Aerobic Fitness in Young People
Importance in the prevention of heart disease and endurance performance
February is American Heart Month, a time when all people can focus on their cardiovascular health.
REMINDER: Cardiovascular disease remains the #1 cause of death in the U.S.
Often times, we think about cardiovascular health and disease in terms of high blood pressure, high cholesterol and heart attacks amongst middle-aged and older adults. However, there is also a need to focus on young people when it comes to cardiovascular health as well.
Research from the Bogalusa Heart Study shows that atherosclerotic plaques emerge in childhood … and even moreso in today’s generation given childhood obesity, sedentary lifestyle and poor dietary habits.
Research also shows that higher levels of aerobic or cardiorespiratory fitness plays a role in the prevention of diabetes, hypertension and hyperlipidemia - even in children and adolescents.
On the other end of the health & performance spectrum, cardiovascular health can also benefit the young athlete in terms of stamina and endurance capacity. However, this blog focuses on the general population of youth. Subsequent posts will surely address aspects of aerobic fitness and endurance training in young athletes - especially since we will be working on updating a book chapter in Endurance Training-Science and Practice edited by Inigo Mujika on the topic in the upcoming months.
What is meant by ‘Aerobic Fitness’?
Let’s first define this term ‘aerobic fitness’. Or what others call cardiorespiratory fitness. And perhaps most often - simply, fitness. I prefer not to use this latter term because ‘fitness’ is actually a much broader and multi-faceted construct. It entails metabolic fitness (glucose, insulin, cholesterol, etc.); muscular fitness (strength, power); morphological fitness (bone, fatness, etc.); motor fitness (balance, coordination, etc.); and cardiorespiratory (heart and lung function, blood pressure, and submax and maximal exercise capacity). Actually, I don’t care for this latter term - cardiorespiratory fitness- all the time either ……
…instead, I prefer ‘aerobic fitness’ when describing the bodies ability to take up/breath in → deliver via heart and cardiovascular function→and utilize oxygen via skeletal muscle oxidative (or aerobic) metabolism. It’s this last part of the oxygen transport system that gets sorely neglected when we use ‘cardiorespiratory’ fitness. And the ability of our exercising muscle to produce energy in the powerhouse of the cell - the mitochondria - is a thing of natural beauty.
Of course, it is this entire system - the oxygen transport system - working efficiently and at a high level that gets us to peak aerobic fitness.
Much of my thinking shared above comes from my exposure to the comparative mammalian physiologists, and especially this book - The Pathway for Oxygen. The schematic on the book cover depicts the lung at the top, the cardiovascular conduit system in the middle, and the muscle/mitochondria at the bottom, or as they refer to it - the oxygen sink.
Here’s a more detailed look into this complex and well-orchestrated journey of oxygen from air to mitochondria.
Assessment of Aerobic Fitness
Next question: how can we assess the whole body oxygen transport system?
Well, we can do it in the laboratory with sophisticated and expensive equipment or we can estimate it in the field.
Wait a minute … what’s it? Here, I am referring to aerobic fitness = Vo2max or maximal oxygen consumption.
Back to the lab. I assume you all have seen a commercial or ESPN clip of an athlete running on a treadmill or cycling or rowing with a contraption on their head holding what looks like scuba mouthpiece with a hose hanging from it.
This method called indirect calorimetry uses what is often referred to as a ‘metabolic cart’ to measure and analyze 2 things: 1) expired respiratory gases (O2 and CO2) and 2) respiratory flow or ventilation. Again, a lot of sophisticated and expensive equipment and well-trained exercise physiologists. So… maybe not so feasible or practical for most folks.
No worries - we do not need to go to the lab to test aerobic fitness. Just grab your sneakers and head to the track or an open field or court. Here is a list of field-based tests of aerobic fitness:
1-Mile Endurance Run / Walk Test
Yo-Yo Intermittent Recovery Endurance Test
These are all proxies of Vo2max and estimate Vo2max based on various parameters (time completed, age, body size, etc.). In general, they all are fairly reliable and accurate alternatives. Not perfect but gets you in the ballpark of the true Vo2max.
In children, some of these tests can be challenging due to instructions, pacing, effort, etc. In one study of 4-6 yr olds, researchers taped “money” onto the front safety bar of the treadmill and at the end of every stage of the test the child got the money. The more stages completed, the more money earned which could be spent at the “store” the researchers set up with gifts. Preschoolers seem quite young to test but in general, maximal exercise testing either in the lab or field starts to work well at age of 12 or so.
Appropriate Levels of Aerobic Fitness for Youth
Ok, we have the test data - an estimated Vo2max of 45 ml/kg/min for a 14 yr old girl. Is that good or bad?
Reference values (or benchmarks) can either be norm-based or criterion-based. Norm-based references will use test scores from a sample of individuals. For example, maybe a large soccer club ran the Yo-Yo test and determined the 25th (poor), 50th (average) and 75th (good) percentiles. However, if the entire group was relatively unfit this is not an “accurate” evaluation.
In turn, the criterion-based reference standards are based on some criterion outcome. In medicine, this is common. Levels of blood pressure, cholesterol, etc. are linked to disability, disease or death. In youth, this gets tricky since these hard outcomes are not common.
In 2011, along with my colleagues Drs. Greg Welk and Kelly Laurson, we developed criterion-reference standards for aerobic fitness in youth as part of the FITNESSGRAM testing program. Using a nationally representative sample of 12 to 18 year olds who participated in the National Health and Nutrition Examination Survey, we first constructed age- and sex-specific percentiles of estimated Vo2max (norm-based references). This first step was necessary to develop the criterion-referenced standards of aerobic fitness using a statistical method called receiver operator characteristic (ROC) curves. Without getting too technical, the hard outcome was defined as having or not having the metabolic syndrome - a clustering of adverse cardiovascular disease risk factors like high blood pressure, cholesterol, glucose and waist circumference. In a nutshell, we linked aerobic fitness values to the metabolic syndrome to create the benchmarks. The statistical analysis provided values at a high risk or Needs Improvement (NI) and at the “Healthy Fitness Zone” (HFZ). The HFZ threshold ranged from 40-44 ml/kg/min for males and 38-40 ml/kg/min for females.
Tracking of Aerobic Fitness into Adulthood
If an adolescent has poor aerobic fitness are they are more likely to remain in this category into adulthood? If so, recall that poor aerobic fitness in adulthood is a darn good predictor of cardiovascular disease morbidity and mortality!
Although it is purported that having higher levels of aerobic fitness early in life may confer advantages to adult aerobic fitness, this conjecture remains controversial as there are few longitudinal studies that have explored it. Imagine measuring aerobic fitness in a group of youngsters and then bringing them back in 20, 30, 40 or more years later. Ya .. challenging. But, we did it using individuals from the Saskatchewan Growth and Development Study which started in the 1960s.
In this study, we located, recruited and tested 78 individuals (59 males and 18 females) of the 207 boys & 149 girls who were now 40-50 years old. The results showed that overall the tracking correlations between adolescent and adult aerobic fitness values were low. However, those in the low aerobic fitness group in adolescence had lower values in adulthood relative to other groups (moderate and high) providing some evidence.
Remember your really fit buddies in high school? Some may still be fit and others not. And how about that kid who was not real active in sports but as an adult become really interested in mountain biking and now shredz ever weekend. A lot happens between adolescence and adulthood - some behaviours stick and others change for a lot of reasons.
But again, those in the low aerobic fitness group in adolescence had lower values in adulthood.
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Is Aerobic Fitness Trainable?
Yes, aerobic fitness is trainable in children and adolescents (5-10% improvement) given training of sufficient frequency, intensity and duration. Exercise intensity appears to be the key variable. Nonetheless, there is wide variability in the response to training, which reflects pre-training levels and genotypic factors among others.
It has been recommended that a mixture of continuous and interval exercise at 85-90% heart rate maximum for a minimum of 3-4 sessions of 40-60 minutes per week for a minimum of 12 weeks to induce changes in Vo2max. However, it is also important to consider that blending continuous and interval exercise (and games) is important since endurance training is often made up of continuous long-distance exercises, which can result in boredom in children.
It’s getting late … and this could get long … so I’ll come back at another time to seriously discuss aerobic and endurance training in young athletes.