Available online at www.sciencedirect.com
Journal of Sport and Health Science 1 (2012) 131e140
www.jshs.org.cn
Review
Young people are fit and active e Fact or fiction?
Neil Armstrong
Children’s Health and Exercise Research Centre, University of Exeter, Exeter EX1 2LU, UK
Received 21 February 2012; revised 10 March 2012; accepted 16 March 2012
Abstract
This paper analyses young people’s habitual physical activity (HPA) and aerobic fitness (AF) in relation to health and well-being, with
reference to previous generations. Data consistently show that more boys than girls experience health-related physical activity (PA) and that both
genders experience a decline in HPA during adolescence. The majority of young people (w60%e75%) do not meet the requirements of current
health-related PA guidelines but youth HPA appears to have stabilised over the last two decades. Although it does not describe all aspects of AF,
_ 2) is recognised as the best single measure of AF. Peak VO
_ 2 is enhanced with age, growth and maturation with
peak oxygen uptake (peak VO
increases of w150% and w80% in boys and girls respectively from 8 to 16 years. Boys’ values are higher than those of girls throughout
childhood and adolescence with the gender difference reaching w35% by age 16 years. There is no compelling evidence to suggest that young
_ 2 or that they are less aerobically fit than children of previous generations. Despite the relative stability of peak
people have low levels of peak VO
_VO2 there has been a decline of w13% over the last 35 years in young people’s ability to perform maximal aerobic exercise which involves the
_ 2 is,
transport of body mass. In their daily lives young people very rarely experience PA of the intensity and duration to enhance AF and peak VO
at best, only weakly related to current levels of HPA during youth. Low levels of HPA and a marked secular decline in maximal aerobic
performance remain major issues in the promotion of youth health and well-being.
Copyright Ó 2012, Shanghai University of Sport. Production and hosting by Elsevier B.V. All rights reserved.
Keywords: Adolescents; Aerobic fitness; Children; Health and well-being; Physical activity
1. Introduction
In studies of young people’s health and well-being the
terms physical activity (PA) and physical fitness are often used
interchangeably but they are not synonymous. PA consists of
behaviors which contribute to total energy expenditure and
involve bodily movements produced by skeletal muscles. In
the context of young people’s health and well-being habitual
PA (HPA) is the behavior of prime interest. HPA has been
defined as, “usual physical activity carried out in normal daily
life in every domain and any dimension”.1
E-mail address: [email protected]
Peer review under responsibility of Shanghai University of Sport
Production and hosting by Elsevier
Physical fitness is a complex phenomenon which can be
described in terms of its health-related and skill-related
components. Health-related fitness includes discrete physiological attributes such as aerobic fitness (AF), muscle strength,
muscle power, and flexibility. All of these attributes are
important in the promotion of health but it is AF which is most
frequently associated with health and well-being during youth.
AF depends on the pulmonary, cardiovascular, and haematological components of oxygen delivery and the oxidative
mechanisms of exercising muscles. It has been defined as, “the
ability to deliver oxygen to the muscles and to utilise it to
generate energy to support muscle activity during exercise”.2
The measurement and interpretation of HPA3 and AF4
during growth and maturation have been extensively
reviewed elsewhere and these aspects will therefore only be
summarised herein. This paper will analyse current levels and
secular changes in HPA and AF in relation to youth health and
well-being and examine the evidence relating HPA to AF
during childhood and adolescence.
2095-2546/$ - see front matter Copyright Ó 2012, Shanghai University of Sport. Production and hosting by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jshs.2012.05.003
132
Studies for review were located through computer searches
of Medline, SPORT Discus and personal databases supplemented with an extensive search of bibliographies of accessed
studies.
2. HPA
PA consists of a complex set of behaviors which can be
subdivided into dimensions such as frequency, duration,
intensity and mode or type of PA. It can also be subdivided
into different domains such as HPA, leisure-time PA, sportstime PA, school-time PA, school break-time PA, and hometime PA. HPA is the most important domain for health
outcomes and is therefore the focus of this paper.
Numerous reviews comparing and contrasting methods of
measuring PA during youth have been published,5e7 including
a recent supplement to Medicine and Science in Sports and
Exercise8 which examines current methodology and explores
the potential of emerging technology to provide new insights
into PA patterns. HPA is estimated from measurement of freeliving PA for a defined length of time but if a true picture of
HPA is required some account must be taken of day-to-day
variation. Early studies adopted a recommendation of
a minimum monitoring period of 3 days9 but recent evidence
suggests 4e9 days of monitoring, including 2 weekend days
might be the minimum period required for a reliable
estimate.10
Young people’s PA patterns are different from those of
adults due to psychological, physiological and biomechanical
changes during growth and maturation and socio-cultural
differences in lifestyles. Although inappropriate for measuring
HPA, direct observation has proved useful for capturing
detailed analyses of short periods of young people’s PA and it
has confirmed that young people’s PA patterns consist of
shorter, more intermittent and often more intense bouts of PA
than those of adults.11
Similarly, the interpretation of young people’s HPA is
complex and their health-related PA is normally classified in
relation to guidelines developed by expert committees on the
basis of published evidence relating PA during youth to health
outcomes.12e16 In the following sub-sections the measurement
and interpretation of HPA will be briefly critiqued to provide
necessary context before examining the current PA patterns of
youth and exploring time trends in HPA.
2.1. Measurement of young people’s HPA
In 1985, LaPorte and his colleagues17 identified more than
30 different methods of measuring PA and although techniques
have been refined over time the measurement tools available
can still be simply classified into subjective and objective
methods. No single method adequately describes all aspects of
HPA and all current instruments have deficiencies. Some
studies have tried to overcome this by using more than one
method to measure young people’s PA but correlations
between subjective and objective methods are at best low to
moderate.18
N. Armstrong
2.1.1. Subjective methods of measuring PA
Subjective methods of measuring PA are based on selfreport and include questionnaires, interviews, and activity
diaries. Actual behavior is not directly measured using selfreport techniques and Sallis19 has defined the data obtained as,
“memories of the behavior of interest that have decayed, been
filtered through perceptions and biases, and have been tainted
by competing memories, social desirability and misunderstanding of instructions”. Children are less time conscious
than adults and to recall details of specific events from the past
places considerable demands on their cognitive abilities. Proxy
reports by parents and/or teachers have been employed in
some studies but accurate recall of young children’s PA by
adults is difficult and confounded by children’s intermittent PA
patterns. Young people’s moderate intensity PA tends to be
non-planned and less memorable than more intense PA and is
therefore often underestimated by self- or parental-recall.
Vigorous PA is generally overestimated, particularly when
carried out in a sporting context where, say, football may be
reported as 90 min of vigorous PA whereas it actually consisted of rest and moderate intensity PA interspersed with short
intermittent bouts of vigorous PA. Comparisons of data from
a range of self-report instruments have indicated wide
discrepancies in estimates of HPA at an individual level.3
Some researchers have estimated energy expenditure from
self-report instruments but as there are no comprehensive
reference values for children and adolescents energy costs of
activities are usually derived from adult values. This methodology introduces additional errors as with young people
energy costs may be underestimated by more than 20% using
adult values as a proxy.20
Although errors may be substantial in estimating an individual’s HPA several large, well-designed, national and
multinational surveys have provided valuable descriptions of
young people’s HPA at a population level.21e23
2.1.2. Objective methods of measuring PA
As PA involves movement of the whole or parts of the body
motion sensors provide an objective means of measuring HPA.
The most common motion sensor used in early studies of HPA
was the pedometer which was first used in this context by
Lauter24 in 1926, although Leonardo da Vinci designed
a pedometer to measure distance by counting steps somewhat
earlier.25 More sophisticated mechanical, electronic and
magnetic counters have gradually replaced pedometers26,27
and in recent years accelerometers have become the motion
sensor of choice in the study of young people’s HPA.5
Pedometers are simple motion sensors which are normally
used to detect and record the number of steps taken over
a period of time. Advantages include the low cost and nonreactivity whereas disadvantages include the inability to
record intensity, duration or frequency of PA and susceptibility
to noise during activities such as cycling. Interpretation of step
counts per day during youth must take into account factors
such as stature and stride length as pedometer data are not
directly comparable across ages during growth and maturation.
Despite problems with assigning the appropriate number of
Physical activity and aerobic fitness during youth
steps recorded to specific PA guidelines pedometers do give an
overall indication of HPA and are useful tools for large scale
studies.28
Accelerometers measure the acceleration of the body or
different parts of the body in one or more dimensions for
a specific time period (epochs). In most studies accelerometry
data are expressed as activity counts per minute (cpm) which
are then translated into estimates of PA. Calibration studies are
generally performed in the laboratory where activity cpm and
energy expenditure are simultaneously measured by accelerometry and indirect calorimetry. Activity cpm equivalent to
cut-off points described by metabolic equivalents (METs) are
often used as thresholds for moderate or vigorous PA. The
challenge is the extrapolation of these data to translate activity
cpm into free-living moderate or vigorous PA. There is no
consensus on appropriate “cut points” indicating different
categories of the intensity of PA and the optimum length of
sampling frequency and epoch are currently subjects of
intensive research programmes. Accelerometry data must
therefore be carefully interrogated when making cross-study
comparisons. Nevertheless, the development of accelerometers
has provided significant advances in our understanding of
young people’s HPA.29
A range of physiological sensors have been used either in
combination or independently to estimate HPA. It has been
suggested that although PA is not directly measured by
physiological sensors the physiological responses derived
from PA may offer more clinically relevant parameters with
which to evaluate relationships between health and PA.30
Heart rate (HR) monitoring, for example, provides an estimate of the stress placed upon the cardio-respiratory system
by PA. The technique of monitoring children’s HR in field
conditions emerged in the early 1970s31 and recent years have
seen the development of sophisticated, self-contained, computerised telemetry systems which have been used widely to
estimate young people’s HPA. As with other techniques data
extrapolating HR to estimates of PA need to be treated with
caution but HR monitoring has provided unique insights into
young people’s HPA. Several factors other than PA can
influence HR, particularly during low intensity PA, but
continuous HR monitoring over extended periods of time
provides an objective means of estimating moderate to
vigorous PA (MVPA). HR monitoring also lends itself to the
application of threshold values with which to interpret established PA guidelines.32
133
literature reviews the ICC proposed that young people should,
in addition to being physically active daily, engage in three or
more sessions per week of activities that last 20 min or more at
a time and that require moderate to vigorous levels of exertion.
Moderate to vigorous activities were defined as those that
require at least as much effort as brisk walking.13 The ICC
guidelines informed most HPA studies of young people in the
1990s.
In 1998, the UK Health Education Authority (UKHEA)
commissioned a similar series of systematic reviews. Derived
from the same evidence-base as the ICC guidelines the
primary UKHEA recommendation was that all young people
should participate in PA of at least moderate intensity for
60 min per day and that young people who currently do little
activity should participate in PA of at least moderate intensity
for at least 30 min per day. This recommendation shifted the
emphasis from vigorous to moderate intensity PA and from
sustained periods of PA to PA accumulated over a day.14 The
UKHEA guidelines have been influential in the interpretation
of young people’s HPA over the last 15 years although more
recent PA guidelines have emphasised the importance of
MVPA15 and vigorous PA.16
Twisk33 explored the pattern of relationship(s) between PA
and health-related outcomes during youth. He critically
reviewed the evidence on doseeresponse relationships and
threshold values between PA and health-related outcomes and
demonstrated that where there is evidence of a relationship
there is minimal evidence of a particular pattern of that relationship. He showed that there are different patterns of relationships for different health-related outcomes and only
marginal scientific evidence to support current PA guidelines.
He argued that at best current guidelines are “evidenceinformed” rather than “evidence-based”.
2.3. Are young people active?
2.2. Interpretation of young people’s PA
Previous sections have outlined the complexities of
measuring and interpreting young people’s HPA. It is clear
that all methods of measuring HPA have deficiencies and that
different instruments measure different dimensions of PA. The
extrapolation of the evidence-base relating youth PA to health
outcomes into PA guidelines has been challenged. Current PA
guidelines appear to be evidence-informed rather than
evidence-based. Conclusions on the number or percentage of
young people who have adopted healthy lifestyles must
therefore be viewed with caution but data trends are remarkably consistent.
The earliest PA guidelines for young people were developed by the American College of Sports Medicine (ACSM)
and based on their guidelines for adults. The ACSM recommended that for optimal functional capacity and health children and adolescents should achieve 20e30 min of vigorous
exercise each day.12 Five years later an invited group of
experts convened an International Consensus Conference
(ICC) and systematically reviewed the scientific literature
relating HPA to health-related outcomes. Founded on their
2.3.1. Studies using subjective methods of measuring PA
Several multinational surveys of aspects of young people’s
health have been sponsored by the World Health Organisation
(WHO). One of the most comprehensive WHO surveys
involved 31 European countries, Canada, Israel, and USA, and
included the self-reported HPA of 162,036 young people aged
11, 13 or 15 years. The national sample sizes varied from 1980
in Malta to 10,612 in Belgium. Informed by the UKHEA PA
guidelines, the participants were provided with a definition of
134
PA as “any activity that increases your heart rate and makes
you get out of breath some of the time”21 and asked to add up
the time spent in PA each day and to record the number of
days they were active for at least 60 min in a typical week.
Thirty-four percent of young people reported that they experienced moderate intensity PA for 60 min or more on at least 5
days per week. There was a wide variation across countries
with proportions of young people meeting the PA guidelines
ranging from 26% in Belgium (Flemish) to 57% in Ireland for
boys and 12% in France to 44% in USA for girls. In all
countries and across all age groups, more boys (mean 40%)
than girls (mean 27%) met the UKHEA PA guidelines
although the gender differences were small in some countries.
A strong trend of HPA decreasing with age was noted.21
Data from 16,410 U.S. adolescents from the 2009 Youth
Risk and Behaviour Surveillance Survey (YRBSS) indicated
that 37% of 15e18-year-olds experienced PA that increased
their HR and made them breathe hard some of the time for
a total of at least 60 min per day, on at least 5 days per week.
More boys than girls (46% vs. 28%) met the PA guideline.34
A survey of 32,005 13e18-year-olds from Hong Kong,
China reported 64% of Chinese boys and 40% of Chinese girls
to achieve 60 min of moderate intensity PA after school on 5
days per week. Similar to studies of western youth a declining
trend of HPA with age was observed.35
A study of 2101 6e18-year-old Russians used the ICC PA
guidelines and reported that although nearly 70% of Russian
youth met the guideline for daily PA fewer than 45% met the
guideline advocating sustained periods of MVPA. A marked
decrease with age in the percentage of young people who
experienced sustained periods of MVPA was noted with none
of 17e18-year-olds meeting this PA guideline.36
Although the use of different methodology means that
comparisons must be carried out cautiously, a recent WHO
sponsored survey of 72,000 youth, aged 13e15 years, from 34
developing countries suggests that self-reported levels of HPA
from developing countries are lower than those from Europe,
China and North America. Only 24% of boys and 15% of girls
were reported to experience 60 min of daily MVPA.23
2.3.2. Studies using objective methods of measuring PA
Studies using pedometers provide limited insights into the
percentage of young people meeting PA guidelines but they
are consistent in reporting boys to be more active than girls at
all ages from 7 to 18 years with HPA declining in both genders
with age.3 One study of Canadian youth used a cut-off point of
15,000 steps per day as a guideline and reported 6%e17% of
girls and 14%e33% of boys to meet this target.37
The percentage of young people reported to be physically
active in studies using accelerometers varies from 0 to 100%
depending on the activity cpm defining the required intensity
of PA.7 For example, 2185 9- and 15-year-olds were recruited
from four European countries and monitored for 3 or 4 days
including, where possible, both weekend days. Using total
activity counts boys were more active than girls at both ages
and the 9-year-olds were more active than the 15-year-olds.
The authors estimated that at 9 years >97% of children and at
N. Armstrong
15 years 82% of boys and 62% of girls experienced 60 min of
moderate PA per day. However, age-related intensities of
1000 cpm at 9 years and 1500 cpm at 15 years were used to
define moderate intensity PA so the comparative data must be
interpreted cautiously.38 The data from this study were
collected from four of the same countries, at the same time, as
the WHO study described earlier.21 It is notable that whereas
the accelerometer data reported 82% of 15-year-old boys and
62% of girls to satisfy the UKHEA PA guidelines the selfreport survey indicated only 28% and 19% of 15-year-old boys
and girls respectively to satisfy the same criterion.
A longitudinal study which monitored 1032 young people
for 4e7 days and used a threshold of 3 METs to mark
moderate PA, reported >96% of Americans to meet PA
guidelines of daily 60 min of moderate PA at 9 and 11 years
but the percentage of active youth fell to 83% at 12 years and
31% at 15 years. Age and gender were the most important
determinants of PA with boys more active than girls and PA
declining with age in both genders.39
A cross-sectional study of 1778 American young people
who were monitored for at least 4 days reported mean activity
cpm to decline with age and boys to have higher average
values than girls. This study used a threshold of 4 METs to
define moderate PA and reported 49% of boys and 35% of
girls aged 6e11 years, 12% of boys and 3% of girls at 12e15
years, and 10% of boys and 5% of girls at 16e19 years to
satisfy PA guidelines.40
Two UK studies monitored 10-year-olds (n ¼ 1862 and
n ¼ 2071) for 3 days, used 2000 activity cpm as the threshold
of moderate PA and reported 76%e82% of boys and 53%e
59% of girls to experience 60 min per day of at least moderate
PA.41,42 However in a study of 5595 British 11-year-olds
monitored for at least 3 days, an intensity threshold for
moderate PA of 3600 cpm was used and calculated to be
equivalent to 4 METs or a “comfortable to brisk” walking
pace. Only 5% of boys and <1% of girls accumulated 60 min
of moderate PA per day. In keeping with other studies boys
were significantly more active than girls.43
Studies involving HR monitoring over at least 3 days
generally include small samples of young people but data from
a number of countries consistently show boys to be more
active than girls and PA to decline with age in both genders.3
A longitudinal study of 11e13-year-olds demonstrated that
with age controlled using multilevel regression modelling an
additional decrement in PA was evident in late maturity.44
In a series of studies over a 10-year period the HRs of 1227
English 5e16-year-olds were monitored for at least 10 h on
each of three schooldays.45e47 Pilot work determined brisk
walking (moderate intensity PA) to generate a steady-state HR
of w140 beats/min and jogging (vigorous PA) to generate
a steady-state HR of w160 beats/min. A re-analysis of the
combined data with the participants classified into three
categories according to type of school indicated that at first
school (mean age 7.2 years) the percentage of time with HR
>139 beats/min was 12% and 9% in boys and girls respectively with corresponding figures in middle school (mean age
10.9 years) 9% and 8%, and in high school (mean age 13.1
Physical activity and aerobic fitness during youth
years) 6% and 5%. Twenty-min sustained periods of either
moderate or vigorous PA, in accord with the ICC PA guidelines, were sparse in all age groups. Forty-seven percent of
girls and 34% of boys attending middle or high schools did not
experience a single sustained 10-min period of moderate PA
over the 3 days of monitoring. Sustained periods of moderate
PA were more common among first school children but 31% of
girls and 11% of boys did not experience a single sustained
10-min period of moderate PA.32
A study of 114 Singaporean 9-year-olds used exactly the
same HR monitoring and analysis techniques and reported that
the percentage of time spent with HR >139 beats/min was 6%
in boys and 5% in girls over 3 days of monitoring. Seventy
percent of girls and 47% of boys did not experience a single
sustained 10-min period of moderate PA.48
2.4. Time trends in young people’s PA
The studies outlined in previous sections demonstrate that
the majority of young people (w60%e75%) do not satisfy
current PA guidelines but are young people less active than
they were in previous decades? Reliable data collected prior to
1990 are sparse but several subjective and objective studies
have reported time trends in HPA over the last 20 years.
2.4.1. Studies using subjective methods of measuring PA
A regional U.S. study of adolescents from 31 Minnesota
schools indicated a decline in the MVPA of girls and a decline
in the MVPA of late, but not early, adolescent boys from
1999e2004.49 However, a rigorous analysis of national
YRBSS data collected over the same time period concluded
that whilst there is some evidence of decreased PA amongst
U.S. adolescents overall changes were small and unlikely to
play a role in reported secular trends in overweight and
obesity.50
In a more comprehensive report of U.S. youth seven published studies of YRBSS data were identified which provided
comprehensive, nationally representative, longitudinal data
covering the period 1991e2007. It was concluded that there
was no clear evidence of young people becoming less active
over this time period. The prevalence of young people experiencing sufficient vigorous PA varied from 66% in 1993 to
64% in 2005 with no change in the percentage of girls (56%)
and the percentage of boys varying from 75% to 73%.22
A WHO study of seven European countries, including data
from 47,201 adolescents, reported general stability or a small
increase in the PA of boys and girls aged 11e15 years from
the mid-1980s to the early 2000s.51 These data on European
children are supported by an Icelandic survey involving
27,426 participants. An overall increase in the proportion of
14- and 15-year-olds reporting vigorous PA was observed over
the period 1992e2006.52
An Australian study of 12e15-year-olds reported data on
1055 participants surveyed in 1985 and 1226 participants
surveyed in 2004. All age and gender groups presented
increases in both the prevalence of PA and in the min per week
spent in MVPA. With a mean increase in PA prevalence of
135
12%e20% and a median increase in time spent in MVPA of
135e175 min the authors concluded that PA participation in
Australian youth had considerably increased over the 19-year
period.53
2.4.2. Studies using objective methods of measuring PA
Studies of time trends in PA using objective methodology
are sparse but data are generally consistent. Two Swedish
studies from the same research group analysed daily step
counts using pedometers, over 4 consecutive days. The first
study, of 7e9-year-olds, presented a significant increase of
10% in girls and 6% in boys in daily accumulated step
counts over the period 2000-2006.54 The second study, of
13e14-year-olds, reported no significant change in step
counts in either boys or girls from 2000 to 2008.55 Another
Swedish study carried out during the same time period used
accelerometers to compare the PA of cohorts of 6e10-yearolds 1.5 years apart and confirmed the stability of children’s
PA levels over time.56 These results were further supported
by a Danish study which compared the percentage of time
8e10-year-olds spent in accelerometer-measured, moderate
PA in 1997/1998 with 2003/2004 and reported no significant
changes in HPA.57
In 1990 HR monitoring was used to estimate the HPA of
11e16-year-olds in the South-West of England45 and the study
was repeated 10 years later using the same methodology.58
The percentage of time spent by girls in moderate PA
(HR > 139 beats/min) increased from 4% to 6% whereas the
boys’ values did not change (6%). Analyses of 5-, 10-, and 20min of sustained periods of moderate PA revealed a strikingly
similar pattern 10 years apart. The authors concluded that PA
levels had remained stable over the decade.
In summary, self-reported HPA data suggest that w30%e
40% of youth satisfy the UKHEA PA guidelines with the
figure lower in adolescents from developing countries. The
interpretation of data collected using accelerometers varies
with the adopted cut point. However, the review underpinning
the International Olympic Committee consensus statement on
“health and fitness of young people through physical activity
and sport” concluded that, using an intensity threshold of 3000
activity cpm, which was defined as broadly equivalent to brisk
walking, <25% of young people satisfy expert guidelines for
health-related PA.59 HR data demonstrate that the ICC PA
guidelines for sustained PA are met by very few young people.
A consistent trend, regardless of methodology, is for HPA to
be lower in girls than in boys and to fall with age in both
genders. Evidence from studies using both self-report and
objective methodology suggests that young people’s HPA has
not declined over time, at least not during the last two decades.
3. Aerobic fitness
_ 2), the highest rate at which
Peak oxygen uptake (peak VO
oxygen can be consumed during exercise, is recognised as the
best single measure of young people’s AF although it does not
describe all aspects of AF.60 Other components of AF such as
_ 2 to rapid changes in exercise
the kinetic response of VO
136
intensity61 and blood lactate accumulation during submaximal exercise62 are less well-documented during youth
_ 2 and there are no data on normal values or time
than peak VO
trends. This paper will therefore focus on the extensive data
_ 2.
base on young people’s peak VO
_ 2
3.1. Measurement of peak VO
_ 2 is well-documented and
The determination of peak VO
normally involves an incremental62 or ramp63 exercise test to
voluntary exhaustion during which respiratory gases and HR
are monitored. Only a minority of young people exhibit the
plateau which conventionally marks maximal oxygen uptake
_ 2 max) in adults but it is well-established that a rigorously
(VO
_ 2 is a maximal index of AF during
determined peak VO
64
_ 2 is as
youth. The determination of young people’s peak VO
_
reliable as VO2 max is in adults with a typical error across
three tests of w4%.65
_ 2 are expensive and
Direct determinations of peak VO
require sophisticated equipment, laboratory facilities and
technical expertise. They are therefore not compatible with the
testing of very large samples of participants and several field
_ 2 have been developed.66
tests designed to estimate peak VO
The most extensively documented test is the 20 m shuttle
run (20mSRT)67 and large databases of young people’s
performance on the 20mSRT have been published.68 The
20mSRT is not, however, a direct measure of AF and performance is dependent on factors such as body fatness,
mechanical efficiency, and fractional use of oxygen with
affective issues (e.g., lack of motivation) and cognitive issues
(e.g., inability to judge pace) also important. Validity coefficients of 20mSRT data relative to directly determined peak
_ 2 expressed in ratio with body mass (mL/kg/min) have
VO
been reported to vary from 0.21 to 0.77.69
_ 2
3.2. Interpretation of peak VO
_ 2 rises almost linearly with chronological
Boys’ peak VO
age and girls’ data show a similar but less consistent trend with
some studies suggesting a tendency to plateau at about 14
years. Muscle mass is the dominant influence in the increase in
_ 2 through adolescence with boys’ peak VO
_ 2 values
peak VO
w10% higher than those of girls at age 10 years and w35%
higher by age 16 years. Over the age range 8e16 years peak
_ 2 increases by w150% and w80% in boys and girls
VO
respectively.4
_ 2 is expressed in relation to body mass
When peak VO
_ 2 remaining
a different picture emerges with boys’ peak VO
remarkably consistent from 8 to 18 years at w48e50 mL/kg/
min and girls’ values showing a decline from w45e35 mL/kg/
_ 2 in
min over the same age range. The reporting of peak VO
ratio with body mass is of interest in the context of health and
well-being but ratio scaling has clouded the physiological
_ 2 during growth and maturation.70
understanding of peak VO
Longitudinal studies using multi-level modelling have
demonstrated that, in addition to age, both growth and matu_ 2.71
ration independently and positively influence peak VO
N. Armstrong
With body mass appropriately controlled for, boys’ peak
_ 2 increases through childhood and adolescence into young
VO
_ 2 increases at least into puberty and
adulthood. Girls’ peak VO
possibly into young adulthood.72
3.3. Are young people fit?
In a population of children and adolescents it is not possible
_ 2 with disease outcomes such as coronary heart
to link peak VO
disease mortality and efforts have been focused on relating AF
to risk factors such as elevated blood lipids, body fatness and
_ 2
high blood pressure. As a result of maturation both peak VO
and coronary risk factors are constantly changing through
adolescence and may not relate to adult values. Not surprisingly, evidence linking young people’s AF to coronary risk
factors is less compelling than that observed in adults although
some studies have reported associations with AF and/or
positive changes with aerobic training.73 There is, however, no
evidence to support the existence of a “threshold level” of
_ 2 which is associated with youth health and wellpeak VO
being.
Nevertheless, several publications have advocated health_ 2 based on expert
related threshold levels of peak VO
74
opinion, extrapolated from cut-off points established for
adults75 or linked to current risk-based values via receiver
operating characteristics.76 Proposed thresholds are similar
and, in mL/kg/min, within the range for children of w35e39
(girls) and w40e44 (boys) and for adolescents of w33e35
(girls) and w40e46 (boys). All these thresholds are
compromised by being expressed in ratio with body mass and
when extrapolated from actual data the participants were
volunteers who may not reflect population values.
Few studies have reported their results in sufficient detail to
estimate the number of young people falling below proposed
threshold levels. Data from the Amsterdam Growth and Health
Longitudinal Study (AGHLS) show the percentage of
adolescents to fall below the threshold suggested by an expert
group drawn from the European Group of Pediatric Work
Physiology74 to increase, in males, from 1% to 8% and in
females from 3% to 17% over the age range 13e17 years. The
higher percentage of older females not meeting the threshold
was partly explained by the sex-specific increase in body fat
during puberty.77 A re-analysis of two large data sets from my
laboratory revealed that of 220 11e16-year-olds 3% of the
boys and 3% of the girls fell below the threshold78 and of 164
pre-pubertal 11-year-olds none fell below the threshold.79
_ 2
3.4. Time trends in young people’s peak VO
It is over 70 years since Robinson80 reported the first study
_ 2 and 60 years since Astrand81 published his
of boys’ peak VO
thesis on AF in relation to sex and age. Since this time peak
_ 2 has become the most researched variable in paediatric
VO
exercise science and medicine and scrutiny of studies, at least
from Europe and North America, reveals a marked consistency
in young people’s AF over time.4 This is quite remarkable
considering the fact that these studies have been designed for
Physical activity and aerobic fitness during youth
specific purposes and performed in a large number of laboratories using different equipment, testing protocols and staff.
Eisenmann and Malina82 examined the available data for
_ 2 in American boys and girls in the 20th century in
peak VO
the context of potential secular changes in AF. Boys and
girls were classified into three age groups and estimated
mean values were determined for boys from the 1930s
through the 1990s and for girls from the 1960s through the
1990s. Mean values were fit by least squares, goodness-of-fit
_ 2 had
regression lines and it was noted that peak VO
remained relatively stable among boys of all ages and in
young girls. In adolescent girls, particularly those 15 years
_ 2 was reported to have decreased
and older, mean peak VO
by w17% over the past few decades. However, scrutiny of
_ 2 of
the data reveals only a w4% range in the mean peak VO
15e19-year-old girls in the 1960s, 1980s, and 1990s (1.95,
2.03, and 1.98 L/min respectively). The girls in the 1970s,
who were, on average, 4.5 cm taller but 1.7 kg lighter than
_ 2 value
those in 1990s samples, showed a mean peak VO
w18%e20% (2.39 L/min) higher than girls from other
decades. This indicates that older girls’ AF increased from
the 1960s to 1970s but then fell back to 1960s values over
the next two decades.
Using a systematic review and meta-analytical strategy
_ 2 data, expressed in
a more recent review identified peak VO
ratio with body mass, for >4000 9e17-year-olds from five
countries. It was reported that over the time period 1962e1994
_ 2 of 0.3%.83
there was a very small mean change in peak VO
These exercises in data gathering are interesting and
consistent but they provide only partial insights into temporal
_ 2. They are not epidemiological studies but
trends in peak VO
compilations of small studies providing local snapshots and
involving volunteer participants who may not reflect the
population from which they are drawn.
Large data sets on the 20mSRT are available and an analysis of the 20mSRT performance of 129,882 6e19-year-olds
from 11 countries over the period 1981e2000 indicated an
annual decline in sample-weighted, mean rates of change of
0.3% to 0.5% in children and 1.0% in adolescents. A
great deal of variability across countries was noted ranging
from a mean increase per year of 0.5% in girls from Greece to
an annual 1.9% decline in U.S. boys.84 In a more recent
publication of 20mSRT performances the same author
reviewed data collected between 1964 and 2008, from
25,245,203 9e17-year-olds, from 28 countries. A large deterioration in young people’s performance was noted with
a mean decline of 13.3% since 1975.83
20mSRT performance is strongly influenced by the body
mass the participant carries over the distance run and there is
compelling evidence of an increase in young people’s body
fatness in recent decades.85 Increases in body fatness appear to
explain w50%e70% of the decline in 20mSRT performance86,87 although other factors such as reduced experience
with maximal sustained efforts and an unwillingness to
continue running when fatigued undoubtedly also play a role.
As most health-related activities including aspects of play and
sport participation involve moving body mass the increase in
137
body fatness without a corresponding increase in AF is a cause
for concern in the context of youth health and well-being.
In summary, there is no compelling evidence to suggest that
_ 2 is low although there
as a population young people’s peak VO
are wide variations within studies of healthy young volunteers
with typical coefficients of variation of w15%.2 Elite young
athletes present values w50% higher than healthy non-athletic
peers but whether this is due to genetics, training or, more
likely, both is unknown.88 Even when expressed in ratio with
_ 2 values have been shown to be stable over
body mass peak VO
recent decades. However, data are consistent in showing that
maximal aerobic performance, operationalised as 20mSRT
performance, has significantly declined over the time period
_ 2 has remained relatively stable. Although this
that peak VO
decline in maximal aerobic performance reflects secular
increases in body fatness rather than decreases in AF, it has
important implications for the health and well-being of young
people.
4. Aerobic fitness and HPA
In a 1994 review of the literature Morrow and Freedson89
located 17 published papers which had investigated the relationship between young people’s AF and their HPA. Studies
_ 2
which used performance measures and predictors of peak VO
from sub-maximal data as criterion measures of AF were
included in the review. A median correlation from all reviewed
studies of r ¼ 0.17 was reported and the authors concluded
that the majority of reports indicated no significant relationship between AF and HPA.
_ 2 from sub-maximal data or
Predictions of peak VO
maximal performance tests which do not collect respiratory
_ 2 in
gases inevitably introduce errors into analyses of peak VO
relation to HPA and results from these studies should be
interpreted with caution. However, some studies have analyzed
_ 2 of children and adolescents
the directly determined peak VO
in relation to their HPA and data stretching back over 35 years
have consistently showed little or no relationship between the
two variables.90
Several recent investigations have used the objective
methods of 3- or 4-day HR monitoring or accelerometry to
estimate HPA and analyzed it in relation to directly deter_ 2. In a series of studies of large samples of
mined peak VO
young people from the UK (n ¼ 123e195) no significant
relationships were reported between HR estimates of moderate
_ 2.91e93 A 3-year longitudinal
and vigorous HPA and peak VO
study of over 200 British children used multilevel modelling to
examine age, gender and maturation influences on moderate
_ 2
and vigorous PA, from the ages of 11e13 years. Peak VO
was investigated as an additional explanatory variable of HPA
once age, gender and maturation had been controlled for and
a non-significant parameter estimate was obtained.44
A study of Hispanic American children and adolescents
assessed the HPA of 424 non-overweight and 473 overweight
4e19-year-olds using accelerometers and explored the rela_ 2 using generalized estimating equations
tionship with peak VO
models. They reported that the strength of the relationship
138
between PA and AF was generally low to moderate, accounting
_ 2.94
for a small percentage of the variation in peak VO
Recent European studies have reported similar findings. A
Swedish study of 82 14e15-year-olds noted no significant
relationships between MVPA estimated from accelerometry and
_ 2 in either boys or girls but observed weak but signifipeak VO
cant correlations between “activity-related energy expenditure”
_ 2 in both boys and girls. However, after controlling
and peak VO
for body fat and maturation, none of the PA variables were
_ 2 in boys. Moreover, when the
significantly related to peak VO
highly active boys were compared to the rest of the boys no
_ 2.95 Another
significant differences were observed in peak VO
study of Swedish children measured the HPA of 248 8e11-yearolds using accelerometers and reported no relationship between
_ 2 and moderate HPA. Aweak but significant correlation
peak VO
_ 2 and vigorous HPA was observed with
between peak VO
_ 2. In this
vigorous PA explaining 9% of the variability of peak VO
study only 71% of children reached 85% of predicted HR max
before voluntarily ending the exercise test. With such low end_ 2 data were
exercise HRs it is unlikely that the recorded peak VO
maximal values and the results need to be interpreted
cautiously.96 However, a study of 592 Danish 6e7-year-olds
_ 2 with accelerometry-determined HPA and
compared peak VO
reported similar results with sustained periods of PA explaining
_ 2.97
9% of the variance in peak VO
Using data from the AGHLS, Kemper and Koppes98 tested
the hypothesis that HPA was beneficial to AF in young male
and female participants (13e27 years). They reported that
a 30% increase in HPA score over a period of 15 years was
_ 2 max but noted that
associated with a 2%e5% increase in VO
the functional implications were small and concluded that, “if
we take into account that the relationship calculated with
autoregression over the period of 23 years resulted in nonsignificant relationships, we must admit that in this observational study no clear relation can be proved between PA and
_ 2 max in free-living males and females”.98
VO
On balance, the evidence suggests that HPA is, at best, only
_ 2 during childhood and adolesweakly related to peak VO
cence. This is not an unexpected finding as the HPA of young
people typically lacks the intensity and duration necessary to
_ 2.88
improve their peak VO
5. Conclusion
The assessment and interpretation of young people’s HPA
is complex. Measurement tools assess different dimensions of
HPA and current health-related PA guidelines are evidenceinformed rather than evidence-based. Nevertheless, PA trends
are consistent and indicate that boys are more active than girls
and that the PA levels of both genders decline as they move
through adolescence. The percentage of young people classified as not meeting health-related PA guidelines varies from
w60% to 75% but youth HPA appears to have stabilised, at
least over the last two decades.
_ 2 during childhood and adolescence is wellPeak VO
documented but other aspects of AF during youth are less
well-understood. There is no compelling evidence to suggest
N. Armstrong
_ 2 are common and data indicate that
that low levels of peak VO
_
youth peak VO2 has remained stable over several decades.
However, the secular increase in body fatness is not being
accompanied by a corresponding increase in AF and young
people’s maximal aerobic performance (20mSRT) has
declined markedly over the last 35 years.
In their daily lives young people very rarely experience PA
_ 2 and
of sufficient intensity and duration to enhance peak VO
there is no meaningful relationship between current levels of
_ 2 during youth.
HPA and peak VO
Within the definitions used in this paper most young people
are fit but not active. Both HPA and AF have stabilised over the
last two decades but the low levels of young people’s HPA and
the marked decline over the last 35 years in maximal aerobic
performance which involves transporting body mass remain
major issues in the promotion of youth health and well-being.
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