Hypertension has been a well-known condition causing cardiovascular disease, stroke, and chronic kidney disease [
Physical training, particularly aerobic exercises, is recommended as an antihypertensive solution leading to significant blood pressure, body weight, body fat, and waist circumference reduction, along with insulin sensitivity and HDL cholesterol level improvement [
However, despite the obvious reduction of cardiovascular mortality in other reports [
The aim of this study is to examine the association of occupational physical activity, leisure-time physical activity, and sedentary lifestyle with hypertension according to the adherence with aerobic exercise in a nationally representative sample of women over 40 years of age, using recent Korea National Health and Nutrition Examination Survey (KNHANES) Data.
In this study, a cross-sectional analysis was performed using the Korea National Health and Nutrition Examination Survey (KNHANES) data collected by the Korea Centers for Disease Control and Prevention (KCDC) from 2016 to 2017. KNHANES is a surveillance system to assess the health and nutritional status of population in the Republic of Korea.
KNHANES includes a health interview, health examination, and nutrition survey. The interview and examination are performed by trained medical staff and interviewers. A multistage stratified cluster sampling was done for the household unit selection. The Institutional Review Board of the KCDC approved the data collection with written informed consent forms from all participants. The details of the survey design and data resource were described on a profiles paper in 2014 [
Hypertension was defined as “participants’ answer on hypertension diagnosed by medical doctor” from health survey questionnaires. Participants with no response were excluded from the analysis. If the participants answered “yes” to the questions for moderate intensity physical activity (at least 2.5 hours per week) or “yes” for vigorous intensity physical activity (at least 1.5 hours per week), they were classified as “aerobic activity adherence group.” Those who failed to be classified as the adherence group were categorized as “nonadherence group.” If the participants answered “yes” to the questions for muscle strengthening exercises (MSE) such as push-up, sit-up, dumbbell, or other types of muscle exercises more than 2 days for the past week, they were classified as “adherence group.”
Sociodemographic characteristics including age, quartiles of equivalent income, and education (less than elementary school, less than middle school, less than high school, and more than university graduation) were included in the questionnaires. Smoking status was classified as never smoker, former smoker, or current smoker, and alcohol consumption was divided into 4 categories (never, no drink within recent 1 year, less than 4 times drink per a month, or more than 5 times per month). Sleeping hours were classified as 7 hours of sleep per day in weekdays as well as in weekends, respectively. For physical activities, each of vigorous or moderate intensity physical activity group was divided into two aspects, work (occupational physical activity, OPA) and leisure (leisure-time physical activity, LPA), with 3 subcategories (never, ≤4 days per week, or 5∼7 days per week), respectively. Sitting hours were classified as ≤6, 7∼12, or ≥13 hours per a day. Frequency of walking per week was divided into 3 categories (never, ≤4 days per week, or 5∼7 days per week).
The data were expressed by numbers and percentages for describing the general characteristics of the study participants. The frequency in the adherence group and the nonadherence group was compared by the chi-square test. To determine the difference for clinical variables in each group, independent two-sample
The mean age of the 4,241 women participants from KNHANES 2016-2017 data was 58.4 (±11.4, range: 40–80 years old). Among them, 29.1% (1,234) of participants had hypertension and 92.7% were never smokers. There were 1,681 (39.6%) women in the aerobic activity adherence group and 2,560 (60.4%) participants were in the nonadherence group. For muscle strengthening exercises adherence, 3,560 (83.9%) participants never did the exercise, 465 (11.0%) did 1–4 day per week, and 215 (5.1%) did more than 5 days per week. Most participants reported no vigorous intensity physical activity for both occupation (99.5%) and leisure time (94.6%). There was a similar trend for moderate intensity physical activity (Table
Characteristics of the women participated in this study.
Items |
|
% | |
---|---|---|---|
Age (years) | 40–49 y | 1148 | 27.1 |
50–59 y | 1210 | 28.5 | |
60–69 y | 1042 | 24.6 | |
≥70 y | 841 | 19.8 | |
|
|||
Income level | Level 1 | 940 | 22.2 |
Level 2 | 1061 | 25.0 | |
Level 3 | 1091 | 25.7 | |
Level 4 | 1142 | 26.9 | |
|
|||
Education level | Elementary | 1355 | 32.0 |
Middle | 568 | 13.4 | |
High | 1276 | 30.1 | |
University | 1036 | 24.4 | |
|
|||
Hypertension | No | 3007 | 70.9 |
Yes | 1234 | 29.1 | |
|
|||
Aerobic activity adherencea (per week) | No | 2560 | 60.4 |
Yes | 1681 | 39.6 | |
Muscle strengthening exercises adherenceb (per week) | Never | 3560 | 83.9 |
≤4 | 465 | 11.0 | |
5∼7 days | 215 | 5.1 | |
|
|||
OPA vigorous intensity (per week) | Never | 4219 | 99.5 |
≤4 | 12 | 0.3 | |
5∼7 days | 10 | 0.2 | |
|
|||
OPA moderate intensity (per week) | Never | 4061 | 95.8 |
≤4 | 108 | 2.5 | |
5∼7 days | 72 | 1.7 | |
|
|||
Physical activity days with moving location (per week) | Never | 1801 | 42.5 |
≤4 | 1101 | 26.0 | |
5∼7 days | 1339 | 31.6 | |
|
|||
LPA vigorous intensity (per week) | Never | 4011 | 94.6 |
≤4 | 162 | 3.8 | |
5∼7 days | 68 | 1.6 | |
|
|||
LPA moderate intensity (per week) | Never | 3436 | 81.0 |
≤4 | 577 | 13.6 | |
5∼7 days | 228 | 5.4 | |
|
|||
Sitting hours (per day) | ≤6 hours | 1831 | 43.2 |
≤12 hours | 2028 | 47.8 | |
≥13 hours | 382 | 9.0 | |
|
|||
Walking (days per week) | Never | 823 | 19.4 |
≤4 days | 1578 | 37.2 | |
≥5 days | 1838 | 43.3 | |
|
|||
Sleeping hours in weekdays (per week) | <7 hours | 2594 | 61.2 |
≥7 hours | 1647 | 38.8 | |
|
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Sleeping hours in weekends (per week) | <7 hours | 1989 | 46.9 |
≥7 hours | 2252 | 53.1 | |
|
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Alcohol consumption (within 1 year) | No experience | 884 | 20.8 |
No drink within recent 1 year | 834 | 19.7 | |
≤4 times (months) | 2147 | 50.6 | |
≥5 times (months) | 376 | 8.9 | |
|
|||
Smoking status | Never | 3933 | 92.7 |
Former smoker | 162 | 3.8 | |
Current smoker | 145 | 3.4 |
aAerobic activity adherence: 2.5-hour moderate intensity physical activity per week or 1.25-hour vigorous intensity physical activity per week. bMuscle strengthening exercises adherence: push-up, sit-up, dumbbell, or other types of muscle exercises more than 2 days during 1 week recently. OPA: occupational physical activity; LPA: leisure-time physical activity.
Aerobic activity adherence showed a significant association with age groups (40–49, 50–59, 60–69, and more than 70 years) (Table
General characteristics and physical activity status due to aerobic activity adherence.
Items | Total | Nonaerobic ( |
Aerobic ( |
| |||
---|---|---|---|---|---|---|---|
|
|
% |
|
% | |||
Age (years) | 40–49 y | 1148 | 624 | 24.4 | 524 | 31.2 | <0.001 |
50–59 y | 1210 | 660 | 25.8 | 550 | 32.7 | ||
60–69 y | 1042 | 640 | 25.0 | 402 | 23.9 | ||
≥70 y | 841 | 636 | 24.8 | 205 | 12.2 | ||
|
|||||||
Income level | Level 1 | 940 | 611 | 23.9 | 329 | 19.6 | <0.001 |
Level 2 | 1061 | 660 | 25.8 | 401 | 23.9 | ||
Level 3 | 1091 | 640 | 25.1 | 451 | 26.8 | ||
Level 4 | 1142 | 643 | 25.2 | 499 | 29.7 | ||
|
|||||||
Education level | Elementary | 1355 | 970 | 38.0 | 385 | 22.9 | <0.001 |
Middle | 568 | 351 | 13.7 | 217 | 12.9 | ||
High | 1276 | 703 | 27.5 | 573 | 34.1 | ||
University | 1036 | 531 | 20.8 | 505 | 30.1 | ||
|
|||||||
Hypertension | No | 3007 | 1689 | 66.0 | 1318 | 78.4 | <0.001 |
Yes | 1234 | 871 | 34.0 | 363 | 21.6 | ||
|
|||||||
Aerobic activity adherencea (per week) | No | 3560 | 2265 | 88.5 | 1295 | 77.0 | <0.001 |
Yes | 465 | 199 | 7.8 | 266 | 15.8 | ||
Never | 215 | 95 | 3.7 | 120 | 7.1 | ||
|
|||||||
OPA vigorous intensity (per week) | ≤4 days | 4219 | 2557 | 99.9 | 1662 | 98.9 | <0.001 |
5∼7 days | 12 | 2 | 0.1 | 10 | 0.6 | ||
Never | 10 | 1 | 0.0 | 9 | 0.5 | ||
|
|||||||
OPA moderate intensity (per week) | ≤4 days | 4061 | 2533 | 98.9 | 1528 | 90.9 | <0.001 |
5∼7 days | 108 | 25 | 1.0 | 83 | 4.9 | ||
Never | 72 | 2 | 0.1 | 70 | 4.2 | ||
|
|||||||
Physical activity days with moving location (per week) | ≤4 days | 1801 | 1592 | 62.2 | 209 | 12.4 | <0.001 |
5∼7 days | 1101 | 710 | 27.7 | 391 | 23.3 | ||
Never | 1339 | 258 | 10.1 | 1081 | 64.3 | ||
|
|||||||
LPA vigorous intensity (per week) | ≤4 days | 4011 | 2550 | 99.6 | 1461 | 86.9 | <0.001 |
5∼7 days | 162 | 10 | 0.4 | 152 | 9.0 | ||
Never | 68 | 0 | 0.0 | 68 | 4.0 | ||
|
|||||||
LPA moderate intensity (per week) | ≤4 days | 3436 | 2412 | 94.2 | 1024 | 60.9 | <0.001 |
5∼7 days | 577 | 138 | 5.4 | 439 | 26.1 | ||
Never | 228 | 10 | 0.4 | 218 | 13.0 | ||
|
|||||||
Sitting hours (per day) | ≤6 hours | 1831 | 1003 | 39.2 | 828 | 49.3 | <0.001 |
≤12 hours | 2028 | 1290 | 50.4 | 738 | 43.9 | ||
≥13 hours | 382 | 267 | 10.4 | 115 | 6.8 | ||
|
|||||||
Walking (days per week) | Never | 823 | 731 | 28.6 | 92 | 5.5 | <0.001 |
≤4 days | 1578 | 1126 | 44.0 | 452 | 26.9 | ||
5∼7 days | 1838 | 702 | 27.4 | 1136 | 67.6 | ||
|
|||||||
Sleeping hours in weekdays (per week) | <7 hours | 2594 | 1505 | 58.8 | 1089 | 64.8 | <0.001 |
≥7 hours | 1647 | 1055 | 41.2 | 592 | 35.2 | ||
|
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Sleeping hours in weekends (per week) | <7 hours | 1989 | 1189 | 46.4 | 800 | 47.6 | 0.465 |
≥7 hours | 2252 | 1371 | 53.6 | 881 | 52.4 | ||
|
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Alcohol consumption (within 1 year) | No experience | 884 | 584 | 22.8 | 300 | 17.8 | <0.001 |
No drink within recent 1 year | 834 | 535 | 20.9 | 299 | 17.8 | ||
≤4 times (months) | 2147 | 1210 | 47.3 | 937 | 55.7 | ||
≥5 times (months) | 376 | 231 | 9.0 | 145 | 8.6 | ||
|
|||||||
Smoking status | Never | 3933 | 2357 | 92.1 | 1576 | 93.8 | 0.091 |
Former smoker | 162 | 103 | 4.0 | 59 | 3.5 | ||
Current smoker | 145 | 99 | 3.9 | 46 | 2.7 |
aAerobic activity adherence: 2.5-hour moderate intensity physical activity per week or 1.25-hour vigorous intensity physical activity per week. bMuscle strengthening exercises adherence: push-up, sit-up, dumbbell, or other types of muscle exercises more than 2 days during 1 week recently. OPA: occupational physical activity; LPA: leisure-time physical activity.
Systolic blood pressure (SBP) and diastolic blood pressure (DBP) showed significant differences between the aerobic activity nonadherence group and aerobic activity adherence group (
Comparison of clinical characteristics according to the aerobic activity adherence.
Aerobic activity nonadherence group ( |
Aerobic activity adherence group ( |
| |||
---|---|---|---|---|---|
Mean | SD | Mean | SD | ||
Mean SBP | 122.05 | 17.73 | 119.25 | 17.42 | <0.001 |
Mean DBP | 74.58 | 9.51 | 75.17 | 9.32 | 0.048 |
BMI | 24.12 | 3.57 | 23.79 | 3.34 | 0.002 |
Total cholesterol | 198.51 | 39.27 | 199.82 | 38.15 | 0.294 |
HDL cholesterol | 52.48 | 12.53 | 54.61 | 12.66 | <0.001 |
Triglyceride | 132.38 | 100.99 | 117.13 | 77.88 | <0.001 |
AST (GOT) | 22.43 | 9.65 | 21.73 | 8.54 | 0.016 |
ALT (GPT) | 19.85 | 12.98 | 18.84 | 12.08 | 0.011 |
SD: standard deviation; SBP: systolic blood pressure; DBP: diastolic blood pressure; BMI: body mass index; GOT: glutamate oxaloacetate transaminase; GPT: glutamate-pyruvate transaminase.
In the aerobic activity nonadherence group, the odds ratio (OR) for hypertension in muscle strengthening exercises (MSE) adherence group (1∼4 days per week) was 0.468 (95% CI: 0.322–0.681,
For sitting hours, ORs for hypertension were 1.211 (95% CI: 1.008–1.455) in 7∼12 hours group and 2.275 (95% CI: 1.704–3.036) in 13 hours or longer group, compared with 6 hours or less sitting hours group in unadjusted model, respectively. They were 1.161 (95% CI: 0.929–1.451) and 1.849 (95% CI: 1.279–2.673) compared with 6 hours or less sitting hours group in the adjusted model with adjustment of age, quartiles of equivalent income, education levels, smoking status, and alcohol consumption, respectively.
A significant association was found between hypertension and days of walking per week. The OR for hypertension was 0.802 (95% CI: 0.644–0.998) in ≤4 days per week group, compared with the never walking group in the unadjusted model. However, no significant association was found with hypertension after adjustment.
Sleeping hours during week or weekend showed significant association with hypertension in the unadjusted model. However, the association disappeared in results from the adjusted model (Table
Association between physical activity and hypertension in the aerobic activity nonadherence group.
Items | Unadjusted OR |
|
Adjusteda OR |
| |
---|---|---|---|---|---|
Muscle strengthening exercises adherenceb (per week) | Never | <0.001 |
0.034 |
||
≤4 | 0.468 | <0.001 |
0.554 | 0.010 |
|
5∼7 days | 0.830 | 0.433 | 0.850 | 0.570 | |
|
|||||
OPA vigorous intensity (per week) | Never | 1.000 | 1.000 | ||
≤4 | 0.999 | 0.999 | |||
5∼7 days | 0.000 | 1.000 | 0.000 | 1.000 | |
|
|||||
OPA moderate intensity (per week) | Never | 0.247 | 0.550 | ||
≤4 | 0.446 | 0.116 | 0.533 | 0.282 | |
5∼7 days | 2.797 | 0.570 | 1.577 | 0.847 | |
|
|||||
Physical activity days with moving location (per week) | Never | 0.131 | 0.173 | ||
≤4 | 1.173 | 0.134 | 1.000 | 0.999 | |
5∼7 days | 1.301 | 0.091 | 1.410 | 0.069 | |
|
|||||
LPA vigorous intensity (per week) | Never | ||||
≤4 | 0.262 | 0.212 | 0.158 | 0.149 | |
5∼7 days | |||||
|
|||||
LPA moderate intensity (per week) | Never | 0.493 | 0.441 | ||
≤4 | 0.782 | 0.254 | 1.318 | 0.281 | |
5∼7 days | 1.247 | 0.757 | 1.821 | 0.477 | |
|
|||||
Sitting hours (per day) | ≤6 hours | <0.001 |
0.005 |
||
7∼12 hours | 1.211 | 0.041 |
1.161 | 0.190 | |
≥13 hours | 2.275 | <0.001 |
1.849 | 0.001 |
|
|
|||||
Walking (days per week) | Never | 0.096 | 0.203 | ||
≤4 | 0.802 | 0.048 |
1.187 | 0.213 | |
5∼7 days | 0.953 | 0.704 | 1.310 | 0.078 | |
|
|||||
Sleeping hours in weekdays (per week) | ≥7 hours | 2.007 | <0.001 |
1.329 | 0.065 |
|
|||||
Sleeping hours in weekends (per week) | ≥7 hours | 0.591 | <0.001 |
0.942 | 0.698 |
|
|||||
Alcohol consumption (within 1 year) | No experience | <0.001 |
0.776 | ||
No drink within recent 1 year | 0.680 | 0.002 |
0.895 | 0.474 | |
≤4 times (months) | 0.491 | <0.001 |
0.912 | 0.497 | |
≥5 times (months) | 0.475 | <0.001 |
0.807 | 0.337 | |
|
|||||
Smoking status | Never | 0.262 | 0.246 | ||
Former smoker | 0.846 | 0.465 | 0.776 | 0.372 | |
Current smoker | 0.690 | 0.134 | 0.637 | 0.144 |
aAdjusted by covariates (controlling age, income level, education levels, smoking status, and alcohol consumption) in multiple logistic regression analysis. bMuscle strengthening exercises adherence: push-up, sit-up, dumbbell, or other types of muscle exercises more than 2 days during 1 week recently. OR: odds ratio; OPA: occupational physical activity; LPA: leisure-time physical activity.
In the aerobic activity adherence group who answered “yes” to the question about aerobic activity adherence, there was no significant association between MSE adherence and hypertension in both unadjusted model and adjusted model with adjustment forage, quartiles of equivalent income, education levels, smoking status, and alcohol consumption.
Frequency of leisure-time physical activity (LPA) level of vigorous and moderate intensity was associated with hypertension in the unadjusted model. The OR for hypertension was 0.537 (
Frequency of occupational physical activity (OPA) level of moderate intensity showed increased association with hypertension in the adjusted model (in ≤4 per week group, OR = 1.931;
Days of walking per week were significantly associated with hypertension. The OR for hypertension was 0.410 (95% CI: 0.243–0.692,
They were 0.532 (95% CI: 0.284–0.997) and 0.530 (95% CI: 0.292–0.963) compared with the never walking group in the adjusted model with adjustment of age, quartiles of equivalent income, education levels, smoking status, and alcohol consumption, respectively.
However, sitting hours per day and sleeping hours during week showed no significant association with hypertension in each model (Table
Association between physical activity and hypertension in the aerobic activity adherence group.
Items | Unadjusted OR |
|
Adjusteda OR |
| |
---|---|---|---|---|---|
Muscle strengthening exercises adherenceb (per week) | Never | 0.879 | 0.527 | ||
≤4 | 0.928 | 0.693 | 1.292 | 0.259 | |
5∼7 days | 1.069 | 0.789 | 1.067 | 0.819 | |
|
|||||
OPA vigorous intensity (per week) | Never | 0.541 | 0.698 | ||
≤4 | 0.890 | 0.891 | 0.558 | 0.572 | |
5∼7 days | 0.300 | 0.270 | 0.422 | 0.516 | |
|
|||||
OPA moderate intensity (per week) | Never | 0.050 |
0.123 | ||
≤4 | 1.515 | 0.123 | 1.931 | 0.048 |
|
5∼7 days | 0.509 | 0.074 | 0.859 | 0.725 | |
|
|||||
Physical activity days with moving location (per week) | Never | 0.623 | 0.861 | ||
≤4 | 1.203 | 0.467 | 1.073 | 0.813 | |
5∼7 days | 1.269 | 0.331 | 1.153 | 0.619 | |
|
|||||
LPA vigorous intensity (per week) | Never | 0.007 |
0.474 | ||
≤4 | 0.537 | 0.030 |
0.852 | 0.627 | |
5∼7 days | 0.314 | 0.017 |
0.538 | 0.251 | |
|
|||||
LPA moderate intensity (per week) | Never | 0.101 | 0.338 | ||
≤4 | 0.701 | 0.032 |
0.798 | 0.256 | |
5∼7 days | 0.906 | 0.652 | 1.163 | 0.552 | |
|
|||||
Sitting hours (per day) | ≤6 hours | 0.323 | 0.845 | ||
7∼12 hours | 1.211 | 0.133 | 1.002 | 0.991 | |
≥13 hours | 1.102 | 0.692 | 0.841 | 0.576 | |
|
|||||
Walking (days per week) | Never | 0.002 |
0.103 | ||
≤4 | 0.410 | 0.001 |
0.532 | 0.049 |
|
5∼7 days | 0.436 | 0.001 |
0.530 | 0.037 |
|
|
|||||
Sleeping hours in weekdays (per week) | ≥7 hours | 1.335 | 0.076 | 1.182 | 0.409 |
|
|||||
Sleeping hours in weekends (per week) | ≥7 hours | 0.656 | 0.007 |
0.844 | 0.380 |
|
|||||
Alcohol consumption (within 1 year) | No experience | <0.001 |
0.859 | ||
No drink within recent 1 year | 0.756 | 0.135 | 1.134 | 0.580 | |
≤4 times (months) | 0.545 | <0.001 |
1.049 | 0.806 | |
≥5 times (months) | 0.403 | 0.001 |
0.863 | 0.662 | |
|
|||||
Smoking status | Never | 0.394 | 0.264 | ||
Former smoker | 1.538 | 0.172 | 1.878 | 0.109 | |
Current smoker | 1.020 | 0.961 | 0.890 | 0.798 |
aAdjusted by covariates (controlling age, income level, education levels, smoking status, and alcohol consumption) in multiple logistic regression analysis. bMuscle strengthening exercises adherence: push-up, sit-up, dumbbell, or other types of muscle exercises more than 2 days during 1 week recently. OR: odds ratio; OPA: occupational physical activity; LPA: leisure-time physical activity.
In the aerobic activity nonadherence group, women who had sedentary lifestyle such as long sitting hours showed increased association with hypertension and women who showed endurance and muscle strengthening exercise adherence were associated with decreased odds ratios of hypertension. Particularly, they showed increased association with hypertension along with increment of sitting hours, which is closely related to the level of occupational physical activity.
Previous studies on the effects of occupational physical activity (OPA) on hypertension are inconsistent [
On the other hand, the mechanism of cardiovascular diseases due to increased sitting hours due to occupation has been explained more clearly. Decreased local contraction of muscles interferes with lipoprotein lipase activity, leading to an increase in triglycerides and a decrease in high-density lipoprotein cholesterol [
In the aerobic activity adherence group, women doing occupational physical activity of moderate intensity showed increased association with hypertension. Frequency of walking, e.g., days of walking per week, was associated with decreased odds ratios of hypertension in this group.
The relationship between physical activity and hypertension has been reported many times. The previous studies also reported impacts on RCT interventions and risk of coronary heart disease. In the study of Twinamasiko, the mean systolic blood pressure was higher in the sedentary work style group [
Several studies reported that the mechanisms leading to reducing blood pressure by physical activity were related to cardiac remodelling, endothelial function, decreased oxidative stress, and the inflammation syndrome [
Also it was demonstrated that the largest isometric training resulted in reductions of blood pressure in hypertensive patients which was similar to aerobic exercise training [
There are several limitations in interpreting the results of this study. First, it was not possible to demonstrate any causal relationship between physical activities and hypertension in women aged from 40 to 80 years because of the cross-sectional design of this study. Secondly, there was no significant association between leisure-time activities and hypertension in neither adherence nor nonadherence group. This may be due to the small number of active women in the nonadherence group. Finally, there was no consideration that it was surgical menopause or natural menopause. Information on the risky components in the work activities such as chemical, mechanical injury, or job stress was insufficient to analyze the association between occupational physical activity and hypertension. A more comprehensive study may be necessary to overcome those limitations.
In the aerobic activity adherence group, women doing occupational physical activity of moderate intensity showed increased association with hypertension. Further research that identifies the dangerous components in occupational physical activity is warranted. Walking seems to have benefits and be appropriate to be recommended. In the aerobic activity nonadherence group, women who had long sitting hours showed increased association with hypertension, and women who showed endurance exercise adherence were associated with decreased odds ratios of hypertension. Decreasing sitting hours and increasing endurance exercise may be helpful for this group.
The data used to support the findings of this study are available from the website of the Korea National Health and Nutrition Examination Survey (
The funding sources had no role in the design, implementation, analyses, interpretation of the data, or decision to submit results.
The authors declared no conflicts of interest.
Mikyung Ryu and Ho Kim participated in the study concept, design, data analyses, interpretation of analyses, and writing of the manuscript. Heejin Kimm participated in the study concept design, data interpretation, and critical appraisal of the content of the paper. Sol Leeand Weon-Chil Baek participated in data analysis, the interpretation of analyses, and critical revision of the paper.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (14245).