Attaining efficient, upright locomotion marks a milestone in the development of an amputee.30 Persons with acquired locomotor dysfunction, such as a lower extremity amputation, spend significant time and effort attempting to regain their lost walking proficiency. For some of these individuals ambulation is difficult and may not be feasible or even practical. Important factors cited for failure to ambulate are the relative high exercise intensity required and associated energy cost. Even though the impairment may prevent completely normal walking, with appropriate treatment intervention, most lower extremity amputees can still achieve an efficient gait within the limits of their disability. For optimum gait efficiency, it is imperative that prosthetic devices keep energy expenditure to a minimum.
The gait of non-amputee subjects has been extensively studied by means of motion and force analysis, as well as energy cost techniques.5,13 Comprehensive descriptive and analytical data concerning normal gait have been obtained. Gait in several categories of disabled subjects has also been studied, though less completely.11,17
Results from these studies indicate that an amputee walking with a leg prosthesis consumes more energy than a non-amputee at comparable walking velocities.12,14,23,32 R.L. Waters indicated that the increased energy cost was a function of the level of amputation.34 Until the development of an energy storing design, the type of prosthetic foot assumed minor concern. The recent introduction of the Flex-Foot a dynamic foot prosthesis, appears to offer some advantages to the conventional prosthetic foot. Wagner's biomechanical analysis revealed improved ankle range of motion and gait symmetry for the Flex-Foot in contrast to the SACH foot.33
Research is currently limited; no information is available concerning differences in energy cost or efficiency of ambulation between these two types of prosthetic feet. The purpose of this study was to investigate differences in self-selected walking velocity, relative exercise intensity, oxygen consumption, and gait efficiency in below-knee amputees during ambulation with the Flex-Foot versus the conventional prosthetic foot.
Walking velocity was repeatedly measured in each subject for graded walking speeds on a motor driven treadmill for the Flex-Foot and conventional prosthetic SACH foot. Selfselected walking velocity was also measured on each individual.
An admissions criterion was that the subject have a Flex-Foot as well as a conventional prosthetic foot. Since we are located in a relatively rural area and the subjects were required to commute daily to and from the test site, a maximum driving distance of 150 miles was adopted. Both of these criteria obviously limited the availability of subjects.
This research is an on-going study, and we hope to expand the number of participants. To date, self-selected walking velocity data has been collected on seven subjects, including the graded test protocol on three subjects. Within this context, our results provide descriptive information. With the inclusion of more subjects, the data will be subjected to comparative statistical analysis.
The subjects of this study were healthy adult males with unilateral traumatic below-knee amputations (mean age = 26.7 Þ7. 1 years, mean weight = 172.7 Þ33.0 pounds). All subjects had the Flex-Foot and a conventional prosthetic foot and were proficient walkers with both types of prostheses. In accordance with the Human Subjects Review Committee of the College of Medicine at The University of Iowa, informed written consent was obtained from each subject prior to participation in the study.
Three one-hour sessions (one orientation and two test periods) on separate days were required of each subject. The initial session was spent completing paperwork, measuring the self-selected walking velocity, and practicing on the treadmill. In the following two sessions, the subjects were tested on the graded speed treadmill protocol using the Flex-Foot and conventional prosthetic foot on alternate days. The testing order was randomized according to the type of prosthesis.
A 15 meter long segmental walkway was used for the self-selected walking velocity measurements. Measurements were taken with an electronic timer with portable lights and photoconductive switches. Five repeated time measurements were taken over the mid five meter section of the walkway at the end of five minutes of self-selected, steady walking with each prosthetic foot. The average of the five time measurements was used to calculate self-selected walking velocity.
In order to standardize walking velocity and to facilitate measurement procedures, the actual walking tests were performed on the treadmill. A progressive graded testing protocol with three to five minutes of walking at each of seven walking velocities (l.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0 mph) was adopted. Heart rate and oxygen uptake measurements were taken at the end of each testing stage. The heart rate measurements were used to caculate %MHR (relative workload of walking expressed as a percentage of age-predicted maximum heart rate), which was our criterion measure of relative exercise intensity. The oxygen uptake values were used for the calculation of gait efficiency (ml 02/Kg*m). Heart rate was monitored by ECG radiotelemetry. The system consisted of three small disposable chest electrodes, a miniature radiotransmitter worn on a belt around the subject's waist, a remote FM receiver, and a standard single channel electrocardiograph recorder which was connected to a digital cardiotachometer. Oxygen uptake was determined by the open-circuit method with a semiautomated online computer system. The method involved timed collection, volume measurement, and electronic gas analysis of the subject's expired air. A printer connected to the computer provided typed summary tables of the oxygen uptake results.
Table 1 presents the results of the self-selected walking velocity tests. Included are the values for our sample of seven subjects as well as for the smaller group of three subjects. For comparison, the data reported by Waters are also included.34 As the table indicates, the Flex-Foot values compared to the conventional foot were higher for both groups of our subjects with mean percent increases of 9% and 7% respectively.
As illustrated, the energy cost of ambulation increased systematically with increases in walking velocity (Figure 1) . In all cases, the oxygen uptake values for our three amputees were higher compared to non-amputees.26 Energy cost differences related to type of prosthetic foot were minimal at the low speeds. However, at walking speeds of 2.5 mph and above, the energy cost of walking with the conventional foot was higher.
As was expected, changes in %MHR (our criterion measure of relative exercise intensity) mirrored the energy cost responses (Figure 2) . Exercise intensity increased systematically with increases in walking velocity. Again, prosthetic foot differences were greatest at the higher walking speeds with a maximum 35% difference at 4.0 mph.
As shown in Figure 3 , gait efficiency for amputee walking paralleled the response curve for non-amputee subjects.26 In all cases, the individual values of energy cost per meter were upwardly displaced for amputee walking. Little difference in gait efficiency was observed between the two types of prosthetic feet at the slower speeds. For speeds equal to and above 2.5 mph, the values for the Flex-Foot were generally lower.
As described in the company's product literature, the Flex-Foot is a dynamic prosthesis designed to store and release energy during the normal course of locomotion.31 Accordingly, the fiberglass and carbon (graphite) pylon compresses during heel contact and extends during heel-off, increasing forward momentum during toe-off. The company maintains that these energy absorption and releasing features should make walking and running with the Flex-Foot easier and, theoretically, should result in reduced energy consumption.
In support of these claims, Wagner reported increased ankle range of motion and generally improved walking biomechanics for the FlexFoot compared to the SACH foot.33 Based on our search of the literature, no definitive information is available concerning the energy cost of ambulation with the Flex-Foot.
Several investigations have shown that people spontaneously self-select an optimally efficient walking speed referred to as the selfselected or free-paced walking velocity.1,17,18,23,24,25,37 The energy cost per meter traveled is higher for speeds slower or faster than the self-selected walking velocity (Figure 3) . For non-amputees, the most efficient average self-selected walking velocity is approximately 80 meters/minute (3 mph) with a range from 74 to 83 m/min (2.8-3.1 mph).11,26 Persons with abnormal gait usually walk slower, but also tend to select the most optimally efficient walking speeds.11'34 However, this optimal speed may not be possible if the total energy cost and relative exercise intensity are excessive.
Wagner indicated that self-selected walking velocity for both Flex-Foot and SACH foot ambulation was below normal values.33 This was in agreement with our findings as well as other reports on below-knee amputee walking.27,34 The 71.4 meters/minute (2.7 mph) value for ambulation with the conventional prosthetic foot for our sample of seven subjects was essentially identical to the self-selected velocity reported by Waters, 71 meters/minute (2.6 mph).34 For our group of three subjects who were quite active and physically fit individuals, the value was higher; self-selected walking velocity for the conventional foot was 80.5 meters/minute (3.0 mph).
In contrast to Wagner, who reported no differences between the prosthetic feet, our subjects produced higher self-selected walking velocities using the Flex-Foot compared to the conventional prosthetic foot.33 Since Wagner did not report any numerical values, no specific comparisons could be made. Variations in testing protocols may help explain the inconsistency. Wagner's velocity measurements were determined from repeated individual, motion analysis walking trials. Our measurements were based on continuous steady state five minute walking tests for which we have previously established within session and between session measurement reliability.28 Subject differences in physical fitness status could have been another contributing factor.
Research on the energy cost of walking with non-amputee subjects in our lab, as well as other studies, has shown a curvilinear increase in oxygen uptake with increases in walking speed.2,8,26 One could speculate that the altered biomechanics in amputee walking would produce corresponding changes in gait efficiency and subsequent elevations in energy cost. Accordingly, oxygen uptake for ambulation in our below-knee amputees was higher than normal, as found in other studies involving amputee walking.12,14,34 Increases in walking velocity tended to augment these differences. Expressed as a percentage above normal values, the elevations ranged from 48% at 1.0 mph to 61% at 4.0 mph.
Of particular interest in the present study were the lower energy cost values observed at the higher walking velocities for Flex-Foot ambulation compared to conventional foot walking. The largest decrease was 2.5 ml 02/kg*min corresponding to a 10% difference occurring at 4.0 mph. These results suggest that the energy storing-releasing design characteristics of the Flex-Foot were of negligible consequence at slow walking speeds, but at speeds equal to and above 2.5 mph walking performance was enhanced.
The relative exercise intensity of gait is the relative workload of walking which can be expressed as a percentage of the person's age-predicted maximum heart rate (%MAP):
or as a percentage of the individual's maximum aerobic power (%MAP):
Relative exercise intensity (%MHR and %MAP) has been used to evaluate gait performance in various patient groups.1,17,23,28,34 It has been stated that ambulation may be too demanding for some disabled individuals.3,7 The general guideline is that the relative exercise intensity for ambulation at the self-selected walking velocity should not exceed 70% MAP or 80% MHR values.17,22 For practical reasons, we did not want to do maximal graded exercise tests on our subjects; we elected to use %MHR as the criterion measure of relative exercise intensity.
The range in %MHR for our amputee subjects was from 48% at 1.0 mph to 80% at 4.0 mph. The average %MHR value at the subject's self-selected walking velocity was approximately 65%. These results suggest that the stress associated with below-knee amputee walking was well within tolerable physiological limits. The reduced %MHR values for ambulation with the Flex-Foot, which occurred at all walking velocities, indicated decreased levels of stress.
Efficiency is technically defined as the ratio of the work output to the work input or the ratio of the power output to the power input:20
Numerous approaches to studying gait efficiency have been described in the literature, but there is little agreement about which approach is best.4,10,18,21,25,35 Part of the problem is related to inconsistency and inadequate definition of terminology. The complexity of the motor task and the inherent difficulty of objectively measuring the energy component of walking are confounding.
The energy output during ambulation can be simply expressed as the product of the person's body weight times the vertical displacement of the body's center of gravity. The energy input is reflected by the energy cost of walking.35
However, displacement of the body's center of gravity during walking is difficult to measure, usually requiring sophisticated cinematographical measurement systems, e.g. high speed l6mm cameras or special videotaping-motion analyzers.6
A more easily measured, alternative criterion of gait efficiency is the term we refer to as "distance efficiency." Gait (distance) efficiency is the energy cost per distance traveled. It is calculated simply from the ratio of the oxygen uptake to the walking velocity and may be expressed in milliliters of oxygen consumed per kilogram of body weight per meter traveled:
In this context, a decrease in calculated gait efficiency reflects improved overall work/exercise efficiency. Although the specific term "distance efficiency" has had limited employment in the literature, the concept behind the term has been used by numerous authors who studied gait.1,8,9,17,19,27,34,36
The gait efficiency graphs that we obtained on our three amputee subjects appear to be quite reasonable. The response curves were a little erratic, probably due to the small sample size. The profile of the graphs was the same for the amputee and non-amputee subjects. The upward displacement of both amputee curves indicated decreased efficiency for amputee compared to normal walking. The separation of the amputee curves suggested improved gait efficiency for Flex-Foot ambulation at speeds of 2.5 mph and above.
Optimal efficiency, i.e., the minimal energy cost per meter traveled, for all three curves occurred at approximately the same speed (3.0 mph), which interestingly corresponded to the self-selected walking velocity of both amputee and normal groups. The optimal gait efficiency value for ambulation with the conventional foot was .24 ml 02/kg*m at 80 meters/minute (3.0 mph) which was higher than the value Waters reported (.20 ml 02/kg*m at 71 meters/min (2.6 mph) for his traumatic below knee amputee subjects).34 Interestingly, the optimal value for the Flex-Foot was .21 ml 02/kg*m at 85.8 meters/minute (3.2 mph). These results suggest that the Flex-Foot accommodates faster walking velocities without compromising gait efficiency.
Subjective feedback from the subjects supports these findings. The comments in general were positive regarding their use of the Flex-Foot. The most common responses were that the Flex-Foot allowed faster walking and improved general balance and stability while walking on uneven ground. Ambulation with the conventional foot was possibly better at very slow walking speeds and during downhill walking. Based on these observations, future research could include the effect of uphill and downhill grade walking.
The present study focused on the gait performance of traumatic below-knee amputees during walking with the Flex-Foot versus a conventional prosthetic foot over a functional range of walking velocities from 1.0 to 4.0 mph. Although the results are based on only a small number of subjects, several conclusions appeared to be warranted:
Ambulation with the Flex-Foot tended to facilitate faster walking approximating more normal values of self-selected walking velocity.
Few differences between types of prosthetic foot were seen in gait performance for slow walking velocities (<=2 mph).
Ambulation with the Flex-Foot at higher walking velocities (>=2.5 mph) tended to conserve energy, resulting in lower relative levels of exercise intensity and enhanced gait efficiency.