INTRODUCTION
One-third of individuals attending an executive health program were found to have low cardiorespiratory fitness (CRF)[1], defined as maximal oxygen consumption (VO2 max) below the 25th percentile for their age and sex [2]. These individuals were more likely to have higher body mass index (BMI), dyslipidemia, and hypertension than those in the excellent to superior category of CRF [1], defined as a VO2 max ≥75th percentile for their age and sex [2]. The clinical significance of low CRF is that it is associated with a two- to five-fold increased risk of morbidity from cardiovascular disease (CVD) and all-cause mortality, independent of other risk factors [2]. Moreover, CRF also correlates inversely with plasma glucose and lipids, blood pressure, and markers of inflammation [3,4]. Furthermore, higher CRF is associated with decreased mortality from CVD [5]. In support of these findings, the Coronary Artery Risk Development in Young Adults (CARDIA) study demonstrated that every 1-minute increment in duration during a treadmill exercise test was associated with an 11% lower risk of fatal or nonfatal CVD and every 5% increment in cardiorespiratory fitness retained through year 20 of the CARDIA study was associated with an 11% lower risk of all-cause mortality, fatal or nonfatal CVD [6]. For these reasons, it has been recommended that CRF be routinely measured in clinical practice as an overall health parameter [7].
Low CRF is a modifiable risk factor that can be improved with exercise training. Studies have shown that sedentary adults participating in endurance exercise training programs lasting five months can improve their cardiorespiratory fitness by 15.9% to 20.3% [8]. Furthermore, these increases in CRF are accompanied by cardiovascular and metabolic health benefits, including a lower body fat percentage, improved glucose metabolism, lower plasma triglycerides, higher HDL cholesterol, and decreased blood pressure [9,10]. This accounts for the lower risk of cardiovascular disease and diabetes in those with greater CRF, and it provides a compelling basis for encouraging individuals to increase their physical activity and participate in exercise training programs.
Sedentary job holders, such as farm machine operators, office managers, and supervisors, have the lowest CRF of all workers in the United States.[11] Similarly to the national trend, we showed that a third of individuals who attended the M Health Fairview Executive Health program at the University of Minnesota had low CRF [1]. To address this health issue, individuals attending the Executive Wellness program receive an individualized unsupervised exercise prescription, assess their cardiovascular and metabolic health, and were reevaluated 1-2 years later. This analysis aims to evaluate the effectiveness of this exercise training program in improving CRF and to determine the association between changes in CRF and changes in body weight, blood pressure, plasma glucose, and lipid levels.
METHODS
Research Design
This study is a retrospective observational analysis of participants in the M Health Fairview Executive Health program at the University of Minnesota. An Institutional Review Board (IRB) exemption was approved by the University of Minnesota IRB committee (11775, December 22, 2020). Data for this analysis were extracted from the electronic medical records of individuals who participated in the Executive Health program at M Health Fairview between June 2016 and May 2023. These records were provided to us in a deidentified manner, in accordance with the Health Insurance Portability and Accountability Act (HIPAA) regulations. The Clinical and Translational Science Institute (CTSI) at the University of Minnesota, along with the Data and Informatics and Best Practices Integrated Informatics Core (BPIC), provided data extraction and processing services. The M Health Fairview Executive Health program was initiated in 2016 to offer single-day comprehensive preventive health evaluations to busy professionals from companies in the metropolitan Twin Cities area of Minneapolis/Saint Paul, Minnesota.
Participants
Participants attending the M Health Fairview Executive Health program self-select between two different program options, the Gold and Maroon services. Those who selected the ‘Gold’ service underwent a graded exercise treadmill test to measure VO2max and received an exercise prescription provided by an advanced-degree exercise physiologist. Patients were included if they were within the age range of 21 to 75 years, without limitations in performing moderate to vigorous physical activity, without overt cardiovascular disease, and had a return visit with VO2max testing. A total of 397 participants consented to have their data used for research purposes, and 65 of them underwent repeat VO2max testing during a self-selected follow-up visit. During the interval between clinic visits, no data were collected regarding adherence to the exercise prescription, nor were there any communications between clinic personnel and patients regarding adherence. Additionally, patients were not provided with any motivational support or encouragement to adhere to the exercise prescription.
Data on age, sex, and VO2max were available for all participants who returned for a follow-up visit. However, age and sex were also available for 98% of those who did not return for a follow-up visit. Data on anthropometric and metabolic variables were randomly missing, a well-known limitation of retrospective analysis [12].
Measures
Demographics, including age, sex, race (white, Black, and Asian), self-declared ethnic group (Hispanic/Latino or non-Hispanic/Latino), and social history (alcohol, tobacco, and drug use), were reported to the clinic personnel and documented in the medical record at the baseline visit. Physician-diagnosed comorbidities were gleaned from medical records. Vitals signs and anthropometrics (i.e., resting heart rate, systolic and diastolic blood pressure) were measured using an automated blood pressure device after 5 minutes of rest. Height and weight were measured following standard clinical procedures. Participants attended their appointment after fasting for at least 8 hours. Laboratory values, including plasma glucose and lipids (total cholesterol, low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), and triglycerides), were measured at the Fairview Hospital Central Laboratory.
The Bruce protocol was used to determine VO2max in mL of O2/kg of body weight/min utilizing an Ultima CPX metabolic cart with gas analyzer Breeze Suite 8.6. (MGC. Diagnostics, St. Paul, MN). A modified Bruce protocol was used when indicated based on the exercise physiologist's judgment. Heart rate was measured continuously during the VO2max test using a Polar T31 or Zephyr HXM monitor, which was connected wirelessly to the metabolic cart via the Breeze Suite. The gas analyzer and flow rates were calibrated before all VO2max testing. Exercise tests were terminated when participants reached volitional exhaustion, a rate of perceived exertion (RPE) ≥ 18. VO2max was confirmed by respiratory exchange ratio (VCO2/VO2) > 1.10 and a heart rate within ten bpm of the estimated maximal heart rate using the Fox equation, where heart rate max = 220 – age [2]. Ventilatory outliers were not removed, and the data remained uninterpolated. VO2 max was calculated using the “mid-5-of-7” averaging method, where the average is taken using the middle five breaths of a rolling 7-breath window.
The exercise prescription was provided on the same day the patient performed their VO2 max test at the clinic lab. The exercise prescription was a one-time session between the patient and the physician. There were no further follow-ups to assess progress, modify the exercise prescription, or provide motivational support. The exercise prescription was tailored to the patient's activity history, interests, and the availability of facilities and locations for performing the chosen physical activities. Each patient chose the type and location of their exercise activities. Barriers to exercise were identified, and solutions were proposed. The exercise physiologist and client established a training plan that detailed the weekly frequency, session duration in minutes, intensity as a percentage of the client's maximal heart rate, and type of physical activity. Following the first clinic visit, there was no further contact with the participants until the participant, of their own free will, decided to attend a second clinic visit at a date of their choosing at the Executive Wellness Clinic.
Statistical Analysis
Data were reported as mean (SD) for continuous variables and frequency (percentage) for categorical variables. Pearson’s Chi-squared test and Student's t-test were used to compare differences between those who returned for a follow-up visit and those who did not for categorical and continuous variables, respectively. Changes between visits 1 and 2 were analyzed using a mixed-effects model adjusted for visit duration. Correlations between changes in VO2max and other variables were assessed with linear regression.
Participants were categorized by body mass index (BMI): lean (<25), overweight (25-30), and obese (≥30 kg/m²). Dyslipidemia was defined as treatment for it or specific LDL, HDL-C, total cholesterol, or triglyceride levels according to the Third Report of the Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults [13]. Prediabetes was indicated by a fasting plasma glucose level of ≥100 but < 126 mg/dL, and diabetes by a fasting plasma glucose level ≥ 126 mg/dL or a physician-diagnosed [14] while hypertension was defined by blood pressure medication use or resting BP ≥140/90 mmHg according to the 2017 Guideline for the Prevention, Detection, Evaluation, and Management of High Blood Pressure in Adults [15].
In this study, VO2 max was categorized as low (<25th percentile), fair to good (≥25th to <75th), and superior to excellent (≥75th percentile) based on Cooper Clinic reference data [16]. Participants were classified by VO2max change after the second visit as decreased, increased (by >10%), or no change (if <10%). The 10% threshold was selected because it exceeds the mean measurement error and the age-related decline in VO2max [17].
Anthropometric and laboratory variables measured on the same day as VO2max were analyzed. The change was calculated as the variable at time 2 minus the variable at time 1. Pearson’s r values were computed for the change in VO2max against anthropometric and laboratory variables. A p-value < 0.05 indicated statistical significance. All analyses were conducted using R version 4.3.0 in RStudio 2023.
RESULTS
Baseline characteristics showed that 65 of 397 participants in the Executive Health program returned for a second clinic visit, with a mean (SD) interval of 2.13 (±1.01) years. Among them, 93% were white, predominantly male (78%), with a mean (SD) age of 50.7 (±9.7) years, as shown in Table 1. Most participants (78%) were never smokers, 17% were former smokers, and none were current smokers. Plasma glucose and lipid levels were within the recommended values set by the American College of Cardiology for individuals without cardiovascular disease, as shown in Table 1 [18]. When comparing demographic, anthropometric, cardiovascular, and metabolic data between those who returned for a follow-up visit and those who did not return, we find that those who returned were a year older, 50.7 (±9.7) vs 49.4 (±9.5), p = 0.03, and more likely to be male, 78% vs 68%, p = 0.03. In all other variables, the two groups of individuals were similar (Table 1).
This was a relatively healthy group, as participants were free of cardiovascular disease (stroke, heart failure, or previous MI). The prevalence of comorbidities was similar between those who returned for a follow-up visit and those who did not, except for the percentage of those with dyslipidemia. When comparing those who returned for a follow-up visit and those who did not, 67% and 61%, p = 0.04 were overweight, 16% and 18%, p = 0.8, were obese; 15% and 10%, p = 0.2, were hypertensive; 26% and 27%, p > 0.9, had pre-diabetes; 1.5% and none, p = 0.2, had diabetes; 60% and 31%, p <0.001, had dyslipidemia, and 14% and 21%, p = 0.3, did not report any comorbidity (Figure 1).
Cardiorespiratory fitness (CRF) at baseline showed that 41.5% (n = 27) of participants had low CRF, 30.8% (n = 20) had fair to good CRF, and 27.7% (n = 18) had superior to excellent CRF (Figure 2A). For patients in the low, fair to good, and excellent to superior categories, mean (SD) VO2max values were 28.3 (±5.8), 37.7 (±3.9), and 44.8 (±5.7) mL/kg/min for each category, respectively (Figure 2B).
Mean group-level VO₂max change (Δ) was not significant, Δ = 0.05 (±3.9) mL/kg/min, p=0.92. Following the first clinic visit, VO2 max decreased by >10% in 12.3% (n = 8) of participants, remained unchanged in 69.2% (n = 45), and improved by >10% in 18.5% (n = 12) (data not shwon). Among those with low CRF, 3.7% experienced a >10% decrease, 55.6% had <10% change, and 40.7% improved by >10%. In the fair to good category, 15% decreased by > 10%, 80% remained unchanged, and 5% improved by > 10%. For those in the superior to excellent category of CRF, 22.2% decreased by > 10%, 77.8% had no change, and none improved by >10% (Figure 3).
Total cholesterol had a marginal mean (SD) increase of 7.30 (±27.6) mg/dL, and LDL-C increased by 7.95 (±14.9) mg/dL, with p-values of 0.06 and 0.02, respectively, between visits. Resting heart rate increased by a mean (SD) of 4.3 (±9.3) bpm between visit 1 and 2, p value = 0.006 (Table 2). Other anthropometric, metabolic, and cardiovascular variables showed no changes. There were no significant correlations between changes in these parameters and changes in VO2max, data not shown.
DISCUSSION
This analysis shows that nearly half of the M Health Executive Health program self-selected participants who returned for a follow-up visit had low CRF at baseline. Most notably, almost two-thirds of individuals showed no improvement in CRF two years after an individualized, unsupervised exercise prescription, which coincided with no changes in anthropometric, cardiovascular, and metabolic health variables. However, about half of those with low CRF at baseline improved by over 10%, which is encouraging for this high-risk group. Improvements in cardiovascular and metabolic health through exercise training likely require more than a one-time assessment and prescription.
Although compliance with the exercise prescription was not measured, low compliance is one important factor contributing to the lack of improvement in CRF and other health parameters during unsupervised exercise training programs [9]. This is supported by evidence showing that unsupervised exercise programs struggle with compliance, the high prevalence of sedentarism in the U.S., and that half of Americans fail to meet minimum guidelines for cardiorespiratory health [9]. These guidelines recommend 30 minutes of moderate-intensity aerobic activity five times a week or 25 minutes of vigorous-intensity activity three times a week [9]. Various obstacles hinder a long-term active lifestyle, including low motivation, self-efficacy, negative past experiences, and environmental barriers such as limited access to facilities and time constraints [19]. For exercise programs to be successful, they must identify these challenges and propose practical solutions to address them.
In addition to low compliance with the exercise prescription, other factors, such as training intensity, variability in response to training, the genetic makeup of the participants, and the effect of detraining on CRF, are potential causes related to the lack of change in VO2max observed in our study. The changes in VO2max following an exercise training program are associated with the volume of exercise, i.e., duration × intensity [20]. However, studies where exercise volume was equal across groups but exercise intensity varied showed that those who exercised at vigorous and near-maximal intensities had a 14.3% and 20.6% increase in VO2max, respectively, compared to a 10% increase in VO2max in those who exercised at moderate intensity [21]. The effect of exercise intensity on the amount of change in VO2max depends on the age and training status of the participants. In an overview of Systematic Reviews and Meta-Analyses, it was reported that those who experienced the most significant increase in VO2max were older and less fit individuals who exercised at longer intervals (2-4 minutes of work/bouts), with high volume (15 minutes of work/session), and for at least four weeks [22]. In our study, the impact of CRF at baseline on change in VO2max was noticeable, as almost half of those with low CRF at baseline experienced an increase in VO2max of more than 10%. In contrast, none of those in the superior to excellent category of CRF showed an increase in VO2max of more than 10%.
The HERITAGE Family study showed that there is ample variability in response to a 20-week endurance exercise training program. The mean (SD) change in VO2max was 5.4 (±2.8) mL/kg/min (17%), with a range of -4.7% to 47.8%. [23] Interestingly, 49% of the variation in the change in VO2max following the exercise training program was associated with 21 single-nucleotide polymorphisms (SNPs) identified during the HERITAGE Family study [24]. This highlights the vital contribution of genetics to baseline VO2max and the ability to improve VO2max in response to exercise training. Detraining is another essential factor that may have contributed to the lack of change in VO2max between the two clinic visits, as it has been shown that only two weeks without exercise training can decrease VO2max by 4%, and the decrement can be substantially larger the longer the time passes without training [25]. Unfortunately, we do not have data on their training intensity, the duration of the training sessions, and their adherence to the exercise prescription provided at the wellness clinic, or their genetic makeup. Nevertheless, a combination of these factors was likely responsible for the variation in the change in VO2max observed amongst participants of this study.
Another potential reason for the decline in CRF over two years is the natural decrease in VO2 max with aging. The Baltimore Longitudinal Study on Aging found that the decline rate in CRF increased from 3-6% per decade in those in their 20s and 30s to 20% per decade in those 70 and older [26]. The Aerobics Center Longitudinal Study confirmed that after age 45, CRF decreases more rapidly, particularly in men [17]. This decline is also greater with each unit of BMI and in smokers; however, this study's cohort had lower obesity rates and smoking prevalence compared to the general population [17]. Although the decline in CRF with age is independent of baseline fitness [17,27], those who engage in regular endurance training maintain higher VO2max levels than sedentary peers of the same age and sex [27].
Despite strong evidence linking increased CRF to better health, a longer lifespan [28] and reduced healthcare costs [29], only 24.2% of U.S. adults met the 2018 aerobic and muscle-strengthening recommendations [30]. Additionally, sedentary behavior, defined as energy expenditure of ≤1.5 METs and linked to a higher risk of cardiovascular disease, remains prevalent [31]. This highlights the urgent need for strategies to motivate individuals to be more active and reduce sedentary behavior.
To understand the consequences of being in the low category of CRF, it is essential to know the energy costs associated with activities of daily living (ADL). These activities include home chores, gardening, and light walking, among others, requiring an energy cost in VO2 between 14 and 17 mL/kg/min or an energy cost in METs between 4 and 5 [32], In our group of participants, those in the low category of CRF had a mean VO2max that was 5 to 10 mL/kg/min above the level required for ADL. This suggests that for some individuals, performing ADLs requires an energy expenditure of approximately 75% of their maximal endurance capacity. As these individuals were in their 50s and mostly men, who have a faster rate of decline in CRF than younger individuals and women [17], a 20% decline in their VO2max would require them to use their maximal CRF when performing activities of daily living.
This study found CRF improvements in a small percentage of participants, mainly those with low baseline CRF. The HERITAGE Family study showed no correlation between baseline VO2max and absolute change after 20 weeks of endurance training; however, a correlation existed between baseline VO2max and percent change, indicating that lower initial values yielded higher relative gains [33]. A meta-analysis indicated that younger individuals who trained longer and had lower baseline CRF were more likely to improve VO2max with high-intensity interval training than continuous endurance training [34].
Strengths and Limitations
This retrospective analysis maintained consistent VO2 max assessment procedures, as the supervisors were the same or trained by the same individual. A limitation is that some participants may have requested to end the test before reaching their actual VO2max; however, all participants met the criteria outlined in the methods for the maximal exercise test. Because this was not a randomized trial, the participants who attended a second visit may not be representative of the general population, which limits the ability to generalize the results. Furthermore, the group of individuals self-selected to return for a second visit, which introduces selection bias in our results. Unfortunately, we don’t have data on the reasons why some individuals returned for a second visit and others did not. These reasons could modify the results reported in this study and limit the generalizability of the findings to the broader population. However, it was noticeable that at baseline, the two groups were similar in CRF level, BMI, and other metabolic and cardiovascular variables. Missing data, a common limitation in retrospective analysis [12], is also present in this analysis. However, data on age, sex, and VO2max are available for all participants who returned for a second visit, which is how data on cardiorespiratory fitness are standardized. Additionally, the lack of compliance data with exercise prescription prevents the analysis of factors affecting participants' changes in CRF, which is crucial for designing effective exercise programs. Future studies examining the effectiveness of exercise training programs in inducing changes in CRF in healthy and disease populations should utilize wearable physical activity tracking devices to assess adherence to the exercise training program, as these devices are effective at improving CRF [35].
CONCLUSIONS
After nearly two years, an unsupervised exercise program did not improve CRF in most participants, regardless of baseline fitness levels. However, almost half of the individuals with a low baseline CRF showed improvement in their VO2max. Future research should identify the strategies these individuals used to enhance their CRF, which could be evaluated and applied to help others improve their cardiorespiratory fitness and reduce health risks.
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