Abstract
The tolerable duration of continuous high-intensity exercise is determined by the hyperbolic Speed-tolerable duration (S-tLIM) relationship. However, application of the S-tLIM relationship to normalize the intensity of High-Intensity Interval Training (HIIT) has yet to be considered, with this the aim of present study. Subjects completed a ramp-incremental test, and series of 4 constant-speed tests to determine the S-tLIM relationship. A sub-group of subjects (n = 8) then repeated 4 min bouts of exercise at the speeds predicted to induce intolerance at 4 min (WR4), 6 min (WR6) and 8 min (WR8), interspersed with bouts of 4 min recovery, to the point of exercise intolerance (fixed WR HIIT) on different days, with the aim of establishing the work rate that could be sustained for 960 s (i.e. 4×4 min). A sub-group of subjects (n = 6) also completed 4 bouts of exercise interspersed with 4 min recovery, with each bout continued to the point of exercise intolerance (maximal HIIT) to determine the appropriate protocol for maximizing the amount of high-intensity work that can be completed during 4×4 min HIIT. For fixed WR HIIT tLIM of HIIT sessions was 399±81 s for WR4, 892±181 s for WR6 and 1517±346 s for WR8, with total exercise durations all significantly different from each other (P<0.050). For maximal HIIT, there was no difference in tLIM of each of the 4 bouts (Bout 1: 229±27 s; Bout 2: 262±37 s; Bout 3: 235±49 s; Bout 4: 235±53 s; P>0.050). However, there was significantly less high-intensity work completed during bouts 2 (153.5±40. 9 m), 3 (136.9±38.9 m), and 4 (136.7±39.3 m), compared with bout 1 (264.9±58.7 m; P>0.050). These data establish that WR6 provides the appropriate work rate to normalize the intensity of HIIT between subjects. Maximal HIIT provides a protocol which allows the relative contribution of the work rate profile to physiological adaptations to be considered during alternative intensity-matched HIIT protocols.
Highlights
In classic epidemiological data it is well established that there are significant health benefits associated with leading a physically active lifestyle (e.g. [1,2,3])
There was no difference in peak [L2] (P.0.050; mean 8.561.3 mM) or peak heart rate (HR) (P.0.050; mean 18868 beats?min21) with work rate at the point of exercise intolerance
D9 recovery averaged 54.767.8%, 48.9610.2% and 48.9611.2% for bouts 2, 3 and 4 respectively, with this recovery not significantly different between bouts (P,0.050). This is the first study to apply the S-tLIM relationship to identify the appropriate work rate for High-Intensity Interval Training (HIIT) to normalize the relative intensity between subjects to the very-heavy intensity domain, identifying that WR6 for a 464 min HIIT session provides the appropriate balance between D9 depletion during the ON bouts, and repletion in the intervening 4 min recovery period that allowed for the completion of the required,4 (3.760.7; i.e. 93%) ON bouts
Summary
In classic epidemiological data it is well established that there are significant health benefits associated with leading a physically active lifestyle (e.g. [1,2,3]). Given the implications of training for improving exercise performance, and in the prevention/rehabilitation of chronic disease, establishing optimal training strategies – to maximize training adaptations and associated health-related benefits, and to improve participation and adherence in the general population – is of critical importance. Key in this regard is the intensity of the exercise. Accumulation of high volumes of continuous, progressively higher intensity exercise is limited by the mechanisms that result in rapid exercise intolerance – i.e. tolerable duration is intensity dependent [18,19] This has led to significant interest in High-Intensity Interval Training (HIIT). Repeated short-duration (i.e. ,30 s) all-out Wingate-style HIIT; i.e. Sprint Interval Training (SIT) is popular, and has been demonstrated to effectively improve endurance capacity and time-trial performance [20,21,22,23], muscle oxidative enzyme activity [20,21,22,23] and aerobic capacity (V_ O2max) ([21,24]), as well as specific health-related parameters such as insulin sensitivity [24,25], blood pressure [24] and vascular function [26] in a time-efficient manner (compared with current moderate-intensity physical activity guidelines; i.e. 150 min/week; [27])
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