Abstract
Background:
Cardiorespiratory fitness, typically measured as peak oxygen uptake (VO2peak) during maximal graded exercise testing (GXTmax), is a predictor of morbidity, mortality, and cardiovascular disease. However, measuring VO2peak is costly and inconvenient and thus not widely used in clinical settings. Alternatively, postexercise heart rate recovery (HRRec), which is an index of vagal reactivation, is a valuable assessment of VO2peak in older adults and athletes. However, the validity of HRRec as a clinical indicator of cardiorespiratory fitness in young, sedentary adults, who are a rapidly growing population at risk for developing obesity and cardiovascular disease, has not been fully elucidated.
Methods:
We investigated the association between cardiorespiratory fitness, measured by VO2peak (mL·kg−1·min−1), and HRRec measures after a GXTmax in 61 young (25.2 ± 6.1 years), sedentary adults (40 females) using 3 methods. We examined the relationship between VO2peak and absolute (b·min−1) and relative (%) HRRec measures at 1, 2, and 3 min post GXTmax, as well as a measure of the slow component HRRec (HRRec 1 min minus HRR 2 min), using Pearson’s correlation analysis.
Results:
VO2peak (36.5 ± 7.9 mL·kg−1·min−1) was not significantly correlated with absolute HRRec at 1 min (r = 0.18), 2 min (r = 0.04) or 3 min (r = 0.01). We also found no significant correlations between VO2peak and relative HRRec at 1 min (r = 0.09), 2 min (r = −0.06) or 3 min (r = −0.10). Lastly, we found no correlation between the measure of the slow component HRRec and VO2peak (r = −0.14).
Conclusions:
Our results indicate that HRRec measures are not a valid indicator of cardiorespiratory fitness in young, sedentary adults.
Keywords: Sedentary lifestyle, physical activity, cardiovascular health, graded exercise test
INTRODUCTION
Accurate assessments of cardiovascular health and fitness are important for predicting at-risk individuals and for developing interventional therapeutic strategies (1). Accumulating evidence has established that clinical assessments of cardiorespiratory fitness improve risk stratification for adverse health outcomes and are a powerful tool for patient management (2–4). The American Heart Association released a statement indicating that cardiorespiratory fitness should be considered a clinical vital sign and should be assessed regularly in the clinic along with other preventative assessments (4). However, direct measures of cardiorespiratory fitness rely on determining peak oxygen uptake (VO2peak) during a graded exercise test conducted in a laboratory setting. Because measuring VO2peak during a graded exercise test can be costly and inconvenient, it is often more practical to estimate cardiorespiratory fitness using simple, non-invasive measures that are easily collected during exercise. One such measure is heart rate recovery (HRRec) following exercise testing, which is an index of vagal reactivation and is a strong predictor of morbidity and mortality in older adults (5–9).
HRRec is used in some clinical settings as a measure of autonomic dysfunction to identify high risk cardiovascular disease patients. However, it is not typically used as a marker of cardiorespiratory fitness. For example, HRRec is predictive of long-term outcomes and survival in patients with coronary artery disease and congestive heart failure, which have known autonomic nervous system dysfunction (7, 8). The HRRec is also an independent risk factor for development of metabolic diseases, suggesting it is an informative marker for at-risk individuals (10, 11). Evidence suggests that HRRec is a valid method to assess cardiorespiratory fitness. Cross sectional studies show that physically active individuals have improved HRRec compared to their sedentary counterparts (12–15). Likewise, both VO2peak and HRRec improve following an exercise regimen in longitudinal studies (16–18). In addition, HRRec and VO2peak are highly associated in studies that include older adults, athletes, and physically active individuals (19–23). Together authors of these studies suggest that HRRec is a valid marker of cardiorespiratory fitness. However, authors of one study examined the association between cardiorespiratory fitness and HRRec in young, healthy, sedentary females and found no association between VO2peak and submaximal HRRec (24). Therefore, despite evidence in other populations, the use of HRRec as an accurate assessment of cardiorespiratory fitness in young and sedentary, but otherwise healthy (non-smoking, non-hypertensive, non-diabetic), adults is unclear.
The purpose of this study was to investigate the validity of using HRRec as an estimate of cardiorespiratory fitness in young, sedentary adults by determining a significant association exists between HRRec and VO2peak during a maximal graded exercise test (GXTmax). It is well-known that sedentary behavior is associated with cardiovascular disease risk factors (25). In the U.S., the amount of time young adults spent in sedentary behaviors increased 12% from 2007 to 2016 (26). Thus, young adults are an emerging at-risk population and valid clinical measures of cardiorespiratory fitness in this group are necessary.
METHODS
The study was reviewed and approved by the University of Kentucky Office of Research Integrity Medical Institutional Review Board (16-0789-F6A), and participants provided written informed consent before inclusion in the study. Participants between the ages of 18 and 45 years were recruited for a previously reported intervention study (27). Each participant completed a Physical Activity Readiness-Questionnaire and Health History Form and were excluded if they had existing contraindications to the GXTmax as specified in the American College of Sports Medicine Guidelines for Exercise Testing and Prescription (28).
Anthropometric and body composition measures, including standing height, body mass, circumference measurements, and a total-body dual-energy X-ray absorptiometry (DXA) scan were performed. Participants were measured in lightweight clothing containing no metal and without shoes. Standing height was determined to the nearest 0.1 cm from a wall-fixed meter stick (Pittsburgh®; Pittsburg, PA) with the participants’ hands positioned on the hips during a maximal inhalation. Body mass was determined to the nearest 0.1 kg using a calibrated electric scale (Escali Corp., Burnsville, MN). Circumference measurements (waist, abdominal, and hip) were taken in triplicate using a fiberglass anthropometric tape (Creative Health Products BMS-8) in accordance with the guidelines established by the Airlie Conference Proceedings (29). The mean of the 3 measures was used for analysis. Body composition was measured using total body DXA scans performed using a GE Lunar iDXA bone densitometer (Lunar Inc., Madison, WI). All female participants had negative urine pregnancy tests, which were taken immediately before DXA scanning. A single trained investigator completed and analyzed all scans using the Lunar software Version 14.10. Total body DXA absolute fat-free mass (kg) and mineral-free lean mass (kg), and absolute (kg) and relative (% of body mass) fat masses were determined for each participant.
GXTmax tests were completed using an indirect calorimetry testing system (Vmax Encore, Vyaire Medical, Yorba Linda, CA) with an integrated ECG (60 Hz sampling rate; Cardiosoft v6.51, GE Healthcare, Chicago, IL) and a treadmill ergometer. Before the exercise test, baseline heart rate and blood pressure were measured while the participant stood on the treadmill. During the test and recovery period continuous cardiovascular measurements (heart rate and ECG) were monitored. During the continuous, progressive (speed and grade) tests, oxygen consumption (VO2) was measured breath by breath and averaged over 1-minute intervals. The GXTmax tests were performed using an incremental treadmill protocol, with 2-min workload stages, developed for a previous study (30). Since prior studies have shown that similar VO2peak and HRmax are achieved regardless of treadmill protocol (ramp vs incremental), we chose to use the incremental protocol that is regularly used in our lab (31, 32). The initial stage of the test began with a walking speed of 5.1 km·h−1 and 0% grade. The test progressed with a 0.6- km·h−1 increase in speed and 2% increase in grade with each subsequent stage. During the final minute of each stage, blood pressure (by manual auscultation) and ratings of perceived exertion (RPE; using the original 6–20 Borg Scale (33)) were recorded. Heart rate was recorded in the last 10 seconds of each stage. The test was terminated in all participants in this study at volitional fatigue (3–4.5 seconds for treadmill to fully stop), and no participants presented contraindications to continuing according to the American College of Sports Medicine guidelines (28). Verbal encouragement was given throughout the test. After completing the GXTmax, five minutes of passive recovery data (heart rate and blood pressure) were taken while the participants’ remained standing on the treadmill. Achievement of VO2peak (ml·kg−1·min−1) was defined as a participant’s ability to obtain a minimum of two of the following criteria: respiratory exchange ratio ≥1.1 (determined by 1-minute averaging), RPE ≥ 17, and/or age-predicted maximal heart rate achieved or exceeded (34). The highest VO2 value observed during GXT was used for analysis. Eleven participants were not included in the analytic data set due to failure to achieve VO2peak.
Our primary analysis included heart rate recovery data at 1-, 2-, and 3-minutes post-exercise. Absolute HRRec (bpm) was defined as the heart rate at 1-min, 2-min and 3-minutes post-exercise subtracted from the maximal heart rate achieved during the graded exercise test. Relative HRRec was defined as absolute HRRec divided by maximal heart rate, multiplied by 100. Also, the difference between 1- and 2-min HRRec was calculated as an index of the slow component of the post-exercise HRRec (35).
Data were analyzed using Statistical Package for Social Sciences (SPSS, Version 26, IBM, Armonk, NY). Descriptive data are presented as means ± standard deviations. Independent sample t-tests were conducted to assess differences in measured variables between males and females. Pearson’s correlation coefficients were used to assess associations between VO2peak and relative and absolute HRRec as well as measures of body composition. Correlation analyses were stratified by sex to assess potential sex differences. Since we recognize that obesity could influence the analysis, we ran a secondary sensitivity analysis to determine if adiposity influenced our findings. Partial Pearson’s correlation coefficients were used to assess associations between VO2peak and relative and absolute HRRec in participants while controlling for body fat %. A one-way repeated measures analysis of variance (ANOVA) was performed to determine if relative and absolute HRRec differed between 1-, 2-, and 3-minutes post-exercise. Significance was ascribed at p<0.05.
RESULTS
All participants reported good health, including no known cardiovascular disease or hypertension, were medication-free (except contraceptives), and did not participate in a structured exercise regimen at the time of the study or participate in greater than 2 hours moderate-vigorous physical activity each week. Eleven of the 72 participants were excluded because they did not achieve VO2peak (details below). The remaining 61 participants included in the analytic data set (40 females) had varying adiposities (BMI: 16.6–37.0; Body fat percentage 12.4–51.7). Males had significantly greater height, body mass, and waist circumference than females (Table 1).
Table 1.
Participant characteristics and anthropometric measures
|
Variable |
Male Mean ± SD (range) |
Female Mean ± SD (range) |
Total Group Mean ± SD (range) |
|---|---|---|---|
| Age (y) | 24.0 ± 4.4 (18.0–31.0) |
25.9 ± 6.7 (18.0–45.0) |
25.2 ± 6.1 (18.0–45.0) |
| Height (cm) | 176.0 ± 6.4* (161.7–188.5) |
164.0 ± 6.3 (152.0–183.2) |
168.1 ± 8.5 (152.0–188.5) |
| Body Mass (kg) | 78.7 ± 16.7* (61.8–127.7) |
67.8 ± 14.7 (42.9–107.6) |
71.6 ± 16.2 (42.9–127.7) |
| BMI (kg∙m−2) | 25.3 ± 4.5 (19.6–35.9) |
25.2 ± 5.4 (16.6–37.0) |
25.3 ± 5.1 (16.6–37.0) |
| Anthropometric | |||
| Abdominal Circumference (cm) | 89.3 ± 11.3 (75.3–110.3) |
85.7 ± 13.2 (63.2–123.6) |
87.0 ± 12.6 (63.2–123.6) |
| Waist Circumference (cm) | 83.8 ± 9.9* (71.4–104.4) |
76.3 ± 11.3 (58.3–107.2) |
78.9 ± 11.3 (58.3–107.2) |
| Hip Circumference (cm) | 102.0 ± 9.7 (89.2–126.8) |
102.6 ± 11.4 (84.5–134.0) |
102.4 ± 10.8 (84.5–134.0) |
| Body Composition | |||
| Body fat (%) | 27.1 ± 7.1*
(12.4–39.8) |
35.5 ± 8.6 (21.4–51.7) |
32.6 ± 9.0 (12.4–51.7) |
| Fat mass (kg) | 21.5 ± 8.7 (7.4–36.1) |
24.4 ± 10.8 (9.1–55.1) |
23.4 ± 10.1 (7.4–55.1) |
| Fat free mass (kg) | 55.5 ± 10.1*
(43.5–90.2) |
41.8 ± 6.1 (31.3–53.7) |
46.5 ± 10.1 (31.3–90.2) |
| Mineral free lean mass (kg) | 52.7 ± 9.5*
(41.4–85.3) |
39.3 ± 5.8 (29.4–50.5) |
44.0 ± 9.6 (29.4–85.3) |
Sex differences P<0.05
During the GXT, all participants in the analytic data set, except 3 females, were above an absolute HRRec of 18 bpm, a cutoff value for abnormal 1-minute HRRec observed in a previous study (Supplemental Table 1; DOI: 10.6084/m9.figshare.14691099) (36). VO2peak was not associated with absolute HRRec at 1-, 2-, or 3-minutes when males and females were examined separately, or when the entire cohort of participants were combined (Table 2; Figure 1). Absolute HRRec was also not significantly associated with VO2peak at 1-, 2-, or 3-minutes when controlling for body fat %. Increased absolute HRRec at 3-minutes was associated with an increased waist circumference for the total study group only. However, absolute HRRec at 1-, 2-, and 3-minutes were not significantly associated with age, BMI, or other anthropometric measures for males, females, or the total study group (Table 2). Males had a significantly greater VO2peak than females (Table 3). The difference between absolute HRRec at 1-min and 2-min was also not associated with VO2peak in males, females, or the total study group.
Table 2.
Pearson correlation coefficients among absolute HRRec, VO2peak, and anthropometric and body composition measures.
|
Variable |
1-min Absolute HRRec | 2-min Absolute HRRec | 3-min Absolute HRRec | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Male (N = 21) |
Female (N = 40) |
Total (N = 61) |
Male (N = 21) |
Female (N = 40) |
Total (N = 61) |
Male (N = 21) |
Female (N = 40) |
Total (N = 61) |
|
| Age | 0.27 | 0.08 | 0.09 | 0.21 | 0.11 | 0.11 | 0.34 | 0.21 | 0.18 |
| BMI | −0.19 | 0.15 | 0.05 | −0.03 | 0.27 | 0.16 | 0.09 | 0.27 | 0.21 |
| Cardiorespiratory Fitness | |||||||||
| VO2peak | 0.15 | −0.03 | 0.18 | −0.08 | −0.07 | 0.04 | −0.19 | −0.04 | 0.01 |
| Anthropometric | |||||||||
| Abdominal Circum | −0.15 | 0.15 | 0.09 | 0.08 | 0.19 | 0.17 | 0.24 | 0.21 | 0.25 |
| Waist Circum | −0.20 | 0.18 | 0.14 | 0.03 | 0.24 | 0.20 | 0.19 | 0.23 | 0.26* |
| Hip Circum | 0.01 | 0.07 | 0.05 | 0.21 | 0.18 | 0.18 | 0.26 | 0.19 | 0.22 |
| Body Composition | |||||||||
| Body fat | −0.08 | 0.05 | −0.10 | 0.26 | 0.14 | 0.09 | 0.33 | 0.21 | 0.16 |
| Fat mass | −0.05 | 0.14 | 0.05 | 0.21 | 0.22 | 0.18 | 0.31 | 0.24 | 0.25 |
| Fat-free mass | 0.03 | 0.24 | 0.26* | −0.03 | 0.28 | 0.19 | 0.06 | 0.13 | 0.19 |
| Mineral-free lean mass | 0.02 | 0.19 | 0.24 | −0.03 | 0.25 | 0.18 | 0.06 | 0.10 | 0.18 |
Circum = Circumference; HRRec = heart rate recovery; VO2peak = peak oxygen uptake
Significant correlation p<0.05
Figure 1. Absolute and relative heart rate recovery are not associated with VO2peak.
Representative exercise recovery HR data from male (A) and female (B) participants. Pearson correlations were used to compare VO2peak from the GXTmax to measures of absolute (C,E,G) and relative (D,F,H) heart rate recovery at 1, 2, and 3 minutes following exercise termination. HRRec = heart rate recovery
Table 3.
VO2peak, HR, and BP responses to maximal graded exercise test
|
Variable |
Male (N = 21), Mean ± SD (range)a |
Female (N = 40), Mean ± SD (range)a |
Total Group (N = 61), Mean ± SD (range)a |
|---|---|---|---|
| Cardiorespiratory Fitness | |||
| VO2peak (ml·kg−1·min−1) | 43.3 ± 6.0*
(34.5–54.4) |
33.0 ± 6.4 (18.3–45.0) |
36.5 ± 7.9 (18.3–54.4) |
| Heart Rate (b∙min −1 ) | |||
| Baseline HR | 83.3 ± 13.8 (63.0–122.0) |
88.8 ± 10.9 (62.0–109.0) |
86.9 ± 12.1 (62.0–122.0) |
| Peak HR | 198.6 ± 6.6* (187.0–210.0) |
191.3 ± 9.1 (157.0–210.0) |
193.8 ± 9.0 (157.0–210.0) |
| Absolute HRRec (b∙min −1 ) | |||
| 1 min | 32.2 ± 8.0 (19.0–47.0)A |
27.9 ± 8.3 (14.0–50.0)A |
29.4 ± 8.4 (14.0–50.0)A |
| 2 min | 54.5 ± 13.4 (29.0–88.0)A |
50.8 ± 11.4 (24.0–79.0)A |
52.1 ± 12.1 (24.0–88.0)A |
| 3 min | 66.5 ± 13.7 (36.0–97.0)A |
62.7 ± 10.5 (39.0–84.0)A |
64.0 ± 11.8 (36.0–97.0)A |
| Relative HRRec (%) | |||
| 1-minute | 16.3 ± 4.1 (9.2–23.5)B |
14.7 ± 4.6 (7.3–26.5)B |
15.2 ± 4.5 (7.3–26.5)B |
| 2-minute | 27.5 ± 6.8 (14.5–44.9)B |
26.6 ± 6.1 (12.6–43.7)B |
26.9 ± 6.3 (12.6–44.9)B |
| 3-minute | 33.5 ± 7.0 (18.0–49.5)B |
32.8 ± 5.8 (20.5–44.8)B |
33.1 ± 6.2 (18.0–49.5)B |
| Baseline BP (mmHg) | |||
| Systolic | 118.1 ± 6.0* (106.0–126.0) |
112.5 ± 7.4 (90.0–130.0) |
114.4 ± 7.4 (90.0–130.0) |
| Diastolic | 76.6 ± 4.2* (68.0–82.0) |
74.0 ± 4.5 (62.0–82.0) |
74.9 ± 4.6 (62.0–82.0) |
| Peak BP (mmHg) | |||
| Systolic | 189.6 ± 13.7* (160.0–220.0) |
164.7 ± 16.1 (140.0–224.0) |
173.2 ± 19.4 (140.0–224.0) |
| Diastolic | 88.2 ± 2.0* (86.0–94.0) |
85.5 ± 1.3 (82.0–90.0) |
86.4 ± 2.0 (82.0–94.0) |
| Recovery BP (mmHg) | |||
| 1-min Systolic | 163.7 ± 18.6* (142.0–216.0) |
145.4 ± 15.1 (118.0–198.0) |
151.7 ± 18.4 (118.0–216.0) |
| 1-min Diastolic | 85.5 ± 2.0* (84.0–90.0) |
83.4 ± 1.9 (78.0–86.0) |
84.1 ± 2.2 (78.0–90.0) |
| 3-min Systolic | 140.9 ± 15.6* (108.0–188.0) |
129.4 ± 10.8 (102.0–164.0) |
133.3 ± 13.7 (102.0–188.0) |
| 3-min Diastolic | 83.0 ± 2.2* (78.0–88.0) |
79.5 ± 3.9 (64.0–84.0) |
80.7 ± 3.8 (64.0–88.0) |
BP = blood pressure; HR = heart rate; HRRec = heart rate recovery;
Like capital letters indicate significant differences based on repeated measures analysis of variance
Sex differences P<0.05
Increased age (r = −0.28; p = 0.03), BMI (r = −0.47; p < 0.01) and waist (r = −0.27; p = 0.04), abdominal (r = −0.43; p < 0.01), and hip circumferences (r = −0.46; p < 0.01) were associated with a lower VO2peak for the total study group. Males, compared to females, had higher HR at peak exercise as well as higher systolic and diastolic blood pressure at baseline, peak exercise, and during recovery (Table 3).
Total body DXA scans were used to determine body composition measures for all participants. Body composition varied in our cohort (%fat range:12.4–51.7%). Increased absolute HRRec at 1-minute was associated with increased fat-free mass in the total study group only. However, fat mass, %fat, and mineral-free lean mass were not associated with absolute HRRec in males, females, or total study group (Table 2). Increased %fat and fat mass were associated with a reduced VO2peak in males and females separately, as well as the total study group. Also increased mineral-free lean mass and fat-free mass was associated with an increased VO2peak for the total study group only (Figure 2). Males had greater fat-free mass and mineral-free lean mass compared to females (Table 1). Additionally, females had greater body fat percentage compared to males (Table 1).
Figure 2. Absolute and relative measures of body composition are associated with VO2peak.
Pearson correlation was used to compare VO2peak from the GXTmax to body fat percentage (A), fat mass (B), mineral-free lean mass (C), and fat-free mass (D).
Since peak HR varied within our cohort (range: 157–210), we also examined the relationship between VO2peak and relative HRRec, which accounts for variability in peak HR. Relative HRRec at 1, 2, or 3 minutes were not significantly associated with measures of VO2peak when males and females were examined separately, or when the entire cohort of participants were combined (Figure 1, Table 4). Relative HRRec was also not significantly associated with VO2peak at 1-, 2-, or 3-mintues when controlling for body fat %. Greater relative HRRec at 3 minutes was associated with increasing age and increasing fat mass for the total study group only. However, relative HRRec measures were not significantly associated with BMI, or any other anthropometric and body composition measure for males, females, or the total study group (Table 4).
Table 4.
Pearson correlation coefficients among relative HRRec, VO2peak, and anthropometric and body composition measures.
| 1-min Relative HRRec | 2-min Relative HRRec | 3-min Relative HRRec | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Male (N = 21) |
Female (N = 40) |
Total (N = 61) |
Male (N = 21) |
Female (N = 40) |
Total (N = 61) |
Male (N = 21) |
Female (N = 40) |
Total (N = 61) |
|
| Age | 0.31 | 0.12 | 0.13 | 0.25 | 0.16 | 0.17 | 0.39 | 0.23 | 0.25* |
| BMI | −0.12 | 0.12 | 0.06 | 0.03 | 0.24 | 0.17 | 0.16 | 0.24 | 0.21 |
| Cardiorespiratory Fitness | |||||||||
| VO2peak | 0.11 | −0.07 | 0.09 | −0.12 | −0.13 | −0.06 | −0.23 | −0.15 | −0.10 |
| Anthropometric | |||||||||
| Abdominal Circum | −0.09 | 0.13 | 0.09 | 0.13 | 0.18 | 0.17 | 0.29 | 0.21 | 0.24 |
| Waist Circum |
−0.13 | 0.16 | 0.12 | 0.09 | 0.23 | 0.19 | 0.25 | 0.22 | 0.23 |
| Hip Circum | 0.08 | 0.05 | 0.05 | 0.26 | 0.16 | 0.19 | 0.32 | 0.19 | 0.23 |
| Body Composition | |||||||||
| Body fat | −0.04 | 0.01 | −0.08 | 0.29 | 0.10 | 0.11 | 0.35 | 0.16 | 0.17 |
| Fat mass | 0.01 | 0.12 | 0.06 | 0.26 | 0.19 | 0.20 | 0.36 | 0.23 | 0.25* |
| Fat-free mass | 0.09 | 0.24 | 0.24 | 0.04 | 0.30 | 0.17 | 0.13 | 0.19 | 0.16 |
| Mineral-free lean mass (kg) | 0.09 | 0.19 | 0.22 | 0.03 | 0.26 | 0.15 | 0.13 | 0.15 | 0.14 |
BMI = body mass index; Circum = Circumference; HRRec = heart rate recovery; VO2peak = peak oxygen uptake
Significant correlation P < 0.05
DISCUSSION
HRRec is recognized as a powerful prognostic measure and predictor of mortality in older adults (5, 6, 9). HRRec and VO2peak are both used to inform clinical practices in older adults because they are associated with health and longevity (2, 3, 9). Furthermore, numerous studies have shown that HRRec and VO2peak are increased in physically active compared to sedentary participants, and after completing various exercise regimens, suggesting an important physiological relationship between HRRec and cardiorespiratory fitness (14, 16, 17, 37). However, the utility of HRRec as an indicator of cardiorespiratory fitness may vary by population and has not been well-studied in young sedentary adults.
Consistent with this study, Tonello et al. also examined the association between cardiorespiratory fitness and HRRec in young (mean age: 34.5yrs) adults not participating in structured exercise and found no association between VO2peak and 1-, 2-, 3- and 5-minute HRRec (24). While our study and Tonello et al. both studied young sedentary adults, the 2 studies used somewhat different methods. Our study included both sexes and determined VO2peak and HRRec from treadmill exercise testing, while Tonello et al. studied only females and used a cycle ergometer. Maximal oxygen uptake measured by cycle ergometer has been shown to be lower than treadmill protocols (38, 39). Also, Tonello et al. utilized a submaximal test to measure HRRec (that induced a HR at 86% age-predicted max), while our participants performed a GXTmax to volitional fatigue for determination of HRRec. Thus, despite distinct methodological differences, the fact that our 2 studies had similar results is strong evidence that HRRec may not be a valid indicator of cardiorespiratory fitness in young, sedentary adults.
Our study examined HRRec at 1-, 2-, and 3-minutes as these are the measures that have been associated with cardiorespiratory fitness in other populations (19, 23, 40). HRRec after exercise is orchestrated by both the parasympathetic and sympathetic branches of the autonomic nervous system (41). Parasympathetic reactivation is predominately responsible for the decrease in heart rate immediately following exercise, while sympathetic withdrawal occurs more gradually (41, 42). For this reason, previous studies have considered HRRec at 1- and 2-minutes an indicator of vagal reactivation (5, 6). In our study of young sedentary adults, HRRec at 1-, 2-, and 3-min was not significantly associated with cardiorespiratory fitness in our total analytic data set or when stratified by sex. Thus, vagal reactivation may not be a reliable indicator of cardiorespiratory fitness in this population.
Our study cohort spanned a large range of adiposities, and 47% of participants were overweight or obese (BMI≥25). Previous studies found that increased obesity is associated with reduced 1-minute HRRec (43, 44). In contrast, our data revealed that an increased waist circumference was associated with a greater absolute 3-min HRRec and increased fat mass was associated with a greater relative 3-min HRRec. However, this unexpected finding may be due to the difference in physiological significance of the 3-minute HRRec measure compared to the 1-minute (i.e., sympathetic withdrawal vs. parasympathetic reactivation). Also, our previous data showed that HRRec following a GXTmax was similar in healthy-weight and obese children, indicating that young individuals with poor body composition can have normal vagal reactivation following exercise (45).
Physical activity status may be another factor influencing the relationship between VO2peak and HRRec. Studies in young adults, which included both physically active and sedentary participants, reported a significant association between VO2peak and HRRec (22, 23, 40). Studies have also found that subjectively- and objectively-measured physical activity were associated with HRRec (23, 24). Although our subjects did not participate in structured exercise, incidental activity may have influenced HRRec, as shown by Tonello et al (24). In fact, fat-free mass, which is affected by sedentary behavior (46), was associated with 1-min HRRec in our cohort. Age may also be an important factor since a significant association between VO2peak and HRRec following a maximal treadmill test was observed in older adults with congestive heart failure (19). Thus, sedentary behavior and young age appear to be important contributing factors when determining if HRRec is a valid indicator of cardiorespiratory fitness.
We also found that VO2peak was associated with body fat and fat-free body composition measures. Although VO2peak is expressed relative to body mass, the composition of body mass varies. In agreement with our findings, previous research reported that greater %fat was associated with reduced VO2peak and greater fat free mass was associated with increased VO2peak (47, 48).
There were some limitations of our study. First, our HRRec measures were collected during passive recovery while participants remained standing. Other studies collected either active recovery measures or passive recovery measures while participants were seated or lying down (6, 23, 36). However, immediately moving participants to a seated or supine position following a maximal exercise test can be difficult in practice. Since our goal was to inform clinical utility, we collected measures while participants remained standing. Second, our sample size was small with varying adiposities. However, since we designed this study to inform on clinical utility of HRRec in the general population, our inclusion criteria included young, relatively healthy, and sedentary individuals, and no exclusion criteria regarding obesity status were implemented. Third, we did not control for dietary supplements that may have been consumed during the study. Fourth, we did not control for the phase of the menstrual cycle when the GXTmax was performed for female participants. It is possible that phase of the menstrual cycle may influence maximal oxygen uptake (49).
Since the clinical utility of HRRec was first introduced in the late 1990s, many studies have investigated HRRec as a measure of cardiovascular health and fitness in a variety of populations (5, 20, 23, 24). HRRec has been shown to be a useful diagnostic and prognostic tool for coronary artery disease, heart failure and mortality in older adults, including cardiovascularly healthy participants and heart failure patients (5–7). However, few studies have been performed in young, sedentary adults, which is a rapidly expanding and at-risk population. Valid, non-invasive measures of cardiorespiratory fitness are needed to identify at-risk individuals at a young age and develop interventional therapeutic strategies.
CONCLUSION
The prevalence of cardiovascular disease among U.S. adults is a staggering 49% of the population (50). We found that HRRec measures were not significantly associated with VO2peak in a sample of young, sedentary, adults. While HRRec measures have been used as a clinical indicator of health and morbidity in other populations, they are not a reliable indicator of cardiorespiratory fitness in sedentary young adults.
Supplementary Material
Funding Source:
This study was supported by a Barnstable Brown Diabetes and Obesity Center Pilot Award, a National Institutes of Health (UL1TR001998) Center for Clinical and Translational Science Pilot Award, the National Center for Advancing Translational Sciences (NIH TL1TR001997), the University of Kentucky Pediatric Exercise Physiology Laboratory Endowment, and the University of Kentucky Arvle & Ellen Turner Thacker Research Fund.
Footnotes
No conflicts of interest
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