ENERGY EXPENDITURE INDEX OF WALKING FOR NORMAL CHILDREN AND FOR CHILDREN WITH CEREBRAL PALSY Jessica Rose James G. Gamble Anthony Burgos John Medeiros William L. Haskell

Information on the energy expended during walking by children with cerebral palsy can provide objective data to help assess ambulatory aids, orthotic prescriptions or surgical intervention (Campbell and Ball 1978, Butler et al. 1984). Generally the rate of oxygen uptake is used to measure energy expenditure, but heart rate is more easily measured and has been shown in adults to be an accurate and convenient estimate of energy expenditure (Poulsen and Asmussen 1963, Astrand and Rodahl 1970, Shephard 1975, McArdle et al. 1976, Ganguli and Datta 1978, Stallard ef al. 1978). Heart rate has also been suggested for estimating energy expenditure by normal and disabled children (Berggren and Christensen 1950, Berg 1970, Campbell and Ball 1978, Sutherland 1978, Ghosh et al. 1980, Butler et al. 1984, Cooper ef al. 1984, Rose et al. 1985). We have shown previously that there is a linear relationship between heart rate and oxygen uptake over a wide range of walking speeds for normal children and for children with cerebral palsy (Rose et al. 1989). The present paper reports further on this linear relationship and compares the results of an energy expenditure index of walking based on oxygen uptake with an index based on heart rate. This comparison was made to determine whether heart rate provided an accurate estimate of energy expenditure.

Material and method Thirty-one children participated in this study: 18 normal children aged between seven and 17 years, 13 children with cerebral palsy (three hemiplegia, 10 diplegia) aged between seven and 16 years (Table I). They were recruited from the orthopaedic clinics at the Children’s Hospital at Stanford, from the California Children’s Services schools for the orthopaedically disabled, and from local public schools. The children with cerebral palsy were capable of walking on a treadmill, with or without the use of handrails, for at least two minutes at 21.5m/min (0.8 mph): 12 of the 13 were able to walk for at least six minutes at speeds up to 35-9m/min. This study was approved by the Human Subjects Review Committee of Stanford University, and informed consent was obtained from the parents or legal guardian. The normal children had no cardiovascular, neurological or orthopaedic problems. Those with cerebral palsy had no cardiovascular disease and had had no surgery within the preceding year. Each child attended one session, during which height, weight and comfortable speed of floor-walking were measured. Heart rate and oxygen uptake were measured at rest, after the child had been sitting quietly for five minutes, and while walking on the level treadmill at increasing speeds. The electrocardiogram was monitored con-

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TABLE I

Demographic data Cerebral palsy (N = 13)

Normal ( N = 18) Sex (F/M)

Age (vs) Height (cm) Weight (kg)

334

617

9/9

Mean 12.5 153.7 47.3

(SO)

Range

Mean

(SO)

Range

(15.5)

7-17 126-178 25'5-76.0

11.2 138.5 37.6

(10.5) (9.8)

7-16 121-159 28-61

(13.7)

tinuously on a three-channel oscilloscope for two minutes at rest and for two minutes during each successive work load. Heart rate was calculated as the average rate for the last 10 seconds of each minute. The children walked for two minutes at each speed, beginning at 21 -5m/min and increasing t o 29.5, 35.9, 51, 64.4, 77.8, 91.2, 104.6, 118 and 131-4m/min, as tolerated. The procedure was stopped if the gait became unsteady, or if the child gave a signal to stop or broke into a run. Oxygen uptake was determined by a microprocessor-based system. First, the volume of inspired air was measured by passing it through a dry-gas meter (Rayfield Equipment Company, Chicago, IL) then through low-resistance Otis McKenow respiratory valve, and finally through a five-foot length of plastic tubing (3.5cm ID) into a three-liter plexiglass mixing chamber. Expired air was withdrawn continuously from the chamber at a rate of 3OOmr/mm and passed through oxygen and carbon dioxide analyzers (Applied Electrochemistry, Inc.). Signals from the analyzers were transmitted through an analog-to-digital converter t o a computer (Apple Twoplus). Minute average values were printed every 30 seconds for expired ventilation volume, oxygen and carbon dioxide percentage in expired air, oxygen uptake (in L/min and in m/kg/min) and respiratory exchange ratio. Values for an energy expenditure index (EEI) based on (a) oxygen uptake and (b) on heart rate were calculated. EEl(o2) was calculated as oxygen uptake per kilogram bodyweight-walking speed (mL/kg-m); EEI(HR) was calculated as heart rate minus resting heart rate, divided by

walking speed (b/m). Values were plotted for each walking speed. The mean economical EEI(O~)and EEI(HR) was the average of the lowest EEI values for each individual, economical walking speed being defined as that associated with the lowest EEI value. Because the EEI(O~) and EEI(HR) remained low for most of the children during several consecutive speeds of walking, a range of economical walking speed was chosen for each individual. This was calculated as the walking speed with the lowest EEI value, plus 3 per cent of the total range of EEI values. Statistical analysis was done using the Student t test (Brown and Hollander 1977), and the conclusions drawn took multiplicity into consideration.

Results Figure 1 shows the EEls based on heart rate and on oxygen consumption as a function of speed of walking for a normal child and for a child with cerebral palsy. Both were representative of their group and their values were close to the mean. Figure 2 shows the relationship of both indices to walking speed for all 18 normal children, and Figure 3 shows this relationship for the 13 children with cerebral palsy. The curves reflect energy expenditure as a function of walking speeds. At slow speeds, EEI values for both groups of children were high, indicating poor economy. As walking speed increased, the EEI values decreased until a range of maximum economy was reached. For normal children who could walk beyond this range of maximum economy, the EEI increased with faster speeds of walking. This pattern occurred in both indices.

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WALKH(I SPEED (mhnln)

WALKING SPEED (Wmln)

Fig. 2. EEI based on (A) oxygen uptake and (B)

Fig. 1. EEI based on (A) oxygen uptake and (B)

. heart rate as function

heart rate as funcfion of walking speed f o r normal child and for child with cerebral palsy.

of walking speed f o r all 18

normal children.

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Fig. 3. EEI based on (A) oxygen uptake and (B) heart rate as function of walking speed for all 13

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WALKIHO SPEED ( m n )

Fig. 4. Means and SDs of EEI values based on (A)

children with cerebral palsy.

oxygen uptake and (B) hean rate for all 31 children studied. Shaded areas represent mean range of economical walking speeds.

The indices showed little variation among normal children, but considerable variation among those with cerebral palsy (see Figs. 2 and 3). Most of the latter were unable to walk at speeds beyond their most economical one: either they could go no faster and had to stop, or they broke out of a walking pattern. The economical

walking speed tended to increase with age in normal children, but the differences were not statistically significant. The economical walking speed for children with cerebral palsy did not vary with age. Figure 4 shows the EEI values for both groups walking at increasing speeds. The

335

TABLE 11

Economical EEI and associated economical walking speed Economical EEI(O3 (mL/kg-m) Mean (SO) Range Normal Cerebral palsy Hemiplegic Diplegia

0.17 0.48 0.21 0.56

(0.02) (0.22) (0.01) (0.18)

336

0.41 1.38 0.49 1.46

(0.12) (0.70) (0.23) (0.54)

84.5 51.2 64.4 45.3

0.13-0.21 0.20-0.93 0.20-0-22 0'35-0.93

Economical EEI(HR) (mL/kg-rn) Mean (SO) Range Normal Cerebral palsy Hemiplegic Diplegic

EEI(O3 economical walking speed (m/min) Mean (SO) Range 64.4-91.2 21.5-77.8 51-0-77.8 21.5-64.4

EEI(HR) economical walking speed (m/min) Mean (SO) Range 74.8 50.8 55.5 45-3

0.25-0.64 0.24-2-58 0.24-0.70 0.70-2.58

mean of the economical EEI indices for the normal children was significantly lower (p

Energy expenditure index of walking for normal children and for children with cerebral palsy.

Energy expenditure indices (EEI) based on oxygen uptake and heart rate were used to compare the economy of walking at various speeds by normal and cer...
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