© 2015 John Wiley & Sons A/S.
Scand J Med Sci Sports 2015: ••: ••–•• doi: 10.1111/sms.12425
Published by John Wiley & Sons Ltd
Cardiorespiratory fitness in groups with different physical activity levels S. M. Dyrstad1, S. A. Anderssen2, E. Edvardsen2,3, B. H. Hansen2 Department of Education and Sport Science, University of Stavanger, Stavanger, Norway, 2Department of Sports Medicine, Norwegian School of Sport Sciences, Oslo, Norway, 3Department of Pulmonary Medicine, Oslo University Hospital, Oslo, Norway Corresponding author: Sindre M. Dyrstad, Dr. Scient, Department of Education and Sport Sciences, University of Stavanger, 4036 Stavanger, Norway. Tel: (+47) 51 83 34 43, Fax: (+47) 51 83 34 50, E-mail: [email protected]
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Accepted for publication 9 January 2015
The aim of the study was to determine how different categorizations of self-reported and objectively measured physical activity (PA) reflect variations in cardiorespiratory fitness (VO2max). A total of 759 individuals (366 women) with a mean age of 48.5 years (SD 14.4) wore an accelerometer (ActiGraph GT1M) for seven consecutive days and answered the short International Physical Activity Questionnaire (IPAQ). VO2max was directly measured during a continuous graded exercise treadmill test until exhaustion. Men and women categorized as highly active by IPAQ had 9% and 13% higher VO2max, respectively, than those reporting a low PA level (P < 0.05). Men
and women meeting the PA recommendation of 150 min/ week of daily moderate intensity PA, measured by accelerometer, had 13% and 9% higher VO2max, respectively, than participants not meeting this recommendation (P < 0.01). No significant differences in average sedentary time, analyzed in total min/day and in bouts of 10 and 30 min, were found between participants with high or low cardiorespiratory fitness. However, women spent less time than men in bouts of sedentary behaviors. Self-reported PA by IPAQ and objectively measured PA by accelerometer were both useful instruments for detecting differences in VO2max.
Both high levels of physical activity (PA) and high cardiorespiratory fitness are inversely associated with diseases and all-cause mortality (Blair et al., 2001; Lee & Skerrett, 2001; Kodama et al., 2009). However, cardiorespiratory fitness is more strongly correlated with coronary heart disease than PA (Talbot et al., 2002). This might be attributable to the fact that cardiorespiratory fitness can be measured more precisely and that high cardiorespiratory fitness has a genetic component that also contributes to good health (Bray et al., 2009). While measures of cardiorespiratory fitness are best obtained by direct measurement of maximal oxygen uptake (VO2max), assessment of PA is mainly performed using subjective methods such as questionnaires, or objectively by activity monitors like accelerometers. The International Physical Activity Questionnaire (IPAQ) is a tool suitable for assessing population levels of PA across countries. IPAQ is thoroughly validated (Lee et al., 2011), and a wide range of studies have used IPAQ as an instrument for measuring PA level in different populations (Bauman et al., 2009). Because of the development of cheaper and more accurate objective assessment instruments of PA, accelerometer has become a commonly used device to assess PA at population level. However, questionnaires are still a frequently used tool to assess PA in large samples because of their relatively low cost and ease of use.
Fogelholm et al. (2006) studied the validity of IPAQ for measuring fitness in soldiers and found that the most active group, according to IPAQ, did not have the highest estimated VO2max. Ottevaere et al. (2011) compared subjective and objective PA data in European adolescents with their fitness level. They concluded that both the subjective and objective instruments were able to detect adolescents with the highest cardiorespiratory fitness, which were the most active adolescents. However, none of these studies used direct VO2max tests, and only adolescents and men were studied. Because a direct measurement of VO2max requires skilled technicians, expensive equipment, motivated subjects, and are time-consuming, measuring VO2max is impractical in larger cohorts. It would therefore be valuable to study if a variation in both self-reported PA and objectively measured PA reflects variation in VO2max, and to evaluate the agreement between VO2max and PA. The main purpose of the present study was therefore to examine how different levels of self-reported and objectively measured PA, including time spent being sedentary, reflect variation in VO2max. Methods This multicenter study involved nine regional test centers throughout Norway. A representative sample of 11 515 adults (aged 20–84
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Dyrstad et al. years) from the areas surrounding each test center was drawn from the Norwegian population registry. Written informed consent was obtained from 3867 participants (34%). These participants received a pre-programmed accelerometer and a questionnaire by mail, and wore the accelerometer for seven consecutive days. From the sample that completed the accelerometer measurements, 1930 were randomly selected and invited to undergo a cardiopulmonary exercise test. A total of 1030 participants accepted the invitation, 904 persons met at the lab, and 759 participants (366 women) with mean age of 48.5 years (SD 14.4) completed the cardiopulmonary exercise test 5–8 months after the questionnaire and accelerometer measurements. The numbers of participants vary in tables from 480 to 729 according to valid data. Most of the dropouts are caused by missing variables from the IPAQ. The design of the study has been described in detail elsewhere (Hansen et al., 2012), and the participants’ cardiorespiratory response during maximal exercise is previously reported (Edvardsen et al., 2013). The study was approved by the Regional Ethics Committee for Medical Research and the Norwegian Social Science Data Services AS.
Objective PA measurement The ActiGraph GT1M (ActiGraph, LLC, Pensacola, Florida, USA) was used to assess participants’ PA levels. Participants with a minimum of four days of at least 10 h of daily recordings were included in the analysis. Data were collected in 10-s epochs, which were collapsed into 60-s epochs for comparison with other studies. The data were reduced using an SAS-based macro (SAS Institute Inc., Cary, North Carolina, USA). Wear time was defined by subtracting nonwear time from 18 h since all data between 00:00 and 06:00 h were excluded to avoid potential bias because of participants forgetting to remove the monitor when going to bed at night. Nonwear time was defined as intervals of at least 60 consecutive minutes with zero counts, with allowance for 1 minute with counts greater than zero (Troiano et al., 2008). For analysis of sedentary behavior, ActiLife 6.11.4 software (ActiGraph, LLC) was used to analyze the accelerometer data. The first sedentary break of each day was ignored to avoid a misclassification of nonwear time during evening as a sedentary break. PA guidelines for Norwegians and Americans recommend 150 min/week of moderate intensity PA or 75 min/week of vigorous intensity aerobic PA, or an equivalent combination of moderate and vigorous intensity PA (U.S. Department of Health and Human Services, 2008; The Norwegian Directorate of Health, 2014). Participants accumulating an average of minimum of 21.4 min of daily moderate to vigorous PA (MVPA), corresponding to 150 min/week, in bouts of 10 min or more (with allowance for interruptions of 1 min) were categorized as being sufficiently active. This definition allowed participants to have longer bouts of activity on certain days and to be less active on other days, and still be categorized as sufficiently active. Since none of the participants met the PA guidelines by 75 min/week of vigorous PA alone, and few participated in vigorous PA, the limit of 150 min of MVPA was used. Average counts per minute (cpm) was expressed as the total number of registered counts (> 100 cpm) for all valid days divided by wearing time. To identify PA of different intensities, count thresholds corresponding to the energy cost of the given intensity were applied to the data set. Sedentary time was defined as all activity below 100 cpm, a threshold that corresponds with sitting, reclining or lying down (Healy et al., 2007; Matthews et al., 2008). Time in lifestyle activity was defined as counts between 760 and 2019 cpm, a threshold that corresponds with slow walking, grocery shopping, vacuuming, and child care (Hagstromer et al., 2010b). Moderate intensity was defined as cpm between 2020 and 5998 [3–6 metabolic equivalent tasks (METs)], and vigorous intensity PA (> 6 METs) was defined as 5999 cpm or more
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(Troiano et al., 2008). Mean minutes/day at different intensities was calculated as the sum of all minutes where the count met the criterion for that intensity, divided by the number of valid days.
Self-reported assessment of PA Self-reported PA over the previous 7 days was obtained by the short, self-administered version of the IPAQ (translated to Norwegian). The sum of time spent walking and in MVPA was used to estimate the total amount of time spent in PA per day, and data cleaning and processing was carried out in accordance to the official IPAQ scoring protocol (IPAQ, 2005). Vigorous intensity PA was assumed to correspond to 8 METs, moderate intensity activity to 4 METs and walking to 3.3 METs. The participants were categorized into three PA levels according to the IPAQ scoring criteria. The “high” level indicates a PA level equivalent to 60 min of MVPA/day and meets any one of the following two criteria: (a) vigorous intensity activity on at least 3 days and accumulating at least 1500 MET min/week; (b) 7 days of any combination of walking, moderate or vigorous intensity activities accumulating at least 3000 MET min/week. The “moderate” PA level is equivalent to 30 min of MVPA on most days, and meets any of the following three criterion: (a) three or more days of vigorous intensity activity of at least 20 min/day; (b) five or more days of moderate intensity activity and/or walking of at least 30 min/day; (c) five or more days of any combination of walking, moderate intensity or vigorous intensity activities achieving a minimum of at least 600 MET min/week. The “low” PA level is indicated if PA meets neither moderate nor high criterion.
Cardiopulmonary exercise test Body weight was measured to the nearest 0.1 kg, with participants wearing light clothes and no shoes. VO2max was directly measured by using a continuous graded protocol of walking uphill on a treadmill until exhaustion (Edvardsen et al., 2013). Gas exchange and ventilatory variables were sampled as the subjects breathed into a Hans Rudolph two-way breathing mask (2700 series; Hans Rudolph Inc., Shawnee, Kansas, USA). During the last part of the cardiopulmonary exercise test, the subject’s effort was encouraged by the technician until voluntary termination. Rating of perceived exertion was obtained using the Borg Scale6–20 (Borg, 1974). Gas exchange variables were reported as 30-s averages. VO2max was accepted if respiratory exchange ratio ≥ 1.10 or the Borg 6–20 score was ≥ 17. Each day, the gas analyzers used were calibrated for volume and gas and corrected for barometric pressure, temperature, and humidity. A detailed description of measurement accuracy between gas analyzers is given elsewhere (Edvardsen et al., 2013).
Statistics A multivariate general linear model, adjusted for appropriate confounders such as age and accelerometer wear time, was used to assess differences in, e.g., VO2max in men and women performing different amounts of PA. To study the variation in VO2max among participants with different PA level, men and women were divided in quartiles based at both accelerometer-measured MVPA (≥ 2020 cpm) and self-reported MET level (walking and MVPA). To study the variation of sedentary behavior in participants with different VO2max, participants were divided in quartiles based on their VO2max level. Correlations between accelerometer-measured PA, self-reported PA, and VO2max were assessed by Spearman correlation coefficient, ρ (rho). To analyze the relationships between VO2max and the correlates of sex (women = 1, men = 2), age (years), body mass index (BMI), and PA, hierarchical regression with enter procedure was applied. Preliminary analyses of
Participants are categorized into low, moderate and high PA levels by using the International Physical Activity Questionnaire (IPAQ), and by meeting/not meeting the PA recommendations of at least 150 min/week with moderate PA, measured by accelerometer. Variables are adjusted for age and accelerometer wear time and are presented as mean values (SE). n for self-reported total activity is lower according to valid data, see methods. *Different from No (P < 0.001, within gender). † Different from the two other groups (P < 0.05, within gender). ‡ Definitions described in “Self-reported assessment of physical activity” in methods. § A minimum of 150 min/week with MVPA > 2019 cpm, measured in 10 min blocks. MET, metabolic equivalent task.
347.3 (6.6) 10299 (161)* 363 (26)* n = 100 343.6 (4.9) 7336 (140) 226 (24) n = 123 361.3 (8.3)† 9965 (287)† 572 (23)† n = 68 338.4 (5.9) 8739 (204) 225 (20) n = 86 339.8 (5.7) 7840 (197)† 85 (23)† n = 69 329.3 (4.3) 7338 (127) 375 (29) n = 174 324.3 (6.8) 8914 (239) 241 (32) n = 67
VO2max (mL/kg/min) Average counts/min Steps/day Self-reported total activity (MET min/day)
314.2 (5.3) 7473 (188)† 121 (27)† n = 94
346.4 (6.9)† 9089 (244) 750 (27)† n = 96
323.2 (7.0) 10475 (192)* 413 (42) n = 83
No n = 200 30.9 (0.4) High n = 68 35.2 (0.7)† Moderate n = 133 31.7 (0.5) Low n = 142 31.2 (0.5) Yes n = 112 43.3 (0.7)* No n = 256 38.2 (0.5)
Accelerometer Meeting PA recommendations§
High n = 96 41.7 (0.8) Moderate n = 100 41.4 (0.8) Low n = 161 38.1 (0.6)†
International Physical Activity Questionnaire PA level‡ International Physical Activity Questionnaire PA level‡
Men with a high self-reported PA level had 9% higher VO2max and 10% higher average cpm than men selfreporting a low PA level (Table 1). Women with a high self-reported PA level had a 13% higher VO2max and 6% more accelerometer-measured PA than women selfreporting a low PA level (Table 1). Men meeting the PA guidelines of 150 min/week of MVPA had a 13% higher VO2max and no significant difference in self-reported activity level in MET min/day than men not meeting the PA guidelines (Table 1). Women meeting the PA guidelines of 150 min/week of MVPA had a 9% higher VO2max and reported 61% higher activity level in MET min/day than women not meeting the PA guidelines (Table 1). Men with the 25% highest accelerometer-measured MVPA level (quartile 4), performed 70 (2) min/day of MVPA (≥ 2020 cpm). Their VO2max was 42.8 (0.8) mL/ kg/min, which was 21% higher than for men with the 25% lowest accelerometer-measured MVPA level (quartile 1), who performed 14 (0.5) min/day of MVPA. Self-reported PA for men in quartile 4 was 932 (24) MET min/day and they had a VO2max of 43.2 (0.9) mL/ kg/min, which was 16% higher than for men in quartile 1, who performed 51 (24) MET min/day (P < 0.05). Similar PA levels were found in women with similar differences in VO2max between groups. The correlation between VO2max and both self-reported and accelerometer-measured PA increased across intensity levels (Table 2). Accelerometer-measured vigorous intensity PA in women had the highest correlation to VO2max, while a correlation < 0.13 was found between VO2max and both total self-reported PA and average cpm in men and women. The biological factors of gender, age, and BMI included in the regression analyses displayed the largest amount of explanatory power, explaining 59% of the variance in VO2max (Table 3). Vigorous PA and moderate PA (in min/day) explained 2.8% and 1.0% of the variance in VO2max, respectively, and increased the total explained variance to 62.5%. Average counts/day (> 99 cpm) and total self-reported time in min/day in activity (walking + MVPA) did not have any significant influence in VO2max.
Table 1. Cardiorespiratory fitness (VO2max) and physical activity (PA) variables for men and women with different PA levels
Results
Variable
Women Men
Accelerometer Meeting PA recommendations§
normal distribution were conducted to ensure that there was no violation of the assumptions of linear regression. The analysis contained three models with the biological variables of sex, age, and BMI in the first, vigorous PA in the second, and moderate PA in the third. Since half of the participants did not perform any vigorous PA at all, participants were split in two. Group one did not perform any vigorous PA, group two performed vigorous PA. Data were adjusted for accelerometer wear time. Demographic data are presented as mean values ± standard error (SE) unless otherwise specified. A P-value of less than 0.05 was regarded as statistically significant. All statistical analyses were performed using PASW Statistics 21 for Windows (IBM Corporation, Somers, New York, USA).
Yes n = 152 33.8 (0.5)*
Physical activity and maximal oxygen uptake
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Dyrstad et al. Table 2. Correlation coefficients (Spearman) between VO2max and PA (self-reported and accelerometer-measured)
Self-reported PA variables
Women VO2max (mL/kg/min) Men VO2max (mL/kg/min)
Accelerometer-measured variables
Moderate (min/day)1a
Vigorous (min/day)2a
Total PA3a
Moderate to vigorous (min/day)1b
Vigorous (min/day)2b
Average counts/day3b
n = 290 0.14* n = 313 0.05
n = 315 0.30** n = 337 0.30**
n = 225 0.09 n = 261 0.08
n = 354 0.23** n = 375 0.29**
n = 354 0.54** n = 375 0.47**
n = 354 0.13* n = 375 0.08
*P < 0.05. **P < 0.01. 1a Self-reported time in moderate activity; 1b≥ 2020 counts/min. 2a Self-reported time in vigorous activity; 2b≥ 5999 counts/min. 3a Self-reported time in activity (walking + moderate + vigorous); 3baverage counts/day (≥ 100). PA, physical activity.
Table 3. Hierarchical regression analysis of variables of maximal oxygen uptake (mL/kg/min), n = 729
Variables
Model 1 B
SE
Model 2 t
Explained B variance (R2)
Constant 64.07 1.73** 37.04 Sex (female vs male) 8.56 0.46** 18.74 Age (year) −0.36 0.02** −23.80 BMI −0.93 0.06** −14.89 VPA, min/day in ≥ 5999 cpm (none vs any) MVPA, min/day in 2020–5999 cpm
58.45 8.22 −0.32 −0.85 3.64
SE
Model 3 t
Explained B variance (R2)
1.82** 32.03 0.44** 18.56 0.02** −20.21 0.06** −13.86 0.48** 7.67
56.97 8.12 −0.33 −0.82 3.01
SE
0.615**
Explained variance (R2)
1.83** 31.12 0.44** 18.53 0.02** −20.81 0.06** −13.59 0.49** 6.16
0.05 0.01** 0.587**
t
4.56 0.625**
**P < 0.001. B, unstandardized beta coefficient; SE, standard error of the mean; t, t-statistics; BMI, body mass index; MVPA, moderate physical activity; VPA, vigorous physical activity (participants performing none VPA vs participants performing VPA).
There were no significant differences in accelerometer-measured sedentary time, analyzed in total min/day and in bouts of 10 and 30 min, across quartiles of VO2max (Table 4). However, in general, women spent 330 (2) and 135 (2) min/day in sedentary behavior when analyzed in 10 and 30 min bouts, respectively. This was 12% and 18% less than for men (P < 0.001; data adjusted for age and wear time). Men and women with the 25% highest VO2max had significant more time in vigorous intensity PA than the other groups (Table 4). Discussion This study examined associations between cardiorespiratory fitness and PA level measured by IPAQ and accelerometer in a large national sample. The main findings were that men and women meeting the PA recommendations, had 5–13% higher VO2max than those not meeting the PA recommendations. Large variations in
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PA reflected relatively small variations in VO2max. The correlations between average cpm/total self-reported PA and VO2max were low, but the correlation between accelerometer-measured vigorous PA and VO2max were around 0.50 for men and women. No differences in sedentary time were found between participants with high and low VO2max.
PA recommendations and cardiorespiratory fitness How the different methods of assessing PA reflect variations in VO2max is useful knowledge since cardiorespiratory fitness is strongly correlated with health. It has been found that for every increase in cardiorespiratory fitness by approximately 3.5 mL/kg/min, the risk of death was reduced by 17% in women (Gulati et al., 2003) and 12% in men (Myers et al., 2002). In the present study, the differences in VO2max between those reporting a low or high PA level were above 3.5 mL/kg/min. However, this difference is far less than earlier reported. Mynarski
Physical activity and maximal oxygen uptake Table 4. Physical activity (PA) measured by accelerometer, for quartiles of maximal oxygen uptake (VO2max) in men and women
VO2max (mL/kg/min) Sedentary activity, < 100 cpm (min/day) Sedentary activity, < 100 cpm (min/day), analyzed in 30 min bouts Lifestyle PA, ≥ 760 cpm (min/day)* Moderate PA, ≥ 2020 cpm (min/day)* Vigorous PA, ≥ 5999 cpm (min/day)* Steps/day
Men
Women
Quartiles groups of VO2max
Quartiles groups of VO2max
1 n = 87
2 n = 81
3 n = 86
4 n = 84
1 n = 77
2 n = 81
3 n = 81
4 n = 80
28.9 (0.4) 564 (8)
35.6 (0.4) 553 (8)
42.3 (0.4) 562 (7)
52.4 (0.4) 561 (9)
23.1 (0.4) 545 (9)
29.2 (0.4) 540 (8)
34.4 (0.4) 532 (8)
41.9 (0.4) 534 (9)
164 (8)
145 (8)
158 (7)
162 (8)
150 (8)
138 (7)
130 (7)
128 (8)
39 (4)
45 (4)
54 (4)
65 (4)‡
35 (4)
42 (3)
50 (3)
57 (4)†
10 (2)
15 (2)
21 (2)‡
29 (2)‡
13 (2)
21 (2)
24 (2)
27 (2)†
0.2 (0.7)
0.9 (0.7)‡
2.9 (0.7)
5.5 (0.7)‡
0.8 (0.5)
0.7 (0.5)
1.2 (0.5)
3.2 (0.5)‡
7407 (295)
8001 (264)
8500 (264)
9413 (301)‡
7547 (303)
8280 (265)
9142 (264)†
9450 (303)†
Variables are adjusted for age and accelerometer wear time and presented as mean (SE). *Measured in 10-min blocks. † Different from group 1 and 2 (P < 0.05). ‡ Different from all groups (P < 0.05).
et al. (2009) found that estimated VO2max in male and female students reporting a high PA level were 13% and 21% higher, respectively, than those reporting a moderate PA level. Corresponding differences for men and women reporting high and low PA in the present study were 10% and 13%, respectively. That is, smaller differences in VO2max were found between high-low PA than the earlier study of high-moderate PA. Since Mynarski et al. (Mynarski et al., 2009) estimated VO2max using a submaximal bicycle test, these differences could be due to their methodical error range. In contrast to the men in this study, women reporting a moderate PA level did not have a significantly higher VO2max than those reporting a low PA level. Even though they had an 11% higher number of steps, the PA intensity was probably too low to have an impact on VO2max. It was also found that the difference in VO2max between participants meeting/not meeting the PA guidelines, accelerometer-measured, was 4% points larger in men as women. This could be a result of a higher amount of vigorous PA in men than women meeting the PA guidelines, or that men not meeting the guidelines performed less intense PA than women. Correlations between PA and cardiorespiratory fitness Large differences in self-reported and accelerometermeasured PA were found between the most and least physically active men and women, but VO2max only varied from 12% to 21%. For example, self-reported PA for the 25% most physically active men was 18 times larger than for the 25% least active men. Yet the difference in VO2max between these groups was only
16%. This indicates that VO2max is more dependent on other factors than participants’ general PA level. This was confirmed by the low correlations (< 0.13; Table 2) and low explained variance, derived from the regression analyses, between VO2max and total self-reported PA/average counts/day. However, fairly good correlations (around 0.5) were found between accelerometermeasured vigorous PA and VO2max, which is similar to findings in Japanese men and women (Cao et al., 2010a,b). This is in line with classic training studies that have found that subjects performing high intensity PA experienced a higher increase in VO2max than subjects performing PA at a lower intensity level (Wenger & Bell, 1986; Swain & Franklin, 2006; Helgerud et al., 2007). The intensity level during PA could also partly explain the paradox that Norwegians are found to be among the least physically active citizens in Europe (Vaage, 2008) yet have a higher VO2max level (Aspenes et al., 2011; Edvardsen et al., 2013) compared with others (Inbar et al., 1994; Nelson et al., 2010). In other words, Norwegian adults may spend less time being physically active than adults from other European countries, but when they are physically active, it might be at a higher intensity level. Another interesting finding was that the correlation between self-reported vigorous PA and VO2max was not higher than around 0.30 in both men and women. Selfreported vigorous PA was 9 min (women) and 17 min (men) higher than accelerometer-measured vigorous PA (data not shown), indicating that participants may overestimate the intensity of PA, a finding also reported elsewhere (Prince et al., 2008; Hagstromer et al., 2010a; Dyrstad et al., 2014).
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Dyrstad et al. Sedentary time No significant differences in sedentary time were detected in quartiles of VO2max. This finding was also the case when sedentary time was analyzed in bouts of 10 and 30 min, and indicates that men and women with high or low cardiorespiratory fitness spent a similar amount of time in sedentary behaviors. The majority of prospective studies of sedentary behavior have shown that greater sedentary time is associated with an increased health risk (Ford & Caspersen, 2012). It is also found that sedentary time is strongly related to both breast cancer and metabolic risk independent of PA (Bankoski et al., 2011; Lynch et al., 2011). It has previously been reported that women spent 4% less time in sedentary behaviors than men (Hansen et al., 2012). Data from the present study show that when sedentary behaviors were analyzed in bouts of 10 and 30 min, women spent 12% and 18% less time in these bouts then men. This indicates that men use more time in longer bouts of sedentary behaviors than women. In the present study, it was also found that women with the 25% lowest VO2max, self-reported 164 min/day less sedentary time than the accelerometer-measured sedentary time (data not shown in results). This difference between self-reported and accelerometer-measured sedentary behavior was 125% higher than for women with the 25% highest VO2max, suggesting that women with high cardiorespiratory fitness could be more aware of their sedentary time than women with the lowest cardiorespiratory fitness. Strengths and limitations of the study The major strength of the study is the large population sample size recruited from a wide age range throughout Norway. Furthermore, VO2max was directly measured on a treadmill by gas analysis during a maximal fitness test and strict end criteria for VO2max were used. The study has some limitations. First, different test laboratories were used. However, all test personnel were well trained and gas analyzers were compared and calibrated. Second, the VO2max measurements were completed 5–8 months after the self-reported and accelerometer-measured PA level, meaning that participants could have increased or decreased their VO2max after the PA registration, potentially skewing the results. However, mean habitual PA level during such a short time period is not likely to change much in a large group not participating in any intervention programs. Third, there are only two categorizations of accelerometer-measured MVPA, while participants were spilt in three groups based on self-reported PA (Table 1). Even though differences between groups then become smaller, this categorization is often used in the literature, and was therefore used in the present study. Fourth, in the present study, it was found that vigorous PA explained 2.8% of the variance in VO2max.
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This was lower than expected and could be explained by a low number of participants performing any significant amount of vigorous PA. Only 53% of the participants performed any vigorous PA at all, and only 1/3 of these performed more than five minutes per day. Accelerometers have also limitations in measuring vigorous intensity PA. A phenomenon called the ‘plateau’ is demonstrated, i.e., the activity counts level off in most accelerometers, including ActiGraph, when running speed exceeds 10 km/h (Brage et al., 2003; Sasaki et al., 2011). Another limitation of the accelerometer intensity data is that the cut points were not based at the participants’ individually aerobic capacity.
Perspectives The IPAQ categorization reflects significant variations in VO2max between low and high self-reported PA level, but not between moderate and high/low PA level. Significant differences in VO2max were also detected between participants meeting/not meeting the PA recommendations measured by accelerometer. These are important findings since VO2max is a significant health marker and the measurement of VO2max is impractical in large cohorts. The present study provides reference values for VO2max for the different categorizations of IPAQ and accelerometer-measured PA. Men and women with high and low cardiorespiratory fitness spent a similar amount of time in sedentary behaviors, while women in general spent less time in longer bouts of sedentary behaviors than men. If the majority of future studies state that sedentary time is a risk factor independent of cardiorespiratory fitness, findings from the present study indicate that fit people should also be targeted in interventions that focus on reducing sitting time. Future studies of sedentary behavior should analyze benefits of reducing sedentary behavior for people with both low and high cardiorespiratory fitness. Key words: Maximal oxygen uptake, sedentary behavior, accelerometer, self-reported physical activity, IPAQ.
Acknowledgements The authors thank all the test personnel at the nine institutions involved in the study for their work during the data collection: University of Tromsø (campus Alta), Hedmark University College, NTNU Social Research AS, Sogn og Fjordane University College, University of Agder, University of Nordland, University of Stavanger, Telemark University College, and Norwegian School of Sport Sciences. The study was funded by The Norwegian Directorate of Health and the Norwegian School of Sport Sciences. The authors declare no conflict of interest.
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Scand J Med Sci Sports 2015: ••: ••–•• doi: 10.1111/sms.12425
Published by John Wiley & Sons Ltd
Cardiorespiratory fitness in groups with different physical activity levels S. M. Dyrstad1, S. A. Anderssen2, E. Edvardsen2,3, B. H. Hansen2 Department of Education and Sport Science, University of Stavanger, Stavanger, Norway, 2Department of Sports Medicine, Norwegian School of Sport Sciences, Oslo, Norway, 3Department of Pulmonary Medicine, Oslo University Hospital, Oslo, Norway Corresponding author: Sindre M. Dyrstad, Dr. Scient, Department of Education and Sport Sciences, University of Stavanger, 4036 Stavanger, Norway. Tel: (+47) 51 83 34 43, Fax: (+47) 51 83 34 50, E-mail: [email protected]
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Accepted for publication 9 January 2015
The aim of the study was to determine how different categorizations of self-reported and objectively measured physical activity (PA) reflect variations in cardiorespiratory fitness (VO2max). A total of 759 individuals (366 women) with a mean age of 48.5 years (SD 14.4) wore an accelerometer (ActiGraph GT1M) for seven consecutive days and answered the short International Physical Activity Questionnaire (IPAQ). VO2max was directly measured during a continuous graded exercise treadmill test until exhaustion. Men and women categorized as highly active by IPAQ had 9% and 13% higher VO2max, respectively, than those reporting a low PA level (P < 0.05). Men
and women meeting the PA recommendation of 150 min/ week of daily moderate intensity PA, measured by accelerometer, had 13% and 9% higher VO2max, respectively, than participants not meeting this recommendation (P < 0.01). No significant differences in average sedentary time, analyzed in total min/day and in bouts of 10 and 30 min, were found between participants with high or low cardiorespiratory fitness. However, women spent less time than men in bouts of sedentary behaviors. Self-reported PA by IPAQ and objectively measured PA by accelerometer were both useful instruments for detecting differences in VO2max.
Both high levels of physical activity (PA) and high cardiorespiratory fitness are inversely associated with diseases and all-cause mortality (Blair et al., 2001; Lee & Skerrett, 2001; Kodama et al., 2009). However, cardiorespiratory fitness is more strongly correlated with coronary heart disease than PA (Talbot et al., 2002). This might be attributable to the fact that cardiorespiratory fitness can be measured more precisely and that high cardiorespiratory fitness has a genetic component that also contributes to good health (Bray et al., 2009). While measures of cardiorespiratory fitness are best obtained by direct measurement of maximal oxygen uptake (VO2max), assessment of PA is mainly performed using subjective methods such as questionnaires, or objectively by activity monitors like accelerometers. The International Physical Activity Questionnaire (IPAQ) is a tool suitable for assessing population levels of PA across countries. IPAQ is thoroughly validated (Lee et al., 2011), and a wide range of studies have used IPAQ as an instrument for measuring PA level in different populations (Bauman et al., 2009). Because of the development of cheaper and more accurate objective assessment instruments of PA, accelerometer has become a commonly used device to assess PA at population level. However, questionnaires are still a frequently used tool to assess PA in large samples because of their relatively low cost and ease of use.
Fogelholm et al. (2006) studied the validity of IPAQ for measuring fitness in soldiers and found that the most active group, according to IPAQ, did not have the highest estimated VO2max. Ottevaere et al. (2011) compared subjective and objective PA data in European adolescents with their fitness level. They concluded that both the subjective and objective instruments were able to detect adolescents with the highest cardiorespiratory fitness, which were the most active adolescents. However, none of these studies used direct VO2max tests, and only adolescents and men were studied. Because a direct measurement of VO2max requires skilled technicians, expensive equipment, motivated subjects, and are time-consuming, measuring VO2max is impractical in larger cohorts. It would therefore be valuable to study if a variation in both self-reported PA and objectively measured PA reflects variation in VO2max, and to evaluate the agreement between VO2max and PA. The main purpose of the present study was therefore to examine how different levels of self-reported and objectively measured PA, including time spent being sedentary, reflect variation in VO2max. Methods This multicenter study involved nine regional test centers throughout Norway. A representative sample of 11 515 adults (aged 20–84
1
Dyrstad et al. years) from the areas surrounding each test center was drawn from the Norwegian population registry. Written informed consent was obtained from 3867 participants (34%). These participants received a pre-programmed accelerometer and a questionnaire by mail, and wore the accelerometer for seven consecutive days. From the sample that completed the accelerometer measurements, 1930 were randomly selected and invited to undergo a cardiopulmonary exercise test. A total of 1030 participants accepted the invitation, 904 persons met at the lab, and 759 participants (366 women) with mean age of 48.5 years (SD 14.4) completed the cardiopulmonary exercise test 5–8 months after the questionnaire and accelerometer measurements. The numbers of participants vary in tables from 480 to 729 according to valid data. Most of the dropouts are caused by missing variables from the IPAQ. The design of the study has been described in detail elsewhere (Hansen et al., 2012), and the participants’ cardiorespiratory response during maximal exercise is previously reported (Edvardsen et al., 2013). The study was approved by the Regional Ethics Committee for Medical Research and the Norwegian Social Science Data Services AS.
Objective PA measurement The ActiGraph GT1M (ActiGraph, LLC, Pensacola, Florida, USA) was used to assess participants’ PA levels. Participants with a minimum of four days of at least 10 h of daily recordings were included in the analysis. Data were collected in 10-s epochs, which were collapsed into 60-s epochs for comparison with other studies. The data were reduced using an SAS-based macro (SAS Institute Inc., Cary, North Carolina, USA). Wear time was defined by subtracting nonwear time from 18 h since all data between 00:00 and 06:00 h were excluded to avoid potential bias because of participants forgetting to remove the monitor when going to bed at night. Nonwear time was defined as intervals of at least 60 consecutive minutes with zero counts, with allowance for 1 minute with counts greater than zero (Troiano et al., 2008). For analysis of sedentary behavior, ActiLife 6.11.4 software (ActiGraph, LLC) was used to analyze the accelerometer data. The first sedentary break of each day was ignored to avoid a misclassification of nonwear time during evening as a sedentary break. PA guidelines for Norwegians and Americans recommend 150 min/week of moderate intensity PA or 75 min/week of vigorous intensity aerobic PA, or an equivalent combination of moderate and vigorous intensity PA (U.S. Department of Health and Human Services, 2008; The Norwegian Directorate of Health, 2014). Participants accumulating an average of minimum of 21.4 min of daily moderate to vigorous PA (MVPA), corresponding to 150 min/week, in bouts of 10 min or more (with allowance for interruptions of 1 min) were categorized as being sufficiently active. This definition allowed participants to have longer bouts of activity on certain days and to be less active on other days, and still be categorized as sufficiently active. Since none of the participants met the PA guidelines by 75 min/week of vigorous PA alone, and few participated in vigorous PA, the limit of 150 min of MVPA was used. Average counts per minute (cpm) was expressed as the total number of registered counts (> 100 cpm) for all valid days divided by wearing time. To identify PA of different intensities, count thresholds corresponding to the energy cost of the given intensity were applied to the data set. Sedentary time was defined as all activity below 100 cpm, a threshold that corresponds with sitting, reclining or lying down (Healy et al., 2007; Matthews et al., 2008). Time in lifestyle activity was defined as counts between 760 and 2019 cpm, a threshold that corresponds with slow walking, grocery shopping, vacuuming, and child care (Hagstromer et al., 2010b). Moderate intensity was defined as cpm between 2020 and 5998 [3–6 metabolic equivalent tasks (METs)], and vigorous intensity PA (> 6 METs) was defined as 5999 cpm or more
2
(Troiano et al., 2008). Mean minutes/day at different intensities was calculated as the sum of all minutes where the count met the criterion for that intensity, divided by the number of valid days.
Self-reported assessment of PA Self-reported PA over the previous 7 days was obtained by the short, self-administered version of the IPAQ (translated to Norwegian). The sum of time spent walking and in MVPA was used to estimate the total amount of time spent in PA per day, and data cleaning and processing was carried out in accordance to the official IPAQ scoring protocol (IPAQ, 2005). Vigorous intensity PA was assumed to correspond to 8 METs, moderate intensity activity to 4 METs and walking to 3.3 METs. The participants were categorized into three PA levels according to the IPAQ scoring criteria. The “high” level indicates a PA level equivalent to 60 min of MVPA/day and meets any one of the following two criteria: (a) vigorous intensity activity on at least 3 days and accumulating at least 1500 MET min/week; (b) 7 days of any combination of walking, moderate or vigorous intensity activities accumulating at least 3000 MET min/week. The “moderate” PA level is equivalent to 30 min of MVPA on most days, and meets any of the following three criterion: (a) three or more days of vigorous intensity activity of at least 20 min/day; (b) five or more days of moderate intensity activity and/or walking of at least 30 min/day; (c) five or more days of any combination of walking, moderate intensity or vigorous intensity activities achieving a minimum of at least 600 MET min/week. The “low” PA level is indicated if PA meets neither moderate nor high criterion.
Cardiopulmonary exercise test Body weight was measured to the nearest 0.1 kg, with participants wearing light clothes and no shoes. VO2max was directly measured by using a continuous graded protocol of walking uphill on a treadmill until exhaustion (Edvardsen et al., 2013). Gas exchange and ventilatory variables were sampled as the subjects breathed into a Hans Rudolph two-way breathing mask (2700 series; Hans Rudolph Inc., Shawnee, Kansas, USA). During the last part of the cardiopulmonary exercise test, the subject’s effort was encouraged by the technician until voluntary termination. Rating of perceived exertion was obtained using the Borg Scale6–20 (Borg, 1974). Gas exchange variables were reported as 30-s averages. VO2max was accepted if respiratory exchange ratio ≥ 1.10 or the Borg 6–20 score was ≥ 17. Each day, the gas analyzers used were calibrated for volume and gas and corrected for barometric pressure, temperature, and humidity. A detailed description of measurement accuracy between gas analyzers is given elsewhere (Edvardsen et al., 2013).
Statistics A multivariate general linear model, adjusted for appropriate confounders such as age and accelerometer wear time, was used to assess differences in, e.g., VO2max in men and women performing different amounts of PA. To study the variation in VO2max among participants with different PA level, men and women were divided in quartiles based at both accelerometer-measured MVPA (≥ 2020 cpm) and self-reported MET level (walking and MVPA). To study the variation of sedentary behavior in participants with different VO2max, participants were divided in quartiles based on their VO2max level. Correlations between accelerometer-measured PA, self-reported PA, and VO2max were assessed by Spearman correlation coefficient, ρ (rho). To analyze the relationships between VO2max and the correlates of sex (women = 1, men = 2), age (years), body mass index (BMI), and PA, hierarchical regression with enter procedure was applied. Preliminary analyses of
Participants are categorized into low, moderate and high PA levels by using the International Physical Activity Questionnaire (IPAQ), and by meeting/not meeting the PA recommendations of at least 150 min/week with moderate PA, measured by accelerometer. Variables are adjusted for age and accelerometer wear time and are presented as mean values (SE). n for self-reported total activity is lower according to valid data, see methods. *Different from No (P < 0.001, within gender). † Different from the two other groups (P < 0.05, within gender). ‡ Definitions described in “Self-reported assessment of physical activity” in methods. § A minimum of 150 min/week with MVPA > 2019 cpm, measured in 10 min blocks. MET, metabolic equivalent task.
347.3 (6.6) 10299 (161)* 363 (26)* n = 100 343.6 (4.9) 7336 (140) 226 (24) n = 123 361.3 (8.3)† 9965 (287)† 572 (23)† n = 68 338.4 (5.9) 8739 (204) 225 (20) n = 86 339.8 (5.7) 7840 (197)† 85 (23)† n = 69 329.3 (4.3) 7338 (127) 375 (29) n = 174 324.3 (6.8) 8914 (239) 241 (32) n = 67
VO2max (mL/kg/min) Average counts/min Steps/day Self-reported total activity (MET min/day)
314.2 (5.3) 7473 (188)† 121 (27)† n = 94
346.4 (6.9)† 9089 (244) 750 (27)† n = 96
323.2 (7.0) 10475 (192)* 413 (42) n = 83
No n = 200 30.9 (0.4) High n = 68 35.2 (0.7)† Moderate n = 133 31.7 (0.5) Low n = 142 31.2 (0.5) Yes n = 112 43.3 (0.7)* No n = 256 38.2 (0.5)
Accelerometer Meeting PA recommendations§
High n = 96 41.7 (0.8) Moderate n = 100 41.4 (0.8) Low n = 161 38.1 (0.6)†
International Physical Activity Questionnaire PA level‡ International Physical Activity Questionnaire PA level‡
Men with a high self-reported PA level had 9% higher VO2max and 10% higher average cpm than men selfreporting a low PA level (Table 1). Women with a high self-reported PA level had a 13% higher VO2max and 6% more accelerometer-measured PA than women selfreporting a low PA level (Table 1). Men meeting the PA guidelines of 150 min/week of MVPA had a 13% higher VO2max and no significant difference in self-reported activity level in MET min/day than men not meeting the PA guidelines (Table 1). Women meeting the PA guidelines of 150 min/week of MVPA had a 9% higher VO2max and reported 61% higher activity level in MET min/day than women not meeting the PA guidelines (Table 1). Men with the 25% highest accelerometer-measured MVPA level (quartile 4), performed 70 (2) min/day of MVPA (≥ 2020 cpm). Their VO2max was 42.8 (0.8) mL/ kg/min, which was 21% higher than for men with the 25% lowest accelerometer-measured MVPA level (quartile 1), who performed 14 (0.5) min/day of MVPA. Self-reported PA for men in quartile 4 was 932 (24) MET min/day and they had a VO2max of 43.2 (0.9) mL/ kg/min, which was 16% higher than for men in quartile 1, who performed 51 (24) MET min/day (P < 0.05). Similar PA levels were found in women with similar differences in VO2max between groups. The correlation between VO2max and both self-reported and accelerometer-measured PA increased across intensity levels (Table 2). Accelerometer-measured vigorous intensity PA in women had the highest correlation to VO2max, while a correlation < 0.13 was found between VO2max and both total self-reported PA and average cpm in men and women. The biological factors of gender, age, and BMI included in the regression analyses displayed the largest amount of explanatory power, explaining 59% of the variance in VO2max (Table 3). Vigorous PA and moderate PA (in min/day) explained 2.8% and 1.0% of the variance in VO2max, respectively, and increased the total explained variance to 62.5%. Average counts/day (> 99 cpm) and total self-reported time in min/day in activity (walking + MVPA) did not have any significant influence in VO2max.
Table 1. Cardiorespiratory fitness (VO2max) and physical activity (PA) variables for men and women with different PA levels
Results
Variable
Women Men
Accelerometer Meeting PA recommendations§
normal distribution were conducted to ensure that there was no violation of the assumptions of linear regression. The analysis contained three models with the biological variables of sex, age, and BMI in the first, vigorous PA in the second, and moderate PA in the third. Since half of the participants did not perform any vigorous PA at all, participants were split in two. Group one did not perform any vigorous PA, group two performed vigorous PA. Data were adjusted for accelerometer wear time. Demographic data are presented as mean values ± standard error (SE) unless otherwise specified. A P-value of less than 0.05 was regarded as statistically significant. All statistical analyses were performed using PASW Statistics 21 for Windows (IBM Corporation, Somers, New York, USA).
Yes n = 152 33.8 (0.5)*
Physical activity and maximal oxygen uptake
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Dyrstad et al. Table 2. Correlation coefficients (Spearman) between VO2max and PA (self-reported and accelerometer-measured)
Self-reported PA variables
Women VO2max (mL/kg/min) Men VO2max (mL/kg/min)
Accelerometer-measured variables
Moderate (min/day)1a
Vigorous (min/day)2a
Total PA3a
Moderate to vigorous (min/day)1b
Vigorous (min/day)2b
Average counts/day3b
n = 290 0.14* n = 313 0.05
n = 315 0.30** n = 337 0.30**
n = 225 0.09 n = 261 0.08
n = 354 0.23** n = 375 0.29**
n = 354 0.54** n = 375 0.47**
n = 354 0.13* n = 375 0.08
*P < 0.05. **P < 0.01. 1a Self-reported time in moderate activity; 1b≥ 2020 counts/min. 2a Self-reported time in vigorous activity; 2b≥ 5999 counts/min. 3a Self-reported time in activity (walking + moderate + vigorous); 3baverage counts/day (≥ 100). PA, physical activity.
Table 3. Hierarchical regression analysis of variables of maximal oxygen uptake (mL/kg/min), n = 729
Variables
Model 1 B
SE
Model 2 t
Explained B variance (R2)
Constant 64.07 1.73** 37.04 Sex (female vs male) 8.56 0.46** 18.74 Age (year) −0.36 0.02** −23.80 BMI −0.93 0.06** −14.89 VPA, min/day in ≥ 5999 cpm (none vs any) MVPA, min/day in 2020–5999 cpm
58.45 8.22 −0.32 −0.85 3.64
SE
Model 3 t
Explained B variance (R2)
1.82** 32.03 0.44** 18.56 0.02** −20.21 0.06** −13.86 0.48** 7.67
56.97 8.12 −0.33 −0.82 3.01
SE
0.615**
Explained variance (R2)
1.83** 31.12 0.44** 18.53 0.02** −20.81 0.06** −13.59 0.49** 6.16
0.05 0.01** 0.587**
t
4.56 0.625**
**P < 0.001. B, unstandardized beta coefficient; SE, standard error of the mean; t, t-statistics; BMI, body mass index; MVPA, moderate physical activity; VPA, vigorous physical activity (participants performing none VPA vs participants performing VPA).
There were no significant differences in accelerometer-measured sedentary time, analyzed in total min/day and in bouts of 10 and 30 min, across quartiles of VO2max (Table 4). However, in general, women spent 330 (2) and 135 (2) min/day in sedentary behavior when analyzed in 10 and 30 min bouts, respectively. This was 12% and 18% less than for men (P < 0.001; data adjusted for age and wear time). Men and women with the 25% highest VO2max had significant more time in vigorous intensity PA than the other groups (Table 4). Discussion This study examined associations between cardiorespiratory fitness and PA level measured by IPAQ and accelerometer in a large national sample. The main findings were that men and women meeting the PA recommendations, had 5–13% higher VO2max than those not meeting the PA recommendations. Large variations in
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PA reflected relatively small variations in VO2max. The correlations between average cpm/total self-reported PA and VO2max were low, but the correlation between accelerometer-measured vigorous PA and VO2max were around 0.50 for men and women. No differences in sedentary time were found between participants with high and low VO2max.
PA recommendations and cardiorespiratory fitness How the different methods of assessing PA reflect variations in VO2max is useful knowledge since cardiorespiratory fitness is strongly correlated with health. It has been found that for every increase in cardiorespiratory fitness by approximately 3.5 mL/kg/min, the risk of death was reduced by 17% in women (Gulati et al., 2003) and 12% in men (Myers et al., 2002). In the present study, the differences in VO2max between those reporting a low or high PA level were above 3.5 mL/kg/min. However, this difference is far less than earlier reported. Mynarski
Physical activity and maximal oxygen uptake Table 4. Physical activity (PA) measured by accelerometer, for quartiles of maximal oxygen uptake (VO2max) in men and women
VO2max (mL/kg/min) Sedentary activity, < 100 cpm (min/day) Sedentary activity, < 100 cpm (min/day), analyzed in 30 min bouts Lifestyle PA, ≥ 760 cpm (min/day)* Moderate PA, ≥ 2020 cpm (min/day)* Vigorous PA, ≥ 5999 cpm (min/day)* Steps/day
Men
Women
Quartiles groups of VO2max
Quartiles groups of VO2max
1 n = 87
2 n = 81
3 n = 86
4 n = 84
1 n = 77
2 n = 81
3 n = 81
4 n = 80
28.9 (0.4) 564 (8)
35.6 (0.4) 553 (8)
42.3 (0.4) 562 (7)
52.4 (0.4) 561 (9)
23.1 (0.4) 545 (9)
29.2 (0.4) 540 (8)
34.4 (0.4) 532 (8)
41.9 (0.4) 534 (9)
164 (8)
145 (8)
158 (7)
162 (8)
150 (8)
138 (7)
130 (7)
128 (8)
39 (4)
45 (4)
54 (4)
65 (4)‡
35 (4)
42 (3)
50 (3)
57 (4)†
10 (2)
15 (2)
21 (2)‡
29 (2)‡
13 (2)
21 (2)
24 (2)
27 (2)†
0.2 (0.7)
0.9 (0.7)‡
2.9 (0.7)
5.5 (0.7)‡
0.8 (0.5)
0.7 (0.5)
1.2 (0.5)
3.2 (0.5)‡
7407 (295)
8001 (264)
8500 (264)
9413 (301)‡
7547 (303)
8280 (265)
9142 (264)†
9450 (303)†
Variables are adjusted for age and accelerometer wear time and presented as mean (SE). *Measured in 10-min blocks. † Different from group 1 and 2 (P < 0.05). ‡ Different from all groups (P < 0.05).
et al. (2009) found that estimated VO2max in male and female students reporting a high PA level were 13% and 21% higher, respectively, than those reporting a moderate PA level. Corresponding differences for men and women reporting high and low PA in the present study were 10% and 13%, respectively. That is, smaller differences in VO2max were found between high-low PA than the earlier study of high-moderate PA. Since Mynarski et al. (Mynarski et al., 2009) estimated VO2max using a submaximal bicycle test, these differences could be due to their methodical error range. In contrast to the men in this study, women reporting a moderate PA level did not have a significantly higher VO2max than those reporting a low PA level. Even though they had an 11% higher number of steps, the PA intensity was probably too low to have an impact on VO2max. It was also found that the difference in VO2max between participants meeting/not meeting the PA guidelines, accelerometer-measured, was 4% points larger in men as women. This could be a result of a higher amount of vigorous PA in men than women meeting the PA guidelines, or that men not meeting the guidelines performed less intense PA than women. Correlations between PA and cardiorespiratory fitness Large differences in self-reported and accelerometermeasured PA were found between the most and least physically active men and women, but VO2max only varied from 12% to 21%. For example, self-reported PA for the 25% most physically active men was 18 times larger than for the 25% least active men. Yet the difference in VO2max between these groups was only
16%. This indicates that VO2max is more dependent on other factors than participants’ general PA level. This was confirmed by the low correlations (< 0.13; Table 2) and low explained variance, derived from the regression analyses, between VO2max and total self-reported PA/average counts/day. However, fairly good correlations (around 0.5) were found between accelerometermeasured vigorous PA and VO2max, which is similar to findings in Japanese men and women (Cao et al., 2010a,b). This is in line with classic training studies that have found that subjects performing high intensity PA experienced a higher increase in VO2max than subjects performing PA at a lower intensity level (Wenger & Bell, 1986; Swain & Franklin, 2006; Helgerud et al., 2007). The intensity level during PA could also partly explain the paradox that Norwegians are found to be among the least physically active citizens in Europe (Vaage, 2008) yet have a higher VO2max level (Aspenes et al., 2011; Edvardsen et al., 2013) compared with others (Inbar et al., 1994; Nelson et al., 2010). In other words, Norwegian adults may spend less time being physically active than adults from other European countries, but when they are physically active, it might be at a higher intensity level. Another interesting finding was that the correlation between self-reported vigorous PA and VO2max was not higher than around 0.30 in both men and women. Selfreported vigorous PA was 9 min (women) and 17 min (men) higher than accelerometer-measured vigorous PA (data not shown), indicating that participants may overestimate the intensity of PA, a finding also reported elsewhere (Prince et al., 2008; Hagstromer et al., 2010a; Dyrstad et al., 2014).
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Dyrstad et al. Sedentary time No significant differences in sedentary time were detected in quartiles of VO2max. This finding was also the case when sedentary time was analyzed in bouts of 10 and 30 min, and indicates that men and women with high or low cardiorespiratory fitness spent a similar amount of time in sedentary behaviors. The majority of prospective studies of sedentary behavior have shown that greater sedentary time is associated with an increased health risk (Ford & Caspersen, 2012). It is also found that sedentary time is strongly related to both breast cancer and metabolic risk independent of PA (Bankoski et al., 2011; Lynch et al., 2011). It has previously been reported that women spent 4% less time in sedentary behaviors than men (Hansen et al., 2012). Data from the present study show that when sedentary behaviors were analyzed in bouts of 10 and 30 min, women spent 12% and 18% less time in these bouts then men. This indicates that men use more time in longer bouts of sedentary behaviors than women. In the present study, it was also found that women with the 25% lowest VO2max, self-reported 164 min/day less sedentary time than the accelerometer-measured sedentary time (data not shown in results). This difference between self-reported and accelerometer-measured sedentary behavior was 125% higher than for women with the 25% highest VO2max, suggesting that women with high cardiorespiratory fitness could be more aware of their sedentary time than women with the lowest cardiorespiratory fitness. Strengths and limitations of the study The major strength of the study is the large population sample size recruited from a wide age range throughout Norway. Furthermore, VO2max was directly measured on a treadmill by gas analysis during a maximal fitness test and strict end criteria for VO2max were used. The study has some limitations. First, different test laboratories were used. However, all test personnel were well trained and gas analyzers were compared and calibrated. Second, the VO2max measurements were completed 5–8 months after the self-reported and accelerometer-measured PA level, meaning that participants could have increased or decreased their VO2max after the PA registration, potentially skewing the results. However, mean habitual PA level during such a short time period is not likely to change much in a large group not participating in any intervention programs. Third, there are only two categorizations of accelerometer-measured MVPA, while participants were spilt in three groups based on self-reported PA (Table 1). Even though differences between groups then become smaller, this categorization is often used in the literature, and was therefore used in the present study. Fourth, in the present study, it was found that vigorous PA explained 2.8% of the variance in VO2max.
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This was lower than expected and could be explained by a low number of participants performing any significant amount of vigorous PA. Only 53% of the participants performed any vigorous PA at all, and only 1/3 of these performed more than five minutes per day. Accelerometers have also limitations in measuring vigorous intensity PA. A phenomenon called the ‘plateau’ is demonstrated, i.e., the activity counts level off in most accelerometers, including ActiGraph, when running speed exceeds 10 km/h (Brage et al., 2003; Sasaki et al., 2011). Another limitation of the accelerometer intensity data is that the cut points were not based at the participants’ individually aerobic capacity.
Perspectives The IPAQ categorization reflects significant variations in VO2max between low and high self-reported PA level, but not between moderate and high/low PA level. Significant differences in VO2max were also detected between participants meeting/not meeting the PA recommendations measured by accelerometer. These are important findings since VO2max is a significant health marker and the measurement of VO2max is impractical in large cohorts. The present study provides reference values for VO2max for the different categorizations of IPAQ and accelerometer-measured PA. Men and women with high and low cardiorespiratory fitness spent a similar amount of time in sedentary behaviors, while women in general spent less time in longer bouts of sedentary behaviors than men. If the majority of future studies state that sedentary time is a risk factor independent of cardiorespiratory fitness, findings from the present study indicate that fit people should also be targeted in interventions that focus on reducing sitting time. Future studies of sedentary behavior should analyze benefits of reducing sedentary behavior for people with both low and high cardiorespiratory fitness. Key words: Maximal oxygen uptake, sedentary behavior, accelerometer, self-reported physical activity, IPAQ.
Acknowledgements The authors thank all the test personnel at the nine institutions involved in the study for their work during the data collection: University of Tromsø (campus Alta), Hedmark University College, NTNU Social Research AS, Sogn og Fjordane University College, University of Agder, University of Nordland, University of Stavanger, Telemark University College, and Norwegian School of Sport Sciences. The study was funded by The Norwegian Directorate of Health and the Norwegian School of Sport Sciences. The authors declare no conflict of interest.
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