The Early Years
The first Canadian studies on children’s physiological responses to exercise were published in the 1960’s and focused on the physical working capacity of healthy children (25, 26, 72, 73). These studies were likely inspired by the ‘grandfather’ of pediatric exercise physiology, Per Olof Astrand, whose PhD dissertation (3) examined the effect of age and sex on exercise capacity in males and females (4-33 year old), and was one the first major publication in our field (Note: Sid Robinson studied exercise response in 5-76 year old males, publishing the first data on exercising children in 1938, (68)). Gordon Cumming published the first studies on the working capacity of healthy Canadian children (25, 26), where working capacity was defined as the power output at a heart rate of 170 b.min-1 (PWC170). Based on those findings, he remarked that the PWC170 of Canadian children was much lower than that of Swedish and American children (2). It was not until the late 1970’s that he implemented the now-commonly-used VO2max as a measure of aerobic fitness in children (Cumming et al. 1978).
In the late 1960’s (72, 73) Roy Shephard examined Canadian children’s fitness in a comprehensive battery of anthropometric and fitness tests. It was part of the Human Adaptability project of the International Biological Programme, which aimed to establish a standardized methodology and comparative values of working capacity of “primitive peoples, athletes, city-dwellers, and schoolchildren” (72). In general, these Canadian pediatric studies of the 1960’s were mainly cross-sectional, often with large sample sizes of healthy, typically-developing children. Their main objective was to characterize children’s ‘normative’ physical capacities for aerobic power and cardiovascular response. These studies reflected a worldwide effort to study the role of exercise in children’s growth and development at the time when pediatric exercise physiology was just a budding field.
The Longitudinal Studies of the 70’s – our ‘Childhood’
The 1970’s saw the emergence of longitudinal studies, which focused mainly on somatic growth and development, but also on age-related changes in motor fitness, among healthy children. The Saskatchewan Growth and Development Study, initiated by Bill Orban and headed by Don Bailey, followed more than 400 seven-year-olds over a 10-year period (1963-1973). The study aimed to establish patterns of normal growth and development of physical fitness parameters (17, 32, 59). Follow-up measurements, led by Bob Mirwald, Bob Faulkner and Adam Baxter-Jones on these same participants in 1998/99 and 2007-2011, identified the long-term effects of exercise during childhood and adolescence on adult (40-50 year olds) health status (79, 80). The study inspired the later Saskatchewan Pediatric Bone Mineral Accrual Study (see below). Together, they are considered as the leading longitudinal growth investigations worldwide establishing patterns of growth, development and physical fitness, and their association with subsequent adult health.
During the same period, another interesting longitudinal study was carried out by Roy Shephard and his graduate student, Andris Rode, in which extensive fitness parameters of a large, community-wide sample of Inuit children (Igloolik, Nunavut) were measured. The children were followed for 20 years during which their health was compared with subsequent generations, as the community became acculturated to the physical activity and dietary patterns of modern society (74).
Another example of the longitudinal investigative approach was the Trois-Rivières Physical Education Study, which examined the effects of additional physical education hours within the school day in primary school children over seven years. The study examined the effects on health, fitness, academic achievement and not surprisingly, found that devoting extra time to physical activity during the school day had no detrimental effects on academic achievement (88) and improves their attitudes toward physical activity as adults (87). This study pioneered the current efforts to establish the effect of exercise on cognitive function in children (18, 71).
The above-mentioned studies were similar to other longitudinal projects around the world that focused on normative growth and development, but were unique in their extended duration and particularly in their focus on health. Interestingly, they advocated for the promotion of physical activity through physical education within the school system (4). Today, there is a debate whether it is more effective to increase physical activity levels among youths through the school system, or perhaps outside of school hours, making it the responsibility of parents and governmental bodies at the various levels.
The Great ’80s – our Early ‘Maturation’
In 1981, the Children’s Exercise and Nutrition Centre was established at McMaster University by the late Oded Bar-Or. This Centre ‘put Canada on the international map’ in terms of pediatric exercise physiology and medicine. It was the world’s first pediatric centre dedicated to the application of exercise for the diagnosis, treatment or prevention of pediatric pathologies. Over a decade past Oded’s passing, the Centre remains internationally renowned for its research on the effects of exercise and training on the health of typically developing children, as well as those with chronic conditions or disability.
Oded’s seminal book, Pediatric Sports Medicine for the Practitioner (7), published in 1983, became the ‘go-to’ resource for pediatricians and sports physicians, researchers, students and practitioners worldwide, and remained so for over 20 years. An updated text was published 21 years later (12). Together, the Centre and the book, likely provided the much-needed impetus for the emergence of studies examining the role of exercise and training in clinical populations. While the role of exercise was well established in adults, especially in cardiac rehabilitation, there was only scant information of its benefits for children with various chronic conditions or disabilities.
The 1980’s saw the emergence of more controlled training intervention studies and a greater interest in other fitness components and physiological systems. For example, resistance training for children was a controversial issue and prompted several Canadian studies (e.g., (28, 29, 66)). Despite numerous position statements (16) the use of resistance training in children still remains controversial in some circles. Additional Canadian studies on children’s muscle strength emanated from a powerful research group led by Joe Blimkie at McMaster University, with studies on children and the effect of growth and development, physical activity and training on muscle strength (19, 20).
From a methodological perspective, there were two important Canadian developments in the 1980’s: Roy Shephard and his doctoral student, Gaston Godin, developed the Godin-Shephard leisure-time physical activity questionnaire (42), aimed to assess physical activity behaviour of children and their parents. The questionnaire has been translated into many languages and used to assess physical activity in different populations around the world. Other physical activity questionnaires designed for children were developed a few years later by John Hay (CSAPPA (45) and HAES (44)). Around the same time, Luc Leger published the 20-meter shuttle run test for children (50) that was to become the most prominent pediatric aerobic field test worldwide. It is a simple test, used to predict maximal oxygen uptake. It is so widely-used that its limitation as a predictive, rather than a measuring tool, is often overlooked.
The Productive ‘90s – continued ‘Maturation’ of our Field
The 1990’s saw an emergence of interest in exercise and the growing child’s bone with the first study, globally, coming from the University of Calgary. It demonstrated that adolescent athletes involved in high impact-type of training, i.e., volleyball, basketball, had greater bone density than those involved in non-impact training, i.e. swimming (43). This study paved the way for many other cross-sectional studies in the 1990’s and early 2000’s, from Canada and beyond, which demonstrated enhanced bone quality and quantity in young athletes compared with non-athletes (e.g., (21, 27, 37, 40)).
The Saskatchewan Pediatric Bone Mineral Accrual Study was a longitudinal study, inspired by the Saskatchewan Growth and Development Study, led by Don Bailey. It examined determinants of bone mineral accrual, including physical activity, in healthy children over a 7-year period (1991-1997), with follow-up measurements up until 2011. It was followed up by the Saskatchewan Young Recreational Gymnast Study, which tracked bone mineral content in over 160 children, 4-6 years old, over a four-year period. Researchers such as Heather McKay and Adam Baxter-Jones, along with the original team of researchers, contributed greatly to our current understanding of the changes that take place in bone mineral content during growth and maturation, as well as the important effect of childhood exercise and physical activity on bone health in later years (e.g.,(5, 15, 33, 54)). In 2000, a unique pre-meeting of the American Society of Bone Mineral Research meeting was organized by Joe Blimkie, titled: “Exercise & Nutrition as Modulating Factors in Skeletal Development of Children in Health & Disease”. At this meeting the pubertal period was highlighted as the critical period in bone development, with the greatest potential benefits of exercise. The road was paved for intervention studies, in Canada and elsewhere, demonstrating the beneficial effects of exercise on growing bone (13, 56, 63). These studies are responsible for the current recognition of osteoporosis as an adult disease with a pediatric origin.
The longitudinal studies of the 1990’s often employed a mixed, cross-sectional-longitudinal design, allowing researchers to examine a wide age range (e.g., 8 to 16 years old), within a relatively short period of time (e.g., 3-5 years). This type of design called for non-traditional data analysis. Thus, multilevel modeling became an essential analysis tool in the field of pediatric exercise science globally, as advocated for and supported by many Canadian researchers (e.g., (14)).
Children’s ability to adequately regulate their core temperature while exercising in the heat first gained attention in the late 1970’s and early 1980’s (outside of Canada). In the 1990’s, this area of study was invigorated and extended to include hydration needs by healthy children and those with cystic fibrosis, by several researchers in the Children’s Exercise and Nutrition Centre, under the leadership of Oded Bar-Or (9, 34-36, 46, 57, 58, 67, 90). At the time, it was widely assumed that children were at a relative disadvantage compared to adults during exercise in the heat (children’s lower sweating rate). This view was revisited over a decade later, concluding that, although children indeed sweat consistently less than adults, they ‘employ’ a different thermoregulatory strategy, relying on their greater body surface-area-to-mass ratio and associated skin blood flow to dissipate heat rather than on evaporative heat loss via sweating. They are therefore, not more susceptible to heat injury than adults (39, 69). In a cold environment, however, children’s larger surface-area-to-mass ratio makes them more prone than adults to heat loss and potential cold injuries than adults (48). To partially compensate, they elevate their metabolic rate in the cold to a greater extent than adults (78).
Canadian researchers, particularly Oded Bar-Or, played a central role in the world-wide awareness of the importance of exercise and physical activity for children with chronic conditions, with editorials and commentaries published in international scientific journals (6, 52, 77). The benefits (and sometimes perils) of exercise for diseases such as asthma (11), and in the management of diseases such as obesity (8), cystic fibrosis (9, 55), cerebral palsy (89), cardiac rehabilitation (53) and others were established.
Building on the established strength of the neuromuscular physiology field in Canada, neuromuscular responses to exercise and training among children received renewed interest by Canadian researchers and others in the 21st century (22, 24, 30, 38, 41, 60, 61). Much of this research was carried out at Brock University, introduced and provided support that children utilize less of their higher-hierarchy (type II) motor units compared with adults (31). This hypothesis, which is slowly gaining international recognition, is attractive because it can explain many observed child-adult differences, not only in strength-related attributes but also in metabolism, endurance performance and aerobic trainability.
Into the 21st Century – A fully Grown Field
The role of exercise and physical activity in children’s health and well-being has been firmly established since before the turn of the century. Canada is one of the leading nations in terms of physical activity recommendations and guidelines for children and adolescents (e.g., (85, 86)), and recently for pre-schoolers (84). These guidelines were adopted, promoted and extended by CSEP for other populations. Much of this was triggered, supported, reinforced, even provoked by Active Healthy Kids Canada, and Mark Tremblay who was its chief scientific officer for many years. Since 2005, Active Healthy Kids Canada has been publishing yearly ‘Report Cards’ on children’s physical activity, which serve to sound the alarm on the low physical activity levels among Canadian youth, but more importantly, to also search for solutions to the problem. These ‘Report Cards’ have now been adopted worldwide.
As the value of exercise and physical activity for healthy, developing children has been well-recognized, pediatric exercise physiology in the 21st century has been dominated by a plethora of studies demonstrating the importance of exercise in combatting childhood obesity. Although it is a global concern, many Canadian researchers have contributed to the recognition of the importance of exercise in the management of childhood obesity (e.g., (1, 10, 75)). Along these lines, the Children’s Hospital of Eastern Ontario (CHEO) established the Healthy Active Living and Obesity Research Group (HALO) in 2007, under the leadership of Mark Tremblay. Years later, Kristi Adamo (HALO, University of Ottawa) received CSEP’s 2015 Young Investigator Award for her contribution to intervention and prevention of childhood obesity. Other pediatric clinical conditions, and the beneficial role exercise may play in contending with these conditions have also emerged, with contributions by Canadian scientists (e.g., Diabetes (62); Juvenile arthritis (76); other inflammatory diseases (65)). It is likely that this trend, highlighting the importance of exercise and physical activity for childhood disease management and prevention will continue and gain momentum in the coming years.
Exercise can elicit a strong (acute) immune response but chronic exercise (training) can strengthen the immune system. This has long been recognized in adults but only in the 21st century did this issue receive attention, especially by Canadian researchers. Early studies published by Nota Klentrou at Brock University highlighted that less-active children and those with higher adiposity, suffer from more upper respiratory tract infections (23, 49). Numerous studies from the renewed Child Health and Exercise Program at McMaster University, under the leadership of Brian Timmons, use cutting edge approaches to further our understanding of the pediatric immune response to exercise (e.g., (81-83)). Worldwide, there are very few laboratories investigating the role of exercise and training on the immune system in children. Notably, Canadian researchers have played a major role in advancing this field.
In the last two decades, the underlying paradigm in pediatric exercise physiology seems to have reversed. The value of exercise and physical activity for our health and well-being is well-recognized. Perhaps understandably, more effort (and funding) is devoted to determining the factors affecting physical activity (dependent variable) and less effort is devoted to studying the effects of physical activity and exercise (independent variables) on children’s physiological systems and the physiological mechanisms that may differentiate children from adults. Pediatric exercise physiologists have investigated the effects of exercise and training on organ systems (e.g., cardiovascular system – O’Leary at Brock, Maureen MacDonald at McMaster), different tissues (e.g., bone – McKay at UBC, Baxter-Jones at Saskatchewan, Falk and Klentrou at Brock, and even cells (e.g., immune cells – Timmons at McMaster). However, pediatric research is limited by ethical constraints of putting children under stress or using invasive procedures. In recent years, mechanistic research is emerging to tackle the effect of exercise on bone turnover during the growing years (e.g., (47)), on inflammation and disease in children (e.g., (70)), and on the effect of growth and maturation on the way children’s muscle function (31, 51, 60, 64).
Canadians have contributed greatly to the field of pediatric exercise physiology over the past fifty years. These contributions have been highlighted in Canadian and international scientific journals. Overcoming the ethical, technical, and even logistical constraints in pediatric exercise research will continue to be a challenge in the upcoming decades. It is up to us to rise to the challenge with continued innovation!
Some of the material in this essay is based on a lecture given by Joe Blimkie at the Golden Horseshoe Pediatric Exercise Group meeting in Hamilton, ON, April 9th, 2010, and on a lecture by Tom Rowland at the Golden Horseshoe Pediatric Exercise Group meeting in St. Catharines, ON, April 14th, 2015. The Golden Horseshoe Pediatric Exercise Group was inspired by the late Oded Bar-Or and was founded in 2006, with the purpose of promoting research, exchange of ideas, and practice in pediatric exercise science. The group meets regularly in the Golden Horseshoe area (Toronto, Hamilton and Niagara).