Free-living Evaluation of Laboratory-based Activity Classifiers in Preschoolers Machine learning (ML) classification models for accelerometer data are a potentially more accurate method to measure physical activity in young children than traditional cut-point methods. However, existing algorithms have been trained on laboratory-based activity trials and their performance has not been investigated under free living conditions. Purpose To evaluate the accuracy of laboratory trained hip and wrist Random Forest (RF) and Support Vector Machine (SVM) classifiers for the automatic recognition of five activity classes: sedentary (SED), light-intensity activities and games (LIGHT_AG), walking (WALK), running (RUN), and moderate-to-vigorous activities and games (MV_AG) in preschool-aged children under free-living conditions. Methods 31 children (4.0 ± 0.9 y) were video recorded during a 20-minute free living play session while wearing an ActiGraph GT3X+ on their right hip and non-dominant wrist. Direct observation was used to continuously code ground truth activity class and specific activity types occurring within each class using a bespoke two-stage coding scheme. Performance was assessed by calculating overall classification accuracy and extended confusion matrices summarizing class-level accuracy and the frequency of specific activities observed within each class. Results Accuracy for the hip and wrist RF algorithms was 69.4% and 59.1%, respectively. Accuracy for hip and wrist SVM algorithms was 66.4% and 59.3%, respectively. Compared to the laboratory cross-validation, accuracy decreased by 11-15% for the hip classifiers and 19-21% for the wrist classifiers. Classification accuracy for SED was 72-78%, 58-79% for LIGHT_AG, 71-84% for MV_AG, 9-15% for WALK, and 66-75% for RUN. Conclusion The accuracy of laboratory-based activity classifiers for preschool-aged children was attenuated when tested on new data collected under free living conditions. Future studies should train and test ML activity recognition algorithms using accelerometer data collected under free-living conditions. Corresponding Author: Matthew Ahmadi, MS, Institute of Health and Biomedical Innovation at QLD Centre for Children’s Health Research, Level 6, 62 Graham Street, South Brisbane, QLD 4101, Australia, Phone: +61 7 3069 7342, Email: matthew.ahmadi@qut.edu.au Funding for this project was provided by an Australian Research Council Discovery Project Grant: DP150100116 - Modelling active play in preschool children using machine learning. Stewart Trost is a member of the ActiGraph Scientific Advisory Board. The authors declare no conflict of interest. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results from the present study do not constitute endorsement by the American College of Sports Medicine. Accepted for Publication: 6 November 2019 © 2019 American College of Sports Medicine

Isometric Exercise Improves Redox Balance and Blood Pressure in Hypertensive Adults Purpose The aim of this study was to investigate the hemodynamic, oxidative stress (OS), and nitric oxide (NO-) responses to a submaximal isometric exercise session (IES) involving large muscle mass. Methods Fourteen hypertensive (HTG: age: 35.9 ± 8.1 years, height: 1.73 ± 0.10 m, total body mass: 78.0 ± 15.8 kg) and 10 normotensive (NTG: age: 41.1 ± 9.4 years, height: 1.71 ± 0.12 m, total body mass: 82.3 ± 22.4 kg) participants performed 2 experimental sessions in the leg press and bench press: i) control session and ii) 8 sets x 1 min contraction at 30% maximal voluntary isometric contraction with 2 min rest interval. Blood pressure (BP) was measured at rest and during 60 min postexercise. Blood samples were collected at rest, immediately after the session, and 60 min postexercise. NO- was obtained through the Griess reaction method. OS parameters were analyzed using commercial kits. A repeated-measures ANOVA with Bonferroni’s post hoc was used to analyze all dependent variables. Results A significant decrease in systolic BP was observed only for HTG at 45 min and 60 min postexercise (baseline vs. 45 min: p = 0.03, Δ% = 4.44%; vs. 60 min: p = 0.018, Δ% = 5.58%). NO- increased immediately postexercise only for HTG (p = 0.008, Δ% = 16.44%). Regarding OS parameters, TBARS presented a significant reduction 60 min after the IES for NTG and HTG; catalase increased in both groups. Conclusions The data showed that only 8 minutes of IES with a large muscle mass elicits an elevated pro-oxidant activity leading to a greater NO- bioavailability, increases antioxidant reaction, and consequently reduces BP in hypertensive patients. Corresponding author: Rafael Reis Olher, Universidade Católica de Brasília. Programa de Pós-Graduação Stricto Sensu em Educação Física. Sala G-116 – QS07 LT1 EPCT, Águas Claras/Taguatinga, DF, Brasil Postal code: 72022-900. Telephone/Fax: +55 61 3356-9350, E-mail: rflolher@gmail.com; mrmoraes70@gmail.com These authors contributed equally to this work: Rafael Reis Olher, Thiago Santos Rosa We are grateful for the financial support of the Foundation for Research Support of the Federal District (FAPDF 193.001607/2017), and this study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001. The authors declare that the results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation, and the results of the study do not constitute an endorsement by the American College of Sports Medicine. The authors declare no competing interests. Accepted for Publication: 14 November 2019 © 2019 American College of Sports Medicine

Sleep Deprivation: Cytokine and Neuroendocrine Effects on Perception of Effort Introduction An increased perception of effort and subjective fatigue are thought to be central to decreased exercise performance observed following disrupted sleep. However, there is limited understanding of the mechanisms which underpin these phenomena. We investigated the role of interleukin-6 (IL-6), the soluble interleukin-6 receptor (sIL-6R) and neuroendocrine factors (cortisol, adrenaline, noradrenaline and brain derived neurotropic factor (BDNF)) in mediating these responses at rest and during exercise. Methods In a randomized order ten healthy active males completed 3 experimental trials following different sleep conditions; a single night of sleep deprivation (DEP), partial sleep deprivation equivalent to 4-hrs of sleep (PART) and normal sleep (CON). The experimental sessions consisted of physiological and perceptual measurements of exercise intensity throughout 45-min moderate intensity and 15-min maximal effort cycling. Cytokine and neuroendocrine factors were assessed at rest and in response to exercise. Results Sleep deprivation resulted in increased resting IL-6, lower blood glucose, increased perceived fatigue and perception of effort, lower free-living energy expenditure, and reduced maximal exercise performance. In contrast, sleep deprivation did not alter physiological, cytokine or neuroendocrine responses to exercise. Variations in the resting concentration of IL-6 were associated with lowered blood glucose, an increased perception of effort, and impaired exercise performance. Resting concentrations of cortisol, adrenaline, noradrenaline, and BNDF showed subtle interactions with specific aspects of mood status, and performance but were not impacted by sleep deprivation. There were minimal effects of partial sleep deprivation. Conclusions These findings demonstrate that cytokine and neuroendocrine responses to exercise are not altered by sleep deprivation but that changes in the resting concentration of IL-6 may play a role in altered perception of effort in this context. Corresponding author: Dr Tom Cullen, Centre for Sport, Exercise and Life Sciences, Coventry University, Priory Street, Coventry, UK, CV1 5FB. Email: ad0189@coventry.ac.uk This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. This research was supported by the National Institute for Health Research (NIHR) Leicester Biomedical Research Centre. The views expressed are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health. The authors report no conflicts of interest in this work. Accepted for Publication: 18 October 2019. This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives License 4.0 (CCBY-NC-ND), where it is permissible to download and share the work provided it is properly cited. The work cannot be changed in any way or used commercially without permission from the journal. © 2019 American College of Sports Medicine

Corticospinal Modulations during Motor Imagery of Concentric, Eccentric, and Isometric Actions PURPOSE It is not known yet whether the neurophysiological specificity of eccentric, concentric and isometric contractions can also be observed when these are mentally simulated. Therefore our aim was to assess corticospinal excitability during motor imagery (MI) of different contraction types and to test whether a passive movement during MI could have additional effects. METHODS Twelve young participants imagined contractions of the wrist flexors, firstly with the arm motionless (static mode) and secondly with a congruent passive movement (wrist extension during eccentric MI and wrist flexion during concentric MI). Motor evoked potentials (MEP) and H-reflexes were elicited in Flexor Carpi Radialis (FCR) at rest and during the three types of MI. As a secondary outcome, the MEPs of one antagonist (Extensor Carpi Radialis, ECR), elicited concomitantly with FCR MEP recording, were also analysed. RESULTS In static mode, FCR MEPs were facilitated during isometric (p=0.046) and concentric (p=0.039) MI, but not during eccentric MI (p=0.902). With passive congruent movements, FCR MEPs were enhanced during all imagined contraction types, including eccentric (p=0.047). FCR H-reflexes increased only during eccentric MI accompanied with wrist extension (p=0.003). ECR MEPs were modulated only when a passive congruent movement was provided (p=0.040). CONCLUSION- Like actual contractions, eccentric MI exhibits specific neural correlates, compared to isometric and concentric MI, which should be considered when using this modality for training. The present results showed that adding passive movements congruent to the eccentric MI task would enhance its impact over corticospinal structures. Corresponding Author: Dr. Sidney Grosprêtre, EA4660-CS3 “Culture, Sport, Health and Society” UPFR Sport, 31, chemin de l’Epitaphe, 25000 Besançon, FRANCE. sidney.grospretre@univ-fcomte.fr ORCID: 0000-0003-1023-5842 This work was supported by the Institut National de la Santé et de la Recherche Médicale (INSERM). This work was supported by the French “Investissements d’Avenir” program, project ISITE-BFC (contract ANR-15-IDEX-0003). CONFLICT OF INTEREST. No conflicts of interest, financial or otherwise, are declared by the authors. The results of the present study do not constitute endorsement by the American College of Sports Medicine. The authors declare that the results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. Accepted for Publication: 6 November 2019. © 2019 American College of Sports Medicine

Cardiorespiratory Fitness, Physical Activity, and Insulin Resistance in Children Purpose Few studies have investigated the independent and joint associations of cardiorespiratory fitness (CRF) and body fat percentage (BF%) with insulin resistance in children. We investigated the independent and combined associations of CRF and BF% with fasting glycaemia and insulin resistance and their interactions with physical activity (PA) and sedentary time among 452 children aged 6–8 years. Methods We assessed CRF with a maximal cycle ergometer exercise test and used allometrically scaled maximal power output (Wmax) for lean body mass (LM1.13) and body mass (BM1) as measures of CRF. BF% and LM were measured by dual-energy X-ray absorptiometry, fasting glycaemia by fasting plasma glucose, and insulin resistance by fasting serum insulin and Homeostatic Model Assessment for Insulin Resistance (HOMA-IR). PA energy expenditure (PAEE), moderate-to-vigorous PA (MVPA), and sedentary time were assessed by combined movement and heart rate sensor. Results Wmax/LM1.13 was not associated with glucose (β=0.065, 95% CI=-0.031 to 0.161), insulin (β=-0.079, 95% CI=-0.172 to 0.015), or HOMA-IR (β=-0.065, 95% CI=-0.161 to 0.030). Wmax/BM1 was inversely associated with insulin (β=-0.289, 95% CI=-0.377 to -0.200) and HOMA-IR (β=-0.269, 95% CI=-0.359 to -0.180). BF% was directly associated with insulin (β=0.409, 95% CI=0.325 to 0.494) and HOMA-IR (β=0.390, 95% CI=0.304 to 0.475). Higher Wmax/BM1, but not Wmax/LM1.13, was associated with lower insulin and HOMA-IR in children with higher BF%. Children with higher BF% and who had lower levels of MVPA or higher levels of sedentary time had the highest insulin and HOMA-IR. Conclusion Children with higher BF% together with less MVPA or higher levels of sedentary time had the highest insulin and HOMA-IR. CRF appropriately controlled for body size and composition using LM was not related to insulin resistance among children. Corresponding author: Eero Haapala, PhD, Biology of Physical Activity, Faculty of Sport and Health Sciences, University of Jyväskylä, PO Box 35, FI-40014 University of Jyväskylä, Jyväskylä, Finland. Tel: +358 40 805 4210, Fax: +35817 162 131, Email: eero.a.haapala@jyu.fi, OrcID: 0000-0001-5096-851X The PANIC Study has financially been supported by Ministry of Education and Culture of Finland, Ministry of Social Affairs and Health of Finland, Research Committee of the Kuopio University Hospital Catchment Area (State Research Funding), Finnish Innovation Fund Sitra, Social Insurance Institution of Finland, Finnish Cultural Foundation, Foundation for Paediatric Research, Diabetes Research Foundation in Finland, Finnish Foundation for Cardiovascular Research, Juho Vainio Foundation, Paavo Nurmi Foundation, Yrjö Jahnsson Foundation, and the city of Kuopio. Moreover, the PhD students and postdoctoral researchers of The PANIC Study have been supported by Program for Clinical Research and Program for Health Sciences of Doctoral School of University of Eastern Finland, Finnish Doctoral Programs in Public Health, Päivikki and Sakari Sohlberg Foundation, Paulo Foundation, Jalmari and Rauha Ahokas Foundation, Aarne and Aili Turunen Foundation, Finnish Medical Foundation, Jenny and Antti Wihuri Foundation, Kuopio Naturalists' Society, Olvi Foundation, and the city of Kuopio. SB was supported by UK Medical Research Council (MC_UU_12015/3) and the NIHR Biomedical Research Centre Cambridge [IS-BRC-1215-20014]. The sponsors had no role in designing the study, the collection, analysis, or interpretation of the data, the writing of the report, or the decision to submit the manuscript for publication. CONFLICT OF INTEREST Authors declare no conflicts of interest. The results of the present study do not constitute endorsement by ACSM. Authors declare that the results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. Accepted for Publication: 5 November 2019. © 2019 American College of Sports Medicine

Metabolic Effects of Three Different Activity Bouts during Sitting in Inactive Adults Purpose This study aimed to determine whether interrupting prolonged sitting with 3 different walking-bout schedules improves glycemic metabolism relative to continuous prolonged sitting in sedentary adults. Methods In a randomized, crossover trial, 16 inactive healthy adults (7 men, 24±3 years old) completed four 26-h laboratory sessions, including 22.5 h in an energy expenditure (EE)-testing calorimeter chamber. The four 9-h intervention periods were as follows: 9-h uninterrupted sitting (SIT); 30-min sitting/3-min brisk (60% VO2max) treadmill walk (WALK3); 45-min sitting/5-min (WALK5); or 60-min sitting/8-min (WALK8). Coprimary outcomes included the difference in the mean interstitial glucose concentration, total areas under the curve (tAUC), and incremental AUC (iAUC) for the entire 26 h and 9 h (intervention period) during three activity-bout conditions compared with SIT. Results Compared with SIT, the 26-h mean glucose concentration was attenuated in WALK8 (Δmean -0.22 mmol·L-1 [95% CI -0.43–-0.001], p= 0.048) without adjustment for EE and attenuated in WALK3 (Δ-0.47 mmol·L-1 [-0.75–-0.10]), WALK5 (Δ-0.47 mmol·L-1 [-0.83–-0.10]), and WALK8 (Δ-0.53 mmol·L-1 [-0.92–-0.13]) after adjustment for EE (all p < 0.01). The 26-h tAUC was reduced in WALK3 (Δ-11.18 mmol·L-1·h [-20.07–-2.29]), WALK5 (2.12.67 mmol·L-1·h [-22.54–-2.79]), and WALK8 (Δ-13.85 mmol·L-1·h [-24.60–-3.10]) (all p < 0.01), as well as the iAUC (all p < 0.05), only after adjustment for EE. The 9-h mean glucose concentration, tAUC, and iAUC decreased in the three activity-break conditions regardless of EE adjustment (all p < 0.05). Conclusions All three walking-bout conditions improved glycemic metabolism compared with SIT, independent of EE, in inactive, healthy adults. Corresponding author: Zhen-Bo Cao, PhD, School of Kinesiology, Shanghai University of Sport, 399 Chang Hai Road, Shanghai, China, 200438, Tel.: +86-21-65507510, Fax: +86-21-65507357. caozb_edu@yahoo.co.jp This study was funded by the Science and Technology Commission of Shanghai Municipality (No. 16080503300) and the National Natural Science Foundation of China (No. 31571226). The authors report no conflicts of interest. The results of the present study do not constitute endorsement by ACSM. These results are presented as clearly as possible, honestly, and without fabrication, falsification, or inappropriate data manipulation. Accepted for Publication: 10 October 2019. © 2019 American College of Sports Medicine

EXERCISE TRAINING REDUCES REWARD FOR HIGH-FAT FOOD IN PEOPLE WITH OVERWEIGHT/OBESITY Purpose There is increasing evidence that exercise training may facilitate weight management via improvements in homeostatic appetite control, but little is known about how exercise training affects food reward and susceptibility to overeating. Methods This study examined changes in food reward and eating behavior traits after a supervised 12-week exercise intervention (10.5 MJ/week) in inactive individuals with overweight/obesity (Exercisers; n=46, 16 males/30 females; BMI=30.6 (SD 3.8) kg/m2 and age=43.2 (SD 7.5) years compared to non-exercising Controls (n=15; 6 males/9 females; BMI=31.4 (SD 3.7) kg/m2 and age=41.4 (SD 10.7) years). Liking and wanting scores for high-fat relative to low-fat foods was assessed with the Leeds Food Preference Questionnaire before and after consumption of an isoenergetic high-fat (HFAT) or high-carbohydrate (HCHO) lunch. Eating behavior traits were assessed using the Three-Factor Eating Questionnaire and Binge Eating Scale. Results A week by group interaction indicated that wanting scores decreased from baseline to post-intervention in Exercisers only (M[INCREMENT]Pre-Post= -4.1, p=0.03, ηp2=0.09, 95%CI= -7.8 to -0.4), but there was no exercise effect on liking. There was also a week by group interaction for binge eating, which decreased in Exercisers only (M[INCREMENT]Pre-Post= -1.5, p=0.01, ηp2=0.11, 95%CI= -2.7 to -0.4). A small reduction in disinhibition was also apparent in Exercisers (M[INCREMENT]Pre-Post= -0.7, p=0.02, ηp2=0.10, 95%CI= -1.3 to -0.1). Conclusion This study showed that 12 weeks of exercise training reduced wanting scores for high-fat foods and trait markers of overeating in individuals with overweight/obesity compared to non-exercising Controls. Further research is needed to elucidate the mechanisms behind these exercise-induced changes in food reward. Corresponding Author: Kristine Beaulieu, School of Psychology, Faculty of Medicine and Health, University of Leeds, Leeds, LS2 9JT, UK. Telephone: +4401133431403. Email: k.beaulieu@leeds.ac.uk Research relating to this study was funded by BBSRC (DRINC) grant no. BB/G005524/1 and European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 266408 ‘Full4Health’. Neither funding body had a role in the design, analysis or writing of this article. Conflict of Interest: The authors declare no conflicts of interest. The results of the present study do not constitute endorsement by ACSM and are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. Accepted for Publication: 19 October 2019. © 2019 American College of Sports Medicine

The Acute: Chronic Workload Ratio Is Associated with Injury in Junior Tennis Players Purpose Session rate of perceived exertion (sRPE) is used to track internal training/competition load in athletes using a metric known as the acute to chronic workload ratio (ACWR). Research reported on team sports have determined that if the acute load is higher than the chronic load athletes are likely to sustain injury. No studies, however, have attempted to investigate internal load and injury in a tennis population despite the rigorous training loads. Therefore, the purpose of this study was to investigate if sRPE ACWR is associated with injury in junior tennis players over a 7-month time period. Methods Forty-two junior tennis players were recruited to participate, 26 were included in the final analysis. Players provided a rating of RPE as an estimate of training intensity every day after training/match sessions. Session RPE, a measure of internal and external training load was calculated by multiplying the training/match session RPE by the session duration in minutes. Players self-reported all injuries. The ACWR was the primary independent variable. Acute load was determined as the total sRPE for one week, while a 4-week rolling average sRPE represented chronic load. Results Seventeen players sustained injuries. The model indicated that ACWR from the previous week (p<0.001) and previous injury history (p=0.003) were significant predictors of injury the following week. In the week preceding injury, the average ACWR was 1.57 (SD 0.90). Conclusion Injured players had on average 1.5 times more training load in the past week compared to the previous 4 weeks. Majority of players that went on to sustain an injury were not prepared for the load endured. These results were similar to previous studies investigating ACWR where an acute increase in load was associated with increased injury risk. Corresponding Author: Natalie L. Myers, PhD, ATC, 601 University Dr. San Marcos, TX 78666, (office): 512-245-2958, (fax): 512-245-8678. natalie.myers@txstate.edu This project was supported financially by Texas State University’s Research Enhancement Program. No conflicts of interest, financial or otherwise, are declared by the authors. The results of this study do not constitute endorsement by the American College of Sports Medicine. The results of this study are presented clearly, honestly, and without fabrication, falsification or inappropriate data manipulation. Accepted for Publication: 2 November 2019. © 2019 American College of Sports Medicine

Physical Activity and Cardiometabolic Risk Factor Clustering in Young Adults with Obesity Introduction There is a paucity of information on the clustering of cardiometabolic risk factors in young adults and how this clustering may vary based on whether or not they perform sufficient levels of physical activity. Methods We analyzed baseline data from 346 young adults (23.3±4.4 y) participating in the Healthy Body Healthy U (HBHU) clinical trial from 2015-2018. Cardiometabolic risk factors were measured according to standard procedures and moderate-to-vigorous intensity physical activity (MVPA) was determined by accelerometry. A cardiometabolic clustering score (CCS; ranging from 0 to 5) was created from five biomarkers according to whether or not a standard clinical risk cut point was exceeded (0=no; 1=yes): abdominal circumference [>102 cm (men) or >88 cm (women)]; HbA1c (≥ 5.7%); HDL-C [<40 mg/dL (men) or <50 mg/dL (women)]; SBP (≥ 130 mmHg); and DBP (≥85 mmHg). Cardiometabolic dysregulation (CD) was defined as a CCS > 3. Multiple logistic regression determined the independent association between level of MVPA and CD, while adjusting for sex, race/ethnicity, sedentary time, and smoking. Results The prevalence of CD was 18% (22% in men; 17% in women). We observed a non-linear graded association between MPA and CD. Participants performing 150-300 min/week of MVPA significantly lowered their odds of CD by 66% (OR=0.34; 95% CI=0.16, 0.75), while those exceeding 300 min/week lowered their odds by 61% (OR=0.39; 95% CI: 0.18, 0.86), compared with those performing <150 min/week, independent of obesity and the other covariables. Conclusion Recommended levels of moderate-intensity physical activity is significantly associated with a lower odds of CD and thus may prevent or diminish the need for expensive pharmaceutical treatment over the remainder of the lifespan. Corresponding Author: Loretta DiPietro, PhD, MPH, The George Washington University, Milken Institute School of Public Health, 950 New Hampshire Ave, 2nd Floor, Washington DC 20052, 202-994-4910 | ldp1@gwu.edu. This study was funded by NIH/NIDDK R01 DK100916 to Dr. Napolitano. The results of this study do not constitute endorsement by ACSM, and are presented clearly, honestly, and without fabrication, falsification, or inappropriate manipulation. The contents of this manuscript have not been previously published elsewhere. No financial disclosures or conflicts of interest have been reported by the authors of this manuscript. Accepted for Publication: 25 October 2019. © 2019 American College of Sports Medicine

Oxygen Demand, Uptake, and Deficits in Elite Cross-country Skiers during a 15-km Race Purpose To quantify the repeated oxygen deficits attained during intermittent endurance exercise by measuring oxygen consumption (V[Combining Dot Above]O2) and oxygen demand (V[Combining Dot Above]O2dem) throughout a simulated roller ski race. Methods Eight male elite cross-country skiers (V[Combining Dot Above]O2,peak 77.4 ± 4.4 mL·min-1·kg-1) raced a 13.5 km roller ski time-trial on a World Cup course. On two additional days, athletes completed (i) 6 sub-maximal loads (à 5 min) and a ~ 4 min maximal trial to establish athlete-specific estimates of skiing economy, V[Combining Dot Above]O2,peak, and maximal ΣO2def (MAOD); and (ii) a simulation of the time-trial on a roller skiing treadmill. During the simulation, external work rate (Pprop) and skiing speed (v) were adjusted to match the Pprop and v measured during the time-trial, and pulmonary V[Combining Dot Above]O2 was measured breath-by-breath. V[Combining Dot Above]O2dem and ΣO2def were calculated using an athlete-specific model for skiing economy throughout the treadmill simulation. Results During the treadmill simulation V[Combining Dot Above]O2 was on average 0.77V[Combining Dot Above]O2,peak, and active V[Combining Dot Above]O2dem, (i.e., excluding the time in simulated downhills), was on average 1.01V[Combining Dot Above]O2,peak. The athletes repeatedly attained substantial oxygen deficits in individual uphill sections of the treadmill simulation, but the deficits were typically small compared to their MAOD (average 14%, range ~0-50%). However, the ΣO2def summed over all periods of active propulsion was on average 3.8MAOD. Conclusion Athletes repeatedly attain substantial oxygen deficits in the uphill segments of a distance cross-country ski race. Furthermore, the total accumulated oxygen deficit of all these segments is several times higher than the athletes’ MAODs. This suggests that rapid recovery of the energy stores represented by the oxygen deficit is necessary during downhill sections, and that this might be an important determinant of distance skiing performance. Corresponding Author: Øyvind Gløersen, Department of Physical Performance, Norwegian School of Sport Sciences, Postboks 4014 Ullevål stadion, 0806 Oslo, Norway, E-mail: o.n.gloersen@nih.no The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. This study was funded by the Norwegian School of Sport Sciences, Oslo, Norway, and the Norwegian Research Council (project 216699). The authors have no conflicts of interest to disclose in relation to this study. The results of the present study do not constitute endorsement by the American College of Sports Medicine. Accepted for Publication: 21 October 2019 This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives License 4.0 (CCBY-NC-ND), where it is permissible to download and share the work provided it is properly cited. The work cannot be changed in any way or used commercially without permission from the journal. © 2019 American College of Sports Medicine