Comparison of Effort Between Children with and without Below-Knee Amputation

Jack R. Engsberg, PhD, Brian R. MacIntosh, PhD and James A. Harder, MD


Children with below-knee amputation (BKA) walk differently from normal children.2,3,5 The differences, particularly the asymmetries, might be expected because the structure of the prosthetic limb is unlike that of the intact one. It is not known whether the differences in gait are beneficial, detrimental or inconsequential with respect to the long and short term health of BKA children. Eventually, chronic higher loading in the nonprosthetic limb may increase the risk for degenerative joint disease. 1,4 In the short term, BKA children may require greater effort to walk and run when compared to the effort needed by normal peers. The greater effort may restrict BKA children from having the same typical experiences as those of normal children, resulting in reduced quality of life.

Effort can be evaluated from both physiological and biomechanical perspectives. Physiologically, effort is evaluated by measuring oxygen uptake (VO2) and heart rate. Pagliarulo and colleagues' determined the effort of 15 adults with unilateral BKA with no pre-existing vascular disease. Subjects walked with a prosthesis and with crutches at various speeds, including their freely chosen speed. While the average freely chosen speeds for both methods of gait were not significantly different, heart rate and VO2 were significantly different. Walking with a prosthesis was accomplished with lower heart rates and lower VO2 (ml/(kg * min)) values than walking with crutches. In addition, VO2 increased when subjects walked faster or slower than their freely chosen speed under either walking condition. Thus, the use of a prosthesis should be encouraged rather than walking with crutches.

Nielsen and coworkers6 measured the heart rate and VO2 of seven adults with unilateral traumatic BKAs during various walking speeds. Three subjects walked with both SACH (solid ankle, cushioned heel) and Flex Foot assemblies. The freely chosen walking speed for the Flex Foot was generally higher than for the SACH foot. VO2 (ml/(kg * min)) was similar for both devices at slower speeds, while at faster speeds slightly less oxygen was consumed when the subjects wore the Flex Foot. Differences in the values of VO2 between the feet were never greater than the differences between either device and the lower VO2 values of normal subjects. The same trends were also displayed for heart rate.

These investigations suggest that 1) although prosthetic gait requires less physiological effort than crutch walking, it nevertheless requires greater effort than walking with two intact lower limbs and 2) the differences with respect to effort for two different prosthetic feet may be small. Whether BKA children display the same or greater physiological effort for walking as adult BKAs has not been reported.

Physiological measurements can provide valuable information to evaluate the effort of BKAs under varying walking conditions; however, they cannot explain why greater effort is required. In contrast, biomechanical measures may offer a mechanical explanation for greater effort. Utilizing both physiological and biomechanical measurements, a relationship may be established to define the cause and effect interaction for effort. This preliminary investigation was intended to compare physiological and biomechanical measures of effort of BKA and normal children during walking.

Methods

Five children volunteered to participate in the investigation. Three were BKAs and two had normal intact lower limbs. One of the BKA children wore a Flex Foot on his prosthesis and the other two had SACH feet. The physical characteristics of the subjects are presented in Table 1 .

After the procedures of the experiment were explained to the children and parents and informed consent forms were signed, the children were permitted to walk on a treadmill for 3 to 5 minutes. Children were also given time to practice breathing into a small mouthpiece.

Prior to data collection, each subject was asked to choose a preferred speed of walking on the treadmill. Subjects increased and decreased the speed of the treadmill by verbal communication with the treadmill operator until they felt comfortable. With the freely chosen speed selected, the treadmill was stopped and the child was prepared for data collection. The subjects then walked for two minutes at each of the following treadmill speeds: 1) preferred, 2) 0.7 m/s, 3) 0.9 m/s, 4) preferred, 5) 1.1 m/s, and 6) preferred.

Effort was evaluated by measuring heart rate, VO2 and vertical displacement of a surface marker approximating the whole body center of mass (COM). Heart rate was monitored with a PE 3000 Sport Tester and VO2 was measured using a Horizon metabolic cart. A sensor unit was attached to the chest to monitor the heart rate and the headgear and mouthpiece were placed on the child to allow for determination of VO2 It had been determined in a preliminary experiment that 2 minutes was adequate for these children to reach a steady level of heart rate and VO2 Thus, while VO2 and heart rate were monitored every 30 seconds, only the final value was used in the analysis. Vertical locations of the reflective surface marker placed on the sacrum were determined for seven to ten step cycles at 45 and 90 seconds with a Motion Analysis video system. Surface marker locations were tracked and vertical displacements (i.e., minimum location to maximum location) of the surface marker were determined for each step cycle. Displacements were normalized by dividing by subject height and an average value was determined. Because only five subjects were involved in the preliminary investigation, individual data were examined but no statistical treatment was performed to determine significant differences between variables.

Results

Freely chosen speeds

The means and standard deviations (sd) for speed, heart rate, VO2 and displacement of COM for the freely chosen walking speed are presented in Table 2 . Results for the freely chosen speed generally did not deviate substantially from the beginning to the end of testing for given subject. Consistency of results provided a measure of testing reliability; thus results for the fixed speeds were viewed as reasonable.

Fixed speeds

Heart rates at the fixed speeds are presented in Figure 1 . For each subject, the heart rate was greatest at the fastest speed and with one exception (Normal 1) was lowest at the slowest speed. At the two greatest speeds the two BKAs with a SACH foot had the highest heart rates and at the fastest speed, all three BKAs had the highest heart rates. VO2 results for fixed speeds are shown in Figure 2 . For every subject, as the speed increased the VO2 also increased. Except for the slowest speed, the two BKAs with the SACH foot had the greatest values. For this exception, Normal 2 had a slightly greater VO2 value than SACH 2. The BKA with the Flex Foot had VO2 values that were closer to those values of Normal 1 than to the other BKAs; the values for Normal 1 were the lowest for the entire group. The results presented in this figure suggest a tendency towards two distinct groups of subjects, particularly at the fastest speed tested. The first group is composed of BKAs wearing a SACH foot. The second group consists of normal children and the BKA child wearing the Flex Foot.

Average vertical displacements of the COM are presented in Figure 3 . The results presented in this figure clearly suggested two distinct groups of subjects. First are the children who had the greatest vertical displacement of the COM, namely the BKAs with SACH feet. The second group, displaying a smaller vertical displacement of the COM, consisted of the two normal children and the BKA with a Flex Foot. At any given speed, the differences separating the members within each group were always less than differences separating the values between the groups.

The physiological measure of VO2 and the biomechanical measure of vertical displacement of the COM were plotted for the three fixed speeds (Fig. 4 ). As with the data presented in Figure 3 , Figure 4 showed the same two groups, although the effect of walking speed was disregarded. The mean and sd for vertical displacement of the COM of the SACH group were 0.044 and 0.005 respectively and for the other group were 0.025 and 0.005 respectively. The mean for the range (i.e., average maximum-average minimum) of VO2 in the SACH group was 5.3 ml/(kg * min) and in the other group 2.9 ml/(kg * min). The mean and sd for VO2 values in the SACH foot group was 17.4 and 2.6 ml/(kg * min) and in the other group 14.5 and 1.7 ml/(kg * min) respectively.

Discussion

Results represent information collected from only five subjects. Interpretation and discussion of the results should be viewed with due consideration to that limitation.

Previous investigators6,7 reported an average speed for walking with a prosthesis of about 1.2 to 1.4 m/s. In this study, the BKA children with SACH feet averaged 0.8 m/s and the child with the Flex Foot had a freely chosen walking speed of 1.3 m/s. The average heart rate at the freely chosen speed in the earlier investigation' was 106 b/m (sd = 10). The average heart rate of 116 b/m in the present research was slightly higher. Finally, the VO2 for the two previous investigations was about 15 to 16 ml/(kg * min) while the V02 for the BKA children in the present investigation was 16.5 ml/(kg * min).

A comparison for VO2 can be made between the fixed speeds of the Nielsen et al. investigation and those in the present study. The VO2 for BKAs and normals of the earlier study ranged from 11 to 16 and 7.5 to 11 ml/(kg * min) respectively, for the three fixed speeds utilized in the present investigation. No difference existed between subjects with the different feet at the 0.7 m/s speed and about a 2 ml/(kg * min) difference existed at the 1.1 m/s speed. The values for the VO2 of the BKA and normal children in the present investigation were very similar to those values presented by Nielsen and associates.

Measures for heart rate and VO2 of BKA children are consistent with those previously found for BKA adults.

A slightly greater consumption of oxygen by BKA children wearing the SACH foot was observed when compared to that of normal children and the child with the Flex Foot terminal device. This relationship also existed for vertical displacement of the COM. Separation of subjects into a SACH foot group and a normal and Flex Foot group appears reasonable. Hence, for the physiological variable of VO2 and the biomechanical variable of vertical displacement of the COM, the average values for the SACH foot group were higher than those of the normals and the child with the Flex Foot. This result varies from that reported by Nielsen et al. where differences separating SACH and Flex Foot values for VO2 were never greater than the differences separating either of those values and the values for normals. The small samples in both investigations prevents resolving the issue.

Measurement of VO2 may provide an appropriate method for differentiating required effort for BKA and normal children and effort between prosthetic feet. VO2 does not explain the observations. The vertical displacement of the COM may provide a partial explanation for the higher metabolic cost of ambulation in BKA children with a SACH foot who had greater displacement than the two normal children and the BKA wearing the Flex Foot. Body weight of the BKAs with SACH feet must be traveling through a greater range of vertical motion than that of the other subjects. It would seem reasonable to suggest that the greater vertical motion may require greater physiological effort.

Conclusions and Recommendation

Tentative conclusions resulting from this investigation are: 1) The freely chosen walking speed, heart rate, and VO2 for BKA children appear to be in general agreement with similar measurements for BKA adults. 2) The VO2 and vertical displacement of the COM seemed to separate subjects into one group consisting of BKAs wearing a SACH foot and a second group consisting of normal children and a child with a Flex Foot. The first group generally displayed greater effort for all walking speeds than the second group. In terms of the physiological and mechanical measures used for determining effort, BKA children wearing the SACH foot terminal device differed from normal children; the BKA child wearing the Flex Foot was similar to the normal children. An investigation should be conducted with a large number of normal children and juvenile BKAs wearing various prosthetic feet to determine if the trends indicated in this investigation could be verified or refuted by statistical analysis.

Acknowledgments

Funding was provided by the Variety Club of Southern Alberta-Tent 61, through the Alberta Children's Hospital. The authors would like to acknowledge the help of A. Godfrey, K. Pritchard and M. Rodberg in the collection of this data.

Human Performance Laboratory, University of Calgary, 2500 University Drive, NW, Calgary, Alberta T2N 1N4, Canada (Drs. Engsberg and MacIntosh). Alberta Children's Hospital, 1820 Richmond Road, SW, Calgary, Alberta T2T 5C7 (Dr. Harder).

Vol. 25, No. Spring 1990

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