EFFECTS OF AN ENDURANCE TRAINING REGIMEN ON ASSESSMENT OF WORK CAPACITY IN PREPUBERTAL CHILDREN * Louis Lussier and Elsworth R. Buskirk Laboratory f o r Human Performance Research The Pennsylvania State University University Park, Pennsylvania 16802
Introduction
The effects of a physical conditioning regimen on prepubescent children were studied. The regimen involved running progressively longer distances over a period of 12 weeks. Most previous work involving physical conditioning of children has centered on subjects of pubescent ages (11 to 13 years) and older,'-' and only three studies were located dealing with children of prepubescent ages.*-l0 The latter data are summarized in TABLE1: No effect of physical conditioning was found on the aerobic capacity of 8- to 11-year-olds in the first three studies listed in TABLE1. The investigators in these three studies utilized interval training procedures. The duration of each exercise bout was short, but identifiable with how children play. Brief-bout interval training is probably closely related to the activity pattern that is normally exhibited by children. Brown et al.' were the only investigators to report significant increases in aerobic capacity with training. They trained prepubertal girls for crosscountry running and utilized distance running, i.e., endurance training, but no control subjects participated in their study and their results could have been confounded by growth-induced changes. Growth during the prepubescent period can be assumed to be linear but growth remains a complicating factor in extended studies of physical conditioning in children. Short training programs have the advantage of minimizing the effect of growth because little growth occurs and any small changes that do occur can be identified with growth, particularly if a control group is employed. Thus, the effects of physical conditioning can be isolated as differences among those subjected to physical conditioning as compared to control subjects who engage in no organized physical activity. Subjects and Methods
A group of 26 children (ages 8 to 12) volunteered and were divided into an exercise group and a control group. All were classified as stage I according to Tanner's standards for genitalia maturity and breast development. Bone age was determined from hand and wrist X rays taken with a General Electric Mobile 225 I11 X-ray Unit. The radiographs were taken at a focal length of 40 in. (1.15 m), 50 kV, 50 mA, 0.2 sec (10 mA.sec). A lead-lined elongated
* This work was supported by Research Grant AM08311 from the National Institute of Arthritis, Metabolism and Digestive Diseases. 734
735
Lussier & Buskirk: Training Children TABLE 1 DEALING WITH THE SUMMARY OF STUDIES IN THE LITERATURE PHYSICAL CONDITIONING OF PREPUBERTAL CHILDREN
Study Bar Or and Zwiren Stewart and Gutin Mocellin and Wasmund lo Brown el al?
Frequency Duration (per of effort * VO,,., week) (min) (ml.kg-'.min-')
.
Duration (weeks)
Sex
n
M+F
92
9
2 4
M
24
8
4
1-3
-0.3
M +F
53
F
12
7 12
1-2 4-5
3-6 >15
= 11.5
1
-0.8
-
* Duration of effort before resting. Several bouts per exercise session were employed by each investigator. wooden pyramid was used to minimize radiation scatter. A developmental age was assigned to each of the 28 ossification centers using the Greulich and Pyle Atlas l 1 and the mean age was calculated as described in Haas et a1.12 Because radiographs for bone age were taken at the time of the final testing, both physiological and chronological ages reported in TABLE 2 represent age at the end of the study. No significant differences between groups were found for either chronological or physiological age. Participation in organized physical activity outside the physical conditioning regimen was essentially the same for both 3, the number of girls in each group was unequal groups. As indicated in TABLE because of the small number of girls who volunteered to participate in the study. After physical examination of the children, familiarization sessions were conducted for the various testing procedures. The children walked on a treadTABLE2 AGE AND HABITUAL PHYSICAL ACTIVITYOF SUBJECTS IN THE EXERCISE AND CONTROL GROUPS Exercise Group n= 16
Variable Chronological age, years Physiological age, years Hours of physical activity per week Fall season Winter season
* Standard deviation.
Mean (+SD) (Range) 10.3 (1.2) (8-12) 9.6 (1.1) (7-11) 4.6 4.4
t
*
Control Group n=10
Mean (+SD) (Range1 10.5 (1.2) (8-12) 9.8 (1.1)
(7-11) 3 .O 4.3
t Excluding time spent in the physical conditioning program.
P NS
NS
736
Annals New York Academy of Sciences TABLE3 SEX
AND AGE DISTRIBUTION OF IN THE EXERCISE AND CONTROL
Exercise Group Sex Boys Girls Age groups * 8-9 years 10-12 years
11 5
SUBJECTS GROUPS Control Group 9 1 4 (5) 6 (5)
* In reference to chronological age. Physiological age groups are represented in parentheses. mill at progressively increasing submaximal workloads (3 mph and 2.5% increments), and maximal effort was elicited through use of repeated 2.5-minute running workloads (modification of the procedure developed by Taylor et u l . 1 3 ) . At least two sessions were required for these tests. Oxygen uptake, heart rate, blood pressure, and cardiac output werq measured at submaximal workloads corresponding to 40%, 53%, 68% of VOzmnx,and oxygen uptake and heart rate during maximal effort. Collectioqs of 45 to $0 seconds of expired air in Douglas bags were used to determine VOzmnxand Vc0,. Analysis of 0, concentration was accomplished using a Beckman Model E-2 paramagnetic analyzer; CO,, with a Lira Model 300 infrared analyzer. Both analyzers were calibrated before each test with standardized gas mixtures. Gas volumes were measured with the use of a Parkinson Cowan gas meter and converted to STPD. The CO, rebreathing method was utilized for cardiac output determination.I4~ End tidal and rebreathing bag CO, levels were analyzed using a Godart Capnograph Type KK infrared analyzer and the results were registered on a Brush Model Mark 280 recorder. Both the analyzer and recorder were calibrated with varying CO, concentrations prepared with a Gefag Type Eg 7382 gas mixer. The points from the rising curve were determined at 1-second intervals, as described initially by Jernerus et al.,'* using a filtered output derived by analog signal processing of the original CO, input. The resulting filtered curve provided a reliable smoothed curve from which to calculate mixed venous CO, concentrations.16 Leveling off of oxygen uptake was used where possible as the criteria to indicate that maximal oxygen uptake had been reached; not all subjects showed an oxygen uptake plateau, and leveling off of heart rate (HR) and inability to complete a workload (2.5 min) were also accepted as indicators of attaining maximal effort. Body densities as determined by underwater weighing,li skinfolds,15 and anthropometry were used to evaluate body composition and growth changes. Body composition measurements were repeated on both groups before and after the conditioning program. Student's paired t-test was used for within-group analysis. Only those differences shown to be significant when the exercise group was compared to the control group, were retained; a noncorrelated t-test was used for the between group analysis. The conditioning program lasted 12 weeks, the children meeting four times per week, after school. The sessions were conducted in a large gymnasium and
Lussier & Buskirk: Training Children
737
on a 200-meter indoor track, All sessions were supervised. Two sessions per week included continuous running for progressively longer periods of time, from 10 to 35 minutes over the 12-week program. The remainder of sessions were organized around running games and activities. A single session lasted 45 minutes. The distance covered per run was calculated knowing the track length and the number of laps run during a given training session. Intensity was evaluated from timed laps and by periodic verification of HR; the child was asked to stop suddenly and HR was counted using a stefhoscope. A comparison was made with the target HR corresponding to 80% Yo,,,, (about 185 bpm, 92% of HR,,,). Results
FIGURE1 illustrates the running results for the overall program: The average cumulative distance totaled 94.5 km (or 58.4 miles), with a range from 63.3 to 126.0 km. The distance increments became quite large toward the end of the 12-week period. There occurred a drop in pace, which reached a low at the 8th week, but this was reversed during the last four weeks. The third curve presents distance per session and reflects both changes in pace and increases in duration of running at a given pace. The distance per session increased gradually throughout the 12-week conditioning period. At monthly intervals, 10-minute run trials were conducted; FIGURE 2 shows the increased distances covered during successive trials (1.94, 2.13, 2.20 km) with paces of 188, 206, 213 m/min, respectively. The distances and paces were
L
DISTANCE/SESSION +
n PACE
DISTANCE
I
2
4
6
8
ID
12
WEEKS
FIGURE1. Mean pace, distance per session, and cumulative distance registered at 2-week intervals for those in the exercise group.
738
Annals New York Academy of Sciences
significantly different from each other (p < 0.5). In addition to these 10-minute trials, a treadmill test involving running for 2.5 minutes at near maximal workloads was performed following a 10-minute warmup. Maximal effort was not elicited in all children. Lower heart rates were recorded during the tests given at 1 and 2 months (196, 194.5), which were significantly different from HR,,, (200). FIGURE 3 shows oxygen uptake per kg body weight. A significant increase in aerobic capacity was evident by the third month of conditioning. The greatest increase in aerobic capacity appears to have taken place during the second month when the children were covering distances of 2.95 and 3.23 km per
I .o
2.0
3.0
MONTHS
FIGURE2. Monthly trials showing the average pace and the longest average distance covered in a l0-minutc period by those in the exercise group.
ca
session at paces of 165-170 m/min. HR,,, and did not change :ignificantly in either group nor did the control group show a change in VO,m,r (TABLES 4 & 5). Submaximal H R decreased significantly in the exercise group but not in the control, group during performance of tyo standard workloads, one walking at 40% Voa , ,, and one running at 80% Yoa (TABLE5 ) . During submaximal workloads of 40%, 53%, and 68% of to,,,,,oxygen uptake, HR, cardiac output (Q), and blood pressure (BP) were measured (TABLE6). With one exception, no differences were found; e.g., H R values were the same at all of the relative workloads except for a drop in the H R at
739
Lussier & Buskirk: Training Children TABLE 4 EFFECTOF
PARTICIPATION IN THE PHYSICAL CONDITIONING REGIMEN. ON AEROBIC CAPACITY ( V o p a x ) AND MAXIMALPULMONARY VENTILATION ( V E m a x )
Variable
Mean Values ( +SE) t Before After
Group *
V0,m.x
E C E
(ml.kg-l-rnin-')
C
55.6 53.1
E C
54.18 (1.55) 55.70 (2.42)
V0,rn.x
(lamin-')
VErnax
(1-min-I)
1.76 (0.07) 1.83 (0.08) (2.07) (1.32)
59.4 53.9
P
A
1.96 (0.08) 1.96 (0.09) (2.28) (1.33)
61.50 (2.16) 57.9 (2.58)
0.20 0.09 3.8 0.8 7.32 2.21
NS 0.05
NS
* E, exercise group, n= 12; C, control group, n= 10. t Standard error. the 40% workload in the exercise group after conditioning. BP changes were not significantly different at any workload (FIGURE 4). Oxygen uptake and ventilation were larger after conditioning at the 68% workload for the exercise group. Because of large individual variations in cardiac output, stroke volume, and arteriovenous oxygen difference, no significant differences in these variables were observed between groups that could be attributed to the conditioning program. It would appear that both an increased stroke volume and arteriovenous oxygen difference contributed to the increase in aerobic capacity. The conditioning program was also without significant effect on the growth of these children: Increases in height, weight, and other anthropometric measurements were the same for both the exercise and control groups (TABLE 7). The suggested greater increase in body diameters and circumferences as well as in body density (TABLE 8) in the exercise group might suggest an increase
TABLE5
EFFECTOF PARTICIPATION IN A PHYSICAL CONDITIONING REGIMEN ON HEARTRATE*
Mean Values (&SE) Variable Submax HR at 40% VO,,.. (walking) Submax HR at 80% V O ~(running) ~ . ~ HR,..
Group t
n
E C E
10
129.3 (1.96) 126.2 (2.91)
119.8 (1.85) 126.7 (3.00)
C
14 10
187.9 (2.41) 188.1 (2.43)
174.4 (2.75) 188.6 (1.86)
E C
11 10
201.3 (1.87) 199.8 (2.43)
200.7 (2.12) 203.2 (2.17)
I5
* Heart rate (HR) in beats amin".
t E, exercise group; C, control group.
Before
After
A 9.5 0.5 13.5 0.5 0.6 3.4
P 0'05
0'01
NS
Variable
C
E
E C E C
E C
E C E C E C
Group *
6.3 6.7 47.8 53.1 129.3 126.2 11.8 11.5 0.73 0.77 22.7 22.4 17.5 17.3
(0.2) (0.2) (1.4) (1.7) (2.0) (2.9) (0.4) (0.3) (0.03) (0.03) (0.8) (0.9) (0.9) (0.9)
6.1 6.7 51.7 52.9 119.8 t 126.7 11.0 11.2 0.67 0.75 20.2 21.0 18.5 16.7
40%
t Significant to the 0.05 level.
* E, exercise group, n=15; C, control group, n=10.
(ml. kg-*.min-') f B ( 1 -min-l)
fh
C(a-v)o, (ml.10Oml-1) emin-l)
H R *(beats.min-l)
SVW)
(1 * min-1)
Q
(sv),
OF PARTICIPATION IN A PHYSICU CONDITIONING REGIMEN ON CARDIAC OUTPUT(Q)STROKEV O L U ~
(0.3) (0.3) (2.8) (2.7) (1.9) (3.0) (0.4) (0.3) (0.03) (0.03) (0.8) (0.9) (1.0) (1.0) 7.6 44.7 50.8 148.0 149.0 13.9 13.3 0.91 1.01 28.6 29.5 20.9 22.3
(0.3) 8.0 (0.6) (1.4) 51.6 (2.3) (1.7) 53.5 (3.7) (1.9) 148.0 (2.2) (2.6) 148.3 (2.4) (0.5) 13.3 (0.5) (0.5) 12.9 (0.4) (0.03) 1.04 (0.04) (0.04) 1.02 (0.06) (1.0) 30.9 (1.1) (1.1) 28.0 (1.5) (0.8) 27.0 (1.3) (1.2) 22.5 (1.2)
Workload (% VO,,.,) 53% 6.7 (0.2) 7.6 (0.4)
Mean Values ( k S E )
8.5 44.0 49.0 173.5 172.7 15.4 15.6 1.16 1.30 36.3 37.9 28.2 31.4
7.8
(0.3) (0.5) 9.4 (0.7) (2.0) 48.9 (1.7) (3.2) 54.9 (3.9) (1.9) 174.8 (2.4) (1.4) 173.0 (1.4) (0.6) 16.0 (0.6) (0.5) 14.1 (0.6) (0.04) 1.35 (0.05) (0.06) 1.30 (0.07) (1.2) 40.3 t (1.1) (1.0) 35.8 (1.6) (0.9) 35.4 t (1.1) (2.0) 30.1 (1.6)
68% (0.3) 8.5
ARTERIovENous OXYGENDIFFERENCE [C(a-v)o,l, HEART RnTE (HR),OXYGEN CONSUMPTION (VO,),AM) VENTILATION (CB5
EFFECT
TABLE6
*'
8
%
Y
W
aa
8-
4
g 3
i2 cn
g U
4 P 0
741
Lussier & Buskirk: Training Children 66
63
-
.-c €
6
-
T
0
‘0 V 0
57
lu .>O
54
51
i
0
20
10
3.0
MONTHS
FIGURE3. Change in aerobic capacity during the 3-month conditioning program. The control group was only tested at the beginning and end of the time period.
160
W v)
z 0 a v)
0 0 0
Control group RO Control Qroup post
Exercise group Pre Exercise group Fbst
SYSTOLIC
I40
W
a
I20
W
a 3 v)
In W a a
IOC
0
0
sm
DIASTOLIC
80
60
.,
i2
& I 48 %
v
02
5’6
$4
2 ;
man
FIGURE4. Blood pressure in relation to the relative workload expressed as a percentage of aerobic capacity.
Annals New York Academy of Sciences
742
TABLE7 EFFECTOF PARTICIPATION IN A PHYSICAL CONDITIONING REGIMEN ON ANTHROPOMETRIC MEASUREMENTS: HEIGHT,WEIGHT,DIAMETERS, AND CIRCUMFERENCES Mean Values ( 2SE) Group * E C E C
Variable Height (cm) Weight (kg) Diameters, sum t (cm) Circumferences, sum t (cm)
E
C E
C
Before 138.7 140.2
After
(1.82) (1.85)
32.5 (1.30) 34.3 (2.18) 118.7 (1.44) 120.0 (2.31) 455.7 (7.89) 467.3 (12.02)
141.1 140.0
P
A
(1.88) (1.82)
33.7 (1.38) 35.8 (2.34) 119.5 (1.23) 120.7 (2.43) 463.7 (7.94) 471.5 (12.79)
2*4 1.8
NS
1*2 1.5
NS
NS
0.7
NS
4.2
~~~
* E, exercise group, n=16;
C, control group, n= 10. t Sum of 8 diameters and sum of 11 circumferences as described by Behnke?'
in lean body mass but the mean difference was not significant. Presumably a program longer than 12 weeks would be needed to show a measurable increase in fat-free weight or lean body mass. The correlation coefficient between initial fitness expressed as Q0, per kg body weight and total distance covered in the training program was r = 0.5 (FIGURE 5 ) . The correlation coefficient between initial fitness and increase in fitness resulting from the program was essentially zero (r = 0.05) suggesting poor predictability from initial fitness of the effect of an endurance conditioning program on the aerobic capacity of prepubescent children.
TABLE8 EFFECTOF PARTICIPATION IN A PHYSICAL CONDRIONING REGIMEN ON BODYCOMPOSITION AS REPRESENTED BY BODYDENSITY AND SKINFOLD MEASUREMENTS Mean Values (+SE) Group * E Wee) C Skinfolds, sum t E (mm) C Variable Body density
Before
n
12 7 16 10
1.053 1.057 88.1 86.4
(0.004) (0,004)
(10.58) (10.32)
* E, exercise group; C, control group. t Sum of
10 skinfolds as described by Allen et d."
After
A
1.055 (0.004) 1.057 (0.005)
0.002 0.006
85.7 85.1
(8.35) (9.94)
2.4 1.3
P
NS NS
743
Lussier & Buskirk: Training Children Discussion
Interval training has been used in most physical conditioning studies involving prepubescent children.x-*” Repeated short bouts of high intensity effort with interspersed rest periods correspond to the observed habitual activity pattern of children, but very little effort has been made to quantify the intensity of their spurts of activity present throughout their games, sports, or spontaneous activity. Contrary to what is observed in adults, studies that have used this type of interval training have failed to show any improvement in aerobic capacity
0 = 0.50 y.17.25’
‘20] 110
r
1
1I
0
1.411
-
/
6 0 6 5
m
/
0
100
0
“1G/ 9 ,.
45
so INITIAL
55
vo2 , CC.
I
kdt.miil
FIGURE5. Initial aerobic capacity and total distance covered over the 12-week physical conditioning program.
of the children. More investigation is required to elucidate why this is so. Presumably their normal pattern of “interval” activity maintains their level of fitness at a reasonably high level. Intensity of work should be monitored and quantified to provide an accurate accounting for study of training effects. Daniels and Oldridge3 showed no increase in aerobic power in young track performers over 22 months of training, but no quantification of the intensity of workouts was mentioned, only yearly mileage. Brown et d.,l in contrast, showed substantial improvement in the aerobic capacity of young girls with training for cross-country running. The mean initial aerobic capacity of the youngest girls (8-9 years) was 36.6
Annals New York Academy of Sciences
744
ml.kgl-min-l and the mean for the older girls was 49 ml.kgl*min-l. The mean increase in aerobic capacity for all of the girls was approximately 12 ml.kgl*min-l. A control group would have been valuable in the study by Brown et ul.' since Klissouras has shown that changes in aerobic capacity are found in controls as well as exercising subjects. Nevertheless, the results of Brown et ul.' indicate that increases in maximal oxygen uptake are produced by endurance work, particularly if the initial aerobic capacities are relatively low. Our results agree with this conclusion, The subjects of Brown et u1.l covered greater distance per session, with more frequent training sessions, than the children in our study, which may partially explain the greater increase in aerobic capacity recorded in their runners. Low initial fitness levels for young boys (1 1-13 years) have been noted.*. 6, ' Changes in aerobic capacity are more readily produced if the initial fitness level is low. The children in this study had relatively high aerobic capacities. The final aerobic capacity of those in the exercise group (59.4 rnl.kg'-min-') is nearly equivalent to that of the subjects of Brown et ul.' (60.3 ml.kg'*min-' for the 10-1 1-year-olds and 62.1 ml-kg-lamin-1 for 12-13-year-olds). Thus, the high initial aerobic capacity probably precluded large increases in fitness for those children who participated in this study. Eriksson and Koch 2 0 showed that the increase in cardiac output associated with greater oxygen uptake at a given workload was due to an increase in stroke volume with little change in $rteriovenous oxygen difference in response to physical conditioning. Their Vo, values were 1.3 to 2 liters per minute higher *an the values predicted from the regression equation calculated from our data, Q = 2.273 5.225 Vo,. The stroke volumes measured in their subjects were 15 to 20 ml greater than those recorded here. Eriksson and Koch *O used a dye dilution method whereas we used the CO, rebreathing method, and their subjects were about two years older than ours. The differences in technique and age of the subjects may be responsible for some of the variation noted. Our data agrees well with those of Bar Or et ul.15 who also used the CO, rebreathing method. Our results show that cardiac output and stroke volume increased moderately with time but that the increase in the exercise group was not greater than that observed in the control group. The coefficients of variation were found to vary from 12% to 15% and this variation was larger than the changes in cardiac output recorded (10% to 13% at the two highest workloads). Thus, there is the suggestion of a moderate increase in cardiac output during submaximal work with growth, but no effect attributable to physical conditioning. The cumulative distance run during the training session over the 12-week period was the product of a number of factors including running efficiency, pace, motivation, and number of sessions attended, as well as health status and social interactions. Perhaps it is not surprising then that initial aerobic capacity explained only 25% of the variance in the cumulative distance covered during training. We had arbitrarily expected that those with high initial aerobic capacity would both run faster and farther during their training sessions. This was only partially true among the children studied. The correlation between initial fitness and change in fitness was insignificantly different from zero ( r = 0.05). The smallest increase in Po*max was less than 1 ml-kgl.min-l in a child with an, initial fitness of 53 ml.kg-'*min-'; on the other hand, the largest increase in VO,maxwas 9.5 ml.kg1.min-' in a child whose initial fitness was 61 ml-kg-'emin-1. Both of these youngsters were
+
Lussier & Buskirk: Training Children
745
high mileage runners compared to their colleagues and maintained fast paces. This difference in response to physical conditioning might be ascribed, in part, to the genetic makeup of each child, perhaps to an individual threshold for initiation of a training response as well as control of the magnitude of the response. In children as in adults, a training threshold must be reached to elicit changes in aerobic capacity. Habitual physical activity levels may alter both the threshold and the magnitude of the response, but the individual variation associated with genetic endowment is apparently predominant.
Further Comment
This study, as well as related ones, provide limited insight into the effects of physical conditioning on prepubertal children, and serious questions remain as to how intensity, duration, and frequency contribute to changes in aerobic capacity in them as compared to postpubescent children and adults. The role of genetic endowment needs further clarification as do the effects of growth. What interrelationships are there between physical activity, growth, and maturation? What effect will endurance training initiated during the period of early growth and development have on their cardiovascular physiology or on the rate of development of atherosclerosis and coronary heart disease? Are there special nutritional requirements for children who engage in endurance training regimens? Since children may be more heat intolerant, what are their environmental and physiological limitations for participation in long distance running? What effects attributable to chronic activity can be found by evaluation of weight-bearing joints? Such questions-and the list could be extensively increased-document the need for more knowledge before youngsters should be advised to participate on a mass basis in long distance training regimens and/or marathon competition.
Summary
The cardiovascular effects of a 12-week endurance training regimen were studied among normally active and healthy prepubertal children. Twenty-six 8- to 12-year-old children (20 boys and 6 girls) volunteered and 10 acted as control subjects. The training regimen consisted of distance running for progressively longer periods (from 10 to 35 min) 2 to 3 times per week, with 2 additional sessions per week devoted to running games. Those who were trained ran a cumulative average distance of 95.6 km (58.9 miles). Intensity of work was assessed from running pace and heart.rate. The target workout intensity was 75% to 80% of aerobic capacity ( Vo, Growth and development accounted for increases in height, weight, body circumferences, and diameters, and fat-free body weight. Heart rate (HR) during submaximal workloads, both running and walking, decreased in the trained group ( p < 0.01) and ( p < 0.05). HR,,, did not change, but Vo, increased significantly (average 7 % ) in the trained group but not in the controls. No significant change attributable to training was found for submaximal cardiac output, stroke volume, or arterio-
746
Annals New York Academy of Sciences
venous oxygen difference. The Vo2,,, value before conditioning was a relatively poor predictor of the magnitude of improvement in functional capacity,
but those with higher initial Vo, ,, logged more cumulative training mileage. It was concluded that prepubertal children respond to an endurance training regimen by improving their running capacity, which is, to a limited extent, associated with increased aerobic capacity.
References
F. DEETER.1972. The effects of cross country running on preadolescent girls. Med. Sci. Sports. 4( 1): 1-5. CUMMING. G. R., A. GOODWIN, G. BAGGLEY & J. ANTEL. 1967. Repeated measurements of aerobic capacity during a week of intensive training at a youth's track camp. Can. J. Physiol. Pharm. 67(45): 805-811. DANIELS,J. & N. OLDRIDGE.1971. Changes in oxygen consumption of young boys during growth and running training. Med. Sci. Sports 3(4): 161-165. DOBELN, W. R. & B. 0. EIUKSSON.1972. Physical Training, maximal oxygen uptake and dimensions of oxygen transporting and metabolizing organs in boys 11-13 years of age. Acta Paediat. Scand. 72(61): 653-660. EKBLOM,B. 1969. Effect of physical training in adolescent boys. J. Appl. Physiol. 27: 350-355. KL~ssourus,V. & G. WEBER. 1973. Training: Growth and heredity. I n Pediatric Work Physiology. Proc. 4th Int. Symp. : 209-216. Wingate Institute, Israel. KOCH,G. & B. 0. ERIKSSON. 1973. Effect of physical training on pulmonary ventilation and gas exchange during submaximal and maximal work in boys aged 11-13 years. Scand. J. Clin. Lab. Invest. 31: 88-94. BAROR, 0.& L. D. ZWIREN. 1973. Physiological effects of frequency and content variation of physical education classes and of endurance conditioning on 9 to 10 year old girls and boys. I n Pediatric Work Physiology. Proc. 4th Int. Symp. : 199-208. Wingate Institute, Israel. STEWART, K.J. & B. GUTIN. 1976. Effects of physical training on cardiorespiratory fitness in children. Res. Quart. 47(1): 110-120. MOCELLIN, R. & U. WASMUND. 1973. Investigation of the influence of a running training program on the cardiovascular and motor performance capacity in 53 boys and girls of a second and third grade primary school class. I n Pediatric Work Physiology. Proc. 4th Int. Symp. :279-288. Wingate Institute, Israel. GREULICH, W. W. & S. I. PYLE. 1966. Radiographic Atlas of Skeletal Development of the Hand & Wrist. 2nd ed. Stanford Univ. Press. Stanford, California. ItLUs, J. D., E. E. HUNT,JR. & E. R. BUSKIRK.1971. Skeletal development of non-institutionalized children with low intelligence quotients. Amer. J. Phys. Anth. 35(3): 455-466. TAYLOR, H. L.,E. R. BU~K~RK & A. HENSCHEL.1958. Maximal oxygen uptake as an objective measure of cardiorespiratory performance. J. Appl. Physiol. 8
1. BROWN, C. H., J. R. HARROWER & M.
2. 3.
4.
5. 6. 7. 8.
9. 10.
11. 12. 13.
73-80. R., G. Lumw & D. THOMSON. 1963. Cardiac output in healthy sub14. JERNERUS, jects determined with a CO, rebreathing method. Acta Physiol. Scand. 59: 390-399. 15. BAROR, O., R. J. SHEPIURD & C. L. Aum. 1971. Cardiac output of 10-13 year old boys and girls during submaximal exercise. J. Appl. Physiol. 30(2): 219-223. B. A. 1976. Effects of a 12 week physical conditioning program on 16.
cardiorespiratory function, body composition and serum lipids of normal and obese middle-aged women. Ph.D. Thesis. The Pennsylvania State University, University Park, Pa.
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747
17. AKERS,R. &
E. R. BUSKIRK.1969. An underwater weighing system utilizing “force cube” transducers. J. Appl. Physiol. 26(5): 649-652. 18. ALLEN,T. H., M. T. PENG,K. T. CHEN,T. F. HUANG,C. CHANG& H. S. FANG. 1956. Prediction of total adiposity from skinfolds and the curvilinear relationship between external and internal adiposity. Metabolism. 5: 346-352. 19. BEHNKE,A. 1961. Quantitative Assessment of body build. Amer. J. Physiol. 201(6): 960-968. 20. ERIKSSON,B. 0. & G. KOCH. 1973. Effect of physical training on hemodynamic response during submaximal and maximal exercise in 11-13 year old boys. Acta Physiol. Scand. 87: 27-39.
Introduction
The effects of a physical conditioning regimen on prepubescent children were studied. The regimen involved running progressively longer distances over a period of 12 weeks. Most previous work involving physical conditioning of children has centered on subjects of pubescent ages (11 to 13 years) and older,'-' and only three studies were located dealing with children of prepubescent ages.*-l0 The latter data are summarized in TABLE1: No effect of physical conditioning was found on the aerobic capacity of 8- to 11-year-olds in the first three studies listed in TABLE1. The investigators in these three studies utilized interval training procedures. The duration of each exercise bout was short, but identifiable with how children play. Brief-bout interval training is probably closely related to the activity pattern that is normally exhibited by children. Brown et al.' were the only investigators to report significant increases in aerobic capacity with training. They trained prepubertal girls for crosscountry running and utilized distance running, i.e., endurance training, but no control subjects participated in their study and their results could have been confounded by growth-induced changes. Growth during the prepubescent period can be assumed to be linear but growth remains a complicating factor in extended studies of physical conditioning in children. Short training programs have the advantage of minimizing the effect of growth because little growth occurs and any small changes that do occur can be identified with growth, particularly if a control group is employed. Thus, the effects of physical conditioning can be isolated as differences among those subjected to physical conditioning as compared to control subjects who engage in no organized physical activity. Subjects and Methods
A group of 26 children (ages 8 to 12) volunteered and were divided into an exercise group and a control group. All were classified as stage I according to Tanner's standards for genitalia maturity and breast development. Bone age was determined from hand and wrist X rays taken with a General Electric Mobile 225 I11 X-ray Unit. The radiographs were taken at a focal length of 40 in. (1.15 m), 50 kV, 50 mA, 0.2 sec (10 mA.sec). A lead-lined elongated
* This work was supported by Research Grant AM08311 from the National Institute of Arthritis, Metabolism and Digestive Diseases. 734
735
Lussier & Buskirk: Training Children TABLE 1 DEALING WITH THE SUMMARY OF STUDIES IN THE LITERATURE PHYSICAL CONDITIONING OF PREPUBERTAL CHILDREN
Study Bar Or and Zwiren Stewart and Gutin Mocellin and Wasmund lo Brown el al?
Frequency Duration (per of effort * VO,,., week) (min) (ml.kg-'.min-')
.
Duration (weeks)
Sex
n
M+F
92
9
2 4
M
24
8
4
1-3
-0.3
M +F
53
F
12
7 12
1-2 4-5
3-6 >15
= 11.5
1
-0.8
-
* Duration of effort before resting. Several bouts per exercise session were employed by each investigator. wooden pyramid was used to minimize radiation scatter. A developmental age was assigned to each of the 28 ossification centers using the Greulich and Pyle Atlas l 1 and the mean age was calculated as described in Haas et a1.12 Because radiographs for bone age were taken at the time of the final testing, both physiological and chronological ages reported in TABLE 2 represent age at the end of the study. No significant differences between groups were found for either chronological or physiological age. Participation in organized physical activity outside the physical conditioning regimen was essentially the same for both 3, the number of girls in each group was unequal groups. As indicated in TABLE because of the small number of girls who volunteered to participate in the study. After physical examination of the children, familiarization sessions were conducted for the various testing procedures. The children walked on a treadTABLE2 AGE AND HABITUAL PHYSICAL ACTIVITYOF SUBJECTS IN THE EXERCISE AND CONTROL GROUPS Exercise Group n= 16
Variable Chronological age, years Physiological age, years Hours of physical activity per week Fall season Winter season
* Standard deviation.
Mean (+SD) (Range) 10.3 (1.2) (8-12) 9.6 (1.1) (7-11) 4.6 4.4
t
*
Control Group n=10
Mean (+SD) (Range1 10.5 (1.2) (8-12) 9.8 (1.1)
(7-11) 3 .O 4.3
t Excluding time spent in the physical conditioning program.
P NS
NS
736
Annals New York Academy of Sciences TABLE3 SEX
AND AGE DISTRIBUTION OF IN THE EXERCISE AND CONTROL
Exercise Group Sex Boys Girls Age groups * 8-9 years 10-12 years
11 5
SUBJECTS GROUPS Control Group 9 1 4 (5) 6 (5)
* In reference to chronological age. Physiological age groups are represented in parentheses. mill at progressively increasing submaximal workloads (3 mph and 2.5% increments), and maximal effort was elicited through use of repeated 2.5-minute running workloads (modification of the procedure developed by Taylor et u l . 1 3 ) . At least two sessions were required for these tests. Oxygen uptake, heart rate, blood pressure, and cardiac output werq measured at submaximal workloads corresponding to 40%, 53%, 68% of VOzmnx,and oxygen uptake and heart rate during maximal effort. Collectioqs of 45 to $0 seconds of expired air in Douglas bags were used to determine VOzmnxand Vc0,. Analysis of 0, concentration was accomplished using a Beckman Model E-2 paramagnetic analyzer; CO,, with a Lira Model 300 infrared analyzer. Both analyzers were calibrated before each test with standardized gas mixtures. Gas volumes were measured with the use of a Parkinson Cowan gas meter and converted to STPD. The CO, rebreathing method was utilized for cardiac output determination.I4~ End tidal and rebreathing bag CO, levels were analyzed using a Godart Capnograph Type KK infrared analyzer and the results were registered on a Brush Model Mark 280 recorder. Both the analyzer and recorder were calibrated with varying CO, concentrations prepared with a Gefag Type Eg 7382 gas mixer. The points from the rising curve were determined at 1-second intervals, as described initially by Jernerus et al.,'* using a filtered output derived by analog signal processing of the original CO, input. The resulting filtered curve provided a reliable smoothed curve from which to calculate mixed venous CO, concentrations.16 Leveling off of oxygen uptake was used where possible as the criteria to indicate that maximal oxygen uptake had been reached; not all subjects showed an oxygen uptake plateau, and leveling off of heart rate (HR) and inability to complete a workload (2.5 min) were also accepted as indicators of attaining maximal effort. Body densities as determined by underwater weighing,li skinfolds,15 and anthropometry were used to evaluate body composition and growth changes. Body composition measurements were repeated on both groups before and after the conditioning program. Student's paired t-test was used for within-group analysis. Only those differences shown to be significant when the exercise group was compared to the control group, were retained; a noncorrelated t-test was used for the between group analysis. The conditioning program lasted 12 weeks, the children meeting four times per week, after school. The sessions were conducted in a large gymnasium and
Lussier & Buskirk: Training Children
737
on a 200-meter indoor track, All sessions were supervised. Two sessions per week included continuous running for progressively longer periods of time, from 10 to 35 minutes over the 12-week program. The remainder of sessions were organized around running games and activities. A single session lasted 45 minutes. The distance covered per run was calculated knowing the track length and the number of laps run during a given training session. Intensity was evaluated from timed laps and by periodic verification of HR; the child was asked to stop suddenly and HR was counted using a stefhoscope. A comparison was made with the target HR corresponding to 80% Yo,,,, (about 185 bpm, 92% of HR,,,). Results
FIGURE1 illustrates the running results for the overall program: The average cumulative distance totaled 94.5 km (or 58.4 miles), with a range from 63.3 to 126.0 km. The distance increments became quite large toward the end of the 12-week period. There occurred a drop in pace, which reached a low at the 8th week, but this was reversed during the last four weeks. The third curve presents distance per session and reflects both changes in pace and increases in duration of running at a given pace. The distance per session increased gradually throughout the 12-week conditioning period. At monthly intervals, 10-minute run trials were conducted; FIGURE 2 shows the increased distances covered during successive trials (1.94, 2.13, 2.20 km) with paces of 188, 206, 213 m/min, respectively. The distances and paces were
L
DISTANCE/SESSION +
n PACE
DISTANCE
I
2
4
6
8
ID
12
WEEKS
FIGURE1. Mean pace, distance per session, and cumulative distance registered at 2-week intervals for those in the exercise group.
738
Annals New York Academy of Sciences
significantly different from each other (p < 0.5). In addition to these 10-minute trials, a treadmill test involving running for 2.5 minutes at near maximal workloads was performed following a 10-minute warmup. Maximal effort was not elicited in all children. Lower heart rates were recorded during the tests given at 1 and 2 months (196, 194.5), which were significantly different from HR,,, (200). FIGURE 3 shows oxygen uptake per kg body weight. A significant increase in aerobic capacity was evident by the third month of conditioning. The greatest increase in aerobic capacity appears to have taken place during the second month when the children were covering distances of 2.95 and 3.23 km per
I .o
2.0
3.0
MONTHS
FIGURE2. Monthly trials showing the average pace and the longest average distance covered in a l0-minutc period by those in the exercise group.
ca
session at paces of 165-170 m/min. HR,,, and did not change :ignificantly in either group nor did the control group show a change in VO,m,r (TABLES 4 & 5). Submaximal H R decreased significantly in the exercise group but not in the control, group during performance of tyo standard workloads, one walking at 40% Voa , ,, and one running at 80% Yoa (TABLE5 ) . During submaximal workloads of 40%, 53%, and 68% of to,,,,,oxygen uptake, HR, cardiac output (Q), and blood pressure (BP) were measured (TABLE6). With one exception, no differences were found; e.g., H R values were the same at all of the relative workloads except for a drop in the H R at
739
Lussier & Buskirk: Training Children TABLE 4 EFFECTOF
PARTICIPATION IN THE PHYSICAL CONDITIONING REGIMEN. ON AEROBIC CAPACITY ( V o p a x ) AND MAXIMALPULMONARY VENTILATION ( V E m a x )
Variable
Mean Values ( +SE) t Before After
Group *
V0,m.x
E C E
(ml.kg-l-rnin-')
C
55.6 53.1
E C
54.18 (1.55) 55.70 (2.42)
V0,rn.x
(lamin-')
VErnax
(1-min-I)
1.76 (0.07) 1.83 (0.08) (2.07) (1.32)
59.4 53.9
P
A
1.96 (0.08) 1.96 (0.09) (2.28) (1.33)
61.50 (2.16) 57.9 (2.58)
0.20 0.09 3.8 0.8 7.32 2.21
NS 0.05
NS
* E, exercise group, n= 12; C, control group, n= 10. t Standard error. the 40% workload in the exercise group after conditioning. BP changes were not significantly different at any workload (FIGURE 4). Oxygen uptake and ventilation were larger after conditioning at the 68% workload for the exercise group. Because of large individual variations in cardiac output, stroke volume, and arteriovenous oxygen difference, no significant differences in these variables were observed between groups that could be attributed to the conditioning program. It would appear that both an increased stroke volume and arteriovenous oxygen difference contributed to the increase in aerobic capacity. The conditioning program was also without significant effect on the growth of these children: Increases in height, weight, and other anthropometric measurements were the same for both the exercise and control groups (TABLE 7). The suggested greater increase in body diameters and circumferences as well as in body density (TABLE 8) in the exercise group might suggest an increase
TABLE5
EFFECTOF PARTICIPATION IN A PHYSICAL CONDITIONING REGIMEN ON HEARTRATE*
Mean Values (&SE) Variable Submax HR at 40% VO,,.. (walking) Submax HR at 80% V O ~(running) ~ . ~ HR,..
Group t
n
E C E
10
129.3 (1.96) 126.2 (2.91)
119.8 (1.85) 126.7 (3.00)
C
14 10
187.9 (2.41) 188.1 (2.43)
174.4 (2.75) 188.6 (1.86)
E C
11 10
201.3 (1.87) 199.8 (2.43)
200.7 (2.12) 203.2 (2.17)
I5
* Heart rate (HR) in beats amin".
t E, exercise group; C, control group.
Before
After
A 9.5 0.5 13.5 0.5 0.6 3.4
P 0'05
0'01
NS
Variable
C
E
E C E C
E C
E C E C E C
Group *
6.3 6.7 47.8 53.1 129.3 126.2 11.8 11.5 0.73 0.77 22.7 22.4 17.5 17.3
(0.2) (0.2) (1.4) (1.7) (2.0) (2.9) (0.4) (0.3) (0.03) (0.03) (0.8) (0.9) (0.9) (0.9)
6.1 6.7 51.7 52.9 119.8 t 126.7 11.0 11.2 0.67 0.75 20.2 21.0 18.5 16.7
40%
t Significant to the 0.05 level.
* E, exercise group, n=15; C, control group, n=10.
(ml. kg-*.min-') f B ( 1 -min-l)
fh
C(a-v)o, (ml.10Oml-1) emin-l)
H R *(beats.min-l)
SVW)
(1 * min-1)
Q
(sv),
OF PARTICIPATION IN A PHYSICU CONDITIONING REGIMEN ON CARDIAC OUTPUT(Q)STROKEV O L U ~
(0.3) (0.3) (2.8) (2.7) (1.9) (3.0) (0.4) (0.3) (0.03) (0.03) (0.8) (0.9) (1.0) (1.0) 7.6 44.7 50.8 148.0 149.0 13.9 13.3 0.91 1.01 28.6 29.5 20.9 22.3
(0.3) 8.0 (0.6) (1.4) 51.6 (2.3) (1.7) 53.5 (3.7) (1.9) 148.0 (2.2) (2.6) 148.3 (2.4) (0.5) 13.3 (0.5) (0.5) 12.9 (0.4) (0.03) 1.04 (0.04) (0.04) 1.02 (0.06) (1.0) 30.9 (1.1) (1.1) 28.0 (1.5) (0.8) 27.0 (1.3) (1.2) 22.5 (1.2)
Workload (% VO,,.,) 53% 6.7 (0.2) 7.6 (0.4)
Mean Values ( k S E )
8.5 44.0 49.0 173.5 172.7 15.4 15.6 1.16 1.30 36.3 37.9 28.2 31.4
7.8
(0.3) (0.5) 9.4 (0.7) (2.0) 48.9 (1.7) (3.2) 54.9 (3.9) (1.9) 174.8 (2.4) (1.4) 173.0 (1.4) (0.6) 16.0 (0.6) (0.5) 14.1 (0.6) (0.04) 1.35 (0.05) (0.06) 1.30 (0.07) (1.2) 40.3 t (1.1) (1.0) 35.8 (1.6) (0.9) 35.4 t (1.1) (2.0) 30.1 (1.6)
68% (0.3) 8.5
ARTERIovENous OXYGENDIFFERENCE [C(a-v)o,l, HEART RnTE (HR),OXYGEN CONSUMPTION (VO,),AM) VENTILATION (CB5
EFFECT
TABLE6
*'
8
%
Y
W
aa
8-
4
g 3
i2 cn
g U
4 P 0
741
Lussier & Buskirk: Training Children 66
63
-
.-c €
6
-
T
0
‘0 V 0
57
lu .>O
54
51
i
0
20
10
3.0
MONTHS
FIGURE3. Change in aerobic capacity during the 3-month conditioning program. The control group was only tested at the beginning and end of the time period.
160
W v)
z 0 a v)
0 0 0
Control group RO Control Qroup post
Exercise group Pre Exercise group Fbst
SYSTOLIC
I40
W
a
I20
W
a 3 v)
In W a a
IOC
0
0
sm
DIASTOLIC
80
60
.,
i2
& I 48 %
v
02
5’6
$4
2 ;
man
FIGURE4. Blood pressure in relation to the relative workload expressed as a percentage of aerobic capacity.
Annals New York Academy of Sciences
742
TABLE7 EFFECTOF PARTICIPATION IN A PHYSICAL CONDITIONING REGIMEN ON ANTHROPOMETRIC MEASUREMENTS: HEIGHT,WEIGHT,DIAMETERS, AND CIRCUMFERENCES Mean Values ( 2SE) Group * E C E C
Variable Height (cm) Weight (kg) Diameters, sum t (cm) Circumferences, sum t (cm)
E
C E
C
Before 138.7 140.2
After
(1.82) (1.85)
32.5 (1.30) 34.3 (2.18) 118.7 (1.44) 120.0 (2.31) 455.7 (7.89) 467.3 (12.02)
141.1 140.0
P
A
(1.88) (1.82)
33.7 (1.38) 35.8 (2.34) 119.5 (1.23) 120.7 (2.43) 463.7 (7.94) 471.5 (12.79)
2*4 1.8
NS
1*2 1.5
NS
NS
0.7
NS
4.2
~~~
* E, exercise group, n=16;
C, control group, n= 10. t Sum of 8 diameters and sum of 11 circumferences as described by Behnke?'
in lean body mass but the mean difference was not significant. Presumably a program longer than 12 weeks would be needed to show a measurable increase in fat-free weight or lean body mass. The correlation coefficient between initial fitness expressed as Q0, per kg body weight and total distance covered in the training program was r = 0.5 (FIGURE 5 ) . The correlation coefficient between initial fitness and increase in fitness resulting from the program was essentially zero (r = 0.05) suggesting poor predictability from initial fitness of the effect of an endurance conditioning program on the aerobic capacity of prepubescent children.
TABLE8 EFFECTOF PARTICIPATION IN A PHYSICAL CONDRIONING REGIMEN ON BODYCOMPOSITION AS REPRESENTED BY BODYDENSITY AND SKINFOLD MEASUREMENTS Mean Values (+SE) Group * E Wee) C Skinfolds, sum t E (mm) C Variable Body density
Before
n
12 7 16 10
1.053 1.057 88.1 86.4
(0.004) (0,004)
(10.58) (10.32)
* E, exercise group; C, control group. t Sum of
10 skinfolds as described by Allen et d."
After
A
1.055 (0.004) 1.057 (0.005)
0.002 0.006
85.7 85.1
(8.35) (9.94)
2.4 1.3
P
NS NS
743
Lussier & Buskirk: Training Children Discussion
Interval training has been used in most physical conditioning studies involving prepubescent children.x-*” Repeated short bouts of high intensity effort with interspersed rest periods correspond to the observed habitual activity pattern of children, but very little effort has been made to quantify the intensity of their spurts of activity present throughout their games, sports, or spontaneous activity. Contrary to what is observed in adults, studies that have used this type of interval training have failed to show any improvement in aerobic capacity
0 = 0.50 y.17.25’
‘20] 110
r
1
1I
0
1.411
-
/
6 0 6 5
m
/
0
100
0
“1G/ 9 ,.
45
so INITIAL
55
vo2 , CC.
I
kdt.miil
FIGURE5. Initial aerobic capacity and total distance covered over the 12-week physical conditioning program.
of the children. More investigation is required to elucidate why this is so. Presumably their normal pattern of “interval” activity maintains their level of fitness at a reasonably high level. Intensity of work should be monitored and quantified to provide an accurate accounting for study of training effects. Daniels and Oldridge3 showed no increase in aerobic power in young track performers over 22 months of training, but no quantification of the intensity of workouts was mentioned, only yearly mileage. Brown et d.,l in contrast, showed substantial improvement in the aerobic capacity of young girls with training for cross-country running. The mean initial aerobic capacity of the youngest girls (8-9 years) was 36.6
Annals New York Academy of Sciences
744
ml.kgl-min-l and the mean for the older girls was 49 ml.kgl*min-l. The mean increase in aerobic capacity for all of the girls was approximately 12 ml.kgl*min-l. A control group would have been valuable in the study by Brown et ul.' since Klissouras has shown that changes in aerobic capacity are found in controls as well as exercising subjects. Nevertheless, the results of Brown et ul.' indicate that increases in maximal oxygen uptake are produced by endurance work, particularly if the initial aerobic capacities are relatively low. Our results agree with this conclusion, The subjects of Brown et u1.l covered greater distance per session, with more frequent training sessions, than the children in our study, which may partially explain the greater increase in aerobic capacity recorded in their runners. Low initial fitness levels for young boys (1 1-13 years) have been noted.*. 6, ' Changes in aerobic capacity are more readily produced if the initial fitness level is low. The children in this study had relatively high aerobic capacities. The final aerobic capacity of those in the exercise group (59.4 rnl.kg'-min-') is nearly equivalent to that of the subjects of Brown et ul.' (60.3 ml.kg'*min-' for the 10-1 1-year-olds and 62.1 ml-kg-lamin-1 for 12-13-year-olds). Thus, the high initial aerobic capacity probably precluded large increases in fitness for those children who participated in this study. Eriksson and Koch 2 0 showed that the increase in cardiac output associated with greater oxygen uptake at a given workload was due to an increase in stroke volume with little change in $rteriovenous oxygen difference in response to physical conditioning. Their Vo, values were 1.3 to 2 liters per minute higher *an the values predicted from the regression equation calculated from our data, Q = 2.273 5.225 Vo,. The stroke volumes measured in their subjects were 15 to 20 ml greater than those recorded here. Eriksson and Koch *O used a dye dilution method whereas we used the CO, rebreathing method, and their subjects were about two years older than ours. The differences in technique and age of the subjects may be responsible for some of the variation noted. Our data agrees well with those of Bar Or et ul.15 who also used the CO, rebreathing method. Our results show that cardiac output and stroke volume increased moderately with time but that the increase in the exercise group was not greater than that observed in the control group. The coefficients of variation were found to vary from 12% to 15% and this variation was larger than the changes in cardiac output recorded (10% to 13% at the two highest workloads). Thus, there is the suggestion of a moderate increase in cardiac output during submaximal work with growth, but no effect attributable to physical conditioning. The cumulative distance run during the training session over the 12-week period was the product of a number of factors including running efficiency, pace, motivation, and number of sessions attended, as well as health status and social interactions. Perhaps it is not surprising then that initial aerobic capacity explained only 25% of the variance in the cumulative distance covered during training. We had arbitrarily expected that those with high initial aerobic capacity would both run faster and farther during their training sessions. This was only partially true among the children studied. The correlation between initial fitness and change in fitness was insignificantly different from zero ( r = 0.05). The smallest increase in Po*max was less than 1 ml-kgl.min-l in a child with an, initial fitness of 53 ml.kg-'*min-'; on the other hand, the largest increase in VO,maxwas 9.5 ml.kg1.min-' in a child whose initial fitness was 61 ml-kg-'emin-1. Both of these youngsters were
+
Lussier & Buskirk: Training Children
745
high mileage runners compared to their colleagues and maintained fast paces. This difference in response to physical conditioning might be ascribed, in part, to the genetic makeup of each child, perhaps to an individual threshold for initiation of a training response as well as control of the magnitude of the response. In children as in adults, a training threshold must be reached to elicit changes in aerobic capacity. Habitual physical activity levels may alter both the threshold and the magnitude of the response, but the individual variation associated with genetic endowment is apparently predominant.
Further Comment
This study, as well as related ones, provide limited insight into the effects of physical conditioning on prepubertal children, and serious questions remain as to how intensity, duration, and frequency contribute to changes in aerobic capacity in them as compared to postpubescent children and adults. The role of genetic endowment needs further clarification as do the effects of growth. What interrelationships are there between physical activity, growth, and maturation? What effect will endurance training initiated during the period of early growth and development have on their cardiovascular physiology or on the rate of development of atherosclerosis and coronary heart disease? Are there special nutritional requirements for children who engage in endurance training regimens? Since children may be more heat intolerant, what are their environmental and physiological limitations for participation in long distance running? What effects attributable to chronic activity can be found by evaluation of weight-bearing joints? Such questions-and the list could be extensively increased-document the need for more knowledge before youngsters should be advised to participate on a mass basis in long distance training regimens and/or marathon competition.
Summary
The cardiovascular effects of a 12-week endurance training regimen were studied among normally active and healthy prepubertal children. Twenty-six 8- to 12-year-old children (20 boys and 6 girls) volunteered and 10 acted as control subjects. The training regimen consisted of distance running for progressively longer periods (from 10 to 35 min) 2 to 3 times per week, with 2 additional sessions per week devoted to running games. Those who were trained ran a cumulative average distance of 95.6 km (58.9 miles). Intensity of work was assessed from running pace and heart.rate. The target workout intensity was 75% to 80% of aerobic capacity ( Vo, Growth and development accounted for increases in height, weight, body circumferences, and diameters, and fat-free body weight. Heart rate (HR) during submaximal workloads, both running and walking, decreased in the trained group ( p < 0.01) and ( p < 0.05). HR,,, did not change, but Vo, increased significantly (average 7 % ) in the trained group but not in the controls. No significant change attributable to training was found for submaximal cardiac output, stroke volume, or arterio-
746
Annals New York Academy of Sciences
venous oxygen difference. The Vo2,,, value before conditioning was a relatively poor predictor of the magnitude of improvement in functional capacity,
but those with higher initial Vo, ,, logged more cumulative training mileage. It was concluded that prepubertal children respond to an endurance training regimen by improving their running capacity, which is, to a limited extent, associated with increased aerobic capacity.
References
F. DEETER.1972. The effects of cross country running on preadolescent girls. Med. Sci. Sports. 4( 1): 1-5. CUMMING. G. R., A. GOODWIN, G. BAGGLEY & J. ANTEL. 1967. Repeated measurements of aerobic capacity during a week of intensive training at a youth's track camp. Can. J. Physiol. Pharm. 67(45): 805-811. DANIELS,J. & N. OLDRIDGE.1971. Changes in oxygen consumption of young boys during growth and running training. Med. Sci. Sports 3(4): 161-165. DOBELN, W. R. & B. 0. EIUKSSON.1972. Physical Training, maximal oxygen uptake and dimensions of oxygen transporting and metabolizing organs in boys 11-13 years of age. Acta Paediat. Scand. 72(61): 653-660. EKBLOM,B. 1969. Effect of physical training in adolescent boys. J. Appl. Physiol. 27: 350-355. KL~ssourus,V. & G. WEBER. 1973. Training: Growth and heredity. I n Pediatric Work Physiology. Proc. 4th Int. Symp. : 209-216. Wingate Institute, Israel. KOCH,G. & B. 0. ERIKSSON. 1973. Effect of physical training on pulmonary ventilation and gas exchange during submaximal and maximal work in boys aged 11-13 years. Scand. J. Clin. Lab. Invest. 31: 88-94. BAROR, 0.& L. D. ZWIREN. 1973. Physiological effects of frequency and content variation of physical education classes and of endurance conditioning on 9 to 10 year old girls and boys. I n Pediatric Work Physiology. Proc. 4th Int. Symp. : 199-208. Wingate Institute, Israel. STEWART, K.J. & B. GUTIN. 1976. Effects of physical training on cardiorespiratory fitness in children. Res. Quart. 47(1): 110-120. MOCELLIN, R. & U. WASMUND. 1973. Investigation of the influence of a running training program on the cardiovascular and motor performance capacity in 53 boys and girls of a second and third grade primary school class. I n Pediatric Work Physiology. Proc. 4th Int. Symp. :279-288. Wingate Institute, Israel. GREULICH, W. W. & S. I. PYLE. 1966. Radiographic Atlas of Skeletal Development of the Hand & Wrist. 2nd ed. Stanford Univ. Press. Stanford, California. ItLUs, J. D., E. E. HUNT,JR. & E. R. BUSKIRK.1971. Skeletal development of non-institutionalized children with low intelligence quotients. Amer. J. Phys. Anth. 35(3): 455-466. TAYLOR, H. L.,E. R. BU~K~RK & A. HENSCHEL.1958. Maximal oxygen uptake as an objective measure of cardiorespiratory performance. J. Appl. Physiol. 8
1. BROWN, C. H., J. R. HARROWER & M.
2. 3.
4.
5. 6. 7. 8.
9. 10.
11. 12. 13.
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