Physical conditioning of children

JOURNAL OF ADOLESCENT HEALTH CARE 3:241-246, 1983

Physical Conditioning of Children* VICTOR L. K A T C H

Exercise-induced changes in muscular and cardiovascular function in pre- and postpubescent children are explained in terms of a "Trigger Hypothesis." This hypothesis predicts that, prepubertally, there will be only small training-induced biological alterations because of the lack of hormonal control. It is suggested, therefore, that emphasis be placed on skill acquisition rather than physiological conditioning during prepuberty. Postpubertal exercise-induced changes are well documented and follow predictable patterns. The principles that govern physiological adaptations to exercise are discussed in terms of energy transfer and the factors that affect training. Duration, intensity, and frequency of performance are detailed. It is recommended that emphasis be placed on these factors when designing a physiologically sound physical training program. KEY WORDS:

Physical training Children Conditioning In this paper, a physical conditioning hypothesis, termed the "Trigger Hypothesis," is presented and discussed, as well as other selected aspects of physical conditioning of children. The focus is on children whose skeletal and physiologic ages range between 6 and 16 years. This age range is critical since it covers the time w h e n most children first become actively involved in organized programs of physical activity and sports. During this time the young child

*Presented at the American Academy of Pediatrics, N e w Orleans, October, 1981

From the Cardiovascular Exercise Laboratory, Section of Pediatric Cardiology, Department of Pediatrics, School of Medicine, and the Kinesiology Department, School of Education, The University of Michiy,an. Address reprint requests to: Victor L. Katch, Director, Cardiovascular Exercise Lab., Department of Pediatrics, C.S. Mott Children's Hospital, Fl123, Box 66, Ann Arbor, MI 48109. Manuscript accepted July 30, 1982.

develops, grows, and perfects his or her physical skills. This developmental process is fascinating not only from a biologic/growth perspective, but also from that of skill/performance.

Conditioning Hypothesis The Trigger Hypothesis is illustrated in Figure 1. It states that there is one critical time period in a child's life (termed the "trigger point") which coincides with puberty in most children, but may occur earlier in some, below which the effects of physical conditioning will be minimal, or will not occur at all. It is suggested that this trigger phenomenon is the result of modulating effects of hormones that initiate puberty and influence functional development and subsequent organic adaptations. This is not to imply that prepubertal changes do not occur. On the contrary, functional changes and organic adaptations occur as a normal consequence of the growth-maturation process (1). It has long been suspected that one's genetic makeup, shown in the lower portion of Figure 1, sets the limit for physiologic performance. Although knowledge about genetic potential is scarce, it can be speculated that its contribution to adaptation to conditioning and performance is major. For example, studies made of 15 pairs of identical and fraternal twins raised in the same city and whose parents were of similar socioeconomic background show that heredity alone accounted for up to 93% of the observed differences in cardiorespiratory endurance (2,3). In addition, the capacity of the short-term energy system of glycolysis and maximum heart-rate were shown to be 81% and 86% genetically determined, respectively (2,3). Subsequent research suggests that muscle fiber composition of identical twins is similar, whereas wide variation exists among fraternal twins (4). It is possible that these estimates represent the upper limit of genetic determination.

© Society for Adolescent Medicine, 1983 Published by Elsevier Science Publishing Co., Inc., 52 Vanderbilt Ave., New York, NY 10017

241 0197-0070/83/010241 + 06503.00

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I

SUPERIOR

PERFORMANCE

~L

IFUNCTIONAL J CHANGES | ,growth

|

--,I

.size

] ,body composition

GENET ~C I 0

PRE PUBERTY

TRIGGER

I 11

I 14

POST PUBERTAL

I ADULT

AGE Figure 1.

Trigger Hypothesis.

The Trigger Hypothesis is based on the assumption that for there to be organic adaptations to training and conditioning certain necessary conditions must occur or be present. These might include an increase in the lean-to-fat ratio, maturation of the neuromuscular system, and certain levels of endocrine function. In this context, the importance that the androgens and growth hormone have in the development of functional capacity, metabolism, and muscular development are especially important. It is known that growth hormone (GH), while stimulating somatic growth, acts on many tissues--in particular, muscle, cartilage, adipose, and other connective tissue. Assuming adequate nutrition, GH stimulates the rate of protein synthesis and enhances the transport of amino acids across cell membranes with resulting nitrogen retention. GH also promotes the retention of minerals, for example, sodium potassium, and magnesium. It enhances the mobilization of fat from adipose tissue depots and utilization of fats for energy. In essence, the metabolic effects of GH enhance the production of pro-

tein, conserve carbohydrates, and mobilize depot fat. Because of these functions, it is a prime agent, working in the presence of physical conditioning to induce organic adaptations (5). A complete description of other hormones and their functions is beyond the scope of this paper, but it can be concluded that it is hormonal regulation that sets the trigger for the organic adaptations and responses to physical conditioning (5).

Skill and Physiologic Training At this point it is important to discuss the difference between skill training and physiologic conditioning. Skill training refers to the development of independent movement patterns, whereas physiologic training refers to the development of such factors as aerobic power, anaerobic power, and anaerobic capacity which include cardiovascular and metabolic alterations that involve energy transfer. (A discussion of the specific energy transfer systems appears in a latter section.) Figure 2 depicts the relationship between skill acquisition, physiologic training, and specific performance capabilities. The solid lines indicate that there

I

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CONDITIONINGOF CHILDREN

SKILL:

TRAINING

.~_ SUPERIOR

NEUROMUSCULAR "SKILL" TRAINING

PERFORMANCE

T

PH siO,OGiC't

PERFORMANCE

243

age 12 years) maintained differences with agematched control subjects 10 years after training had ceased. These data, as well as data from other studies (10), suggest that both pre- and postpubertal training, in some way, may prime the body for accepting potential changes that will have long-term resistance to decav.

J"

TRAINING

Figure 2. Relationship between skill training, physiologic training, and superior physical performance. The solid lines indicate that there is a known positive transfer.

is a known positive transfer; that is, if one increases skill, one necessarily improves physical performance. It is self-evident that skill development is necessary for improved performance. Similarly, if one increases physiologic potential, physical performance should also improve. However, this need not occur unless there have been concomitant increases in skill acquisition. We have all observed individuals who may possess superior strength or endurance capabilities yet lack the skill to use these capabilities in specific athletic events. It has been shown that prepubertal athletes differ little in their metabolic adaptation processes from nonathletes of the same age, yet can be remarkably different in athletic performance. This speaks in favor of the notion that high-level performance is more dependent on the level of skill development than on superior physiologic adaptations (6-8). Based on the above, it is suggested that physical training programs for prepubertal children should concentrate more on skill acquisition (throwing, jumping, kinesthesis, catching, etc.) than on specific physiologic conditioning (endurance, power, strength). For postpubertal children, physiologic conditioning along with skill training is not only desirable, but required for championship performance. No definitive answer is available to the interesting question as to whether prepubertal children who are physiologically conditioned will demonstrate greater physiologic changes after puberty than would occur if conditioning activities had been experienced only after puberty. Work in this area is in its infancy, and the longitudinal studies needed to answer this and other important questions have not been undertaken. However, in a related study, Erikkson et al. (.9) demonstrated that 30 girl swimmers studied over a 10-year period (initial studies made when girls had been swim-training for 2.5 years beginning at

Physical Training and Energy Pathways The major objective of physical training is to cause biologic adaptations to improve performance in specific events. From a physiologic standpoint, training can be classified according to the predominate energy pathway that is used. This is illustrated in Figure 3. Brief power activities lasting about 4 6 seconds rely almost exclusively on immediate energy generated from the breakdown of the stored intramuscular phosphates, adenosine-triphosphate (ATP), and creatine phosphate (CP). Consequently, power athletes like sprinters must gear their training to improve the capacity of this specific energy transfer system. As all-out exercise progresses up to 60 seconds and power output becomes somewhat reduced, the major portion of energy is still generated via nonaerobic pathways. These metabolic reactions involve the short-term energy system of primarily anaerobic glycolysis and subsequent lactic acid for-

Figure3. Energy pathways and physical performanceactivities. [Adapted from McArdle et al. (13).] TIME 0 4 eec sec

10 zec

4 min

5÷ min

STRENGTH--POWER power-lifter

~

POWER shot-put

TYPES OF PERFORMANCE POWER--ENDURANCE 200 meter dash ENDURANCE-3OOmeter to marathon

I iMMEDIATE AND HORT--TERM-NON IDATIVE SYSTEM

I.-~ AEROBIC OXIDATIVE SYSTEM

ENERGY PATHWAYS

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mation. As exercise intensity diminishes and duration extends to 2-4 minutes or longer, reliance on energy from phosphate stores and anaerobic glycolysis decreases, whereas the aerobic production of ATP becomes increasingly more important. Prolonged exercise progresses on a "pay-as-you-go" basis with more than 93% of the energy requirement being generated by aerobic reactions. It seems clear that an efficient training program is one that allocates a proportionate commitment to training the specific energy systems involved in the desired activity (11). In a sense, physiologic adaptations are very specific to the type of training that is performed. This principle, which can be termed the Specificity Principle, is illustrated in Figure 4. Shown are the three major types of performance and their respective training indexes. The solid lines indicate that it is known that there is a positive transfer between training and performance. The dashed lines indicate that there is no evidence for positive transfer; that is, adaptations in the biologic systems that support strength or power performance will not aid performance in endurance activities. Indeed, spe-

cific training elicits specific adaptations creating specific performances. It is remarkable that this specificity principle is not more widely accepted when the evidence in the exercise physiology literature overwhelmingly supports it (11-13).

Overload, Individual Differences, and Reversibility Specificity is only one of four major principles that may be applied to aerobic training and conditioning; the other three are overload, individual differences, and reversibility (Table 1). Overload refers to exercising at a level above normal. This is achieved by manipulating combinations of training frequency, intensity, time, and type of activity (often termed the "FITT" principle). By placFigure4. 5pecificity in performance and training. [Adapted after Edington and Edgerton (19).]

SPECIFICITY STRENGTH"~_ ....

._,,J POWER ~_ L---I

4 ENDURANCEI PERFORMANCE

Table 1. Principles of Training Overload "FITT" (Frequency, intensity, time, type) Specificity "SAID" (Specified adaptations to imposed demands) Individual Differences (Individual variation in response and performance) Reversibility (Transient gains)

ing a specific overload on the physiologic system, gross and cellular changes occur; for example, changes in muscle fiber characteristics and the ability to transport and use oxygen. This adaptation results in improved physical performance if coupled with the appropriate skill training. Individual differences refer to the fact that people will respond and perform differently to similar stimuli. Though training adaptations are general, there are individual differences in these responses and hence individual prediction of responses is complicated, and in many cases unsuccessful (12). The final training principle is reversibility. It has been documented that when a person stops exercising there is a tremendous detraining effect. After only 2 weeks of detraining, significant reductions in work capacity can be measured, and almost all of the training improvements are lost within several weeks. Table 2 shows data from two studies where subjects were first trained and then detrained. In the first study (14) detraining was 20 days of complete bed rest whereas in the second (15) detraining consisted of 84 days of no systematic physical activity. In the first study, the five subjects confined to bed for 20 consecutive days experienced a 27% decrease in maximal oxygen uptake (VO2 max) accompanied by a 24% decrease in maximum stroke volume. This corresponds to about a 1% decrease in physiologic function each day. In the second study the decrease was less but still approximated a 0.5% decrement per day. The important point from these data is that the beneficial effects of exercise training are transient and reversible. For this reason, conditioning activities should be continuous.

Factors Affecting Training

ENDURANCE TRAINING

Optimal physical conditioning requires a consideration of four factors: initial level of fitness, exercise intensity, exercise frequency, and exercise duration.

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Table 2. Detraining Data from Two Studies StudY

N

Sex

Day D-T

Sa|tin (14)

5

M

20

Drinkwater (15)

7

F

84

If someone starts at low levels of functioning, there is room for considerable improvement. This is especially true during the adolescent years when some children initiate their first serious exercise training. As a general guideline, aerobic fitness improvements of 5%-25% can be expected from systematic training in postpubertal individuals (16). For strength training, it is not unusual to see 100%-200% improvement during the adolescent years, and even up to age 40 years (17). Postpubertal changes in muscle hypertrophy, consequent to heavy weight training, can be remarkable for some. Witness to this fact are the remarkable morphologic changes in teenage body builders, both males and females, even in the absence of drug therapy. Training-induced physiologic changes depend primarily on the intensity of the overload. Exercise intensity reflects both the time and extent of overload imposed on the specific energy system. Generally, the greater the relative training intensity the greater the training adaptation, within certain limits. Although there may be a "minimal" threshold intensity below which a training effect will not occur (16), it is reasonable to suggest that there may also be a "ceiling" threshold above which there are no further gains in adaptation, These lower and Upper limits may depend on the person's initial capacity and current state of training. It is interesting to speculate as to what length of time is necessary before adaptations are seen. This will, of course, depend on the initial levels of strength and endurance. However, it is not unusual that one should expect changes and improvements within i 2 weeks of starting a conditioning program if the intensity is sufficient (18). Several studies (16,18) report continuous, week-

Variable

Pre

Post

VO~ Max (l/min) Stroke Volume (ml/bt) CO (l/rain) VO2 Max (ml/kg) VeMax (l/rain) 02 Pulse (ml/bt)

3.3

2.4

116

a(';~) -27

88

- 24

20

14.8

- 26

47.8

40.4

- 15,5

77.5

69.5

- 10.3

12.7

i0.9

- 14.2

by-week improvements in VO2 max. Such improvement eventually stabilizes as a p e r s o n approaches their genetically determined maximum. The exact duration before this leveling off occurs is unknown, especially for individuals in a very intense program. For strength adaptations, the same appears to be true. There is an immediate and progressive adaptation so long as the intensity is sufficient. The frequency as well as the intensity Of the conditioning activity will also affect physiologic adaptations. It has been shown that an extra investment of time, in terms of training more than 3-4 days per week~ may not be very prOfitable in producing maximum changes in physiological function (16). The reason for this phenomenon is not clear; however, it is tempting to suggest that physical conditioning sets the stage for biologic alterations which occur during the resting phase between training sessions. This position would argue that optimum rest between exercise sessions is just as important as the physical act of exercise in producing biologic adaptations. The duration of each exercise session necessary for optimal improvement has not been determined. However, if maximum intensity levels are used the duration will be self-determining, depending on the specific energy SyStem being trained. As a general rule, for adults, endurance exercise a minimum of 25-30 minutes is of sufficient duration to restflt in approximately 300-kcal expendituremwhich is probably the minimal level of er~etgy expenditure necessary for adaptations to take place (16), For children and adolescents there is instlfficient data to make a definitive statement. For strength training it is suggested that too much per Session is undesirable; a good strength training workout, at any level, at any age, need not exceed 40 minutes when maximum

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intensity levels are used. This is perhaps one of the great myths in weight training: that long durations of time are necessary to Obtain maximum results. On the contrary, with maximum intensity training it would be undesirable and unproductive to overtrain. In summary, training adaptations follow predictable patterns after the onset of puberty. Such patterns are most likely not operable during prepuberty. It makes sense to Concentrate on neuromuscular skill training for young children, and include physiologic Conditioning when puberty is reached. Appreciation is expressed to Catherine Moorehead~Steffens for her expert help. Thanks are also extended to Professor P. Freedson, Exercise Science; University of Massachusetts.

Reference 1. Tanne/" JM: Growth at Adolescence, ed 2. Oxford: Blackwell Scientific Publications, 1962 2. Klissouras V: Heritability of adaptive variation. J AppI Physiol 31:338-342, 1971 3. Klissotiras V: Predictiola of athletic performance: Genetic considerations. Can J App1 Spoi-t Sci 1:195-203, 1976 4. Sjordin B: Lactate dehydrogenase in human skeletal muscle. Acta Physiol Scand Suppl 436, 1976 5. Malina R: Physical activity, growth and functional capacity, in Johnston FE, Roche AF, Stisanne C (eds): Human Physical Growth and Maturation: Methodologies and Other Factors. New York, Plenum Press, 1980 6. Mirwald RL, Bailey, DA: Longitudinal comparison of aerobic power in active and inactive boys aged 7 to 17 years. Ann Hum Biol 8(5):405-414, 1981

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7. Kobayashi K, et al.: Aerobic power as related to body growth and training in Japanese boys: A longitudinal study. J Appl Physiol 44:666-672, 1978 8. Schumucker B, Hollman L: The aerobic capacity of trained athletes from age 6 to 7 years and on. Acta Paediatr Belg 28:92-101, 1974 9. Erikkson BO, Lundin A, Saltin B: Long term effect of previous swim training in gifts. A 10 year follow-up of the "Girl Swimmers." Acta Paediatr Scand 67:285-292, 1978 10. Sprynarova S: Longitudinal study of the influence of different physical activity programs on functional capacity of boys 11 to 18 years. Acta Paediatr Belg Suppl 28:204-213, 1974 1l. Pechar GS~ et al.: Specificity of cardiorespiratory adaptation to bicycle and treadmill training. J App1 Physiol 36:753-76t, 1974 12. Katch V, Henry FM: Prediction of running performance from maximal oxygen debt and intake. Med Sci Sports 4:87-191, 1972 13. McArdle B, Katch FI, Katch V: Exercise Physiology: Energy, Nutrition and Human Performance. Philadelphia: Lea & Febiger, 1981 14. Saltin B, et al.: Response to exercise after bed rest and after training. Circulation 38: Suppl 7, 1968 15. Drinkwater, B, Horvath S: Detraining effects on young women. Med Sci Sports 4:91-95, 1972 16. Pollock M: The quantification of endurance training programs, in Wilmore J (ed): Exercise and Sport Science Review. New York: Academic Press, 1973, vol 1 17. Clarke DH: Adaptations in strength and muscular endurance resulting from exercise, in Wilmore J (ed): Exercise and Sport Science Reviews. New York: Academic Press, 1973, vol 1 18. Hixson RC, Bomze HA, Holloszy JO: Linear increase in aerobic power induced by a strenuous program of endurance exercise. J Appl Physiol Respir Environ Exercise Physio142:372376, 1977 19. Edington DW, Edgerton VR: The Biology of Physical Activity. Boston: Houghton-Mifflin, 1976