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The purpose of this study was to evaluate the effects of a microcycle of high-intensity interval training (HIT) sessions with multiple short work intervals followed by an active recovery period, compared to a similar duration of regular training, on determinants and indicators of endurance performance in well-trained cyclists. The participants in the BLOCK group performed a 6-day HIT microcycle including five HIT sessions (5 × 8.75-min 30/15 s short intervals) followed by a 6-day active recovery period with reduced training load, while the regular training group (REG) performed 12 days of their regular training, including four HIT sessions. Physiological testing was performed before and after the training periods. From pre- to post- intervention, BLOCK demonstrated significantly larger improvements than REG in mean power output (PO) during the last min of the maximal oxygen uptake (VO 2max ) test (PO VO2max ) (3.7 vs. 0.7%, p = 0.009, and effect size (ES) = 1.00) and mean PO during the 10-s sprint (2.8 vs. 1.9%, p = 0.028, and ES = 0.63). No significant differences between BLOCK and REG were observed for VO 2max , PO at 4 mmol·L -1 [blood lactate] (PO 4mmol ), 15-min maximal mean power output (PO 15-min ), and gross efficiency (p = 0.156-0.919). However, there was a tendency for larger improvements in the performance index (calculated from the main performance indicators PO VO2max , PO 4mmol , and PO 15-min ) in BLOCK compared to REG (2.9% vs. 1.2%, p = 0.079, and ES = 0.71). A 6-day high-intensity short-interval microcycle followed by a 6-day active recovery period induces improvements in endurance performance indicators compared to regular training, demonstrating its potential as an efficient strategy for endurance training in well-trained cyclists., (© 2024 The Author(s). European Journal of Sport Science published by Wiley‐VCH GmbH on behalf of European College of Sport Science.)

Background: Skeletal muscle mitochondria and capillaries are crucial for aerobic fitness, and suppressed levels are associated with chronic and age-related diseases. Currently, evidence-based exercise training recommendations to enhance these characteristics are limited. It is essential to explore how factors, such as fitness level, age, sex, and disease affect mitochondrial and capillary adaptations to different exercise stimuli., Objectives: The main aim of this study was to compare the effects of low- or moderate intensity continuous endurance training (ET), high-intensity interval or continuous training (HIT), and sprint interval training (SIT) on changes in skeletal muscle mitochondrial content and capillarization. Secondarily, the effects on maximal oxygen consumption (VO 2 max), muscle fiber cross-sectional area, and fiber type proportion were investigated., Methods: A systematic literature search was conducted in PubMed, Web of Science, and SPORTDiscus databases, with no data restrictions, up to 2 February 2022. Exercise training intervention studies of ET, HIT, and SIT were included if they had baseline and follow-up measures of at least one marker of mitochondrial content or capillarization. In total, data from 5973 participants in 353 and 131 research articles were included for the mitochondrial and capillary quantitative synthesis of this review, respectively. Additionally, measures of VO 2 max, muscle fiber cross-sectional area, and fiber type proportion were extracted from these studies., Results: After adjusting for relevant covariates, such as training frequency, number of intervention weeks, and initial fitness level, percentage increases in mitochondrial content in response to exercise training increased to a similar extent with ET (23 ± 5%), HIT (27 ± 5%), and SIT (27 ± 7%) (P > 0.138), and were not influenced by age, sex, menopause, disease, or the amount of muscle mass engaged. Higher training frequencies (6 > 4 > 2 sessions/week) were associated with larger increases in mitochondrial content. Per total hour of exercise, SIT was ~ 2.3 times more efficient in increasing mitochondrial content than HIT and ~ 3.9 times more efficient than ET, while HIT was ~ 1.7 times more efficient than ET. Capillaries per fiber increased similarly with ET (15 ± 3%), HIT (13 ± 4%) and SIT (10 ± 11%) (P = 0.556) after adjustments for number of intervention weeks and initial fitness level. Capillaries per mm 2 only increased after ET (13 ± 3%) and HIT (7 ± 4%), with increases being larger after ET compared with HIT and SIT (P < 0.05). This difference coincided with increases in fiber cross-sectional area after ET (6.5 ± 3.5%), HIT (8.9 ± 4.9%), and SIT (11.9 ± 15.1%). Gains in capillarization occurred primarily in the early stages of training (< 4 weeks) and were only observed in untrained to moderately trained participants. The proportion of type I muscle fibers remained unaltered by exercise training (P > 0.116), but ET and SIT exhibited opposing effects (P = 0.041). VO 2 max increased similarly with ET, HIT, and SIT, although HIT showed a tendency for greater improvement compared with both ET and SIT (P = 0.082), while SIT displayed the largest increase per hour of exercise. Higher training frequencies (6 > 4 > 2 sessions/week) were associated with larger increases in VO 2 max. Women displayed greater percentage gains in VO 2 max compared with men (P = 0.008). Generally, lower initial fitness levels were associated with greater percentage improvements in mitochondrial content, capillarization, and VO 2 max. SIT was particularly effective in improving mitochondrial content and VO 2 max in the early stages of training, while ET and HIT showed slower but steady improvements over a greater number of training weeks., Conclusions: The magnitude of change in mitochondrial content, capillarization, and VO 2 max to exercise training is largely determined by the initial fitness level, with greater changes observed in individuals with lower initial fitness. The ability to adapt to exercise training is maintained throughout life, irrespective of sex and presence of disease. While training load (volume × intensity) is a suitable predictor of changes in mitochondrial content and VO 2 max, this relationship is less clear for capillary adaptations., Competing Interests: Declarations. Funding: No funding was received for this work. Conflict of interest: Knut Sindre Mølmen, Nicki Winfield Almquist, and Øyvind Skattebo declare that they have no conflicts of interest relevant to the content of this review. Ethics approval: Not applicable. Consent to participate: Not applicable. Consent for publication: Not applicable. Availability of data and material: The dataset, statistical analysis codes, and SAS scripts are available from the study’s GitHub repository. To request access, please contact the corresponding author with your GitHub username, and access will be granted upon reasonable request. Authors contribution: K.S.M. and Ø.S. conceived and designed the study, with input from N.W.A.; K.S.M. conducted the literature search and initial study selection; K.S.M. and N.W.A. performed research article screening and data extraction; Ø.S. did the statistical analyses and data visualization; K.S.M., N.W.A., and Ø.S. contributed to the interpretation of the findings; K.S.M. and Ø.S. wrote the first draft of the manuscript; All authors provided critical revision, feedback, comments, and input on the manuscript. All authors read and approved the final manuscript., (© 2024. The Author(s).)

Purpose: Altitude training is a common strategy used with the intent to increase hemoglobin mass (Hb mass ) in athletes. However, if the Hb mass is increased during altitude camps, it seems to decline rapidly upon returning to sea level. This study aimed to examine the efficacy of three weekly heat training sessions over a 3.5-wk period after a 3-wk altitude camp on the maintenance of Hb mass in elite cyclists., Methods: Eighteen male cyclists (maximal oxygen consumption: 76 ± 5 mL·min -1 ·kg -1 ) underwent a 3-wk altitude training camp at ~2100 m above sea level. After the camp, participants were divided into one group performing three weekly heat sessions that were subtracted from their usual training (HEAT) while the other continuing usual training (CON). Training characteristics were recorded during the intervention, whereas hematological measurements were recorded before the camp as well as 2 d and 3.5 wk after the altitude camp., Results: The 3-wk altitude camp led to an overall increase in total Hb mass of 4.1%. Afterward, HEAT maintained Hb mass (0.2%, P = 0.738), whereas CON group experienced a significant reduction (-3.3%, P < 0.001) (ΔHEAT vs ΔCON, P < 0.001). Moreover, HEAT increased plasma volume (PV) by 11.6% ( P = 0.007) and blood volume (BV) by 5.8% ( P = 0.007), whereas CON only showed an increase in PV (5.5%, P = 0.041). Exercise intensity and training load were not different between groups during the maintenance period., Conclusions: This study suggests that incorporating three weekly heat training sessions into the usual training routine preserves a moderately increased Hb mass in elite cyclists after an altitude camp., (Copyright © 2024 by the American College of Sports Medicine.)