The Influencers and Influences of the Very Important Cardiorespiratory Fitness in Youth

Since the seminal findings of Blair et al.,¹ a substantial body of evidence confirms cardiorespiratory fitness (CRF) as a strong independent predictor of all-cause and disease-specific mortality.² Indeed, the American Heart Association (AHA) recently advocated for CRF to be routinely tested in practice as a clinical ‘vital sign’.³ The AHA scientific statement entitled ‘Cardiorespiratory Fitness in Youth – An Important Marker of Health’ seeks to raise awareness amongst clinicians on the importance of CRF for predicting current and future health in otherwise healthy youth. It reviews current knowledge on the association between CRF and health outcomes in youth, neatly nominating the identified ‘influencers’ of CRF, and the ‘influences’ that CRF exerts on a multitude of health-related outcomes. These outcomes transcend the physical, to aspects such as mental health and cognitive function. The statement covers how CRF can be assessed, including accessible options for practitioners beyond exercise physiology settings; timely pragmatic information following the AHA advocacy for routine CRF testing.

What is CRF, and what is the status of CRF in youth?

CRF is defined as the integrated ability to deliver oxygen from the atmosphere to skeletal muscles to then be used for physical movement.³ It’s therefore unsurprising that an identified widespread decline in CRF of youth over time has raised concerns,^4,5 although some critique exists.⁶

Why is CRF in youth very important?

It is clear from extensive epidemiological evidence that gaining and maintaining good CRF is important to general youth health.^2,7,8 The linear inverse relationship between CRF during youth years and all-cause mortality, and cardiovascular disease across the lifespan is well established. In youth, lower CRF is related to a compendium of insidious pathophysiology manifesting in conditions such as nonalcoholic fatty liver disease,⁹ and general metabolic syndrome.¹⁰ Further, and contemporarily relevant, the nexus between CRF and cerebrovascular, cognitive and mental health in youth is of interest.¹¹ Hence the imperative to monitor and rectify CRF deficit during youth is compelling.

How can CRF be assessed?

As with many physiological and physical function assessments, the spectrum of CRF assessment protocols spans from burdensome laboratory tests, to procedures designed for simple, accessible use or population monitoring, with concomitant trade-offs between convenience and accuracy. The gold standard criterion for CRF assessment is direct gas analysis for peak oxygen uptake, albeit subject to its own issues when applied to youth.¹² The procedure requires specialist equipment, training in exercise physiology, and close individual attention to the subject who must perform exercise to exhaustion. At the other end of the CRF assessment continuum, self-assessment questionnaires such as the five question International Fitness Scale¹³ can be completed in around five minutes. Although it’s considered the least accurate method, it nonetheless provides an acceptable proxy for epidemiological purposes. The CRF assessment spectrum ‘middle-ground’ offers multiple options, each nuanced for different settings, and each with their own respective strengths and limitations. The trade-off between ease of implementation and accuracy and reliability is an ongoing conundrum for practitioners. For assessing groups with expediency, the 20 meter shuttle run test¹⁴ is a popular (the most widely used test used to estimate CRF in youth worldwide¹⁵) and validated option with moderate to high validity. It has been used to derive maximal oxygen uptake although it is advised that a more suitable approach is to simply use the raw number of 20 meter ‘laps’ completed as the outcome metric, given issues with the variety of formulae used.¹² Additionally, willingness to run 20 meter laps at ascending pace until volitional exhaustion is requisite for an accurate 20 meter shuttle run test result; therein an inherently confounding factor, including potential gender and social influences on youth in such settings.¹² Considerate of space and equipment restrictions, so-called ‘step tests’ provide an example of a reasonably valid CRF assessment option.16 An added advantage is that step tests are sub-maximal (not requisite of maximal effort), thereby attenuating motivation as a test factor; arguably representing a non-intimidating option for less fit participants. Step tests still require some assessor training but provide a potentially useable option should clinicians adopt the mandate to routinely monitor CRF as a vital sign, including individual patient progress over time. Irrespective of criterion validity, when monitoring of change in an individual’s CRF is intended the chosen assessment protocol must be consistently repeatable. Certainly, reaching agreement, consistency and transparency with protocols and reporting would provide enhanced diagnostic and prognostic value.

How can CRF be improved?

In adults, it is known that moving from the lowest quintile CRF to the next-lowest quintile group is associated with the most striking health benefits.³ Equivalent evidence is not yet clear for youth, signaling an important future research direction, but it could be reasonably speculated that a similar scenario would emerge. Although CRF is influenced by non-modifiable factors such as prematurity, age, sex, and hereditary factors, encouragingly it is most certainly modifiable. Of the modifiable factors, some are arguably more difficult to effect, such as socioeconomic and environmental issues. Although physical activity is universally acknowledged as important to general health, its direct influence on CRF in youth appears tenuous;¹² at any rate, less than one quarter of youth appear to be meeting recommended physical activity levels,¹⁷ despite clear global guidelines.18 Structured or semi-structured exercise is a subset of physical activity, and it’s unequivocal that youth of all ages can experience positive health adaptations from a variety of exercise training modalities.^6,19 Prescription involves an interplay of frequency, intensity, time, and type. All are important but one key prescription parameter now known as a potent component in adults and youth alike is exercise intensity. Intensity in terms of ‘aerobic’ exercise activities is typically expressed in research as a percentage of maximal oxygen uptake. Aerobic exercise intensity can also be prescribed with more accessible tools such as perceived effort scales, or simple heart-rate monitoring. Testimony of the potency of exercise effort on CRF improvement is the emergent evidence on the relative efficacy of high intensity interval training for youth,^20,21 whereby repeated brief intense exercise bouts are performed, interspersed with low intensity recovery phases. The public health applicability of HIIT is still debated,²² given conjecture that the requisite effort may be unpalatable to otherwise low-active individuals, although that is not empirically established. Nonetheless, it is clear that a little effortful exercise training effort is potent for youth CRF.

Conclusion

CRF is very important for youth, because it influences physical, physiological, mental, and cognitive outcomes during youth and then adulthood. The influencers of CRF are many, but it is well known that CRF in youth can be meaningfully modified, particularly through exercise. CRF is a vital contributor to quality and quantity of life for our youth; we must therefore continue the imperative to provide empirically underpinned ways to influence the influencers of their CRF.