Use of Heart Rate for Estimating the Moderate-to-Heavy Intensity Transition During Prolonged Exercise
The moderate-to-heavy intensity transition is commonly measured in individual athletes for purposes of training intensity regulation and load monitoring. The moderate-to-heavy intensity transition is typically expressed using an external work rate metric, such as power output, and an internal work rate metric, such as heart rate. These assessments are often performed in well-rested athletes; however, data from these assessments are used to regulate intensity during prolonged exercise where progressive physiological changes occur over-time. The degree to which these changes occur may vary depending on the so-called 'durability' of the individual athlete. In the context of exercise, durability refers to the time of onset and magnitude of any deterioration in physiological profiling characteristics over-time during prolonged exercise. The acute effect of prolonged exercise on the moderate-to-heavy intensity transition power output and associated heart rate has not been established. Therefore, the aim of this research was to determine the effect of prolonged exercise on moderate-to-heavy intensity transition power output and heart rate. It was hypothesised that the power output at the moderate-to-heavy intensity transition would decrease as a result of acute prolonged exercise, but that proportional cardiovascular drift would occur such that the heart rate associated with the transition would be preserved over-time. Fourteen endurance-trained cyclists and triathletes took part in the present investigation (13 males, 1 female, VO2peak 59.9 ± 6.8 mL.kg-1.min-1, training volume 9 ± 3 h.week-1), which consisted of two laboratory visits. Following a characterisation trial in which the power output at the first ventilatory threshold (VT1) was estimated, participants undertook a five-stage incremental step test to determine the power output and heart rate at the moderate-to-heavy intensity transition before and after two hours of cycling at 90% of the estimated power output at VT1. In line with the stated hypothesis, power output at the moderate-to-heavy intensity transition significantly decreased following acute prolonged exercise when determined using expired gases (VT1, 217 ± 42 W vs. 196 ± 42 W, P < 0.0001) and blood lactate concentrations (LoglogLT, 212 ± 47 W vs. 190 ± 47 W, P < 0.0035). This was attributable to loss of efficiency (VT1, -8 ± 10 W; LoglogLT, -7 ± 9 W) and rates of metabolic energy expenditure (VT1, -14 ± 11 W; LoglogLT, -15 ± 22 W) at the transition. However, in contrast to the hypothesis, the heart rate associated with the moderate-to-heavy intensity transition increased following acute prolonged exercise (VT1, 142 ± 9 beats.min-1 vs. 151 ± 12 beats.min-1, P < 0.001; LoglogLT, 140 ± 13 beats.min-1 vs. 150 ± 15 beats.min-1, P < 0.006). These results demonstrate that the external work output at the moderate-to-heavy intensity transition decreases during prolonged exercise, and that heart rate at the moderate-to-heavy intensity transition increases during prolonged exercise. Therefore, individual assessments of athlete 'durability' are warranted.