling time trial in women.The final study examined the sensitivity of the modified D-max LT2 to adapt to a brief highintensity interval training (HIIT) intervention in already-endurance trained men (n = 9) and women (n = 8). Although not significant, women (but not men) showed a trend towards LT2 power output improvement (2.2%; p = 0.08) post-HIIT. Mean 40 km time trial performance significantly improved in both men (-1.7%) and women (-2.6%), with no difference in the magnitude of change between sexes. Furthermore, although LT2 power output explained 77% of the improvement in 40 km cycling performance in women, none of the measured variables explained the performance improvement in men, suggesting that mechanisms other than those responsible for the LT2 were involved. The LT2-performance relationship was intensity-dependent in both men and women, shown by significant correlations at post-HIIT but not baseline. Some of the sex-related differences may, at least in part, be explained by the influence of oestradiol, perhaps by the exacerbation of the sex-based differences in substrate metabolism and subsequent effects on [La -] and fatigue, and/or greater cardiovascular stress in men than women.Collectively, these studies improve understanding of the modifie...