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Physiological Comparisons of Physically Active Persons with Transtibial Amputation Using Static and Dynamic Prostheses versus Persons with Nonpathological Gait during Multiple-Speed Walking

Miao-Ju Hsu, MA, PT
David H. Nielsen, PhD, PT
John Yack, PhD, PT
Donald G. Shurr, MA, PT, CPO
Suh-Jen Lin, MS, PT

ABSTRACT

The level of physical activity may be a potential correlate of accommodation to amputee walking. The purpose of this study was to compare the energy cost, gait efficiency, and relative exercise intensity of physically active persons with transtibial amputation during walking with the solid ankle cushioned heel (SACH) foot, the Flex-Foot (FF), and the Re-Flex Vertical Shock Pylon (VSP) prosthesis versus persons with nonpathological gait. Subjects were healthy, physically active males, including five subjects with unilateral transtibial amputation and 18 control subjects with nonpathological gait. A repeated-measures design involving multiple-speed treadmill walking (53.64, 67.05, 80.46, 93.87, and 107.28 m/min) was used. Between-group analyses involved separate repeated-measures analysis of variance according to foot type with a priori pairwise contrasts at each test speed. Between-group SACH and FF analyses showed nonsignificant differences in energy cost and gait efficiency for 53.64, 67.05, and 80.46 m/min, but significantly increased energy cost and decreased gait efficiency for 93.87 and 107.28 m/min. Between-group Re-Flex VSP analyses indicated nonsignificant differences for all test speeds. The relative exercise intensity of subjects with transtibial amputation for all foot types was significantly higher than that for controls across all walking speeds. Physically active individuals with transtibial amputation may have enhanced adaptability optimizing gait performance regardless of foot type at both slower and higher walking velocities with the Re-Flex VSP.

Key Words: energy expenditure, gait efficiency, SACH foot, Flex-Foot, Re-Flex VSP prosthesis

Previous research has shown elevated energy cost, exercise intensity, and decreased gait efficiency in people with transtibial amputation.1-8 Various prostheses have been developed to aid individuals with transtibial amputations in enhancing gait performance and minimizing energy cost. Investigation of the effect of the prosthetic foot design has shown mixed results.6, 9-12 Variations in test procedures (such as single-speed versus multiple-speed testing) and variations in subject characteristics (such as level of physical activity and motor adeptness) may help explain interstudy inconsistencies.

Research in people with nonpathological gait and persons with transtibial amputation has shown curvilinear relationships between energy cost and increasing walking speed. However, the energy cost is higher in persons with transtibial amputation at the same walking speed.1-3, 6, 8 Ganguli et al.2 reported that people with transtibial amputation spent 33% more energy walking at 50 m/min than people with nonpathological gait. As reported in previous publications,1,6,11,13 gait efficiency is defined as energy cost per distance traveled. Studies on gait efficiency in people with nonpathological gait and in persons with transtibial amputation have shown a concave parabolic response curve with increasing walking velocity.1,6,13 The lowest value (optimal efficiency) appears to occur at the subject's free pace or self-selected walking velocity (S-SWV), which is also considered the optimal walking speed.1,6

Waters et al.7 reported that the energy cost of walking at S-SWV in subjects with transtibial amputation was similar to or slightly less than that for people with nonpathological gait, but at a slower S-SWV. These results suggest that individuals with transtibial amputation adapt by selecting a slower S-SWV so that the energy cost will not exceed normal limits, which is also less efficient. Nielsen et al.6 found that between-group differences (subjects with transtibial amputation versus control subjects) in energy cost and relative exercise intensity appeared to be speed-dependent, with increasing between-group separation at higher walking speeds.

Various prostheses have been designed to enhance the functional biomechanics of persons with transtibial amputation and to minimize the physiological demands. One of the earlier prosthetic foot devices was the solid ankle cushioned heel (SACH) foot, which is still used on selected amputee groups because of its lower cost. Because of the design of the limited elastic characteristics, the SACH provides only static support and does not allow any dynamic function. Newer innovations, dynamic elastic response feet, differ from the SACH in that they are designed to more closely simulate normal ankle movement during gait as well as store and release energy. Examples of dynamic feet are the Seattle foot, the Carbon Copy II, and the Flex-Foot (FF).

Research on physiological comparisons based on foot type has shown controversial results. In an investigation of persons with transtibial amputation, Torburn et al. 12 observed nonsignificant differences between foot type (FF, Carbon Copy II, Seattle, Sten, and the SACH) in energy cost and gait efficiency at S-SWV.13 However, Nielsen et al.6 reported decreased energy cost, relative exercise intensity, and improved gait efficiency with the FF versus the SACH in persons with transtibial amputation during multiple- speed walking from 67.05 m/min to 107.28 m/min.

The desire of individuals with transtibial amputation who have active lifestyles to participate in recreational sports and competitive athletics has prompted additional innovations, including the development of variations in the pylon design of the lower extremity prosthesis. The Re-Flex Vertical Shock Pylon (VSP) prosthesis is one of the newest of these innovations. It uses a piston cylinder-type pylon with a vertical leaf spring attached to the shank in combination with a dynamic foot (FF). The manufacturer claims that the design of the pylon will provide better shock absorption and energy release with improved performance for jogging and running as well as walking.14 However, efficacy research is limited.

In addition to prosthesis design, individual physical exercise capacity may be another important factor affecting the physiological responses of transtibial amputee gait.15-17 Pitetti et al.16 showed that aerobic exercise training in persons with lower extremity amputation was effective in improving cardiovascular fitness and in decreasing the energy cost of walking. Although not investigated directly, these results suggest that the more physically active individual with a transtibial amputation may have an enhanced potential for more normal and economical walking.16 If so, one could hypothesize that the laboratory-measured outcome differences in gait performance between physically active individuals with transtibial amputation versus individuals with nonpathological gait would be minimized. One could also hypothesize that prosthesis design may be an interactive factor. However, at present, no research has been done to substantiate these conjectures.

The purpose of this study was to compare the energy cost, gait efficiency, and relative exercise intensity of physically active persons with transtibial amputation versus persons with nonpathological gait during multiple- speed treadmill walking with the SACH, the FF, and the Re-Flex VSP. Analyses between foot types are not included in this article because they have been reported in our earlier publication.11

Methods

Subjects

All subjects were screened to assess general health. Subjects with known medical problems (cardiovascular, neuromuscular, or other significant abnormalities) were excluded. Written informed consent was obtained from each subject in accordance with the Human Subjects Review Committee of the College of Medicine at the University of Iowa before admittance into the study. Subjects included five persons with nonvascular unilateral transtibial amputation and eighteen control subjects with nonpathological gait. Subject descriptive data are included in Table 1 and Table 2 for the subjects with transtibial amputation and control subjects, respectively.

In the transtibial amputee group, the Day Activity Score inventory18 was used to evaluate physical activity level. A criterion score >10 indicates a high level of physical activity. On the basis of Day Activity Scores ranging from 12 to 45 (Table 1), all of our subjects with transtibial amputation were considered physically active. The amputee subjects were proficient walkers with the SACH, FF, and Re-Flex VSP and were able to walk at rates of up to 107.28 m/min. In the subjects with nonpathological gait, physical activity score was assessed with a subjective questionnaire. On the basis of a mean physical activity score of 3.89 on a 1 to 5 scale, with 5 as the highest level of physical activity (Table 2), the control subjects were considered as physically active.

Procedures

A repeated-measures design involving multiple-speed treadmill walking was used. A preliminary practice session was provided to all subjects. All necessary paper work was also completed during this session. One test session was required for the control subjects. Three test sessions on separated days were required for transtibial amputees with randomized test order according to foot type. To ensure acclimation to the specific prosthesis for testing purposes, the subjects with transtibial amputation were asked ahead of time to wear the test prosthesis at least 1 day in advance of testing.

All ambulation tests were performed on a motor driven treadmill (TM310 Trackmaster, JAS, Carrollton, TX). The test protocol consisted of a 2-minute stage of resting data collection, followed by a 5-minute practice session at 53.64 m/min, and then five 4-minute exercise stages at functional walking speeds of 53.64, 67.05, 80.46, 93.87, and 107.28 m/min. A 2- to 3-minute cool-down period was provided at 67.05 m/min. Testing was terminated if the subject felt fatigued or felt he or she could not safely walk at the next higher speed.

Physiological measurements included oxygen consumption determined by the open-circuit method with a computerized metabolic cart, and heart rate monitored by electrocardiographic (ECG) radiotelemetry. The physiological measurements were monitored continuously; however, only the last 1-min steady-state values were averaged and used for the data analysis.

Energy cost was quantified through direct measurement of oxygen uptake. A Medgraphics Cardio2 metabolic cart (Medical Graphics, St. Paul, MN) was used to determine oxygen consumption, based on individual breath by breath analysis. The metabolic cart, CO2 and O2 analyzers, and the airflow tube-pneumotach, were calibrated before each test session according to the manufacturer's specified calibration protocols. Gas volume measurements were corrected to reference values of standard temperature, pressure and dry air (STPD) based on existing laboratory environmental conditions. Heart rate was determined by an ECG radiotelemetry system, including a model 78101A Hewlett Packard FM receiver, a model 78100A Hewlett Packard miniature battery operated transmitter, a model 78330A Hewlett Packard cardioscope (Hewlett Packard, Palo Alto, CA) and a model 611 Quinton single channel cardiotachometer (Quinton Instrument, Bothell, Washington). The ECG recording system was interfaced with the Medgraphics metabolic cart. A modified chest manubrium V5 (CM5) recording electrode lead system was used. Procedures to record the heart rate were conducted as previously reported.6 The energy cost per meter traveled (mLO2/kg/m) was used as the criterion for gait efficiency.6,13 Percentage of age-predicted maximum heart rate (%APMHR) (exercise heart rate/age predicted maximum heart rate × 100) served as the index of relative exercise intensity.

Statistical Analysis

The statistical analysis was performed using the Statistical Analysis System for Windows (WINSAS) library program provided by The University of Iowa. A level of p < .05 was adopted for the determination of statistical significance. Means and standard deviations were calculated for each variable. Separate two-way analysis of variance (ANOVA) was used to test main effects and interaction across groups versus speed of ambulation according to each type of prosthesis. Assuming nonparallel between-group responses, prespecified a priori speed-specific analyses were conducted with the Slice comparison procedure.

Results

Energy cost, gait efficiency, and relative exercise intensity were measured in five persons with unilateral transtibial amputation during multiple foot type (SACH, FF, and the Re-Flex VSP) ambulation and 18 people with nonpathological gait over a functional range of walking from 53.64 m/min to 107.28 m/min with 13.41 m/min increments. Group means and standard errors for energy cost, gait efficiency, and relative exercise intensity are graphically presented in Figure 1 , Figure 2 , and Figure 3 , respectively. The two-way ANOVA revealed nonsignificant group by speed interactions for all three physiological measurement analyses for each foot type. The prespecified, between-group, speed-specific Slice comparisons (the control subjects versus each foot type in the subjects with transtibial amputation) are presented in Table 3 , Table 4 , and Table 5 , respectively.

Oxygen uptake normalized to body weight served as the criterion measure of the energy cost and represented the physiological workload demands of the physical exercise. As illustrated in Figure 1, energy cost increased with walking speed in a curvilinear fashion for all subjects. Although there were foot type variations, in general, the subjects with transtibial amputation appeared to show elevations in energy cost across all walking speeds compared with the subjects with nonpathological gait. The between-group differences appeared to be more exaggerated with increases in walking speed. However, the Slice comparisons indicated that the SACH and FF between-group differences were not statistically significant at the three lower speeds (53.64, 67.05, and 80.46 m/min) but were significantly different at the two higher speeds (93.87 m/min and 107.28 m/min) (Table 3). The Re-Flex VSP between-group analyses showed nonsignificant differences between the subjects with transtibial amputation and the subjects with nonpathological gait across all walking speeds (Table 3).

As illustrated in Figure 2, gait efficiency (mLO2/kg/m) showed a parabolic concave pattern in all subjects. Optimal gait efficiency (minimum value) occurred at 80.46 m/min for the subjects with nonpathological gait as well as the subjects with transtibial amputation for all foot types. With all foot types, the subjects with transtibial amputation appeared to have less efficient gait (taller bar graphs) compared with the subjects with nonpathological gait. However, again, the Slice comparisons for the SACH and the FF between-group analyses revealed nonsignificant between-group differences at the three lower speeds (53.64, 67.05, and 80.46 m/min) but significant differences at the two higher speeds (93.87 m/min and 107.28 m/min) (Table 4). The Re-Flex VSP between-group statistical analyses revealed nonsignificant differences across all walking speeds.

As illustrated in Figure 3, %APMHR increased with speed in a curvilinear fashion for all subjects with elevated values in the subjects with transtibial amputation and mirrored between-group separation at the three higher walking speeds for all three foot types. As indicated in Table 5, all between-group comparisons were statistically significant with the largest mean differences for the SACH and the FF compared with the control subjects.

Discussion

The effect of prosthesis design on the physiological responses of gait in individuals with transtibial amputation has been a primary research interest; however, published reports have been equivocal.6,9,10,12,19 Lack of standardization of procedures, i.e. test velocity, and between-study variation in subject level of physical activity may be contributing factors to these inconsistent results.

It is claimed that dynamic elastic response feet enhance gait performance through their energy storing and releasing design characteristics and better simulate ankle-foot movement compared with the conventional prosthetic foot such as the SACH foot. In our earlier publication,11 we investigated the between-foot type comparisons in physiological responses (energy cost, gait efficiency, and relative exercise intensity) for the SACH foot and two dynamic feet (the FF and the Re-Flex VSP) in persons with transtibial amputation during multiple-speed walking (53.64-107.28 m/min) and running (120.69-147.51 m/min). The results indicated significantly reduced energy cost and relative exercise intensity and improved gait efficiency for the Re-Flex VSP compared with the FF and the SACH during all walking and running speeds. Increases in ambulation velocity appeared to augment the performance of the Re-Flex VSP. The results were attributed to the enhanced energy storing and releasing properties of the Re-Flex VSP automated spring pylon.

The primary focus of the current study was to analyze the between-group differences in the physiological responses of physically active persons with nonpathological gait versus persons with transtibial amputation. The analyses were based on multiple-speed walking tests with independent comparisons according to prosthetic foot type (SACH, FF, and Re-Flex VSP). The gait of the subjects with transtibial amputation, similarly to that of the subjects with nonpathological gait, showed a positive curvilinear relationship between energy cost and speed that supports the findings of previous studies.1-8 Molen5 reported an average of 20% increased energy cost in persons with transtibial amputation compared with persons with nonpathological gait during walking from 50 m/min to 90 m/min. In the present study, we observed a general speed-dependent elevation trend, but less dramatic between-group (amputee versus normal walking) differences in energy cost. For the FF and the SACH, between-group analyses showed nonsignificant differences at the three slower walking speeds (from 53.64-80.46 m/min), and significant differences at the two faster speeds (93.87 m/min and 107.28 m/min). The Re-Flex VSP between-group differences were nonsignificant across all walking speeds (53.64-107.28 m/min). Ganguli et al. 2 reported a 33% increase in energy cost during walking at 50 m/min in persons with transtibial amputation compared with persons with nonpathological gait. In contrast, the present study found nonsignificant 1.4% (Re-Flex VSP) to 4.9% (SACH) increases in energy cost in the subjects with transtibial amputation compared with the subjects with nonpathological gait at a similar walking speed of 53.64 m/min.

Gait efficiency expressed as energy cost per meter traveled showed similar parabolic response curves for amputee and normal walking subjects. These results agreed with the findings of earlier gait studies.1,6,13 Previous research indicated that the optimal gait efficiency in persons with nonpathological gait usually occurs at S-SWV (83.14 m/min), which is usually at a higher speed compared with transtibial amputee gait with values reported in the literature ranging from 48.28 m/min to 69.73 m/min.1,4,8,20,21 In contrast, in the present study, optimal gait efficiency in the nonpathological gait and in the transtibial amputee gait occurred at approximately the same speed (~80.46 m/min).

A possible explanation for the inconsistencies in energy cost and gait efficiency between previous reports and our present study may be that the subjects in our study were all physically active, with two of the subjects being involved in competitive sports. As reported in Table 1, the Day Activity Scale group mean in the subjects with transtibial amputation was 32 units, with a range from 12 to 45. On the basis of a criterion score of 10, all subjects fell within the high activity category.18 Compared with earlier reports, our subjects with transtibial amputation were more physically active and appeared to be better adept in adjusting to their physical handicap. The between-group differences in energy cost and gait efficiency were subsequently minimized. Particularly at the slower walking speeds, the type of prosthesis appeared to be of minor importance. The Re-Flex VSP appeared to provide greatest accommodation and allowed the subjects with transtibial amputation to keep energy cost and gait efficiency within normal ranges even while walking at the higher speeds.

Age-predicted maximum heart rate (%APMHR) was used as an index of relative exercise intensity. The persons with transtibial amputation and those with nonpathological gait showed a similar positive curvilinear relationship between the %APMHR and speed. However, the %APMHR was higher in persons with transtibial amputation across all functional walking speeds. These results were consistent with the study by Nielsen et al.6 In the current study, for the walking velocities from 53.64 to 107.28 m/min, %APMHR for SACH walking ranged approximately from 47% to 66%; for FF walking from 47% to 65%; and for Re-Flex VSP walking from 46% to 62%, which were all within tolerable physiological limits.6,22 However, the results reported by Nielsen et al. 6 showed that for walking at the same speeds as our study (53.64-107.28 m/min), the %APMHR for SACH walking ranged approximately from 55% to 80% and the %APMHR for FF walking ranged from 52% to 72%.6 In general, our subjects with transtibial amputation appeared to have lower exercise heart rate responses. These differences suggested that our subjects with transtibial amputation had better cardiovascular fitness; this was also confirmed by a relatively high level of physical activity scores.

Compared with our subjects with nonpathological gait, the %APMHR in the physically active subjects with transtibial amputation was still significantly higher across all walking speeds. The loss of muscle mass associated with transtibial amputation may explain the observed elevated relative exercise intensity. Research on arm exercise compared with leg exercise revealed that arm exercise with smaller muscle mass evoked higher heart rate responses.23,24

In summary, the current study showed consistent findings in energy cost, gait efficiency and relative exercise intensity versus speed response curves in persons with transtibial amputation and persons with nonpathological gait compared to previous research. Optimal gait efficiency in our subjects with nonpathological gait occurred at a speed that agreed with previous reports. However, unlike other reports, optimal gait efficiency for the subjects with transtibial amputation occurred at a higher speed that was approximately the same as that seen for the subjects with nonpathological gait. In contrast to other investigations, the results of the present study showed nonsignificant differences of energy cost, and gait efficiency during all walking speeds for the Re-Flex VSP in persons with transtibial amputation compared with persons with nonpathological gait. Nonsignificances were also found for the FF and the SACH at the three lower speeds (53.64, 67.05, and 80.46 m/min). The high level of physical activity in our subjects with transtibial amputation may help explain the discrepancies between the current study and previous reports. It is possible that highly active persons have enhanced motor adeptness, which translates into increased adaptability to physical disability in individuals with transtibial amputation.

Recommendation

The sample size in the present study was small, and additional research is recommended with more subjects to corroborate the findings of the current study. A biomechanical study involving a combined kinetic and kinematic analysis would be necessary to identify the important factors necessary to more definitively explain the results.

Conclusions

In addition to the type of foot design, the level of physical activity appears to be a contributory factor affecting physiological responses in individuals with transtibial amputation and may possibly be one of underlying explanations to equivocal results of gait studies. Based on our empirical findings, foot type appears to be of minor importance for lower-speed ambulation in physically active persons with transtibial amputation. However, the Re-Flex VSP appears to be the prosthesis of choice for high-velocity walking.


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