Reliability of acute training responses elicited by exhaustive work intervals prescribed with the delta concept

Reliability of acute training responses elicited by exhaustive work intervals prescribed with the delta concept  Arthur H. Bossi 1 , Wouter P. Timmerman 1 , Louis Passfield 1,2 , James G. Hopker 1 1 School of Sport and Exercise Sciences, University of Kent, Chatham, Kent, England 2 Faculty of Kinesiology, University of Calgary, Calgary, Alberta, Canada  Correspondence: Arthur Henrique Bossi School of Sport and Exercise Sciences University of Kent at Medway Medway Building Chatham Maritime Chatham, Kent ME4 4AG England asnb3@kent.ac.uk +447398944056       Background: Biomolecular research has suggested that the chronic adaptation to an exercise programme is modulated by the extent individual training sessions produce homeostatic stress. Hence, training intensity must be carefully prescribed to ensure the expected stimulus for adaptation is provided. To this end, the delta concept (%I”) has been proposed as an intensity prescription method to minimising inter-individual variability of physiological and perceptual responses—expressed by: WI‡ prescribed = WI‡ GET + [(WI‡ VI‡O2max – WI‡ GET ) · %I”]  where WI‡ prescribed is the set work rate, WI‡ GET is the work rate associated with the gas exchange threshold, WI‡ VI‡O2max is the work rate associated with the maximal oxygen uptake and %I” is the targeted intensity. Surprisingly, the inter-individual variability of acute training responses has not been investigated during high-intensity interval training (HIIT), despite HIIT being commonly performed in the laboratory and field settings. Moreover, the intra-individual variability of acute HIIT responses has not been established in cycling, which is vital to understand whether the anticipated training stimulus is achieved whenever a session is performed.  Purpose: We explored the levels of inter- and intra-individual variability of acute training responses elicited by exhaustive work intervals prescribed with %I”.  Methods: Eighteen male and four female cyclists [age: 36 ± 12 years, height: 178 ± 10 cm, body mass: 75.2 ± 13.7 kg, VI‡O 2max : 52 ± 5 ml·kg -1 ·min -1 , peak power output (PPO): 4.72 ± 0.48 W·kg -1 ] volunteered for this study. They performed a ramp test in the first visit to determine VI‡O 2max , PPO, WI‡ GET and WI‡ VI‡O2max . The next four visits consisted of a standardised 21-min warm-up and a HIIT session performed to exhaustion [i.e. 4-min work intervals at 70%I” (WI‡ prescribed = 4.00 ± 0.43 W·kg -1 , 84.7 ± 0.4 %PPO), interspersed with 2-min active recovery at 0.2·WI‡ prescribed ]. Breath-by-breath gas exchanges and heart rate (HR) were continuously measured, with ratings of perceived exertion (RPE) and blood lactate concentration ([La]) obtained after each work interval and at exhaustion. Time at >90%VI‡O 2max was quantified as absolute values and as a percentage of the time to exhaustion. One-way repeated measures analysis of variance was used to test for systematic changes between HIIT sessions. Statistical significance was set at P ≤ 0.1. Reliability estimates [typical error (TE), coefficient of variation (CV), and intraclass correlation coefficient (ICC)] were obtained through Hopkins spreadsheet and are reported with 90% confidence limits. Based on the acquired data, we used G*Power software to perform sample size estimations for a two-tailed matched paired t-test, with alpha error probability set at 0.05 and power at 0.80.  Results: Time to exhaustion, absolute and relative time at >90%VI‡O 2max , peak HR, peak RPE, and peak [La] were not different between HIIT sessions (all F ≤ 2.10, P ≥ 0.13, I· 2 p ≤ 0.09). Dependent variables and their reliability estimates are reported in Table 1. Sample size estimations are reported in Table 2.  Table 1. Reliability data [with 90% confidence limits] for the dependent variables.  mean ± SD TE ICC intra-individual CV (%) inter-individual CV (%)  Time to exhaustion (s) 1219 ± 618 244 [210-294] 0.86 [0.76-0.92] 31.0 67.0  Absolute time at >90%VI‡O 2max (s) 502 ± 366 137 [118-165] 0.87 [0.79-0.93] 67.0 139.3  Relative time at >90%VI‡O 2max (%) 57.0 ± 22.0 14.2 [12.2-17.1] 0.61 [0.43-0.77] 63.7 109.2  Peak HR (b·min -1 ) 179 ± 11 2 [1.7-2.4] 0.97 [0.94-0.98] 1.2 6.2  Peak RPE 19.6 ± 0.8 0.3 [0.3-0.4] 0.85 [0.75-0.92] 1.9 4.7  Peak [La] (mmol·L -1 ) 14.3 ± 2.6 2.0 [1.7-2.4] 0.45 [0.25-0.65] 15.0 20.4  SD, standard deviation; TE, typical error; ICC, intraclass correlation coefficient; CV, coefficient of variation; HR, heart rate; RPE, ratings of perceived exertion; [La], blood lactate concentration  Table 2. Required sample size to detect baseline changes for a given variable analysed with two-tailed matched paired t-test, with alpha error probability set at 0.05 and power at 0.80.  2% 5% 10% 20% 30%  Time to exhaustion (s) 1411 228 59 17 9  Absolute time at >90%VI‡O 2max (s) 2854 458 116 31 15  Relative time at >90%VI‡O 2max (%) 2421 389 99 27 13  Peak HR (b·min -1 ) 8 4 3 3 2  Peak RPE 14 5 3 3 2  Peak [La] (mmol·L -1 ) 724 119 32 10 6  HR, heart rate; RPE, ratings of perceived exertion; [La], blood lactate concentration  Discussion: Although participants consistently achieved peak values of HR, RPE, and to a lesser extent [La], there was substantial inter-individual variability in time to exhaustion, and both absolute and relative time at >90%VI‡O 2max . Importantly, inter-individual variability was much higher than intra-individual, suggesting a greater day-to-day consistency would still produce marked heterogeneity between participants. This raises questions over the validity of %I” to normalise acute HIIT responses. The levels of intra-individual variability also cast doubt on the assumption that a similar stimulus for adaptation is triggered every time a standard HIIT session is performed. Besides the effect on sample size estimations, achievable only if studies aim to detect large changes, this result also suggests athletes may not need to overly adhere to the prescribed power output during HIIT.  Conclusions: In contrast to previous suggestions based on continuous exercise, %I” does not produce consistent inter- and intra-individual acute HIIT responses. Future studies should consider alternative methods for training intensity normalisation. Whether day-to-day consistency in training stimulus is a pre-requisite for optimal adaptation following HIIT is another question that merits investigation.  Acknowledgements: A.H.B. is a CNPq (Conselho Nacional de Desenvolvimento CientA­fico e TecnolA³gico) scholarship holder [200700/2015-4]. Â

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