Comparison of Energy Cost and Gait Efficiency During Ambulation in Below-Knee Amputees Using Different Prosthetic Feet - A Preliminary Report
David H. Nielsen, L.P.T., Ph.D.
Donald G. Shurr, L.P.T., C.O.
Jane C. Golden, L.P.T., M.S.
Kenneth Meier, C.P.
Introduction
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.
Method
Design
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.
Subjects
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.
Procedures
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.
Results
Self-Selected Walking Velocity
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.
Energy Costs
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.
Relative Exercise Intensity
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.
Gait (Distance) Efficiency
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.
Discussion
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.
Summary
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.
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