Author(s): BONE, J., RENWICK, J.R.M., GIBALA, M.J., Institution: MCMASTER UNIVERSITY, Country: CANADA, Abstract-ID: 1724

INTRODUCTION:
High intensity interval training (HIIT) intensity prescription is commonly expressed relative to maximal heart rate or oxygen uptake (1). An alternate approach involves anchoring intensity to the metabolic threshold critical power (CP; 1). CP denotes a ‘fatigue threshold’ that distinguishes the intensity at which physiological responses to exercise can (CP) be stabilized (2). Work prime (W’) denotes the finite, exhaustible amount of work that can be performed above CP. Characterizing intensity relative to CP and W’ depletion may reduce variability in exercise tolerance between individuals (3) but this could be influenced by habitual activity and training status. We investigated this issue using a HIIT protocol in which intensity was prescribed based on CP and two different depletion rates of W′. We hypothesized that habitually active individuals who train regularly would have higher exercise tolerance compared to those who are insufficiently active.
METHODS:
Healthy adults were recruited into one of two groups based on a participant classification framework (4): Less Trained [LT, n=18 (13 males, 5 females)] did not meet the typical weekly guideline for aerobic physical activity; More Trained [MT, n=17 [12 males, 5 females)] met the guideline and trained regularly for competition. LT and MT differed in peak oxygen uptake (V̇O2peak; (37.0 ml·kg-1·min-1 [95% CI = 33.7 – 40.3] vs 61.0 ml·kg-1·min-1 [95% CI = 57.7 – 64.3] and CP (124 W [95% CI = 108 – 139] vs 232 W [95% CI = 216 – 249]), respectively (both p<0.001), but not W’ (16.1 kJ [95% CI = 14.2 – 17.9] vs; 17.7 [95% CI = 15.8 – 19.6] p=0.18). Participants performed two trials in a randomized, crossover, single-blind manner. Each involved a 5-min warmup followed by 4x4-min bouts, with bouts set at a standardized workload designed to deplete 10% (HIIT10) or 50% (HIIT50) of total W’, with 3-min recovery between. Exercise tolerance was estimated by assessing the proportion of the 4x4-min work bouts completed in each trial and expressed as %W’ depleted (W’D%W’) and in kJ (W’DkJ). Data were analyzed with linear mixed models (LMM) with Group*Condition*Bout interactions and a random intercept for participant.
RESULTS:
LMM analyses revealed that MT had greater W’D%W’ (42.8 %W’ [95% CI = 39.0 – 46.5] vs 32.7 %W’ [95% CI = 29.1 – 36.3]; p=0.043) and W’DkJ (8.1 kJ [95% CI = 7.1 – 9.1 kJ] vs 4.2 kJ [95% CI = 3.2 – 5.1 kJ]; p=0.012) during bout four in HIIT50 compared to LT. Neither W’D%W’ nor W’DkJ were different across groups in HIIT10 (p>0.05 for both).
CONCLUSION:
Habitual activity and training status affected exercise tolerance during acute HIIT prescribed at CP+50% W’ but not CP+10% W’. Classifying intensity based on CP+%W’ may enhance the precision of HIIT prescription but exercise tolerance is impacted by participant classification and W’ depletion rate.
Reference PMID: (1) 37804419; (2) 27031742; (3) 36710454; (4) 34965513
Supported by NSERC, Canada.