It is well accepted that maximal oxygen uptake (V˙O2max) is important for endurance performance. In elite endurance athletes, every step of the oxygen transport system is adapted to chronic exposure to endurance training (2,6,16,26). However, the diversity of accumulated adaptations associated with chronic exercise in different sports has not been fully examined. Such specific adaptations are likely most pronounced in elite athletes with high training volumes and are influenced by exercise intensity, duration, and frequency (22), as well as by differences in the contributions of exercising muscle mass to the upper and lower limbs. To the best of our knowledge, no study has systematically compared the long-term physiological effects of upper-body, lower-body, and whole-body exercises since the mid-1960s, when Saltin and Astrand (28) compared V˙O2max (mL·min−1·kg−1 and L·min−1) in different endurance disciplines, including cross-country (XC) skiing, orienteering, and flatwater kayaking.
XC skiing is a whole-body endurance exercise, and world-class XC skiers have shown some of the highest V˙O2max values (in both mL·min−1·kg−1 and L·min−1) ever reported in the literature (1,28,29). In comparison, studies on international-level runners or orienteers versus flatwater kayakers, where the lower and upper extremities, respectively, are mainly responsible for propulsion, indicate that these athletes have lower levels of V˙O2max compared to elite XC skiers (10,34,36,40). Differences in V˙O2max may be associated with differences in cardiac output, blood volume (BV), hemoglobin mass (Hbmass), role of flow-mediated endothelial function in the distribution of blood to working muscle, and effectiveness of oxygen uptake and mitochondrial respiration in muscle (2,11). However, cross-sectional comparisons of sport-specific physiological adaptations lack matching for performance levels. Additionally, comparisons of possible underlying mechanisms for chronic adaptations to lower-body, upper-body, and whole-body exercises have not yet been examined.
Norwegian elite athletes perform at high international levels in XC skiing, orienteering, and flatwater kayaking. Thus, they are unique research models for investigating the effects of exercise, with varying contributions of lower body, upper body, and whole body in humans. Assuming that the genetic component does not differ substantially in its contribution to successful elite athletes of different sports, physiological adaptations of V˙O2max, BV, Hbmass, and brachial endothelial function (FMD) may provide further insight into the upper limits of humans’ ability to adapt to such training. Additionally, body composition, hematological variables, and distribution of training volume and intensity may provide insight into potential mechanisms related to physiological diversities among athletes.
Therefore, the primary aim of this study was to compare V˙O2max, BV, Hbmass, and FMD in international-level endurance athletes primarily using whole-body (XC skiing), lower-body (orienteering), and upper-body (flatwater kayak) exercises in their training. It was hypothesized that XC skiers would have higher V˙O2max values than orienteers and flatwater kayakers, and that the higher V˙O2max values corresponded with higher BV and Hbmass.
Forty-three male elite endurance athletes competing at the national or international level in XC skiing (n = 17), orienteering (n = 15), or flatwater kayaking (n = 11) volunteered to participate in the study. In all three sports, the included athletes ranged from having podium places in world cup events to being among the top 15 in the national championship; an expert panel in the Norwegian Olympic Committee judged the three groups to have comparable international performance levels. Exclusion criteria for participation were significant blood loss (≥500 mL) within the last 3 months, anemia (≤13 g·dL−1), 10 d or more of altitude training during the past 3 months, kidney failure (reduced production of erythropoietin), cardiovascular or pulmonary disease, or medication limiting maximal endurance performance.
All experimental procedures were approved by the Regional Committees for Medical and Health Research Ethics. All subjects received oral and written information about the study before they signed an informed consent form.
In this cross-sectional design, measurement of anthropometry, brachial endothelial function, Hbmass, and BV was performed, and blood samples were collected. The tests were performed in the morning after a 12-h fasting period; the subjects drank 500 mL of water 2 h before this investigation. The international-level XC skiers, orienteers, and flatwater kayakers who took part in the study were tested directly following the end of the primary competition phase of each sport. Additionally, V˙O2max was measured on a separate day within 14 d of the original tests, and we ensured that all of the athletes showed comparable fitness on both testing days. High-intensity exercise; training sessions lasting more than 1.5 h; intake of caffeine, alcohol, vitamins, and minerals; and use of tobacco were avoided in the 24-h period before testing on both days. Additionally, type, volume, and intensity of training over the 6-month period before testing were analyzed based on training diaries.
Maximal oxygen uptake
After an initial low-intensity warm-up, V˙O2max was measured in an incremental test until exhaustion, using a direct ergospirometry system with a mixing chamber (Oxycon Pro; Erich Jaeger GmbH, Hoechberg, Germany). The ergospirometry system was calibrated using high-precision gas (16.00% ± 0.04% O2 and 5.00% ± 0.1% CO2; Riessner-Gase GmbH & Co., Lichtenfels, Germany), and the inspiratory flowmeter was calibrated using a 3-L-volume syringe (Hans Rudolph Inc., Kansas City, MO, USA). The test was performed on a treadmill (Woodway USA Inc., Waukesha, WI, USA) at a fixed inclination of 10.5% and with an increase in speed of 1 km·h−1 every minute until exhaustion. A plateau in oxygen uptake (despite increased workload) and a respiratory exchange ratio ≥1.05 were used as criteria for determining V˙O2max (14). The mean of the three highest consecutive 10 s was used to determine V˙O2max. HRpeak was measured by an HR monitor (Polar RS400; Polar Electro Oy, Kempele, Finland), and HRpeak was defined as the highest recorded value during the test. Peak oxygen pulse (mL per beat) was calculated as V˙O2max (mL·min−1) divided by HRpeak (39).
Carbon monoxide (CO) rebreathing spirometry (Blood Tec, GbR, Bayreuth, Germany), as described by Prommer and Schmidt (24) and Schmidt and Prommer (31), was used to determine BV, Hbmass, red cell volume (RCV), and plasma volume (PV). Subjects inhaled a bolus of CO gas (99.9%) at doses of 1 mL·kg−1 body mass and rebreathed the gas mixture for 2 min. Capillary blood samples were collected after 15 min of seated rest before the test and again at 6 and 8 min after rebreathing of CO. In addition, a venous ethylenediaminetetraacetic acid (EDTA) sample was collected for hematocrit analysis. Capillary blood samples were analyzed for HbCO% with the ABL800 FLEX analyzer (Radiometer Medical ApS, Brønshøj, Denmark) at 0, 6, and 8 min of the test. In addition, end-tidal CO concentration was measured with a CO gas tester (Draeger®, Luebeck, Germany) before and 4 min after CO inhalation. Gas volume and CO concentration were measured with the spirometer after the procedure. BV, Hbmass, RCV, and PV were calculated with Spico Calculation software version 2.0 (Blood tec GbR, Bayreuth, Germany). The method is regarded to be valid and reliable for estimating Hbmass, with a typical error of 1.7% and a coefficient of variation of ∼2.2% (7,31).
Endothelial function was measured as flow-mediated dilatation (FMD) of the brachial artery, using high-resolution vascular ultrasound (a 14-MHz Doppler probe; Vivid 7 and Vivid I; GE Vingmed Ultrasound AS, Horten, Norway) according to current guidelines (5,25). The measurements were conducted in a quiet and temperature-stable room (22°C–24°C) after 10 min of rest in supine position. Baseline images had been taken before we made an occlusion on the forearm’s distal part by inflating a pneumatic cuff (SC10; D.E. Hokanson Inc., Bellevue, WA, USA) to 250 mm Hg for 5 min. The cuff was then deflated to create a high-flow state and increased shear stress in the artery. A longitudinal image of brachial artery internal diameter was recorded continuously for 3 min following cuff release. An integrated ECG was used to secure assessment of the diameter according to the cardiac cycle. Diameter was calculated as an average of three measures synchronized with the R-wave peak and was measured from intima to intima. Mean flow velocity was measured at baseline and 15 s after cuff release (9,35). FMD was calculated as a percentage increase in arterial diameter at 30, 60, and 90 s after deflation from baseline vessel size. Peak shear rate was calculated as the difference between mean flow velocity at baseline and mean flow velocity 15 s after cuff release, divided by baseline diameter. FMD was normalized by dividing FMD by peak shear rate, multiplied by 1000 (35). Ultrasound images were analyzed using EchoPACtm (GE Vingmed Ultrasound AS).
Blood samples were collected by venous puncture in fasting state (≥12 h), and EDTA and serum Vacutainers were collected. Hematocrit, hemoglobin, mean cell volume, mean cell hemoglobin (MCH), MCH concentration (MCHC), leucocytes, erythrocytes, and thrombocytes were analyzed in plasma by the Sysmex XE-2100 analyzer (Sysmex Co., Kobe, Japan). Furthermore, EDTA and serum were frozen (−80°C) for later biochemical analysis. Albumin, iron, total iron binding capacity, and transferrin were analyzed in serum by the Roche Modular P analyzer (Roche Diagnostics, Basel, Switzerland). Ferritin, folate, and cobalamin were analyzed by the Roche Modular E analyzer (Roche Diagnostics). Erythropoietin was analyzed by the Siemens DPC Immulite 2000 (DPC-Siemens, La Garenne-Colombes, France). All analyses were conducted according to standard procedures at the Department of Medical Biochemistry, St. Olavs University Hospital (Trondheim, Norway). Oxidized LDL was measured in plasma with Mercodia oxidized LDL ELISA (Mercodia, Uppsala, Sweden). Plasma nitrite and nitrate were analyzed in plasma/serum according to the procedures previously described in detail by Peacock et al. (23).
Bioelectrical impedance analysis (InBody 720; Biospace Co. Ltd., Seoul, Korea) was used to assess body mass and body composition.
Distribution of training intensity
Training history over the 6-month period before measurements was recorded based on athletes’ training diaries and classified into intensity zones according to the session goal method, as previously employed by Sandbakk et al. (30). Endurance training was registered by an HR monitor and categorized into three intensity zones using the Norwegian Olympic System’s intensity scale: low-intensity endurance training (LIT; 60%–81% of HRmax), moderate-intensity endurance training (MIT; 82%–87% of HRmax), and high-intensity endurance training (HIT) (>88% of HRmax). Speed/resilience and strength training were also recorded in time from the first session to the last session and includes recovery periods. Speed/resilience training was sport-specific, with sessions shorter than 30 s and long recovery periods in all sports. Because laboratory measurements were performed right after each discipline’s competition phase, training hours were recorded during a period with relatively low training volume during the competitive season.
With a mean difference in total BV of 0.7 L and an SD of 0.4 L between expected values in the disciplines of XC skiing versus orienteering and kayaking (10), it was estimated that 12 subjects were needed for each group (80% power and α level set to 5%).
Variables are presented as mean with 95% confidence interval (CI). Histograms with normality curves, error bars, and Q–Q plots were used to explore assumptions of normality. Homogeneity of variances was checked with Levene’s test. One-way ANOVA was used for comparisons between the three groups. Welch and Brown–Forsythe robust tests of equality of means were used if the assumption of the homogeneity of variance was violated. Post hoc comparisons were made using Tukey HSD. Independent-samples t-test was used to look at differences in training volumes between subjects age ≤20 yr and subjects age ≥21 yr. Level of significance was set to α < 0.05. All statistical analyses were performed using IBM SPSS Statistics software program version 20 (SPSS Inc., Chicago, IL, USA). GraphPad Prism version 6 (GraphPad Software Inc., San Diego, CA, USA) was used for graphic illustrations.
Anthropometric characteristics and body composition (Table 1) demonstrate ∼8% higher body mass index and fat-free mass (FFM) in kayakers compared to orienteers (P < 0.05). Upper-body FFM was ∼17% and ∼11% higher in kayakers compared to orienteers and skiers, respectively (both P < 0.01), with the corresponding ratio of upper-body to total-body FFM being ∼8% and ∼4% higher, respectively (both P < 0.01; Table 1). In the latter case, XC skiers showed a ratio ∼4% higher than that of orienteers (P < 0.01). The ratio of leg to total-body FFM was ∼9% and ∼6% higher in orienteers compared to kayakers and skiers, respectively (both P < 0.01; Table 1).
Maximal oxygen uptake
XC skiers demonstrated 9.9% higher V˙O2max (mL·min−1·kg−1) compared to orienteers (P < 0.01), with the corresponding V˙O2max (mL·min−1·kg FFM−1) being 12.5% higher than that in kayakers (P < 0.05; all values are presented in Fig. 1). V˙O2max (L·min−1) was 11.3% and 10.3% lower in orienteers compared to XC skiers and kayakers, respectively (both P < 0.01; Fig. 1). Peak oxygen pulse (mL per beat) was 11.9% higher in XC skiers than in orienteers (P < 0.05; Table 1).
BV and total Hbmass
As shown in the overview of BV variables in Table 2, there were no differences in total BV between the three groups, but XC skiers and orienteers had 10.4% and 9.9% higher BV than kayakers when normalized for total body mass (both P < 0.05). When normalized for FFM, orienteers and XC skiers demonstrated 11.8% and 10.6% higher BV, respectively, than kayakers (both P < 0.01).
XC skiers had 9.2% higher Hbmass relative to body mass compared to kayakers, whereas the corresponding values normalized for FFM were 10.6% and 9.9% higher for XC skiers compared to orienteers and kayakers, respectively (all P < 0.05; Table 2).
Hematological variables are presented in Table 3. There were no differences in hematological values between groups, except for MCHC, where XC skiers had 2.3% and 3.0% higher values than orienteers and kayakers, respectively (both P < 0.01).
Brachial artery diameter and flow are presented in Table 4. Kayakers had 15.0% greater arterial diameter than orienteers at baseline, 13.5% greater arterial diameter after 30 s, and ∼12% greater arterial diameter at 60 and 90 s (all P < 0.05). XC skiers had 11.8%, 13.5%, 10.8%, and 12.0% greater arterial diameter than orienteers at baseline, 30 s, 60 s, and 90 s, respectively (all P < 0.05). There were no significant differences in FMD, even after normalizing FMD for shear rate.
Distribution of training intensity
The training distribution shown in Table 5 demonstrates that kayakers performed 29% and 73% more total training volume than XC skiers and orienteers, respectively, whereas skiers demonstrated a 34% higher training volume than orienteers (all P < 0.01). Correspondingly, XC skiers and kayakers had 42% and 39% more LIT than orienteers, whereas kayakers had almost twice as much MIT and HIT, threefold higher speed/resilience, and threefold more strength training compared to the two other groups (all P < 0.01).
Due to significant group differences in mean age, an analysis of total training hours was also conducted, separating the total sample size into two different age groups (≤20 and ≥21 yr). This showed no significant difference between the two groups.
The purpose of this study was to compare sport-specific physiological adaptations in highly trained endurance athletes primarily using whole-body, lower-body, or upper-body exercise. Here, we showed higher V˙O2max values (mL·min−1·kg−1 and mL·min−1·kg FFM−1) in XC skiers and greater arterial diameters in the arms of skiers and kayakers as sport-specific physiological adaptations to chronic endurance training in these exercise modes. This is further supported by the sport-specific diversity in body composition between groups, where kayakers and orienteers differed in the relative distribution of upper-body and lower-body FFM. However, variations in none of these variables were related to differences in BV or Hbmass.
The mean V˙O2max values for XC skiers were ∼78 mL·min−1·kg−1 and 5.8 L·min−1, in line with previous studies of international-level male XC skiers (12,15,28,30). As hypothesized, skiers showed significantly higher values than orienteers in absolute values and when normalized for total body mass or FFM. This corresponds well with the findings of Saltin and Astrand (28) in the mid-1960s and might be a specific physiological adaptation to whole-body exercise employed in XC skiing compared to lower-limb exercise in orienteering. In contrast, V˙O2max did not significantly differ between XC skiers and kayakers. The high absolute V˙O2max values of kayakers were expected due to their high body mass, but their V˙O2max values, especially normalized for body mass, were surprisingly high compared to those reported in previous literature (28,36). To further understand these findings, we analyzed how blood variables, vascular function, body composition, and training differed between the three sports.
In correspondence with the high V˙O2max values found in XC skiers, we expected higher BV among skiers than in the other groups because V˙O2max has previously been positively associated with BV (4,10,17,32). However, our data showed no difference in BV between XC skiers and orienteers, with both groups having significantly higher body-mass-normalized values than kayakers. The same general pattern of lower values for kayakers was also seen for Hbmass, RCV, and PV normalized for total body mass and FFM, whereas XC skiers and orienteers did not differ in any case. Overall, our data show BV and Hbmass values in XC skiers and orienteers that are 20%–35% and 20%–30% higher than those in untrained individuals and are in line with previous findings in elite endurance athletes (10,32). The similar BV variables in orienteers and XC skiers, but the higher V˙O2max values in the latter, imply that other explanatory mechanisms are associated with higher V˙O2max values (e.g., enhanced cardiac chamber compliance) (19,20). This is likely because oxygen pulse, which is considered to be an indicator of stroke volume (39), was significantly higher in XC skiers than in orienteers. Still, the significant difference in BV and Hbmass between XC skiers and kayakers indicates that physiological adaptations concerning blood constituents may differ between sports consisting of whole-body or upper-body exercise. Whether the more significant anaerobic contribution to kayak performance (3,21,36) (compared to XC skiing) or other factors related to upper-body exercise are responsible for these diversities requires further elucidation.
XC skiers and kayakers were found to have significantly larger brachial artery diameters compared to orienteers. This difference in brachial artery diameter found between different sport disciplines is the direct effect of sport-specific adaptation. It has previously been shown that arterial diameter is induced through training (8,27). Thus, XC skiers and kayakers who train their upper limbs substantially more than orienteers display larger brachial artery diameters. Furthermore, our study showed enhanced FMD in XC skiers and orienteers when we compared them with values found in the general population (33). However, kayakers—who had the largest arterial diameters among our examined groups—had FMD lower than normal values. This finding is in accordance with previous studies suggesting an inverse correlation between arterial diameter and FMD in athletes (8,27). Why this same phenomenon was not present in XC skiers is unclear because this group also presented enlarged arterial diameters compared to orienteers. A possible explanation might be that kayakers’ brachial arteries are even more exposed to shear stress through their more predominant use of upper-body exercise than XC skiers, who use their whole bodies. The expected associations between FMD and nitrite content as indicators of nitric oxide availability (18,37,38) were not found here, indicating that differences in vascular resistance and blood flow were caused by other factors.
With the current design, we assumed that the physiological diversities between sports were mainly associated with differences in chronic exposure to training. Because XC skiers, orienteers, and flatwater kayakers were matched at comparable international performance levels, we suggest that the participating subjects’ training were optimized for specific demands in their sports. This means that the higher V˙O2max values in XC skiers, the differences in BV variables, the larger arterial diameters in skiers and kayakers, and the differences in body composition between sports are likely related to chronic exposure to the main exercise modes used in their training and the corresponding intensity distribution. Although the muscle mass employed in running should be sufficient to stimulate cardiac output maximally (13), the higher V˙O2max values in XC skiers may be an effect of higher training volume and greater amount at high intensity. Although kayakers’ lower BV may be an effect of the lower muscle mass employed in upper-body exercise, their training must have stimulated other factors that enhance V˙O2max effectively. Kayakers had significantly more total training hours than the two other groups during this time of the season, mainly due to higher amounts of MIT, HIT, and strength training. Still, the annual training volume of XC skiers may be similar to that of kayakers because skiers compete more frequently and perform much of their training volume in the 6 months following the period in which this study was performed. Still, the high training volumes in kayakers compared to orienteers may be explained by the fact that kayaking is a non-weight-bearing activity, which is different from running. The lower muscle mass activated during upper-body exercise may also allow for faster recovery and, thereby, higher training volumes. This interaction between training mode and optimal intensity distribution is of high interest for future examination. In this context, one interesting aspect that may have influenced the higher training volumes of kayakers is that the upper body does not profit from lifestyle activities to the same extent as the lower body (i.e., we constantly use the legs to move). Hence, kayakers might require more training hours and tolerate them better compared to athletes participating in weight-bearing activities. However, note that the training distribution for all three sports is taken from the competition phase, which is characterized by high amounts of movement-specific training but relatively low training volumes (i.e., one third of the total annual training volume).
The current study demonstrates specific physiological adaptations to chronic endurance training in sports with a predominance of upper-body, lower-body, and/or whole-body exercise modes among performance-matched international-level athletes. A higher V˙O2max value was found in XC skiers compared to orienteers, which may be linked to the greater muscle mass and greater training volumes employed in this sport. The V˙O2max values in kayakers did not differ from those in the other groups, although kayakers had lower BV and Hbmass than XC skiers. The latter may be linked to lower exercising muscle mass, whereas kayakers compensate with other factors that are possibly associated with their high total training volume and high-intensity training volume. The significantly greater arterial diameters in kayakers and XC skiers may reflect their adaptations to chronic upper-body endurance exercise, which is further supported by the sport-specific diversity in the relative distribution of upper-body and lower-body mass between these groups of athletes. Exploring other possible mechanisms related to higher V˙O2max (e.g., heart size, cardiac chamber compliance, and peripheral muscle factors) in athletes specially trained for upper-body, lower-body, or whole-body exercise may provide further insight into the upper limits of humans’ ability to adapt to such training.
The authors would like to thank the athletes and their coaches for their cooperation and participation in this study; Professor Jostein Hallén and Hege Wilson Landgraff, Ph.D. (Department of Physical Performance, Norwegian School of Sport Science, Oslo, Norway), for assistance during data collection in Oslo; and Ingerid Arbo and Atefe Tari (K.G. Jebsen Center for Exercise in Medicine, Department of Circulation and Medical Imaging, Norwegian University of Science and Technology, Trondheim, Norway) for help in the laboratory.
This study was financially supported by the Department of Circulation and Medical Imaging, Norwegian University of Science and Technology.
The authors declare no conflicts of interest.
All authors assisted in the writing of the manuscript and were involved in study design and/or data acquisition, analysis, and interpretation.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
1. Åstrand P-O, Åstrand P-O. Textbook of Work Physiology: Physiological Bases of Exercise
. 4th ed. Champaign (IL): Human Kinetics; 2003. p. v, 649 p.
2. Bassett DR Jr, Howley ET. Limiting factors for maximum oxygen uptake and determinants of endurance performance. Med Sci Sports Exerc
. 2000; 32(1): 70–84.
3. Bishop D. Physiological predictors of flat-water kayak performance in women. Eur J Appl Physiol
. 2000; 82(1–2): 91–7.
4. Convertino VA. Blood volume
: its adaptation to endurance training. Med Sci Sports Exerc
. 1991; 23(12): 1338–48.
5. Corretti MC, Anderson TJ, Benjamin EJ, et al. Guidelines for the ultrasound assessment of endothelial-dependent flow-mediated vasodilation of the brachial artery: a report of the International Brachial Artery Reactivity Task Force. J Am Coll Cardiol
. 2002; 39(2): 257–65.
6. Costill DL. Metabolic responses during distance running. J Appl Physiol
. 1970; 28(3): 251–5.
7. Gore CJ, Hopkins WG, Burge CM. Errors of measurement for blood volume
parameters: a meta-analysis. J Appl Physiol
. 2005; 99(5): 1745–58.
8. Green DJ, Rowley N, Spence A, et al. Why isn’t flow-mediated dilation enhanced in athletes? Med Sci Sports Exerc
. 2013; 45(1): 75–82.
9. Harris RA, Nishiyama SK, Wray DW, Richardson RS. Ultrasound assessment of flow-mediated dilation. Hypertension
. 2010; 55(5): 1075–85.
10. Heinicke K, Wolfarth B, Winchenbach P, et al. Blood volume
and hemoglobin mass
in elite athletes of different disciplines. Int J Sports Med
. 2001; 22(7): 504–12.
11. Holloszy JO, Coyle EF. Adaptations of skeletal muscle to endurance exercise and their metabolic consequences. J Appl Physiol Respir Environ Exerc Physiol
. 1984; 56(4): 831–8.
12. Holmberg HC, Rosdahl H, Svedenhag J. Lung function, arterial saturation and oxygen uptake in elite cross country skiers: influence of exercise mode. Scand J Med Sci Sports
. 2007; 17(4): 437–44.
13. Holmer I, Stein EM, Saltin B, Ekblom B, Astrand PO. Hemodynamic and respiratory responses compared in swimming and running. J Appl Physiol
. 1974; 37(1): 49–54.
14. Howley ET, Bassett DR Jr, Welch HG. Criteria for maximal oxygen uptake
: review and commentary. Med Sci Sports Exerc
. 1995; 27(9): 1292–301.
15. Ingjer F. Development of maximal oxygen uptake
in young elite male cross-country skiers: a longitudinal study. J Sports Sci
. 1992; 10(1): 49–63.
16. Joyner MJ, Coyle EF. Endurance exercise performance: the physiology of champions. J Physiol
. 2008; 586(1): 35–44.
17. Kanstrup IL, Ekblom B. Blood volume
and hemoglobin concentration as determinants of maximal aerobic power. Med Sci Sports Exerc
. 1984; 16(3): 256–62.
18. Kelm M. Nitric oxide metabolism and breakdown. Biochim Biophys Acta
. 1999; 1411(2–3): 273–89.
19. Levine BD. V˙O2max
: what do we know, and what do we still need to know? J Physiol
. 2008; 586(1): 25–34.
20. Levine BD, Lane LD, Buckey JC, Friedman DB, Blomqvist CG. Left ventricular pressure–volume and Frank–Starling relations in endurance athletes. Implications for orthostatic tolerance and exercise performance. Circulation
. 1991; 84(3): 1016–23.
21. Michael JS, Rooney KB, Smith R. The metabolic demand of kayaking: a review. J Sports Sci Med
. 2008; 7 (1): 1–7.
22. Neal CM, Hunter AM, Galloway SD. A 6-month analysis of training-intensity distribution and physiological adaptation in Ironman triathletes. J Sports Sci
. 2011; 29(14): 1515–23.
23. Peacock O, Tjonna AE, James P, et al. Dietary nitrate does not enhance running performance in elite cross-country skiers. Med Sci Sports Exerc
. 2012; 44(11): 2213–9.
24. Prommer N, Schmidt W. Loss of CO from the intravascular bed and its impact on the optimised CO-rebreathing method. Eur J Appl Physiol
. 2007; 100(4): 383–91.
25. Pyke KE, Tschakovsky ME. The relationship between shear stress and flow-mediated dilatation
: implications for the assessment of endothelial function. J Physiol
. 2005; 568(Pt 2): 357–69.
26. Ramsbottom R, Nute MG, Williams C. Determinants of five kilometre running performance in active men and women. Br J Sports Med
. 1987; 21(2): 9–13.
27. Rowley NJ, Dawson EA, Hopman MT, et al. Conduit diameter and wall remodeling in elite athletes and spinal cord injury. Med Sci Sports Exerc
. 2012; 44(5): 844–9.
28. Saltin B, Astrand PO. Maximal oxygen uptake
in athletes. J Appl Physiol
. 1967; 23(3): 353–8.
29. Sandbakk O, Holmberg HC. A reappraisal of success factors for Olympic cross-country skiing. Int J Sports Physiol Perform
. 2013; 9(1): 117–21.
30. Sandbakk O, Holmberg HC, Leirdal S, Ettema G. The physiology of world-class sprint skiers. Scand J Med Sci Sports
. 2011; 21(6): e9–16.
31. Schmidt W, Prommer N. The optimised CO-rebreathing method: a new tool to determine total haemoglobin mass routinely. Eur J Appl Physiol
. 2005; 95(5–6): 486–95.
32. Schmidt W, Prommer N. Effects of various training modalities on blood volume
. Scand J Med Sci Sports
. 2008; 18(1 Suppl): 57–69.
33. Skaug EA, Aspenes ST, Oldervoll L, et al. Age and gender differences of endothelial function in 4739 healthy adults: the HUNT3 Fitness Study. Eur J Prev Cardiol
. 2012; 20(4): 531–40.
34. Smekal G, Von Duvillard SP, Pokan R, et al. Respiratory gas exchange and lactate measures during competitive orienteering. Med Sci Sports Exerc
. 2003; 35(4): 682–9.
35. Tarro Genta F, Eleuteri E, Temporelli PL, et al. Flow-mediated dilation normalization predicts outcome in chronic heart failure patients. J Card Fail
. 2013; 19(4): 260–7.
36. Tesch PA. Physiological characteristics of elite kayak paddlers. Can J Appl Sport Sci
. 1983; 8(2): 87–91.
37. Totzeck M, Hendgen-Cotta UB, Rammos C, et al. Higher endogenous nitrite levels are associated with superior exercise capacity in highly trained athletes. Nitric Oxide
. 2012; 27(2): 75–81.
38. Tsikas D. Methods of quantitative analysis of the nitric oxide metabolites nitrite and nitrate in human biological fluids. Free Radic Res
. 2005; 39(8): 797–815.
39. Wasserman K. Principles of Exercise Testing and Interpretation: Including Pathophysiology and Clinical Applications
. 5th ed. Philadelphia (PA): Wolters Kluwer Health/Lippincott Williams & Wilkins; 2012. p. xiii, 572 p.
40. Wehrlin JP, Zuest P, Hallen J, Marti B. Live high–train low for 24 days increases hemoglobin mass
and red cell volume in elite endurance athletes. J Appl Physiol
. 2006; 100(6): 1938–45.