Cardiorespiratory Benefits of Exercise

Abstract

Abundant evidence proved that the amount of habitual exercise and the level of cardiorespiratory fitness (CRF) are inversely related to the risk of cardiovascular morbidity and mortality. In this chapter, you can learn about the cardiorespiratory benefits of exercise, involving: (1) delay the development of cardiovascular disease (CVD) affecting many of the standard cardiorespiratory diseases risk factors, such as plasma lipids, especially high-density lipoprotein cholesterol, fasting glucose levels, blood and hypertension control; (2) improve the cardiac output (CO) and the CRF of different ages. However, certain kind of exercise might not be applicable to cardiac patients, since high-intensity, high-volume exercise may increase all-cause mortality among these patients. At present, the American College of Sports Medicine (ACSM) recommends that aerobic exercise (AE) and resistance exercise (RE) two or three times a week is related to better physical function at different ages, improvement of muscle strength, body composition and, especially, CRF.

1. Introduction

According to the statistics data, noncommunicable diseases (NCDs) such as cardiovascular disease (CVD), diabetes, and cancer are still on the rise [1, 2]. NCD resulted in more than 70% of all deaths globally, and there were about 41 million people killed in 2016 [3]. The majority of these deaths were due to CVD [3]. CVD accounts for 44% of these deaths, which translates to about 30% of all deaths [3]. CVD includes coronary artery disease, hypertensive heart disease, stroke, and thrombosis [4]. The risk factors for CVD are varied, such as high levels of plasma lipids and fasting glucose, lack of physical activity, and especially hypertension [4, 5].

Hypertension is one of the most fatal but preventable CVD worldwide [6]. The increased blood pressure is the main risk for death and disability which roughly accounts for 10% of medical healthcare expenditure [7, 8]. Around 40% of adults aged 25 years or older have hypertension, with an estimated 51% of stroke and 45% of heart disease caused by increased blood pressure [7, 9]. Hypertension may also contribute to complications in other prominent conditions such as coronary artery disease, thrombosis, and diabetes mellitus [10]. Moreover, the United Nations developed a target to decrease the incidence rate of nine chronic disease by 2025, of which four were related directly or indirectly to hypertension [9]. According to the statistics data, approximately 50% of hypertension was attributable to the lack of physical activity, 30% to high dietary salt, 15% to low dietary potassium, and 5% to excess alcohol intake [9]. For instance, physically inactive middle-aged women have been reported to have a 52% increase in all-cause mortality and a doubling in cardiovascular-related mortality [11]. In the general population, improving cardiorespiratory fitness (CRF) is a key countermeasure in the prevention of cardiorespiratory diseases and mortality [12, 13]. A study has shown that physical activity was associated with a risk reduction of more than 50% of CVD [14].

Due to the diversity of CVD, studies usually assess only some of the risk factors [15]. It is hard to issue specific recommendations for each CVD [15]. The American College of Sports Medicine (ACSM) suggested aerobic exercise (AE) of moderate intensity for five or more days per week with 30–60 min per session, or three or more vigorous-intensity exercise sessions with 20–60 min per session [16].

Whether physical activity had an effect on cardiovascular fitness and the magnitude of this effect would depend primarily on the frequency, intensity, type, and duration of the exercise [17, 18]. Physical activity can be classified in different ways, such as AE and resistance exercise (RE). AE is the type of exercise in which oxygen is utilized by working muscles [19]. It increases heart rate and energy consumption by dynamic repetitive contractions of major muscle groups [19]. AE is usually performed at a moderate intensity for 30–45 min in the form of continuous running, cycling, jogging, or swimming [20]. RE is the physical activity in which effort is spent against a specific resistive force and which is especially designed to increase muscle strength and endurance [21, 22]. AE has more benefits than RE on the increase of cardiovascular fitness [23]. For instance, AE usually affects all of the risk factors for CVD, including hypertension, plasma lipids, fasting glucose, and so on [24]. However, regular participation in RE can promote cardiovascular fitness by reducing body fat, increasing metabolic rate, lowering blood pressure and cholesterol levels, and increasing glucose tolerance [23, 24]. Moreover, RE can also improve CVD risk factors such as glucose metabolism and insulin sensitivity [24]. The higher muscle strength level is associated with better cardiometabolic, lower all-cause mortality, and fewer CVD events [1, 12]. Thus, AE and RE can be a good choice to promote cardiovascular fitness.

In conclusion, CVD remain the leading cause of death in the industrialized world. Fortunately, as an important part of health-related fitness, the level of cardiovascular fitness is inversely related to the risk of cardiovascular morbidity and mortality. Both AE and RE can increase cardiovascular fitness to a certain extent. Finally, before formulating an exercise plan, an appropriate health manager (such as a primary therapist, internist, or cardiologist) should be consulted to ensure the safety of exercise.

There were strong evidence supporting the cardiovascular health benefits associated with daily physical activity and exercise. Population-based studies have demonstrated that high levels of daily physical activity were associated with a lower risk of cardiovascular events and CVD mortality [25]. Exercise has positive physiologic effects on cardiovascular health, including but not limited to its impact on plasma lipids, especially high-density lipoprotein (HDL) cholesterol, insulin resistance, and hypertension control [26].

2. Risk factors for cardiovascular disease

2.1 High-density lipoprotein cholesterol

The primary driver of CVD development in humans appears to be the elevated level of blood cholesterol [27]. The same lipoprotein particles carry dietary and endogenous lipids. Chylomicrons transport dietary lipids, whereas low-density lipoproteins (LDLs) and high-density lipoproteins (HDLs) transport endogenous lipids. Triglycerides (also known as triglyceride-rich lipoproteins [TRLs]) made in the liver and intestines are transported to capillary beds by very low-density lipoproteins (VLDLs), where they serve as a source of energy for the target tissues [28]. The main method for moving cholesterol from a lesion site back to the liver is through HDL cholesterol. HDL reduces the development of plaque by a process known as “reverse cholesterol transfer” [29]. In addition, VLDL particles cannot pass through the endothelium wall due to their size, while LDL particles can pass through. However, in circulation, VLDLs have the potential to undergo hydrolysis along the luminal surface of capillaries, resulting in the production of free fatty acids and TRL remnants, which are higher in cholesterol than LDLs [29]. These leftovers are capable of being ingested by macrophages without being oxidized and are thus thought to exert a potent atherogenic effect [28]. Therefore, by modifying the lipid “profile,” CVD risk can be decreased (i.e. lowering serum levels of LDL and triglycerides, and increasing HDL cholesterol).

Evidence from the last several decades supported the notion that exercise training has a favorable effect on the blood lipid profile [30]. One comprehensive review found that moderate to high intensity of AE increased the HDL cholesterol level (mean change across the studies reviewed was +4.6%) [31], in which diet was held constant for the participants. In a large randomized controlled trial, Kraus et al. [32] investigated the effects of AE of different volume and intensity on blood lipids. They randomly assigned 111 men and women with mild to moderate dyslipidemia into four groups: a high volume (jogging about 32 km/week) of high-intensity exercise (65–85% of peak oxygen consumption) and a low volume (jogging about 19.2 km/week) of high-intensity exercise (40–55% of peak oxygen consumption). In comparison with groups that engaged in low intensity/volumes of exercise, the high-intensity/high-volume exercise group showed a significant increase in the level of HDL cholesterol (+0.11378 mmol/l) and a decrease in triglycerides (−0.7344 mg/dl), but no significant change in the level of LDL cholesterol. Additionally, O’Donovan et al. [33] found that previously sedentary but healthy males who underwent 24 weeks of high-intensity (80% of aerobic capacity) AE had a significant decrease in LDL cholesterol levels, but not after low-intensity (60%) AE. A more recent metaanalysis [30] found that high-intensity aerobic interval training (i.e. periods of high-intensity exercise interspersed with periods of active/passive recovery) was more effective than moderate-intensity continuous exercise at raising HDL cholesterol levels in subclinical or clinical populations, independent of dietary or pharmaceutical interventions (e.g. healthy or obese individuals taking medications). Meanwhile, RE (i.e. strength exercises using body weight or external resistance) with greater volume of exercise (e.g. more sets and repetitions), but not necessarily greater intensity (e.g. higher loads), has been proven to significantly reduce the level of LDL cholesterol and triglycerides [24].

In addition, exercise enhances the skeletal muscle’s capacity to utilize lipids as the main dietary source, which leads to a decrease in plasma lipid levels [34]. The upregulation of lipoprotein lipase, which hydrolyzes triglycerides into free fatty acids and encourages cellular uptake of TRL remnants, or lecithin cholesterol acyltransferase, which is involved in HDL cholesterol formation and, consequently, reverse cholesterol transport, may be used to partially achieve this goal. According to one previous research, an energy expenditure threshold (i.e. roughly 1000 kcal) must be met to elicit enhanced lipoprotein lipase activity in well-trained men [35]. This threshold may differ for different people based on their exercise history, disease condition (healthy vs. hypercholesterolemic), exercise intensity, age, sex, and type of exercise.

2.2 Insulin resistance

Insulin resistance is a significant predictor of CVD in patients with type 2 diabetes, since it is linked to the cluster of CVD risk factors mentioned earlier (high cholesterol and high blood pressure) [36]. The β cells of the pancreas secrete more insulin when an individual is insulin-resistant due to impaired glucose metabolism [37]. Vascular smooth muscle cells grow and proliferate when insulin levels are high [38]. This is due to the fact that elevated insulin level triggers the inflammatory pathways [39], which reduces nitric oxide levels and stimulates the secretion of endothelin-1; this in turn encourages vasoconstriction and atherogenesis [40]. Exercise might not be sufficient as a stand-alone treatment, as suboptimal diets are the primary lifestyle factors for insulin resistance. However, there were evidence showing significant beneficial impact of exercise on the insulin resistant state [41].

A series of research provided strong evidence for the efficacy of exercise in the treatment of type 2 diabetes. Boulé et al. [42] reviewed seven randomized controlled trials which compared the treatment effects of AE intervention (an average of 50 min per session and 3 sessions per week for 20 weeks) and control group (non-exercise) on patients with type 2 diabetes. As expected, the exercise intervention led to an approximate 12% increase in aerobic fitness compared with that in the controls. Exercise with higher intensity produced greater improvements in aerobic fitness and blood glucose control as defined by the reduction in glycated hemoglobin (HbA1c). However, exercise volume expressed as total weekly energy expenditure neither predicted the change in aerobic capacity nor in HbA1c. Later, this research group [43] further suggested that a threshold of 150 min of weekly structured exercise must be achieved for the significant reduction in HbA1c. An updated meta-analysis reported that patients with a higher level of HbA1c at the beginning of an intervention would experience a greater reduction in HbA1c, besides the magnitude of the reduction was associated with the volume of weekly AE or combined exercise (CE), but not the volume or intensity of RE [44]. However, Church et al. [45] demonstrated that patients with type 2 diabetes (HbA1c levels above 6.5%) involved in moderate-intensity AE (i.e. 150 min/week at 50–80% of the maximum intensity) combined with RE twice weekly for 9 months presented superior HbA1c decrement compared to those engaged in aerobic or resistance exercise alone.

Together, these data suggested that a minimum dose of approximately 150 min of accumulated exercise over the course of a week may be required to significantly influence the insulin resistant state. Additional beneficial effects may be achieved by increasing the intensity of AE. Resistance training (RT) may be most impactful when practiced in combination with an AE.

The mechanisms of exercise ameliorating diabetes have been well studied [46]. Exercise is emphasized as a therapeutic cornerstone for individuals with metabolic illnesses such as diabetes mellitus, as exercise-stimulated glucose absorption is preserved in insulin-resistant muscle [46]. Through the simultaneous enhancement of three crucial processes, including delivery, transport across the muscle membrane, and intracellular flow through metabolic pathways (glycolysis and glucose oxidation), exercise increases the absorption of glucose by up to 50 times [46]. Due to the complexity of the signaling pathways that regulate glucose uptake to ensure the maintenance of muscle energy supply during physical activity, the available data suggested that no single signal transduction pathway can fully account for the regulation of any of these important procedures [46]. Increased blood flow proportionate to exercise intensity caused an increase in glucose supply to working muscles, and an increase in skeletal muscle perfusion was related to the increase in glucose uptake [47]. Furthermore, it has been reported [48] that following just one cycling exercise session (i.e. 45–60 min at 60–70% of one’s maximum effort), glucose transporter 4 (GLUT-4) concentrations were elevated by about 70% in comparison to baseline in both apparently healthy people and patients with type 2 diabetes. The glucose transporter known as GLUT-4 is responsible for facilitating the passage of plasma glucose into muscle and fat cells. Finally, it was discovered that GLUT-4 activity was closely related to the cell metabolism of glucose. During exercise, glycogen is the primary energy source. In order to produce the adenosine triphosphate required for exercise, glycogen is gradually hydrolyzed to blood glucose as activity continues and glycogen stores become exhausted [46]. In conclusion, exercise is an efficient treatment for CVD by simultaneously boosting the elements that enhance glucose delivery, transport, and metabolism.

2.3 Hypertension

Hypertension is one of the most fatal but preventable CVD worldwide [49]. It tends to coexist with hypercholesterolemia [50]. Hypertension is closely associated with inactive lifestyle. Exercise was shown to delay the development of hypertension. AE was recommended by the American Heart Association/American College of Cardiology [51], the European Society of Hypertension/European Society of Cardiology [52], and the Canadian Hypertension Education Program [53] as the first-line treatment for the prevention, treatment, and control of elevated blood pressure or hypertension. Numerous randomized controlled trials supported an average drop in blood pressure of 5–7 mm Hg following AE programs [54]. These governing bodies all agreed that exercise should be undertaken on the majority of days of the week, if not every day. It was founded on the discovery of post-exercise hypotension, or the sudden drop in blood pressure that occurs after a single, severe episode of AE [55]. This drop in blood pressure can sustain for a full day [56]. The drop of the resting blood pressure following long-term AE training was correlated with the drop of the blood pressure after acute exercise [57]. Furthermore, the magnitude of drop of the blood pressure appears to be dose-dependent, which was supported by Eicher et al. who measured ambulatory blood pressure in 45 pre- or stage 1 hypertensive men after they completed low-intensity (40% of peak oxygen consumption), moderate-intensity (60% of peak oxygen consumption), and high-intensity (100% of peak oxygen consumption) exercise, respectively [58]. High-intensity exercise resulted in a blood pressure drop of 11.7/4.9 mm Hg, followed by moderate-intensity exercise (5.4/2.0 mm Hg), and low-intensity exercise (2.8/1.5 mm Hg). A group of persons with resistant hypertension (e.g. blood pressure > 140/90 mm Hg despite using three antihypertensive medicines) who exercised at a moderate intensity three times per week for 8–12 weeks showed a significant decrease in ambulatory blood pressure (6/3 mm Hg) [59]. Even middle-aged hypertension individuals who were deprived of antihypertensive medications experienced a significant drop in resting blood pressure (−16/−12 mm Hg) after moderate-intensity RT.

In conclusion, suggestions in practice for hypertension were as follows: (1) a dose-response relationship exists between AE intensity and blood pressure; (2) moderate-intensity exercise can be used to achieve clinically significant reduction in blood pressure among individuals with established hypertension but resistant to drug therapy; (3) reductions in blood pressure are observed across different exercise modalities [56].

The principal mechanism of exercise maintaining or lowering blood pressure was believed to be the reduction in total peripheral resistance. Exerciseinduced vascular and autonomic adaptations have been proposed to potentially provide major contributions to blood pressure control [60]. First, hypertensive patients show enhanced sympathetic control, which causes vasoconstriction of arterial beds and thus raises total peripheral resistance [61]. This could be induced by the heightened sensitivity of baroreceptor, which are located in the carotid sinus and aortic arch and are in charge of detecting changes in blood pressure [57]. Baroreceptor’s sensitivity is influenced by exercise [62]. For instance, after 4 months of cycling exercise (three times per week at 70% of maximum capacity), the improvements in baroreflex control was corresponded to the reduction in blood pressure and muscle sympathetic nerve activity in a group of hypertensive patients, and their sensitivity was reset to those observed in normotensives [63]. Second, through the local vascular control mechanisms, exercise improves vascular function [64]. In particular, exercise reduces the bioavailability of endothelin-1, a vasoconstrictor, and increases the bioavailability of nitric oxide, a powerful vasodilator [65]. The endothelial cells secrete both of these two chemicals. Endothelial function improved as a result of exercise training, which improved the nitric oxide vasodilator [66]. A recent study [67] observed similar improvements in endothelial function among patients with prehypertension or hypertension who engaged in 8 weeks of aerobic, resistance, or combined exercise. Endothelin-1 concentrations were observed to be higher in older, normotensive women who had previously been sedentary (aged 61–69) than in younger women (aged 21–28) [68]. However, after 3 months of cycling activity (5 days per week), the concentrations were markedly lowered along with the blood pressure. Third, physical activity increases the compliance of large elastic arteries, the aorta, and carotids for instance, which attenuates the fluctuations of the pressure for each heartbeat. Left ventricular hypertrophy would arise from an increase in ventricular afterload caused by a decrease in arterial compliance. Following 3 months of AE training (4–6 days per week, at 70–75% of maximum capacity), an improvement in arterial compliance of the carotid artery was also noticed in middle to old age sedentary normotensive men (50 1 years) [69]. In contrast, people with isolated systolic hypertension did not experience any changes in systemic arterial compliance following 8 weeks (3 days per week) of moderate-intensity (65% of maximal capacity) cycling [70]. The carotid artery stiffness in a group of elderly hypertensives (68.2 ± 5.4 years) was unaffected by 20 weeks (3 days per week) of moderate-intensity (70% of the maximum capacity) AE [71]. Stewart et al. [72] found that while diastolic blood pressure improved following 6 months of combined aerobic and RE in a group of old (55–75 years) hypertensive patients, no changes in systolic blood pressure or aortic stiffness were observed. These findings may be related to genetic, dietary, or long-term training habits and did not provide insight into the effects of training in previously sedentary patients with isolated systolic hypertension. These conflicting data suggested that, in the future, we need to further investigate the effect of exercise on elastic vascular adherence.

3. Benefits of exercise on cardiovascular disease

3.1 Cardiac output

3.1.1 Four determinants of cardiac output

Cardiac output (CO) is the amount of blood pumped out of the left or right ventricle per minute, namely the product of heart rate and output of each stroke. It is expressed with liter/minute. If the heart rate was 75 times per minute, the CO would be 5–6 L in men and slightly lower in women. CO varies with people’s metabolism rate and activity level. It increases with muscle movement, emotional agitation, pregnancy, etc., and is determined by the heart rate, myocardial contractility, preload, and afterload (Figure 1).

Figure 1.

Cardiac output. By Figdraw (www.figdraw.com).

3.1.2 Heart rate

Heart rate refers to the frequency of heart contraction beats, or the number of beats per minute (bpm). When a person is at rest, heart rate is 60–100 times per minute (60–100 bpm). Heart rate will increase during exercise, and athletes with better cardiopulmonary function will have a slower heart rate than people without exercise experience. The age-predicted HRmax eq. (220-age) is commonly used as a basis for prescribing exercise programs. It is considered as a standard for achieving maximal exertion and as a clinical guide during diagnostic exercise testing. A direct percentage of HRmax or a fixed percentage of heart rate reserve (HRmax − heart rate at rest) is used as a basis for prescribing exercise intensity in both rehabilitation and disease prevention programs [73].

Heart rate in adults depends on the activity of the sinoatrial node cells and constantly varies under the influence of a number of nonmodifiable and modifiable factors. The sinoatrial node cells are innervated by the parasympathetic fibers of the vagus and sympathetic nervous thoracic efferents and regulate the heart rate [74]. It was reported that increased all-cause mortality and cardiovascular risk events are associated with high heart rate, more so in men than in women. An increase of 10 beats per minute in a person’s heart rate is associated with a 20% increase in cardiac death [75]. Heart rate recorded in elderly men has a strong predictive value in the survival to a very old age.

3.1.3 Contractility

Contractility is the inherent strength and vigor of the heart’s contraction during systole. According to Starling’s law, the heart will eject a greater stroke volume at greater filling pressures. Heart contraction is initiated by the action potential propagated from sinoatrial node cells [76]. Following which, Ca²⁺ influx through mainly L-type Ca²⁺ channels in the surface membrane promotes further release of stored Ca²⁺ from the sarcoplasmic reticulum (SR) via the SR Ca²⁺ release channel (the ryanodine receptor, RyR) by a process known as Ca²⁺-induced Ca release. The two Ca²⁺ fluxes mentioned earlier combine to initiate contraction [77].

3.1.4 Preload

Preload is the filling pressure of the heart at the end of diastole. The greater the preload, the greater volume of blood will be in the heart at the end of diastole. Preload builds up during diastolic filling and stretches cardiomyocytes. Left atrial pressure (LAP) determines the volume of the heart at the end of diastole, which relates to the filling pressure (preload). The relationship of stroke volume index to LAP is usually plotted as Starling’s law. Contractility and afterload modify this relationship.

3.1.5 Afterload

Afterload is the force against which the myocardial fibers must contact during the ejection phase of systole [78]. In response to a decrease in CO, the body’s homeostatic system will attempt to maintain blood pressure by increasing vascular resistance in the system. When the preload reserve is reduced and the ventricles are hypocontractible, a decrease in afterload generally results in an increase in output [79]. By increasing output, hypotension will be countered, resulting in good clinical results.

3.2 Cardiorespiratory fitness

CRF refers to the capacity of the circulatory and respiratory systems to supply oxygen to skeletal muscle mitochondria for energy production needed during physical activity [80]. It is an intermediate variable between physical activity behavior and health outcomes and reflects the ability of many body organs (such as heart, lungs, and muscles) to generate energy during physical activity and exercise [81]. Maximal oxygen uptake (VO_2max) is considered the gold standard for CRF and can be estimated using either a maximal graded cardiopulmonary test or an indirect calculating method [82]. CRF is associated with CVD and is a strong independent predictor of all-cause mortality in adults [80]. Higher levels of CRF were associated with lower cardiovascular morbidity and all-cause mortality [83].

3.2.1 Physical activity and cardiorespiratory fitness

Physical activity and CRF were associated with health and quality of life, and even small improvements in physical fitness were associated with reduced CVD and all-cause mortality [84]. Based on the importance of physical activity to CRF, the current physical activity guidelines recommend that all adults engage in at least 150 min of moderate-intensity exercise or 75 min of vigorous-intensity exercise per week [84]. To improve CRF, current evidence suggests that physical exercise must achieve a minimum intensity of at least 45% of the oxygen absorption reserve in the general population and at least 70–80% in athletes [83]. Vigorous exercise was associated with greater improvements in VO_2max than moderate-intensity exercise [84].

3.2.2 Training type and cardiopulmonary fitness

In recent years, many studies have identified moderate-intensity continuous training (MICT), high-intensity interval training (HIIT), and high-intensity functional training (HIFT) as interventions for CRF [85, 86]. MICT is a traditional way to increase physical activity, and its effectiveness depends on training for longer periods of time [86]. HIIT is defined as intervals of alternating high-intensity and low-intensity activity, or short bursts of passive recovery [82]. HIIT is recommended because of its short duration (20–30 min) and high efficiency in improving physical fitness and health parameters [86]. HIFT is an exercise modality that emphasizes functional, multi-joint movements that can be tuned to any fitness level and stimulate muscle replenishment more than traditional exercise. As a relatively new training form, HIFT is often compared to HIIT, yet the two are distinct [85].

3.2.3 Cardiac output and cardiopulmonary fitness

Research evidence suggested that higher CRF is associated with higher CO through augmented stroke volume or heart rate, and lower systolic blood pressure, pulmonary arterial pressure, and vascular resistance [87]. In cardiopulmonary exercise testing (CPET), peak oxygen uptake (VO₂peak) is the gold standard for CRF measurement, and research evidence showed that the decline of VO₂peak is mainly related to a lower maximum CO [87].

3.3 Improve the cardiac output and the cardiorespiratory fitness

3.3.1 Preschool children (ages 3–5 years)

Regular physical activity in preschoolers is essential for normal growth and development, providing immediate and long-term benefits for physical and mental health, while the World Health Organization recommends that typically developing children between the ages of 3 and 5 years should be physically active for 3 h a day [88].

An important point of enhancing cardiopulmonary fitness is to execute moderate- to high-intensity physical activity (MVPA) [89], and aerobic training has been proved to effectively improve the VO_2max of preschool children [90]. According to a study of meta-analysis, preschool children are mainly focused on improving coordination and perception, such as 40 min of moderate-to-higher intensity physical activity (brisk walking, jogging, jumping, squatting, crawling, and other alternative exercises) [91].

In conclusion, moderate- to high-intensity physical activity and aerobic training can improve the CRF of preschool children.

3.3.2 Children and adolescents (ages 6-17 years)

Enhancement of CRF has been identified as a key goal for reducing cardiovascular metabolic risk in children and adolescents [92]. A scientific statement from the American Heart Association indicates that high-intensity exercise, including high-intensity interval training, can improve cardiopulmonary fitness in adolescents [80]. At the same time, a study showed that HIIT is a better training methodology to improve CRF among healthy children and adolescents compared to MICT based on the characteristics and efficiency [82]. Secondly, a systematic review study found that HIIT influenced neuromuscular and an AE performance in children and adolescents, including jumping performance and the number of sit-ups [93]. Therefore, more and more research evidence suggested that HIIT may improve the cardiopulmonary fitness of children and adolescents.

3.3.3 Young adults (ages 18-44 years)

As an indicator of CRF, high VO_2max indicated a relatively great level of aerobic capacity which was associated with better physical performance [94]. In the general adult population, HIIT is highly effective as an intervention to enhance CRF, but not significantly for the adult population with exercise habits [94]. There are also studies on RT as an intervention program to enhance CRF. However, the effect of cardiovascular adaptation from RT was controversial, and some researches believed that RT had no benefits on the increase of CRF [95]. A study suggested that RT could not promote CRF mainly because of insufficient training intensity [96]. However, Keith believed that in the long run, high-intensity RT produces more physiological adaptations and contributes to observed improvements in cardiovascular health, such as increased mitochondrial enzymes, mitochondrial proliferation, and conversion of type ii X muscle fibers to Type ii A muscle fibers, and vascular remodeling [97]. Therefore, high-intensity RT can improve CRF. Currently, endurance exercise is well recognized to improve CRF and cardiometabolic risk factors.

In the general adult population, HIIT, high-intensity RT, and endurance exercise can improve CRF.

3.3.4 Middle-aged (ages 45-59 years) adults

HIIT has been promoted as a superior, time-saving exercise strategy to enhance CRF in the middle-aged [98]. A systematic review of evidence suggested that both interval training and MICT could significantly improve CRF in middle-aged and elderly people; however, MICT has no significant effect on improving VO_2max in middle-aged compared with HIIT and sprint interval training. This may be due to their exercise intensity and a substantially reduced training volume and lower time commitment [99]. Therefore, compared with MICT, HIIT is better for promoting maximal oxygen uptake in middle-aged adults. Circular resistance training also showed significant effects on improving circular resistance training, strength, and optimizing body composition in middle-aged [100]. In the middle-aged adults, HIIT, MICT, and circular resistance training can all produce adaptability, so the above training methods can be considered for exercise programs involving the middle-aged [101]. However, HIIT is still the preferred training method.

3.3.5 Younger-old (60-70) adults

With the increase of age, it was documented that the cognitive function of the elderly group declined. Besides, attention control was also affected (such as talking, listening, and writing while driving), which led to the decline of executive function (such as driving and walking) [102]. In fact, higher CRF has been shown to be related to brain structural modification, changes in functional connectivity and higher cerebral blood volume that could enhance cognitive function [102]. Physical training, especially AE (40 min or so) for 8–72 weeks, has been shown to be effective in improving cognitive function and regulating CRF in older adults [103]. Secondly, a systematic review suggested that HIIT and MICT can improve the CRF in younger-old adults, but HIIT is more effective at improving brachial artery vascular function than MICT, perhaps due to its tendency to positively influence CRF [104]. In RT, multi-articular RT has a better effect on CRP than single-articular RT [97]. These positive effects may be due to training intensity because high-intensity RT increases basal metabolic rate and fat oxidation. The multi-articular RT belongs to high intensity [97].

In conclusion, physical exercise, HIIT, and multi-articular RT are more likely to enhance CRF in younger-old adults.

3.3.6 Older-old (70+) adults

Aging is associated with a decline in CRF and VO_2max (maximum oxygen intake per unit of time). In healthy older adults over 70 years, aging accelerates the decline in VO₂peak values (the highest point in oxygen uptake) by an average of 20–25% per decade. Furthermore, cognition, physical abilities (such as muscle strength, balance, and cardiovascular endurance), body function, and independence generally decline with aging [105]. At the same time, increased age is associated with increased risk of injuries during daily living activities and CVD [106].

Recent studies with small sample size and less intervention time showed that HIIT not only increased CRF in healthy elderly people but also had a positive effect on muscle strength, oxidative stress, inflammation, and insulin sensitivity [107]. RT has not been considered as a training method to improve CRF, but some studies have found that RT may be viable means to improve cardiopulmonary endurance in the elderly [108]. This may be due to increased oxidative enzyme activities or by increasing leg strength so that muscle weakness does not preclude achievement of VO₂peak [108]. MICT and RT were superior to HIIT in enhancing executive cognitive function in the elderly, while HIIT was more conducive to improving physical endurance [109].

In conclusion, HIIT is still an optimal training method to improve CRF in the elderly, while aerobic training, MICT, and RT can also include in the intervention program.

As outlined above, AE, RE, and CE were suggested as the effective exercise intervention for people with CVD risk. Detailed information related to the exercise prescription would be discussed in this section. Exercise prescription includes six components: frequency, intensity, time, type, volume, and progression (FITT-VP). Frequency describes how often one executes exercise, intensity signifies how hard the exercise is, time refers to the total time spent on exercise (length of each training session, daily or weekly exercise), type stands for what kind of exercise one chooses to participate, volume is determined by a combination of frequency, intensity, and time, and progression means the adjustment of the frequency, intensity, and time of the exercise to gradually achieve the exercise goal [20, 110].

4. Exercise prescription for the improvement of cardiovascular fitness

4.1 Aerobic exercise

In AE, the large muscles move in a rhythmic manner for a sustained period. AE causes the heart rate to increase and breathing to become more labored [19]. AE is usually performed in a moderate intensity for 30–45 min in the form of continuous running, cycling, jogging, or swimming [20].

4.1.1 Frequency of aerobic exercise

To promote or maintain health/fitness, preschool-aged children should be physically active throughout the day to enhance growth and development [111]. Children and adolescents aged 6 through 17 years should include physical activity at least 3 days a week [111]. The ACSM recommended moderate-intensity AE for most adults at least 5 days per week, or at least 3 days of vigorous AE per week, or a combination of moderate and vigorous exercise 3–5 days a week [110]. Some adults can improve their health with moderate to vigorous activity only 1–2 days a week [110]. However, irregular vigorous exercise may increase the risk of cardiovascular events, and therefore 1–2 days of exercise a week was not recommended for most adults [20]. This recommendation for most adults also applies to older adults. It is important to note that older people should determine their level of effort in physical activity according to their physical condition [20].

4.1.2 The intensity of aerobic exercise

The intensity of AE can be divided into absolute intensity and relative intensity. Absolute intensity refers to the amount of energy consumed during the activity, regardless of people’s CRF or aerobic ability [110]. It is expressed in the metabolic equivalent (MET) of task units; 1 MET is equivalent to resting metabolic rate or energy consumption during wakefulness and meditation [20]. Relative intensity refers to the degree of effort required to carry out an activity relative to a person’s ability [20]. Since the measurement of absolute intensity does not take individual factors into account, such as weight, gender, and fitness level, it may lead to misclassification of exercise intensity [112]. Therefore, relative intensity is more appropriate in the evaluation of exercise intensity, especially for the elderly [20]. There are several commonly used methods for estimating relative exercise intensity during AE: oxygen uptake reserve (VO₂R), heart rate reserve (HRR), percent of the maximum HR (%HRmax), %VO_2max, and MET [113]. Table 1 shows the approximate classification of AE intensity commonly used in practice.

Aerobic exercise
Relative intensity				Absolute intensity
Intensity	%HRR or %VO2R	%HRmax	%VO2max	METs
Very light	<30	<57	<37	<2
Light	30–39	57–63	37-45	2.0–2.9
Moderate	40–59	64–76	46-63	3.0–5.9
Vigorous	60–89	77–95	64-90	6.0–8.7
Near-maximal to maximal	≥90	≥96	≥91	≥8.8

Table 1.

Classification of exercise intensity: relative and absolute intensity for aerobic exercise.

The table was adapted from the American College of Sports Medicine [112]. HRmax: maximal heart rate; HRR: heart rate reserve; MET: metabolic equivalent; VO_2max: maximal volume of oxygen consumed per minute; VO₂R: oxygen uptake reserve.

In cardiovascular regulation and disease prevention, low-, moderate-, and vigorous-intensity exercise have all exhibited some degrees of health benefit [114]. A significant dose-response relationship exists between exercise intensity and overall cardiovascular benefit [115]. Compared with moderate intensity, vigorous-intensity exercise takes less time to obtain the same benefits of improving CRF and preventing CVD. For example, exercise at moderate intensity for 30 min produces roughly the same as that of 15 min of vigorous-intensity exercise [20]. The study found that exercise performed at higher relative intensity led to a greater increase in aerobic capacity and greater cardiac protection than exercise at moderate intensity [116]. However, vigorous activity can also acutely and transiently increase the risk of sudden cardiac death and myocardial infarction in susceptible people [117]. Consequently, it was recommended that people of different ages should engage in moderate (40–59% HRR or VO₂R) to vigorous (60–89% HRR or VO₂R) AE; people in poor health should undergo low- (30–39% HRR or VO₂R) to moderate-intensity AE to improve CRF and prevent CVD [20]. It is noted that children, the elderly, and the frail should exercise under the guidance of caregivers, doctors, and professional trainers to ensure safety [118].

4.1.3 Time of aerobic exercise

Children younger than 6 years undergo periods of rapid growth and development. The recommended duration of exercise for preschool-aged children is 3 h per day of activity of all intensities [111]. School-aged children and adolescents are in the critical periods for developing movement skills, learning healthy habits, and establishing a firm foundation for lifelong health [20]. Aerobic physical activity for 60 min or more per day at moderate or vigorous intensity was recommended for the majority of them [20]. Most adults are recommended to exercise at least 30–60 min at moderate intensity per day (>150 min per week), or at least 20–60 min at vigorous intensity per day (>75 min per week), or execute a combination of moderate- and vigorous-intensity exercise [110]. Older adults should achieve 150 min of moderate-intensity AE per week [112]. The recommended amount of exercise can be completed by continuous or cumulative time over the course of a day with multiple activities, but at least 10 min each time [119]. Even if the duration of exercise is below the minimum recommended amount, there may be benefits for some people, especially for sedentary people [118].

4.1.4 Type of aerobic exercise

Periodic, large-muscle-involved, low-skill-required, at least moderate-intensity of AE was recommended for most people to promote health and CRF [110]. Sports that require other skills and a higher level of fitness are only recommended for those with appropriate skills and fitness [112]. Table 2 categorizes AE according to different intensity and skills required.

Exercise type	Recommended population	Example
Aerobic exercise requiring minimal skill or physical fitness	Most people	Walking, jogging, recreational biking, water aerobics, slow dancing, and skip rope
High-intensity aerobic exercise requiring a minimum of skill	People who exercise regularly and/or at least moderately fit	Jogging, running, rowing, water aerobics, spinning, elliptical, stair climbing, and speed dancing
Aerobic exercise that requires skill	Skilled people and/or at least moderately fit	Swimming, cross-country skiing, and ice skating
Leisure sports	People with regular exercise and/or at least moderate fitness level	Tennis, badminton, basketball, soccer, downhill skiing, and hiking

Table 2.

Aerobic exercise to improve cardiorespiratory fitness [112, 115].

4.1.5 Volume of aerobic exercise

Findings from epidemiological and randomized clinical trials showed that health/fitness benefits increase with physical activity [110]. Although it was not clear whether there was a maximum or minimum volume of exercise to obtain health/fitness benefits, a total energy expenditure of not less than 500–1000 METs-min/week was strongly associated with lower CVD morbidity and mortality [110]. Therefore, a reasonable volume of exercise recommended for most people is ≥500–1000 METs-min/week. This volume of exercise was equivalent to approximately 150 min/week of moderate intensity exercise or physical activity per week [110]. It was noted that smaller volume of exercise (e.g. 4 kcal/kg or 330 kcal/week) may also provide health/fitness benefits for some individuals, especially those with low fitness. Therefore, it was not possible to establish a recommendation of minimum volume of exercise [20].

4.1.6 Progression of aerobic exercise

The progression of the AE program depends on the frequency, intensity, and time (FIT) of the exercise [110]. When implementing program progression, this can be accomplished by the increase of either one or the free combination of the FIT principles of exercise prescription that the exerciser can tolerate [110]. At the beginning of the exercise program, it is recommended to gradually increase the time/duration of exercise (e.g. the duration of each training session) [110]. A more reasonable progression recommended for the general adult population is to extend the time of each training session by 5–10 min every 1–2 week for the first 4–6 week of the program. After regular exercise at least 1 month for older adults and those with lower fitness, the FIT can be gradually increased over the next 4–8 months until achieving the recommended frequency and intensity [20]. The exerciser’s response should be observed after any adjustment to the exercise prescription for adverse reactions, such as shortness of breath, fatigue, and muscle soreness after exercise, and the exercise volume should be reduced when the exerciser was unable to tolerate the adjusted program [120].

4.2 Resistance exercise

RE refers to the active exercise of muscles when overcoming external resistance [121]. RE makes the muscles of the body work or withstand the force or weight exerted, thus improving muscle strength and endurance [20]. RE usually used self-weights, extra weights, air resistance equipment, and elastic bands or dumbbells for resistance to strengthen muscle groups in various parts of the body [20]. Regular participation in RE promotes cardiopulmonary fitness by reducing body fat, increasing metabolic rate, and lowering blood pressure and cholesterol levels [24]. RE improves CVD risk factors such as glucose metabolism and insulin sensitivity [122]. Thus, higher muscle strength level is associated with better cardiometabolic, lower all-cause mortality, and fewer CVD events [123].

4.2.1 Frequency of resistance exercise

Optimal RE frequency depends on several factors such as volume, intensity, and type of exercise, level of conditioning, fatigue recovery ability, and number of muscle groups trained per workout session [124]. According to previous research, RE is appropriate for most people for 2–3 days per week for the major muscle groups (e.g. upper limb, lower limb, chest, back, and core) in the body [125]. Following a period of RE, progression to intermediate level of training is recommended. This could be exercise at a frequency of 3–4 days per week (if trained for the whole body, 3 days a week was required; if trained in a split way, 4 days a week was required) [126]. A rest period of 48–72 h between sessions is needed to optimally promote the cellular/molecular adaptations that stimulate muscle hypertrophy and are associated gains in strength [127].

4.2.2 The intensity of resistance exercise

The recommendation for intensity of RE was as follows: (1) preschool-aged children use body weight exercises to ensure safety and allow for technical development; (2) <60% of 1 repetition maximum (RM) for children and adolescents; (3) 60–70% of 1 RM interval training for beginners; (4) 80% of 1 RM for experienced strength trainers; (5) 40–50% of 1 RM for older, very deconditioned, or frail individuals; (6) 40–50% of 1 RM for sedentary people [121].

4.2.3 Type of resistance exercise

Research has proven that different training type can be effective in RE for children and adolescents, including the use of one’s own weight as resistance, the use of elastic bands, medicine balls, free weight equipment, and child-sized training equipment [128]. For most adults, many RE tools can be used to effectively improve muscle fitness, including free load, ropes, air resistance equipment, and elastic bands [121]. Older adults can use different types of fitness equipment, including weight training equipment, free weights (barbells and dumbbells), or household items such as plastic bottles filled with water [119]. RE should include multi-joint or compound exercises, which can mobilize multiple muscle groups to participate in sports (such as horizontal push, shoulder push, pull-down, arm flexion and extension, prone push-up, sit-up/knee flexion, kick, and squat) [129]. Moreover, it should also include single-joint exercises, such as biceps curls, triceps extensions, quadriceps extensions, calf bending, heel lift, and core muscle group exercises (such as plate support and bridge) [124]. However, in order to maximize the training effect, we should focus on multi-joint exercise. Multi-joint movement is more complex, allowing more muscles to be involved in it and lift a heavier load [130]. It is recommended to separate the whole-body training and upper and lower limbs training. Separate training of muscle groups includes training of large muscle groups before small muscle groups, multi-joint training before single joints, and high-intensity training before low-intensity training [131]. In addition, pay attention to proper breathing during strength training, usually exhaling when lifting the weight and inhaling when putting it down [127].

4.2.4 Volume of resistance exercise

The volume of training can be accomplished by varying the number of practices per class, the number of repetitions per set, or the number of practices per set [121]. Research has shown that preschool-aged children and adolescents mostly use 1–3 sets of 6–12 repetitions for exercises [111]. For beginners and most adults, RT with 2–4 sets of 8–12 repetitions would be sufficient [20]. For older adults, RT with 1–3 sets of 8–12 repetitions in the beginning could effectively improve strength [132]. For RE of all ages, rest intervals of 2–3 min are most effective for achieving the desired increases in muscle strength and hypertrophy [110].

4.2.5 Progression of resistance exercise

After adapted the muscles to the original load through a RE program, muscle strength and volume can be continued to increase through overload or greater stimulation [127]. Exercisers can gradually increase the resistance, the number of repetitions, and frequency to reach the progression [127]. In general, increase the load by 2–10% (low percentage for small muscle mass exercises and high percentage for large muscle mass exercises) when the individual can complete the required workload once or twice in two consecutive sessions [110]. Note that when children and adolescents perform RE, the focus should be on the proper exercise technique, but not on how much weight to lift [20]. The elderly and frail should receive a health check from a doctor or provider before participating in RE. If necessary, seek supervision and guidance from a qualified fitness instructor to ensure exercise safety [110].

4.3 Combined exercise

The combination of aerobic and resistance exercise has been shown to have an additive effect on the enhancement of CRF and the reduction of the risk of CVD, allowing individuals to achieve greater and more comprehensive CVD health benefits [133]. Studies recommended at least two or more CE sessions per week for children and adolescents [134]. For most adults, CE is better performed at least 2–3 times per week for 30–60 min (15 or 30 min for AE and RE, respectively). AE can be conducted at 50–80% HRR for exercises such as cycling, elliptical, and treadmill. RE should be performed at 50–80% 1RM for two sets of 8–12 repetitions. The movements include muscle groups of the whole body, for example, leg press, hamstring curl, quadriceps extension, chest press, abdominal crunch, lower back extension, and torso rotation [135]. The FITT-VP of CE can refer to the aforementioned information.

5. Conclusion

CVD remains the leading cause of death in the industrialized world. Luckily, increased level of exercise and improved aerobic fitness can dramatically reduce CVD risk. Evidence from randomized controlled trials studies suggested that structured exercise confers cardio protection, delays the development of CVD, and influences many CVD risk factors, such as plasma lipids, especially high-density lipoprotein cholesterol, fasting glucose levels, and hypertension. A general understanding of these benefits can be supportive for physical activity and exercise promotion in health care settings.

CRF is an important part of health-related fitness. AE, RE, and CE can all improve CRF to a certain extent. Moreover, there are a wide range of forms of AE, RE, and CE, which can meet the needs of different groups for CRF enhancement and CVD prevention. Exercisers can choose the appropriate exercise for themselves based on the FITT-VP recommendations provided earlier. When exercising, start with a reasonable amount of exercise and make the process gradually. The elderly and the infirm should exercise under the guidance of doctors and professional coaches to ensure safety.

Fund

The research is funded by the program for Overseas High-level Talents at Shanghai Institutions of Higher Learning under Grant No. TP2019072, Natural Science Foundation of the Higher Education Institutions of Jiangsu Province under Grant No. 17KJB320008, Shanghai Key Lab of Human Performance (Shanghai University of Sport) under Grant No. 11DZ2261100, and Key project of “Science and Technology Winter Olympics” in national key research and development program (2021YFF0307105).