Sex differences and training adaptations in relation to the lactate threshold and endurance exercise performance

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. Firstly, research is yet to examine the capacity of the modified D-max to: (a) elicit a workload sustainable for 30 min with a steady-state blood lactate concentration ([La]; an indicator of MLSS), and; (b) a maximum workload during endurance events, in endurance-trained athletes.The first study within this thesis examined the capacity of the modified D-max LT2 to correlate with 40 km cycling time trial performance, reflect a cycling power output that was sustainable for 30 min whilst eliciting a steady-state [La], and elicit mechanical and physiological responses similar to the second ventilator threshold (VT2) and mean values achieved during a 40 km cycling time trial. It was found that the power output and VO2 at LT2 were significantly correlated with 40 km cycling performance, but the strongest combination of variables included both maximal and fractional utilisation variables. Furthermore, LT2 was not significantly different to VT2, and elicited similar physiological responses to mean values achieved during a 40 km cycling time trial. However, only 50% of the 10 participants successfully maintained exercise at their LT2 power output with a concomitant steady-state [La], suggesting the modified D-max LT2 is not a strong indicator of MLSS.Despite physiological differences between men and women, few studies have exclusively explored female responses or have compared these effects between sexes in exercise science research. Of those that have, the concentrations of ovarian hormones are rarely considered, despite their potential to influence [La], LT2 and endurance performance. A small number of studies have found significant relationships between the modified D-max LT2 and endurance cycling performance in women, albeit without consideration, control or 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 VO2 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 VO2max) and fractional utilisation (VO2 and power output at LT2) variables produced the strongest prediction of performance in women (r2 = 0.87) and men (r2 = 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 high-intensity 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 modified D-max method of LT2 determination in endurance-trained men and women. The findings suggest that whilst the modified D-max LT2 is a good correlate of 40 km cycling performance in both men and women, combining it with maximal variables provided stronger predictions of cycling performance. Furthermore, LT2 was not a strong indicator of MLSS in men or women; its greater capacity to explain training-related performance improvements in women than men illustrates inconsistency between sexes, and thus, limited generalisability. Therefore, although it is recommended that sports scientists continue to use the modified D-max method of LT2 determination to predict 40 km cycling performance in both men and women (preferentially in conjunction with other laboratory-derived measures for stronger prediction capacity), it is suggested for use in women only when evaluating cycling performance improvements in response to a short HIIT program.