Children and Exercise XXV

The proceedings of the 25th Pediatric Work Physiology Meeting

Georges Baquet, Serge Berthoin

• Publisher: Routledge
• Year: 2010
• ISBN:
  9781136992636
• Edition: 1
• Topic: Medicine
• Subtopic: Pediatric Medicine

Children and Exercise XXV presents the latest research in the field of paediatric exercise sciences, focusing on the interaction between physical activity, exercise or sport on the one hand, and nutrition, metabolism regulation, cardio-respiratory function or muscle function on the other.

Including contributions from leading international experts, the book is arranged into six thematic sections addressing:

• metabolic syndrome and nutrition

• hormonal and inflammatory regulations

• cardio-respiratory function

• children's performance

• fitness assessment

• physical activity.

Offering a critical review of current topics and reports of contemporary research, this is a key text for all researchers, teachers, health professionals and students with an interest in paediatric sport and exercise science, sports medicine and physical education.

The papers contained within this volume were first presented at the twenty-fifth Paediatric Work Physiology meeting, held in Le Touquet, France, in September 2009.

Part I
Keynote Lectures

CHAPTER NUMBER 1
The 2009 Josef Rutenfranz Lecture “Child as a Source of Mechanical Power”

E. Van Praagh
Blaise Pascal University, Clermont-Ferrand, France

1.1 INTRODUCTION

Since the early aerobic fitness studies conducted by S. Robinson (1938) on males aged 6 to 91 and P.O. Astrand (1952) on both females and males aged 4 to 33, more recent work done by pediatric exercise scientists were very comparable to these initial studies. Tomkinson and Olds (2007) showed that there has been a decline in pediatric aerobic performance since 1970, a pattern which is not observed in pediatric anaerobic performance. The following questions will be addressed: Is there evidence of an age-related decline in physical activity (PA)? Are children more motivated by short-term high intensity exercise than long-term low intensity exercise? How to measure short-term bouts of exercise during growth? What are the main biological determinants?

1.2 AGE-RELATED DECLINE IN PHYSICAL ACTIVITY?

Several studies have demonstrated a decline of PA in children (Sallis, 2000). It is commonly believed that boys are more physically active than girls. Nyberg et al. (2009) reported that this decline may start already at the age of 6 yr. Thompson et al. (2003) showed if PA decreased with increasing chronological age in boys and girls, there were no gender differences in the longitudinal pattern of PA when the confounding effects of biological age were controlled. More recently Sherar et al. (2007) confirmed that to fully understand gender disparities in PA, consideration must be given to the confounding effects of physical maturity.

1.3 SHORT-TERM EXERCISE OR LONG-TERM EXERCISE?

Most of the scientific pediatric literature is devoted to the study of prolonged maximal power output (Baquet et al., 2003), but comparatively little attention has been given to maximal intensity exercise lasting only a few seconds. This is surprising, considering that in almost all daily tasks, games in the playground or multiple sprint sports (such as team ball games, racket sports, sprint- and running events), the child is primarily more involved in short-term high-intensity exercise (HIE) than in long-term activities. Already in 1938, Sid Robinson noted after a strenuous

max test: “the fact that the younger boys do not produce higher lactates may depend on an unwillingness to continue work after it ceases to entertain them”. Showing that PA is not only directed by biological factors, but that psychological factors such as motivation have also an important role during the development period. Reporting that many problems persist in the accurate assessment of energy expenditure, especially in non-laboratory settings under natural conditions, Bailey et al. (1995) developed a direct observational approach for quantifying patterns of frequency, duration, and intensity of PA. The most striking outcome was the short duration of activity events, especially those in the high-intensity range. Recent data showed that, using high-frequency accelerometry during a 7-days PA assessment in prepubertal girls and boys, ninety-six percent of very-high physical activity (VHPA) presented a duration of less than 10 s. (Baquet et al., 2007). Moreover, it was also recently shown that the main component of activity that differs between girls and boys and between high- and low-active children is the frequency of the most intense bouts (Rowlands et al., 2008).

1.4 HOW TO MEASURE ‘ANEROBIC FITNESS’ DURING GROWTH?

1.4.1 Invasive methods

The first studies using the needle biopsy technique in the pediatric population were done in the US (Brooke and Engel, 1969) and in Sweden (Eriksson et al., 1971). Muscle tissue was obtained during surgical intervention (trauma or orthopaedic operations). Few studies on muscle storage of phosphagens have shown that the content of the peripheral energy-delivering substrates is the same for both children and adults. In several textbooks and scientific reports, it is still assumed that the rate of anaerobic glycolysis is limited in children because of their lower phospho-fructokinase (PFK) activity. However, this assumption is only discussed on the basis of the results of PFK at rest (Eriksson et al., 1971). It is interesting that during some 30 years, it was speculated that the lower glycolytic ability during short-term HIE of the child (e.g. lower lactate values, lower short-term power outputs) was due to lower glycogen content and glycolytic enzyme activities at rest. Within the limited available evidence from muscle biopsy studies, it seems necessary to consider other metabolic factors than only the muscle enzyme activities at rest or post-exercise.

1.4.2 Non-invasive method: 31P-NMRS

The first NMR study of muscle bioenergetics during exercise was reported by E. Purcell and F. Bloch (1946). Both won the Nobel Prize in Physics in 1952 (for review, Sapega et al., 1987). In children, the use of nuclear magnetic resonance spectroscopy (NMRS) now provides a safe and non-invasive means of monitoring intracellular inorganic phosphate [Pi], PCr, ATP and pH at rest, during exercise and recovery (Cooper and Barstow, 1996). Zanconato et al. (1993) established the profiles of Pi/PCr and pH in the calf muscle of children and adults performing, at the same intensity, a progressive (plantar flexion) exercise. The minimal drop in pH seen in children and the fact that they achieved an end-exercise Pi/PCr value of only 27% of adult values, are consistent with several reports of the relatively low muscle and blood lactate responses to short-term HIE in children. During short-term HIE and recovery, intramuscular high-energy phosphate kinetics is attenuated in children compared with adults. Although NMRS has proved to be a unique tool for investigating muscle metabolism during exercise, ‘progressive plantar flexion’ may be a poor representation of whole body anaerobic responses.

1.4.3 Wilkie’s statement

During childhood and adolescence, direct measurements of the rate or capacity of anaerobic pathways for energy turnover presents several ethical and methodological difficulties. To quote Wilkie (1960), “In children, exercise scientists instead of attempting to quantify anaerobic energy yield by ATP or glycolysis, are more inspired to measure the resulting mechanical output during short-term exercise, which is the truly useful product”.

This statement makes the assessment of anaerobic performance more “powerful”, since only the subject’s maximal performance will be considered as the criterion. Therefore, during growth, the measurement of mechanical output during short-term high-intensity exercise is a reasonable and useful alternative for elaborating innovative techniques and procedures.

1.4.4 Short-term mechanical power output during growth

Assessment of short-term power raises several methodological problems: (for review see Van Praagh and Doré, 2002).

1. Since power is the product of force and velocity, the external load (e.g. body mass during jumping or load on the cycle ergometer) must closely match the capability of the active muscles so that they operate at their optimal velocity (Wilkie, 1960). Clearly, this is a difficult condition to fulfil or to guarantee in freely accelerating or decelerating cycling or running sprint efforts. Several activities have been proposed for the measurement of short-term power output, including vertical jumping, running or cycling. Of these activities, only cycle ergometry allows precise measurement of power independent of body mass as the imposed load.

2. If ‘true’ short-term peak power output (STPPO) is to be measured, the duration of the test must be as short as possible, because power output decreases rapidly as a function of time (Wilkie, 1960; Van Praagh et al., 1991). The measurement of “true” STPPO requires measurements of instantaneous values of force and velocity.

3. Anaerobic glycolysis and aerobic contribution are limited during instantaneous power tests, although the aerobic fraction in young people is higher than in young men (Van Praagh et al., 1991; Beneke et al., 2007). According to Ferretti et al. (1994) only exercises lasting a few seconds can be considered as truly ‘alactacid’.

1.5 WHAT ARE THE MAIN BIOLOGICAL DETERMINANTS?

STPPO and ‘anaerobic fitness’ are quantitative traits influenced by several factors such as age, gender and training. However, STPO is also determined by innate biological and mechanical variables. Although age – and gender-related differences in the adaptive response to STPO might be related to the maturation of muscle metabolism pathways, little is known about the underlying mechanisms. Differences found between children and young adults during STPPO testing are mainly attributed to size-dependent factors (e.g. muscle size) and size-independent factors (e.g. genetics, hormonal factors). Although in progress, little attention has been devoted to understanding the biological basis of physical activity (Rowland, 1998). However, more and more candidate genes are now being identified and it is clear that heredity contributes to the physical activity phenotype (Eisenmann and Wickel, 2009).

1.6 CONCLUSION

Considering that children are spontaneously more involved in short-burst activities than in long-term low-intensity exercises, short-term peak power output (STPPO) is a fundamental aspect of the child’s physical capacity. There is a general agreement that STPPO increases during growth and maturation and is significantly higher in boys than in girls during and after the adolescent growth spurt. However, in this particular pediatric exercise science area, there has been much relevant applied research, but there is a need for more fundamental research to obtain further knowledge regarding the underlying mechanisms implied in short-term high-intensity physical activity.

1.7 REFERENCES

Åstrand, P.O., 1952, Experimental studies of the physical working capacity in relation to sex and age. Copenhagen, Munksgaar.

Bailey, R.C., Olson, J., Pepper, S.J., Porszasz, J., Barstow, T.J. and Cooper, D.M., 1995, The level and tempo of children’s physical activities: an observational study. Medicine and Science in Sports and Exercise, 27, pp. 1033–1041.

Baquet, G., Van Praagh, E. and Berthoin, S., 2003, Endurance training and aerobic fitness in young people. Sports Medicine, 33, pp. 1127–1143.

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Table of contents

• Contents
• Preface
• Acknowledgments
• Acknowledgments
• Acknowledgments
• Acknowledgments
• Part I Keynote Lectures
• Part II Metabolic Syndrome and Nutrition
• Part III Hormonal and Inflammatory Regulations
• Part IV Cardiorespiratory Functions
• Part V Children’s Performances
• Part VI Fitness Assessment
• Part VII Physical Activity
• Correspondence Addresses
• Author Index
• General Index