Exploring the Association between Vascular Dysfunction and Skeletal Muscle Mass, Strength and Function in Healthy Adults: A Systematic Review

Background: The prevalence of vascular dysfunction increases with advancing age, as does the loss of muscle mass, strength and function. This systematic review explores the association between vascular dysfunction and skeletal muscle health in healthy adults. Methods: EMBASE and MEDLINE were searched for cross-sectional and randomized controlled studies between January 2009 and April 2019, with 33 out of 1246 studies included based on predefined criteria. Assessments of muscular health included muscle mass, strength and function. Macrovascular function assessment included arterial stiffness (pulse wave velocity or augmentation index), carotid intima-media thickness, and flow-mediated dilation. Microvascular health assessment included capillary density or microvascular flow (contrast enhanced ultrasound). Results: All 33 studies demonstrated a significant association between vascular function and skeletal muscle health. Significant negative associations were reported between vascular dysfunction and -muscle strength (10 studies); -mass (9 studies); and -function (5 studies). Nine studies reported positive correlations between muscle mass and microvascular health. Conclusions: Multiple studies have revealed an association between vascular status and skeletal muscle health in healthy adults. This review points to the importance of screening for muscle health in adults with vascular dysfunction with a view to initiating early nutrition and exercise interventions to ameliorate functional decline over time.


Introduction
The aging process is responsible for a variety of detrimental physiological changes within the human body, including losses of skeletal muscle mass, strength and function (termed sarcopenia) [1,2]. Over time, there is a progressive decline in the number and size of muscle fibers resulting in a total decrease in muscle mass of around 40% between the ages of 25 and 80 years [3]. The numerous negative consequences of sarcopenia are well-established and include an increased risk of falls, fractures, hospitalization, frailty, decreased quality of life and even death [4,5]. As such, sarcopenia has been shown to pose a significant economic burden to health care systems [6]. The prevalence of sarcopenia in community dwelling older adults is estimated to range from 7 to 73.3% in long-term care homes and between 22% and 87% in assisted-living facilities [7,8]. In addition, the progression of sarcopenia is complex and multifactorial, with environmental influences such as physical inactivity, nutrient deficiencies and oxidative stress all postulated to have a role [1,9].
Aging is also a major risk factor for cardiovascular (CV) disease, accounting for 17.3 million deaths per year and projected to be 23.6 million by 2030 globally [10,11]. Multiple studies show a relationship between low muscle mass and increased risk of CV-related mortality [12][13][14]. CV disease includes diseases associated with both the heart and the vasculature, with vascular disease encompassing both macro-and microvascular dysfunction.
Even in disease-free aging, it is postulated that age-related declines in macrovascular blood flow to appendicular regions could play a substantial role in determining muscle health. Macrovascular flow is generally assessed via conduit artery function by measuring arterial blood flow via Doppler ultrasound, arterial stiffness (measured by pulse wave velocity (PWV) or augmentation index (AI)), carotid intima-media thickness, and/or flow-mediated dilation (FMD), which is an estimate of endothelial function. It has been demonstrated that older individuals exhibit 20-30% reductions in limb conduit artery blood flow under both post-absorptive [15] and postprandial conditions [16] when compared to younger adults, possibly due to endothelial dysfunction [17,18]. Such blunted blood flow responses may contribute to age-related declines in anabolic responses to feeding by reducing the delivery and/or utility of insulin and amino acids (AA) in muscle [19,20]. In addition, more recently, Rodriguez and colleagues examined pulse wave velocity as an indicator of arterial stiffness in a meta-analysis and demonstrated that lower muscle mass is significantly associated with increased arterial stiffness [21].
In addition to macrovascular blood flow, 'microvascular/capillary' blood flow or perfusion is a critical mediator of insulin and AA delivery to the muscle [22][23][24]. Microvascular blood flow is not routinely measured in clinical practice, but for research purposes can be assessed using contrast enhanced ultrasound (CEUS) [25]. Capillary density and endothelial function are also good indicators of microvascular function-both have been shown to decline with age and improve in response to exercise training [26,27]. It has been suggested that reduced microvascular blood flow may contribute to the anabolic resistance of muscle [28] which is observed with advancing age [20,29,30], however, data on this is not yet conclusive [31].
This systematic review compiles studies that examine both skeletal muscle and vascular health in healthy adult populations. However, the prevalence of hypertension in otherwise healthy adults is estimated to be~29% with progressive increases due to age [32]. Consequently, we expanded the search to include hypertensive adults without other health complications. While the included studies demonstrate various relationships between skeletal muscle health and vascular health, it remains unclear as to whether there is a causal relationship between the two. Given that there are some shared risk factors for declines in both systems, the relationship between them should be explored in a systematic way [21]. One previous systematic review attempted to do this but focused specifically on arterial stiffness assessed through PWV and did not explore other indices of macro or microvascular status [21]. Therefore, the aim of this systematic review is to determine if there is a relationship between skeletal muscle health and both macro-and microvascular function.

Literature Search
The electronic databases EMBASE and MEDLINE were searched for original articles between January 2009 and April 2019. Evidence from cross-sectional and randomized controlled studies was included. All relevant articles were assessed for pre-defined criteria regarding study outcome, population and design. Database searching identified 2341 records of which 1095 were duplicates, leaving 1246 unique records for title and abstract screening. Following screening, 1182 records were excluded based on lack of assessment of an outcome of interest (n = 1101), inclusion of non-healthy participants (n = 74) and animal/cell studies (n = 7), leaving 64 records for full-text review. Of these, 43 records were excluded primarily due to inclusion of diseased participants (n = 32), leaving a total of 21 records. Through hand-searching full-text articles and references, we identified 12 more records which satisfied all criteria for inclusion. As such, a total of 33 records were included in this systematic review. A flow diagram is illustrated in Figure 1.
Nutrients 2020, 12, x FOR PEER REVIEW  3 of 19 which satisfied all criteria for inclusion. As such, a total of 33 records were included in this systematic review. A flow diagram is illustrated in Figure 1.

Inclusion Criteria
The primary inclusion criteria were based on the assessment of certain outcomes, specifically validated measurements of vascular function in combination with validated measurements of skeletal muscle health. Detailed descriptions of the included validated markers for skeletal muscle health assessments were split into three general categories: (1) muscle mass, (2) muscle strength and (3) muscle function and are described in Section 2.5. Arterial function measurements and markers that were included are described in Section 2.6.
Animal and cell-culture studies were not considered, nor were human studies involving diseased populations (diabetes, peripheral arterial disease, atherosclerosis, chronic obstructive pulmonary disease, kidney disease, cancer, or heart failure). Due to the prevalence of hypertension in otherwise healthy adults, the inclusion criteria did allow studies involving hypertensive adults with no other disease. Studies with professional athletic populations were not considered.

Data Extraction
Titles and abstracts resulting from the literature search were evaluated by two independent investigators. Disagreements were resolved by consensus, or by consulting with a third investigator if required. Studies were grouped according to skeletal muscle health assessments and vascular function assessments. Data were extracted and collated on the following study characteristics: reference, study design, participants (human, number of subjects, age, disease status, and gender ratio), skeletal muscle health measurements and vascular function measurements.

Inclusion Criteria
The primary inclusion criteria were based on the assessment of certain outcomes, specifically validated measurements of vascular function in combination with validated measurements of skeletal muscle health. Detailed descriptions of the included validated markers for skeletal muscle health assessments were split into three general categories: (1) muscle mass, (2) muscle strength and (3) muscle function and are described in Section 2.5. Arterial function measurements and markers that were included are described in Section 2.6.
Animal and cell-culture studies were not considered, nor were human studies involving diseased populations (diabetes, peripheral arterial disease, atherosclerosis, chronic obstructive pulmonary disease, kidney disease, cancer, or heart failure). Due to the prevalence of hypertension in otherwise healthy adults, the inclusion criteria did allow studies involving hypertensive adults with no other disease. Studies with professional athletic populations were not considered.

Data Extraction
Titles and abstracts resulting from the literature search were evaluated by two independent investigators. Disagreements were resolved by consensus, or by consulting with a third investigator if required. Studies were grouped according to skeletal muscle health assessments and vascular function assessments. Data were extracted and collated on the following study characteristics: reference, study design, participants (human, number of subjects, age, disease status, and gender ratio), skeletal muscle health measurements and vascular function measurements.

Macrovascular Studies
Macrovascular studies focus on dysfunction at the level of the large conduit arteries, which transport blood away from the heart. Most studies included in this review focused on macrovascular analyses with detailed descriptions of the included macrovascular studies presented in Table 4. Body composition, specifically muscle mass, appears to have a strong association with arterial health. Given that PWV is the most commonly used assessment of arterial stiffness, six studies used PWV as a measurement of arterial health. These studies were conducted in middle-aged and older adults and revealed a negative correlation between PWV and muscle mass [42,43,46,48,54,58]. Using the radial augmentation index as a measurement of arterial stiffness, a single study found an inverse relationship between limb muscle mass and arterial function [44]. These correlations are indicative of greater arterial stiffness in individuals with lower muscle mass. These publications collectively suggest that skeletal muscle mass has an association with arterial health, specifically arterial wall elasticity.
In addition to skeletal muscle mass, skeletal muscle strength is an important indicator of overall muscle health. Hand-grip strength has been correlated with changes in muscle function and joint disability scores [63], disease activity states [64] and even all-cause mortality [65]. Multiple studies in this review reported that increased CIMT, a common measurement of arterial stiffness, is associated with impaired hand-grip strength [35,37,59]. Using other measurements of arterial stiffness, such as radial augmentation index [57] and pulse wave velocity [36], other studies show the same association between higher arterial stiffness and lower grip strength. One included study demonstrated that higher PWV is associated with lower absolute and relative muscle strength assessed using selected weight machines [39]. This observation is important as it directly demonstrates whole body strength loss and not solely hand-grip strength impairment. One included study demonstrated that decreased muscle strength is associated with endothelial dysfunction [53]. The authors of this study suggest that endothelial function plays an important role in overall muscle health. Additionally, increased arterial stiffness measured through radial augmentation index is associated with impaired leg power during leg press exercise [41]. These studies provide evidence for an association between skeletal muscle strength and arterial dysfunction.
Impaired skeletal muscle function also shows an association with macrovascular dysfunction. Various mobility tests are predictive of the onset of disability, hospitalization and mortality [66][67][68][69][70]. This association is important given that many included studies in this review demonstrate the correlation between arterial stiffness and poor performance on tests of muscle function. For example, increased PWV is associated with increased fatigue during a 400-m walk test [40] and increased CIMT is associated with poor performance on the 40-foot walking speed test [38], 12-min walk test [35] and the timed up and go test [35]. In addition, studies show impairments in muscle flexibility with arterial dysfunction. Increased arterial stiffness is associated with poor performance on multiple flexibility tests including sit and reach [51], and arm extensibility tests [50]. Furthermore, increased arterial stiffness is associated with poor performance on a common test of muscle function, the sit-to-stand test [38]. These studies suggest that the overall muscle function is impaired with arterial dysfunction.
Most included studies examined healthy adult populations. Nonetheless, the prevalence of hypertension in otherwise healthy adults is estimated to be~29% [32] and increases as people age. Therefore, we examined relevant studies that included hypertensive participants [55][56][57][58][59][60]. All six studies involved middle-aged and older adults. Five of the six studies reported that arterial stiffness had a significant negative association with muscular strength [57,59,60] and muscle mass [58]. These studies suggest an additional burden of muscle dysfunction in hypertensive populations.
Exercise interventions that increase muscle mass, strength and function were also found to increase overall macrovascular health. One study included a group of middle-aged women that were subjected to 12 weeks of aerobic training [52]. Following the training program, participants experienced a concomitant decrease in arterial stiffness and increase in muscular function through increased strength on various leg exercises and increased aerobic capacity on the cycle ergometer [52]. One group combined both aerobic and resistance exercise into 2 times per week workouts lasting 10 weeks [49]. Upon completion of the study, participants experienced a decrease in arterial stiffness and an increase skeletal muscle strength through significant increases in leg press, chest press, shoulder press, leg curl and seated row exercises [49]. One study subjected young, middle-aged and older participants to progressive resistance exercise training three times a week for twenty weeks [34]. Upon completion of the training, researchers reported significant improvements in skeletal muscle mass and strength, as well as leg blood in response to feeding and exercise [34]. Lastly, a 12-week stair climbing exercise intervention in hypertensive older women increased skeletal muscle strength measured by 8-RM on a leg extension machine and decreased arterial stiffness [60]. This study is noteworthy as it demonstrates that it is possible to improve skeletal muscle health together with macrovascular health despite the presence of hypertension. Overall, these studies show the concomitant positive response of the skeletal muscle and arterial function to exercise, suggesting an intimate connection between the two.

Microvascular Studies
Arteries branch into arterioles which further branch into capillaries, which are primarily responsible for distributing blood carrying nutrients and oxygen to muscle tissues beds. Thus, it is critical to examine the relationship between the muscle microvasculature and skeletal muscle health. Both higher capillary density and/or greater microvascular flow within the muscle (muscle perfusion) offers potential for greater diffusion of substrates, oxygen, hormones, and nutrients, thereby enhancing skeletal muscle mass and function. Detailed descriptions of the included microvascular studies are presented in Table 5. Capillary density, a common marker of microvascular health, appears to have an association with skeletal muscle mass regardless of age. Most studies examined this association through muscle biopsy analysis. These studies demonstrate that increased capillary density is correlated with increased muscle mass across age groups [33,47,62], as well as with increase muscle strength [26,27]. Correspondingly, decrease capillary density was observed in sarcopenic individuals along with concomitant decline in exercise capacity [47,61]. Microvascular flow and function were also found to decline in populations with functional impairments such as aging and chronic disease [43,61].
Two studies in this review included microvascular blood flow analysis in hypertensive participants. One study showed that hypertensive older adults have significantly lower capillary to fiber ratio and muscle mass compared to healthy older controls [56]. Furthermore, adults with hypertension have a reduced tissue oxygen saturation compared to healthy controls and require a two-fold increase in blood pressure to produce equal amount of muscle torque compared to controls [55]. These studies suggest a microvascular impairment in hypertensive populations.
Exercise interventions that increase muscle mass, strength and function were also found to increase microvascular flow and capillary density. In addition to the included cross-sectional studies, several randomized controlled trials that involved an exercise intervention were included. These studies are important as they demonstrate the intimate relationship between vascular health and skeletal muscle health. Two studies examined microvascular function via capillary density and reported significant improvements following both resistance [27] and aerobic [26] training. Following 24 weeks of aerobic exercise, older adults experienced a concomitant increase in skeletal muscle strength and capillary density [26]. Another study demonstrated that older adults following 12 weeks of resistance exercise training experienced an increase in skeletal muscle mass, strength and capillary density [27]. One study examined nutrient delivery to the muscle following a bout of aerobic exercise and reported significant improvements in muscle protein synthesis as well as microvascular blood flow [20]. These data suggest that various types of exercise can significantly impact both the skeletal muscle function and microvascular health of the participants.

Discussion
Sarcopenia, the progressive loss of muscle mass, muscle function and physical performance, was previously associated with cardiovascular disease [12][13][14]. Given that many of the risk factors for sarcopenia and cardiovascular disease are shared, it is not surprising that there is an association between measures of vascular dysfunction and muscle health. Existing reviews solely target arterial stiffness as an indicator of vascular dysfunction and its' association with skeletal muscle health [21]. This systematic review takes a broader approach and examines other validated indices of vascular function including microvascular function. The focus of the review is on relatively healthy adults and we observed an inverse association between skeletal muscle health and vascular dysfunction. Our data indicate that the inverse relationship between vascular dysfunction and skeletal muscle is consistently observed in the hypertensive populations as well. Given the high prevalence of hypertension (63.1%) in adults over the age of 60 [32], these data suggest that it may be important to start to screen for and address muscle health issues in adults with hypertension. Not unexpectedly, exercise interventions demonstrated both macrovascular and microvascular health benefits, in addition to improving indices of skeletal muscle health, indicating the importance of habitual exercise for healthy aging.
Given that most of the included studies are cross-sectional, the mechanism of the association between vascular dysfunction and muscle strength and function is not clear. Rodriguez and colleagues completed an extensive review of the musculoskeletal system and vasculature and proposed a conceptual disease model [71]. On the microsystemic level, systemic inflammation, local inflammation, low calcium and low vitamin D intake, as well as impaired glucose metabolism promote cellular stress and damage. If the cellular stress and damage is chronic, it is manifested in damage to both the vascular system, impacting endothelial function and eventually microvascular health and the musculoskeletal system [71]. Arterial stiffness and perhaps microvascular dysfunction would occur in combination with sarcopenia on the macrosystemic level. It has been shown that muscle perfusion, an indicator of peripheral microvascular health, decline with age [25]. This decline can lead to a decrease in 'nutritive' flow to the muscle, impacting the availability of nutrients need for muscle function [22][23][24]. Reduced 'nutritive' blood flow may contribute to the anabolic resistance of muscle observed with ageing [29,30], eventually leading to loss of muscle mass, strength and function. What is not known is if the relationship between vascular dysfunction and muscle health is bi-directional and this needs to be further explored in prospective studies.
There are some important limitations of the included studies that must be considered. First, there is a heterogeneity in assessment of arterial health. For example, arterial dysfunction in the included studies is measured through pulse-wave velocity, CIMT, augmentation index and flow-mediated dilation. This variation makes it difficult to complete a meta-analysis, though we felt it was important to include these studies to demonstrate the relationship between arterial dysfunction and skeletal muscle health regardless of outcome measurement. Second, like the variation in arterial function assessments, there is a heterogeneity in assessment of skeletal muscle health and function. While muscle mass and strength assessments are relatively consistent, muscle function assessments varied greatly from 40-foot walking speed tests [38] to arm extensibility tests [50]. Lastly, the number of studies evaluating microvascular health is comparatively low and more research is needed in this field.
The exclusion criteria for each study varied which impacts several variables including nutrition status, daily physical activity and use of medications. Though a few of the studies excluded participants based on a low BMI [4,25,59] and low daily protein intake [26], the rest of the studies did not examine the nutrition status of the participants. Nutrition has been suggested to play an important role in arterial stiffness, specifically through the dietary intake of vitamin D and calcium. An observational study conducted on 131 participants suggested that insufficient intake of vitamin D is associated with increased arterial stiffness [72]. A large study of 12,097 men and women determined that higher calcium intake from food was associated with decreased risk for stroke and non-fatal cardiovascular disease [73]. Despite these data, a detailed meta-analysis of vitamin D supplementation and the impact on arterial stiffness determined that there is inconsistent evidence to suggest a connection between the two factors, which was attributed to large heterogeneity in study design [74]. Similarly, other studies demonstrated no relationship between calcium supplementation and markers of vascular disease [75,76]. Most studies included in this systematic review did not measure daily physical activity. Given the importance of physical activity for maintaining skeletal muscle mass and improving cardiovascular function [77,78], an accurate measurement of daily activity is important for a complete assessment of the participants. Lastly, the use of medications was poorly reported in most studies and this can impact a variety of factors related to vascular dysfunction including blood pressure, blood triglycerides and blood cholesterol levels.
Although the focus of this systematic review has been on relatively healthy subjects, the association between skeletal muscle health and vascular dysfunction has been demonstrated in other populations. A recent meta-analysis examined macrovascular function through pulse wave velocity and demonstrated that lower muscle tissue is associated with higher arterial stiffness in populations with diabetes and kidney disease [21]. This association has been demonstrated in healthy children as well, in which researchers determined that arterial stiffness (measured by CIMT) is negatively associated with muscular strength [79]. Furthermore, following isometric handgrip training, young healthy adults improved their brachial artery flow-mediated dilation [80]. Overall, the connection between skeletal muscle function and vascular dysfunction is observed in other populations and may be exacerbated in populations with disease.

Conclusions
In conclusion, we described over 30 studies that demonstrate an inverse relationship between vascular dysfunction and skeletal muscle health. This association is observed both on the macrovascular and the microvascular levels. Given the cross-sectional nature of most of the studies included, it is impossible to say if impaired vascular health causes skeletal muscle dysfunction or vice versa. Nevertheless, we included some exercise intervention trials that demonstrate concurrent improvements in skeletal muscle health and vascular function [26,27,49,52,60]. More studies are necessary to determine if vascular dysfunction is predictive of skeletal muscle dysfunction. Furthermore, the field requires a standardized assessment of macrovascular dysfunction and should also start to include assessment of microvascular dysfunction. The clinical significance of this association between vascular health and muscle health cannot be overlooked, considering the heavy clinical and economic burden of vascular dysfunction and sarcopenia, independently. If skeletal muscle health impairment is predictive of future cardiovascular events or vice versa, early screenings will allow for early preventative interventions to help improve long-term outcomes as the population ages. It is possible that interventions targeting vascular dysfunction may have a long-term benefit on muscle health or vice versa, but this needs to be systematically tested in prospective randomized clinical studies.