Habituation of Children to Treadmill Walking and Running: Metabolic and Kinematic Criteria

Pediatric Exercise Science, 1995, 7, 162-1 75 0 1995 Human Kinetics Publishers, Inc. Habituation of Children to Treadmill Walking and Running: Metabolic and Kinematic Criteria Gail Frost, Oded Bar-Or, James Dowling, and Catherine White This study examined habituation to treadmill walking or running in children. Twenty-four boys and girls, ages 7-1 1, completed six 6-min trials of treadmill exercise at one of these speeds: (a) comfortable walking pace (CWP), (b) CWP + 15%, (c) running at CWP + 3 km.hr-i, or (d) running as above + 15%. The six trials were repeated in a second visit. The a priori criterion for habituation was a decrease in steady state values of oxygen uptake (VO,), heart rate (HR), respiratory exchange ratio (RER), and stride rate (SR) or an increase in stride length (SL) and hip joint vertical amplitude (HA) from one trial to the next. There was no consistent pattem indicating habituation for the group. Many trials and more than one day of testing do not appear to improve the stability of the metabolic or kinematic variables. The lack of consistency in individual responses suggests that monitoring subjects' habituation individually is important. The motor-driven treadmill is often used in metabolic and kinematic studies of locomotion because it provides a convenient method of collecting data on a walking or running subject. Clinicians may use a treadmill protocol to assess cardiovascular health and fitness in patients, or the effectiveness of a surgical intervention, rehabilitation, or a training program. Their interpretations may be inaccurate or misleading if the subject has not had sufficient treadmill exposure to make the adjustments necessary to achieve a stable, consistent gait pattern with minimal stride-to-stride variability. It has been suggested that these adjustments occur in two phases (3). Accommodation, or the development of an essentially normal and fairly stable gait pattem, may occur in the initial exposure. Habituation is not achieved, however, until kinematic analysis of gait reveals no significant within-day or between-day differences from stride to stride (3, 11). Few studies examining the process of treadmill habituation are available (3, 11, 15, 16) and dissimilar subject groups, protocols, and analysis techniques make definitive conclusions difficult. Recommendations for the amount of practice required to achieve accommodation vary from 2 min in walking subjects (16) to 8 min in a group of experienced runners (11). Likewise, a range of G. Frost, 0. Bar-Or, and C. White are with the Children's Exercise and Nutrition Center, Chedoke Hospital, Eve1 Building, 4th Floor, PO Box 2000, Station A, Hamilton, ON Canada L8N 325. J. Dowling is with the Dept. of Kinesiology at McMaster University, Hamilton, ON Canada L85 4K1.

Habituation to Treadmill - 163 practice times from 45 min (11) to 60 min (16) has been suggested to allow habituation to occur. These studies focused on the kinematics of habituation to treadmill locomotion in adults. None examined metabolic changes occurring during this process, and none considered how habituation occurs in children. Knowledge of the habituation process, both kinematic and metabolic, is useful for determining the number of lab visits necessary and for interpreting test results. Likewise, the researcher investigating questions about gait or exercise needs this information in order to create the most effective study design. It was the purpose of this study to clarify both the kinematic and metabolic aspects of treadmill habituation in children. The study did not analyze patterns of accommodation. Subjects Methods Twenty-four children (15 boys, 9 girls), ages 7 to 11 (M = 9.06 f 1.42 years), volunteered for the study. Informed consent was obtained from child and parent prior to each child's participation. The study was approved by the Ethics Committee of the Faculty of Health Sciences, McMaster University. The children (height = 134.88 f 1139 cm, mass = 32.98 rtr 9.28 kg, sum of four skinfolds = 33.36 f 15.65 mm) were healthy, physically active, and had no experience in treadmill walking or running. The children were randomized into one of four groups: (a) walking, comfortable pace (n = 7); (b) walking+, comfortable pace + 15% (n = 4); (c) running, comfortable walking pace + 3 km.hr-' (0.83 m.s-i) (n = 8); or (d) running+, as above + 15% (n = 5). Mean height, mass, and age values were not significantly different between groups. Study Design Subjects made two visits to the lab. During Visit 1, anthropometric, health, and physical activity data were collected and the walking or running speed to be used for the treadmill bouts was determined. Each child was instructed to walk one lap of a 400-m outdoor running track at his or her own comfortable walking pace-"the pace you usually use when walking to school, the store, or a friend's house, for example." They were asked to keep their chosen pace constant, not slowing down or speeding up as they walked. The first and third 100-m segments of the lap were timed; the times were averaged and were used to calculate the walking speed. Calculations were then performed to establish treadmill speeds for children in the walking+, running, and running+ groups. Each child received an instructional demonstration of mounting and dismounting the moving treadmill belt, and of treadmill walking. They then walked on the treadmill at a slow speed for 15-20 s. This ensured some measure of safety while maintaining naivetk to treadmill locomotion (11). Each child then performed six 6-min bouts of walking or running on a previously calibrated Quinton 2472 treadmill at his or her assigned speed. Between-bout recovery time was 8 min or until heart rate (HR) was less than 100 beats-min-l.

164 - Frost, Bar-Or, Dowling, and White During Visit 2, the treadmill portion of the protocol was repeated. The maximum interval between visits was 5 days. To control for circadian variation, both visits for each subject were scheduled for the same time of day. Data Collection The children were connected, through a mouthpiece and a low dead space valve, to a custom-configured open circuit system (Ametek S-3AlI O2 analyzer, Hewlett Packard 78356A C02 analyzer, Ametek R1 Flow Control flow meter), which was calibrated with gases of known concentration. Expired gas was collected continuously, with oxygen consumption (VO,) and respiratory exchange ratio (RER) values recorded at 20-s intervals. HR was monitored throughout each trial and stored at 5-s intervals using a PE4000 Sport tester monitor (Polar Electronics, Finland). Kinematic data were collected by videotaping (Panasonic Camcorder AG450, at 60 framess-i) the children during the final minute of each exercise bout. A reflective marker was placed over the right greater trochanter to estimate hip joint location. Data Analysis The three final-minute values for each of VO, and RER were averaged, as were the corresponding HR values, for further analysis. Ten strides from the final minute videotape were used to determine stride rate (SR) and stride length (SL). Five strides were digitized (Peak Performance Technologies, Inc.) to provide vertical hip amplitude (HA) data. The data were analyzed using analysis of variance (ANOVA) with repeated measures. Tukey's HSD post hoc test was used to identify the comparisons that were significantly different. In addition, mean VO, data from Minutes 5 and 6 of each bout were compared to assess whether the children were at steady state..the criterion for steady state was a difference of less than 2 ml-kg-'.min-i in V02 between the 5th and 6th minute of each exercise bout. Results The ranges of speeds for the four groups were as follows: (a) walking, 1.18-1.65 m.sec-' (M = 1.34 f 0.18 m.secw', (b) walking+, 1.21-1.70 m.sec-' (M = 1.52 f 0.19 m.sec-i), (c) running, 1.92-2.41 m.sec-i (M = 2.15 + 0.17 m.sec-i), and (d) running+, 2.28-2.68 m-sec-' (M = 2.50 f 0.14 m.sec-i). The comparison of mean VO, from Minutes 5 and 6 to assess the achievement of steady state revealed no differences of 2 ml,kg-l.min-l or more in the group data. However, when the same criterion was applied to individual subjects' data, steady state was achieved in only 82% of the total trials. Between-Day Changes For the metabolic and kinematic variables studied, Table 1 shows the means and standard errors of all six trials across days for each speed group. Statistically significant differences were found between days in mean RER values for the

Table 1 Day 1 and 2 Values for Metabolic and Kinematic Variables, by Speeds Walking Walking+ Running Running+ Day 1 Day 2 Day 1 Day 2 Day 1 Day 2 Day 1 Day 2 Variable M SEM M SEM M SEM M SEM M SEM M SEM M SEM M SEM VO, 21.41 0.43 22.10 0.52 23.59 0.98 24.47 0.98 39.39 0.85 37.60 0.77 43.71 0.90 43.07 0.70 (mbkg-l.min-i) RER 0.85 0.01 0.87* 0.01 0.86 0.01 0.90* 0.01 0.91 0.01 0.92 0.01 0.89 0.01 0.91* 0.01 Heart rate 123 2 123 2 137 4 137 4 183 3 176 3 181 2 177 2 (beats.min-') Stride length 1.16 0.02 1.17 0.02 1.26 0.03 1.25 0.03 1.43 0.03 1.42 0.03 1.74 0.03 1.70 0.03 (m) Stride rate 68.57 0.64 68.20 0.59 72.39 0.44 73.13 0.62 89.34 0.78 90.58 0.87 87.21 0.85 89.28 0.75 (strides.min-i) Hipamplitude 3.52 0.12 3.69 0.14 4.69 0.19 4.81 0.27 6.82 0.24 6.80 0.26 7.83 0.22 7.53 0.22 B (cm) 5 3 a "Significantly different from Day 1, p <.05. 5 g

166 - Frost, Bar-Or, Dowling, and White walking, walking+, and running+ groups. For all four speed groups, Day 2 RER values wefe higher than Day 1: There were no significant between-day differences in mean V02 or mean HR. V02 increased with increasing speed, but HR was similar for the running and running+ groups. The running+ group had less variability in HR results than any of the other three groups. The kinematic variables showed no significant between-day differences. For mean SL, SR, and HA values, within-group variability was less in the running+ group than in the running group. Between-Trial Changes Means and standard errors for each variable across trials are shown in Figures 1 and 2 and in Tables 2-4. Statistically significant differences were found in RER, for the walking+, running, and running+ groups (Figure 1) between the final trial(s) on Day 1 and the first trial on Day 2. A pattern was exhibited by all groups, in that highest values were recorded for Trial 1 on both days, followed by a decline and apparent leveling off. For all groups, the Day 2 Trial 1 value was the highest recorded. There were no significant differences in VO~ between trials (Figure 2). The running+ group showed the pattern of highest Trial 1 value, followed by decline to a plateau, on both days. This group also had less variability than the running group (with the exception of Day 2 Trial I), especially on Day 2. No significant differences were found in HR across trials (Table 2), however differences of 10 (running group) and 5 (running+ group) beats-min-' were noted between Day 1 Trial 1 and Day 2 Trial 1 values. The variability of the HR measures was lowest in the running+ group. Between-trial ANOVA for SL revealed no significant differences (Table 3). There was, however, a tendency toward an increase in SL from Trial 1 to Trial 6 on Day 1 for all speed groups. All but running+ began Day 2 with SL values similar to the last value from Day 1. For the running+ group, Day 2 Trial 1 values were lower than initial values on Day 1. The walking, walking+, and running+ groups maintained a plateau in SL values on Day 2. The running group values declined to a plateau on Day 2. SR values were inversely related to SL results. There were no significant differences in HA across trials (Table 4). HA tended to increase from Trial 1 to Trial 6 on both days for the walking, walking+, and running+ groups. The largest HA values were found in the running+ group, but the variability of these results was less than that of the running group. HA variability measures for all speed groups tended to be higher on Day 2. Individual Subject Responses Examination of individual subjects' data for each of the variables studied revealed several different response patterns. Patterns of VOP and SL changes across trials among individual subjects are presented in Table 5. Similar variability was seen in HR, RER, SR, and HA patterns. Discussion Based on the available literature, the a priori criterion for treadmill habituation among these children was a decrease (V02, RER, HR, SR) or an increase (SL,

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168 - Frost, Bar-Or, Dowling, and White Trial Number Figure 2 - $02 (M f SEM) values across trials for walking (O), walking+ (@), mnning (A), and running+ (A) groups. HA) in steady state values from one trial to the next, leading to a plateau. In addition, it was expected that the variability of each of these measures would decrease and that several trials would be needed for these changes to occur. While this pattern was clearly seen for several individual subjects, both walking and running, it was not found for any group as a whole. Multiple trials or more than one day of testing did not appear to improve the stability of the metabolic or kinematic variables examined. Metabolic Variables The only metabolic variable measured that showed significant between-day or between-trial differences was RER (see Figure 1). Although decreases were observed across trials in V02 and HR in some speed groups, they were not significant. It is difficult to interpret the patterns seen in RER values: high Trial 1 results followed by decline to a plateau, with the same pattern repeated on Day 2. No attempt was made in the present study to control or assess subjects' diet, and no measurements of blood glucose or glycerol were made. It is possible that substrate utilization changed from Trial 1 to Trial 6 each day and that this change was reflected by the changes in RER. This is supported by the findings of Martinez and Haymes (9) who observed a drop in RER during 30 min of treadmill running (70% ~ 0~max) in 9-year-old girls. Martinez and Haymes (9) attributed this drop to a gradual increase in the use of fat as the energy source. The initial RER value each day in the present study may also have been influenced by the subjects' anticipation of, or anxiety about, their participation in the experiment.

Table 2 Values for Heart Rate (beats.min-') Across Trials Day 1 trials Day 2 trials 1 2 3 4 5 6 1 2 3 4 5 6 M SEM M SEM M SEM M SEM M SEM M SEM M SEM M SEM M SEM M SEM M SEM M SEM Walking 126 5 124 5 122 6 124 5 123 5 125 5 123 6 124 5 123 6 123 6 122 6 121 6 Walking+ 137 11 137 11 137 12 135 10 136 10 139 10 139 12 135 13 139 13 135 12 135 13 138 12 Running 183 8 182 9 183 8 182 8 183 8 183 8 173 7 175 8 177 7 178 6 177 8 178 6 Running+ 183 5 181 5 181 4 180 5 179 6 181 6 178 5 180 5 177 5 176 4 176 5 175 5 Table 3 Values for Stride Length (m) Across Trials Day 1 trials Day 2 trials I D, 1 2 3 4 5 6 1 2 3 4 5 6 Cr e M SEM M SEM M SEM M SEM M SEM M SEM M SEM M SEM M SEM M SEM M SEM M SEM s. 0 Walking 1.13 0.05 1.15 0.05 1.17 0.05 1.17 0.06 1.18 0.05 1.16 0.06 1.17 0.05 1.16 0.05 1.17 0.05 1.16 0.05 1.17 0.05 1.17 0.05 2 Walking+1.230.091.260.09 1.260.09 1.270.091.270.08 1.260.08 1.240.101.250.101.240.101.260.101.250.091.270.09 Running 1.430.07 1.430.07 1.430.07 1.430.071.450.071.440.071.440.071.430.07 1.420.071.41 0.07 1.400.07 1.400.07 2 - Running+ 1.71 0.08 1.71 0.09 1.74 0.08 1.77 0.09 1.76 0.09 1.77 0.08 1.69 0.08 1.70 0.07 1.70 0.07 1.69 0.07 1.70 0.07 1.71 0.06 1

Table 4 Values for Hip Amplitude (cm) Across Trials P, Day 1 trials Day 2 trials g 1 2 3 4 5 6 1 2 3 4 5 6 $ iit M SEM M SEM M SEM M SEM M SEM M SEM M SEM M SEM M SEM M SEM M SEM M SEM Walking 3.29 0.32 3.39 0.24 3.67 0.31 3.51 0.28 3.67 0.31 3.61 0.41 3.45 0.39 3.84 0.31 3.78 0.35 3.63 0.39 3.82 0.30 3.63 0.37 Walking+ 4.38 0.55 4.77 0.37 4.83 0.46 4.48 0.55 4.65 0.58 5.04 0.57 4.64 0.83 4.79 0.81 4.98 0.68 4.88 0.70 4.89 0.71 4.71 0.76 Running 6.88 0.73 6.84 0.65 6.71 0.55 6.71 0.63 6.94 0.60 6.81 0.55 6.98 0.71 6.79 0.62 6.71 0.64 6.77 0.69 6.65 0.69 6.88 0.65 Running+ 7.44 0.79 7.27 0.50 7.97 0.50 8.22 0.49 8.06 0.52 8.02 0.49 7.26 0.39 7.41 0.42 7.57 0.49 7.55 0.63 7.83 0.55 7.57 0.85

Habituation to Treadmill - 171 Table 5 Percentages of Individual Responders by Pattern for VO, and Stride Length (Day 1) for Walking Groups Combined and Running Groups Combined Patterns observed Trials 1-6 % responders walking % responders running Decrease Increase No change Irregular pattern Increase Decrease No change Irregular pattern Response patterns---oxygen uptake 40 30 0 30 Response patterns-stride length 45 9 27 18 VO~ was not significantly different across trials for any speed group (see Figure 2). Previous studies that have examined V02 from repeated submaximal exercise bouts in adult subjects are contradictory. Morgan and colleagues (10) reported nonsignificant differences and high day-to-day reliability in VO, in a group of trained male runners who performed submaximal treadmill runs on two separate days. In contrast, Daniels et al. (4) reported significant between-test differences in submaximal V0, among their trained male subjects. Armstrong and Costill (1) used a crossover design in which cyclists and runners each performed bouts of cycling and running at selected submaxjmal work rates over an 8-9 day period. Significant between-test decreases in V02 were found, but not at all work rates. Among prepubertal boys, no significant differences in V02 were found when three submaximal running speeds were repeated on two different days (13). Varying amounts of time were allowed for treadmill habituation before data were collected in these studies. There are at least three other factors that must be qonsidered when assessing changes in VO,, the first being variability in resting VO,. As shown by Bailey et al. (2), VO~ measured every 15 min for up to 6 hours in postabsorptive, resting adults rose gradually by about 1% per hour. In addition, superimposed on this rise were regular oscillations with subject-dependent periods of 1-2 hr and amplitudes of 7-20% of mean V02. No measure of resting metabolic rate was used in the present study; therefore, it is impossible to estimate the effects of changes in this variable on the children's V02 results. Second, applying our criterion of differences less than 2 ml-kg-'-min-i in VO, to individual subjects' data revealed that steady state was achieved by the 6th minute in an average of only 82% of trials (94% in the running+ group). The fact that not all subjects reached metabolic steady state in all trials may have confounded the group patterns that were observed. Finally, the stability and reproducibility of submaxim+ cardiorespiratory measures must be considered. Williams et al. (17) measured VO, at submaximal

172 - Frost, Bar-Or, Dowling, and White speeds in moderately trained runners, five times per week for 4 weeks, and found no significant between-day differences. In addition, reliability tests performed to determine the number of times a subject should be tested to get accurate values revealed only a small increase in the amount of explained variance with 5 consecutive days of testing (98%) compared with 2 consecutive days (90%). There are many potential confounding factors that may influence these measures, including training status, diet, running mechanics, circadian variation, ambient temperature, footwear, and length of time for treadmill habituation (4, 17). The first three factors mentioned were not controlled for in the present study; however, the others were. A11 but two of the subjects completed both visits at the same time of day, lab temperature ranged from 21-26 "C, and with the exception of one subject, each child wore the same pair of shoes for both visits. HR was not significantly different among trials in the present study, although differences of 5-10 beats.min-' were noted between Day l and 2 initial values for both running groups (Table 2). Unnithan (13) found no significant differences in HR among active, prepubertal boys during repeated submaximal treadmill running. While the subjects in the present study were not as homogeneous a group, they were of the same age range and many were as active. Davies and colleagues (5) did find a significant decrease in HR in their adult subjects after the first four tests of an eight-test submaximal series performed on a cycle ergometer, followed by a more gradual decline. The submaximal tests were performed every other day, with maximal exercise being done on the alternate days. Because the HR decrease was not accompanied by changes in VO,, Davies et al. (5) suggested that the HR changes were due to circulatory adjustments to the unaccustomed exercise, possibly increased stroke volume and/or O2 extraction at the cellular level. Although we cannot completely exclude the possibility that the lower initial HR on Day 2 in the running groups reflects some training effect, it is unlikely that a single 36-min exercise dose would be sufficient to induce such an effect. The variability of HR was lowest in the fastest speed group, a result that has also been reported in prepubescent boys performing submaximal running at three different speeds (13). When group data were considered, the children in the present study had reached steady state at Minute 6, according to our criterion. This time frame is in agreement with the work of Unnithan (12), who found no significant differences in V02 between Minutes 3 and 6 in a group of active 12-year-old boys running at submaximal speeds similar to those used in the present study. The absence of patterns reflecting a habituation process in either V02 or HR values taken from Minute 6 prompted the question of whether boredom (for the walking groups) or fatigue (for the running groups) could be influencing the results. When VO, and HR data from Minute 4 of each trial are examined, however, there are still no patterns of systematic change from trial to trial that would indicate that habituation is occurring. Kinematic Variables No significant between-day or between-trial differences were found for the kinematic variables measured, although some patterns were apparent. The increase in mean SL from Trial 1 to Trial 6 on Day 1 (Table 3) is in agreement with the findings of Wall and Charteris (15), who reported large increases in SL for the

Habituation to Treadmill - 173 first 2.5 min of a 10-min treadmill walking bout followed by gradual increases until the end. Novice adult subjects who performed 15 min of treadmill running daily for 10 days showed significant increases in SL over the first 2 days (1 1). The same study also reported an accompanying significant increase in vertical displacement of the center of gravity (VDCOG) in these subjects, using total body center of gravity (COG) calculated from a linked-segment model. The children in the present study displayed a tendency toward increased HA values from Trial 1 to Trial 6 on both Day 1 and Day 2, with the exception of the running group (Table 4). It has been suggested that novice treadmill subjects, particularly children, may show their apprehension by using a cautious walking or running style with shortened SL, increased SR, and lower COG at the outset (1 1, 14). Our results would tend to confirm this. SL achieved by the final trial on Day 1 was maintained in the initial trial on Day 2, with the exception of the running+ group, whose Day 2 Trial 1 values were lower than any recorded on Day 1. While this may imply a learning effect from the first day's practice, it is difficult to explain the results of the running+ group, which contained 3 boys with some running experience. It could be expected that experienced runners would be more likely to maintain a comfortable SL acquired by practice. All but the running group displayed a leveling-off pattern of SL values for Day 2. This plateau was also seen after two 15-min trials, in a group of novice adult treadmill runners (11). The largest values for HA occurred in the running+ group, as expected, but the coefficient of variation of these values was less than that of the running group. More experienced runners in the running+ group and the larger size of the running group may account for this result. The variability of HA measures for both walking groups is greater on Day 2, and one possible explanation for this is boredom. The task was clearly explained to the children, and constant encouragement was given; however, some subjects, particularly the younger ones, had occasional difficulties concentrating on what was required of them. Possibly because running necessitated more concentration, the variability for the running groups was not that different between days. Psychological Aspects of Habituation This study did not attempt to assess the psychological aspects of habituation to treadmill walking or running, and yet they are probably quite important. None of the children had previous treadmill experience, and very few had ever been to an exercise lab. Thus, participation in the experiment, although voluntary, was a potentially intimidating prospect for many of the children. Studies of children's anxiety about, and reactions to, a visit to the doctor (7) or minor surgery (6, 8) reveal a complex pattern of emotional and physiological responses. For example, mean HR recorded just prior to the administration of anesthetic in a group of 4- to 10-year-old children was 21 beats.min-' above baseline levels recorded earlier (8). Our subjects were volunteers who knew they could drop out, with no penalty, at any time. Great care was taken to explain each procedure and piece of equipment and offer reassurance and encouragement. It is still possible, however, that the metabolic and cardiovascular variables measured, especially HR, were influenced by the children's psychological state.

174 - Frost, Bar-Or, Dowling, and White It is tempting to suggest that the observed decrease in mean HR between Trial 1 Day 1 and Trial 1 Day 2 in the two running groups is due to subjects' decreased anxiety level once they were familiar with the protocol and their ability to complete it. It is not clear, however, why this did not also occur in the waking groups. If the differences were due to decreased anxiety, observable changes would more likely have been seen in the walking groups, where sympathetic changes due solely to exercise would be of lesser magnitude (5). Two other measures would have been helpful in assessing the psychological aspects of treadmill habituation. Resting HR taken at the beginning of each visit would have given some indication of the level of physiological arousal of the children, without the contaminating effects of exercise. Ideally, the use of an instrument that combined physiological measures, children's self-reports on their feelings about participation in the study, and observations of the children's behavior by researchers would have given more information about the extent of psychological habituation. Such multifaceted rating scales have proven useful in other areas of research with children (6). The lack of a clear pattern of habituation among all subjects in the present study may have several causes. This study differs from others that examined the process of treadmill habituation primarily in the age and experience of its subjects. Although previous experiments used naive treadmill performers, all were adult, and those used in the running study (1 1) were experienced overground runners. In each case, the subject groups were much more homogeneous than the group of children in this study. In addition, the rapid adjustments that occur in the first minute of treadmill exposure were not examined. The present study attempted to bring the children to metabolic steady state before kinematic data were determined. If differences exist between adults and children in this initial, accommodation phase, such differences may have influenced the data from which our observations were made. The great variability in individual responses, some of which are summarized in Table 5, does not allow us to offer general recommendations regarding treadmill habituation in children. While some subjects fulfilled our criteria for habituation, others showed different response patterns. This lack of consistency suggests that monitoring subjects' habituation individually is important. References 1. Armstrong, L., and D. Costill. Variability of respiration and metabolism: Responses to submaximal cycling and running. Res. Quar. Exerc. Sport 56:93-96, 1985. 2. Bailey, D., D. Harry, R. Johnson, and I. Kupprat. Oscillations in oxygen consumption of man at rest. J. Appl. Physiol. 34:467-470, 1973. 3. Charteris, J., and C. Taves. The process of habituation to treadmill walking: A kinematic analysis. Percept. Motor Skills 47:659-666, 1978. 4. Daniels, J., N. Scardina, J. Hayes, and P. Foley. Variations in VO, submax during treadmill running. Med. Sci. Sports Exerc. 16:108, 1984. (Abstract) 5. Davies, C., W. Tuxworth, and J. Young. Physiological effects of repeated exercise. Clin. Sci. 39:247-258, 1970. 6. Elliott, C., S. Jay, and P. Woody. An observational scale for measuring children's distress during medical procedures. J. Pediatr. Psychol. 12543-551, 1987.

Habituation to Treadmill - 175 7. Hyson, M. Going to the doctor: A developmental study of stress and coping. J. Child. Psychol. Psychiatry 24:247-259, 1983. 8. Lumley, M., B. Melarned, and L. Abeles. Predicting children's presurgical anxiety and subsequent behavior changes. J. Pediatr. Psychol. 18:481-497, 1993. 9. Martinez, L., and H. Haymes. Substrate utilization during treadmill running in prepubertal girls and women. Med. Sci. Sports Exerc. 24:975-983, 1992. 10. Morgan, D., P. Martin, G. Krahenbuhl, and F. Baldini. Variability in running economy and mechanics among trained male runners. Med. Sci. Sports Exerc. 23:378-383, 1991. 11. Schieb, D. Kinematic accommodation of novice treadmill runners. Res. Quar. Exerc. Sport 57: 1-7, 1986. 12. Unnithan, V. Factors Affecting Submaximal Running Economy in Children. Unpublished doctoral dissertation, University of Glasgow, Scotland, 1993. 13. Unnithan, V., K. Thomson, T. Aitchison, and J. Paton. Reproducibility of cardiorespiratory measurements during submaximal and maximal running in children. Brit. J. Sports Med. in press. 14. van Ingen Schenau, G. Some fundamental aspects of the biomechanics of overground versus treadmill locomotion. Med. Sci. Sports Exerc. 12:257-261, 1980. 15. Wall, J., and J. Charteris. The process of habituation to treadmill walking at different velocities. Ergonomics 23:425-435, 1980. 16. Wall, J., and J. Charteris. A kinematic study of long-term habituation to treadmill walking. Ergonomics 24:53 1-542, 198 1. 17. Williams, T., G. Krahenbuhl, and D. Morgan. Daily variation in running economy of moderately trained male runners. Med. Sci. Sports Exerc. 23:944-948, 1991.