Unraveling the mystery of isocaloric endurance training - Influence of exercise modality, biological sex, and physical fitness.

BACKGROUND:A sedentary lifestyle with low energy expenditure (EE) is associated with chronic diseases and mortality. Barriers such as "lack of time" or "lack of motivation" are common reasons why physical exercise is neglected in the general population. To optimize EE in the time available, time-efficient but also enjoyable types of exercise are required. We therefore used an isocaloric approach to systematically investigate the effects of six different endurance exercise modalities on metabolic, mechanical, cardiorespiratory, and subjective variables in relation to biological sex and physical fitness. METHODS:Out of 104, 92 healthy participants (21 recreationally trained and 18 trained females, 25 recreationally trained and 28 trained males) were subjected to physiological exercise testing to determine the exercise intensities for six exercise modalities, i.e., three different high-intensity interval training (HIIT) protocols (5 × 4 min, 15 × 1 min, 30 × 30 sec intervals), threshold (THR), speed endurance production (SEP), and low-intensity continuous endurance training (LIT). One of three HIIT sessions served as the reference for the subsequent isocaloric exercise modalities which were completed in randomized order. Metabolic and mechanical variables, i.e., EE during exercise, time to isocaloric EE (Tiso), relative and absolute fat contribution, post-exercise oxygen consumption (EPOC), mechanical energy, as well as cardiorespiratory and subjective variables, i.e., heart rate, oxygen uptake response, rating of perceived exertion, and enjoyment were assessed. Data were analyzed using a 6 × 2 × 2 repeated-measures ANOVA. RESULTS:All three versions of HIIT and THR achieved the same EE during exercise for the same training duration. We found that LIT had a 1.6-fold (p < 0.001) and SEP a 1.3-fold (p < 0.001) longer Tiso compared to HIIT with no effects of biological sex (p = 0.42, pη2 = 0.01) or physical fitness (p = 0.09, pη2 = 0.04). There was a main effect of exercise modality on EPOC (p < 0.001, pη2 = 0.76) with highest values for HIIT 30 × 30 (p = 0.032) and lowest for LIT (p < 0.001). The highest relative and absolute amounts of fatty acids were measured during LIT (p < 0.001), and the lowest values were obtained during HIIT modalities. HIIT 30 × 30 was the most enjoyable version of HIIT (p = 0.007), while THR was the least enjoyable exercise modality (p = 0.008). CONCLUSION:HIIT modalities are time-saving and enjoyable, regardless of sex and physical fitness. The results illustrate the relationship between exercise modality and metabolic, physiological, and subjective responses, and are thus of great interest to healthy individuals seeking time-saving and enjoyable exercise options.