CARDIORESPIRATORY AND NEUROMUSCULAR FITNESS

2.5.1 Definition of cardiorespiratory and neuromuscular fitness

CRF means the integrated ability of the circulatory, respiratory, and muscular systems to supply oxygen to skeletal muscles and oxidative mechanisms within skeletal muscles to utilize oxygen for energy production during sustained PA (222).

Maximal oxygen consumption has been considered as the best indicator of CRF.

Lower levels of CRF have been associated with increased levels of individual and clustered metabolic risk factors among children and adolescents (223,224). It has to be noted that oxygen uptake is equivalent to energy expenditure (225) and that the interpretation of CRF depends on whether it is scaled by body weight or lean body mass (226). Scaling CRF by lean mass would avoid confounding by adiposity.

NMF refers to the ability of the nervous system to control and coordinate the functions of skeletal muscles during PA. NMF can be roughly divided into three components that are muscular endurance, muscular strength, and motor performance (117,120).

2.5.2 Assessment of cardiorespiratory and neuromuscular fitness CRF can be measured using laboratory-based tests (e.g. treadmill or cycle

ergometer) or field-based tests (e.g. distance run and shuttle run tests). Laboratory tests include incremental exercise tests until volitional exhaustion (222). The use of laboratory-based tests in population studies is often limited due to their high cost, technical expertise requirements, and time constraints (227). Field-based tests provide a practical alternative because of the low cost and ease of administration (228).

NMF can also be measured using laboratory-based tests (e.g. dynamometer or force-plate) and field-based tests (e.g. the standing long jump test). (229,230)

2.5.3 Associations of cardiorespiratory and neuromuscular fitness with cardiometabolic risk factors

Body fat content and lean body mass

The relationship between low CRF and adiposity is well recognized among children and adolescents (226,231–235). However, the inverse associations between CRF and cardiometabolic risk factors are largely explained by adiposity rather than

published by Lubans and coworkers in 2010 pointed to the presence of a positive relationship between fundamental movement skills and CRF and an inverse association between these skills and body weight status among children and adolescents aged 3-18 years (238). Low muscle mass and strength have also been found to impair metabolic health in children and adolescents (239). However, longitudinal evidence on these associations is currently limited and complicated by the varying methods which have been applied to assess CRF and NMF. In a recent systematic review of longitudinal studies, about half of the included articles reported inverse associations of CRF in childhood and adolescence with future BMI, body fatness, and MetS. However, many of the included articles did not account for important confounding factors, such as adiposity (240), but it has to be noted that for practical reasons, it is sometimes necessary to scale CRF by total body weight, for example in work ability assessments. In summary, a systematic review by Ruiz and coworkers concluded that there is strong evidence that there are inverse associations of CRF and NMF with overall adiposity among youth (241).

Glucose and insulin metabolism

There is evidence that higher CRF is associated with better insulin sensitivity among children and adolescents (223,242–244). However, a recent study showed that CRF was not associated with insulin resistance when appropriately controlling for body size and composition, suggesting that the association between CRF and insulin resistance in previous studies was largely explained by adiposity and not maximal aerobic power (236). Fraser and coworkers claimed that the lower values of CRF and NMF in childhood and adolescence were related to higher insulin resistance and poorer β-cell function about 20 years later in adulthood (245). In a prospective study in a population sample of adolescents, less isometric muscle strength and poorer CRF at the age of 15 years were associated with higher insulin resistance and worse β-cell function in young adulthood (246). Li and coworkers recently reported that lower NMF, measured by handgrip strength, was related to higher insulin resistance and an elevated glucose level in children and adolescents aged 12-19 years (247). Despite the methodological limitations related to the assessment of CRF in earlier studies among growing children and youngsters, there is some evidence that higher CRF and NMF are beneficial in the early prevention of insulin resistance and adverse glucose homeostasis. Nevertheless, further longitudinal and especially intervention studies are warranted to

investigate the associations of CRF and NMF with glucose and insulin metabolism among children and adolescents.

Lipid metabolism

Several studies have identified favorable relationships of CRF and NMF with measures of lipid metabolism among children and adolescents (248–251).

Evidence on the associations of NMF with the measures of lipid metabolism is less conclusive than that for CRF, but particularly improved muscular strength has been linked with beneficial changes in blood lipids and lipoproteins (241). It is still a matter of debate whether CRF and NMF are important predictors for a favorable lipid metabolism among children because the mechanisms underlying these associations remain to be clarified. There is clear evidence that PA, at least to some extent, enhances lipid metabolism and is directly associated with CRF and NMF, but the causal relationships of CRF and NMF with measures of lipid metabolism and the underlying biological mechanisms are still unresolved.

Blood pressure

According to a recent systematic review, there is no conclusive evidence that a better CRF in childhood and adolescence is associated with lower blood pressure levels in later life (240). On the contrary, in their systematic review, Ruiz and coworkers concluded that a better CRF in childhood and adolescence was strongly associated with lower blood pressure later in life (227).

The findings are also contradictory when related to NMF. A systematic review and meta-analysis of longitudinal studies did not detect a relationship between better NMF in childhood and lower blood pressure levels in later life (252). Ruiz and coworkers did find a weak link between better NMF and lower blood pressure in childhood and adolescence (241). The contradictory conclusions are probably due to methodological differences in these studies. Ruiz and coworkers

emphasized that the results of their review should be interpreted with caution due to heterogeneity in the methods used to assess CRF and NMF and blood pressure, adjustment for possible confounding factors, follow-up periods, and the age of the participants (241).