Endurance training is an effective way to improve cardiorespiratory fitness (CRF) and positively impact on risk factors for cardiovascular disease [1–3]. Yet, it is also known that adaptions to a standardized training greatly differ between individuals. Persons showing only minor or no adaptions to training stimuli are frequently termed nonresponders. Therefore, consideration of individual variability in training response has become increasingly important in training studies in recent years. The Heritage study was one of the first examples for large scale intervention studies that has reported a large heterogeneity in responses of VO2max after a standardized training intervention [4]. The phenomenon of little or no training response has been investigated by other researchers since then [5–7]. Ross and colleagues compared eight endurance training studies and found a range of adaptions from minus 33% to plus 118%, independent of exercise duration, intensity and trial sample size [8].
It is not clear if and how the rate of nonresponders can be reduced by adjusting training modalities (e.g. training volume or intensity). The dose-response relationship of endurance training is influenced by training volume, intensity and training frequency [9–11]. Higher doses of training have proven to elicit more pronounced training adaptions [12, 13]. However, the extent to which modifications of either intensity or volume are the leading factor is a question with large relevance to exercise training prescriptions to achieve the intended training adaptions. There is evidence that training protocols with higher intensities such as high-intensity interval training (HIIT) are more effective in eliciting increases in VO2max than moderate intensity training [10, 14, 15].
Due to the high variability of adaptions to endurance training, it has been suggested that the efficacy of training interventions has to be analysed beyond mere comparison of main effects like VO2max [16, 17]. In this regard, a common approach is to compare the rates of nonresponders between groups [6, 18]. In terms of interindividual variations in training response, Bonafiglia and colleagues [12] found that higher doses of training produce higher rates of response. In a direct comparison of moderate intensity training with energy-matched HIIT, Maturana and colleagues [7] found a greater effect on VO2max and a lower nonresponder rate for HIIT. Montero and Lundby [18] observed that higher training dose through increased volume is an effective approach to achieve a meaningful response in VO2max for participants showing no response after an initial training intervention. However, this inevitably leads to more time consumption. For a standardized EE, participants who performed training with higher intensities were less likely to show a nonresponse than participants training with lower intensities [6]. However, it should be noted that in this study the training frequency was 5•wk− 1 and therefore presumably higher than in most training beginners. These findings indicate that for nonresponders who are unable to perform more or longer training sessions, training with higher intensities might be a promising alternative to achieve greater and continuous training adaptions.
A methodological difficulty is the identification of nonresponders, as there is no uniform established definition of high-, low- or nonresponse. Individuals are termed responders when their individual response exceeds a certain threshold. The threshold for nonresponse has for example been as set as a VO2max improvement ≤ 0 L•min− 1 [19], or less than 5% [20, 21]. Moreover, some studies use a coefficient of variation of 5.6%[5] derived from the literature. Although these thresholds are a straightforward method to categorize responders, they do not consider the true training-induced response. In this context, more attention has been given to the necessity of distinguishing the true training-induced response from within-subject variability and measurement errors of the specific setting [16]. Accordingly, some authors have suggested to use specific study designs such as repeated testing or reliability trials to analyse true training responses [22, 23]. A more individualized approach to take the within-subject variation into account when interpreting and categorizing the response is calculating the technical error of measurement (TEM) and using it as threshold for adaptions [23, 24]. Another methodological challenge in testing the independent effect of training intensity lies in the necessity to control for the overall training dose (volume x intensity). This can be achieved by estimating and standardizing the total energy expenditure (EE) between different training modalities.
In this study, we sought to compare response rates between 26 weeks of moderate intensity training (55% HRreserve) and training with an increase in training intensity (to 70%HRreserve and to 95%HRmax) after 10 and 18 weeks, respectively. The differences on the group level are subject in another publication and not discussed in detail here [25]. It was hypothesized that nonresponse rate can be reduced or even eliminated through an increase in intensity with energy expenditure held constant. Besides the VO2max as main target parameter, this analysis further includes resting and submaximal HR, as we expected different patterns of response for maximal (VO2max) and submaximal (heart rate) parameters.