Perception of Walking Difficulty by
Below-Knee Amputees Using a
Conventional Foot Versus the Flex-Foot
Pamela A. Macfarlane, Ph.D.
David H. Nielsen, L.P.T., Ph.D.
Donald G. Shurr, L.P.T., CO.
Kenneth Meier, C.P.
Introduction
The body of research comparing gait in
below-knee (BK) amputees using different
prosthetic designs is limited. Few advantages
of one design over another were found when
patellar tendon bearing (PTB) prostheses
with the two most common conventional feet
(CF) were compared (3,4). The only significant finding was that the ankle angle at foot
flat was greater during walking with the single axis foot than it was with the SACH foot.
Recent research on BK prostheses has focused on a newer dynamic "energy-storing"
design called the Flex-Foot (FF). The FF
tends to facilitate faster walking and decrease energy expenditure at similar speeds
compared to a CF (6). Walking with the FF
was associated with an improved range of
ankle motion and a more symmetrical gait
than walking with a SACH foot (7). Subjective Feedback from amputees using the FF
indicated that it improved their balanCF and
stability on uneven ground but that the CF
might be better for slow and downhill walking (6).
To compare the FF with the CF we conducted a comprehensive study which included biomechanical, physiological and walking
difficulty measures while subjects walked on
a motor driven treadmill on three grades and
at three speeds at each grade. The focus of
this paper is limited to the amputee subjects'
assessment of walking difficulty. Other
aspects of the study will be considered
in a subsequent paper. The specific purpose
was to investigate the effects of walking
grade and walking speed on the subject's
perception of walking difficulty with the FF
compared to the CF. We hypothesized that
the subjects would find walking with the
FF significantly easier than walking with the
CF.
While biomechanical and energy-cost
measures are objective and sensitive methods can be used to compare prosthetic gait, a
measure of how easy or difficult the subjects
find using the prostheses over a functional
range of walking conditions is important and
could be a factor to consider when prescribing a prosthetic foot.
MethodDesign
Subjects walked at three walking speeds
over three grades on a motor driven treadmill using the CF and the FF. Self-selected
walking velocity (S-SWV) was determined
according to foot type for each subject during overground walking.
Subjects
The seven paid, volunteer male subjects in
this study were healthy, active, unilateral,
traumatic, BK amputees. Descriptive data
of the subjects are presented in Table I
. Each
subject was identified by a certified prosthetist as being a good ambulator with both FF
and CF. (To be admitted into the study subjects had to have a FF as well as a CF.) Since
we are located in a relatively rural area and
the subjects had to commute to the laboratory, a maximum driving distanCF of 150 miles
was adopted. These criteria limited the availability of subjects. Before testing for this
study each of the subjects was habituated to
treadmill walking at the different speeds
with both prosthetic feet. S-SWV was also
determined during this preliminary session.
Subjects were screened and signed informed written consent forms in accordance
with the Human Subjects Review Committee of the College of Medicine at The University of Iowa. Day's "Assessment and Description of Amputee Activity Score" was
completed with each subject to determine
how active each was on a daily basis (2).
Procedures
Subjects' S-SWV was measured with each
prosthetic foot over a level 15 meter walkway using an electronic timer controlled by
two photoconductive switches which were
five meters apart in the midsection of the
walkway. General procedures followed have
been previously described (6).
The subjects completed two similar motorized treadmill testing sessions, one walking with an FF, the other with a CF. Each
session consisted of three graded exercise
tests (GXTs). The testing order of prosthetic
foot was determined by random selection
without replacement which resulted in three
subjects using the CF first and four subjects
using the FF first. Only one testing session
per day was completed, however, three subjects could only be scheduled for one day
due to their travel and work schedules. A
certified prosthetist considered each of these
subjects to be very active and easily able to
handle both testing sessions on the same day.
These subjects were given at least a three-hour recovery period between testing sessions.
Each testing session consisted of a GXT at
zero grade (level), 30 minutes of seated recovery, a GXT at -8.5 percent grade (decline), 30 minutes of seated recovery and a
third GXT at +8.5 percent grade (incline).
Each GXT consisted of a three-minute accommodation/warm-up period then three-minute stages at slow (2.0 mph 53.6 m/min), medium (2.5 mph 67.0 m/min), then
fast (3.0 mph 80.5 m/min) speeds, followed by a two-minute slow cool down. The
grades and speeds were selected to represent
a functional range of walking conditions attainable by all subjects. The Uniform Building Code (5) recommends public buildings
limit the grade on ramps and walkways to a
maximum of 8.5 percent. Using the S-SWVs
as a guide, the 3 mph (80.5 m/min) was considered to be the fastest speed safely attainable by all subjects using both prosthetic feet
on all grades.
At each stage, after walking for three minutes, the subject was asked to evaluate the
relative ease or difficulty he was experiencing walking under that condition. This was
done by asking the subject to select a number which best represented his ease or difficulty of walking as described on a scale
shown in Figure 1
. The scale was based on
the 20-point Borg rating of perceived exertion (RPE) scale (1) and adapted to reflect
walking difficulty. The Borg RPE scale is a
reliable and valid instrument to obtain subjective feedback from the subjects during exercise testing and has been extensively used
in normal and selected patient groups. The
RPE scale assesses physiological parameters
while the modification of the scale was used
for feedback related to the subjects' biomechanical gait. Validity of the modified scale
was checked by comparing the subjects' test
responses to the interview data collected at
the end of the test. The scale results represented their comments well. Before testing,
subjects were familiarized with the use of
this scale.
During the second testing session, and after the completion of each grade condition
while the subject was resting, he was asked
to verbally respond to a questionnaire with
the researcher recording the responses. Subjects were asked which of the two prosthetic
feet they preferred for walking at each of the
three speeds on the grade they had just completed. Subjects were also asked to explain
reasons for their choices.
Data Analysis
A simple computer procedure was used to
rank the ordinal rating data from the walking
difficulty scale in ascending order (from easiest to most difficult). A four-way analysis of
variance (ANOVA) with subjects and three
factors (foot type, grade, and speed) was
performed on the ranked data. Boferroni
adjusted t-tests were used for follow up analysis. Such analyses using current computer
packages on ranked data are considered
comparable to classic non-parametric techniques (8). A p-value of less than 0.05 was
the criterion selected for statistical significance. As a screening procedure for the
ranked data, the absolute values of the residuals from the four-way ANOVA were tested
for homogeneity of variance and were found
to be nonsignificant (among the subjects
p>0.538; among treatment combinations
p>0. 152).
One subject was unable to walk with his
CF at the fast speed on the incline which
resulted in the omission of this comparison
from the data analysis; however, all other
data were collected. The responses from the
questionnaire on the preferred prosthetic
foot were summed for each grade and speed
condition. No statistical analysis was performed on these data but the results were
qualitatively evaluated.
ResultsWalking Difficulty Scale
Figure 2
represents the group means and
standard deviations under all walking conditions for the results of the responses to the
walking difficulty scale. As illustrated with
each grade and speed condition, walking
with the CF was perceived to be more difficult than walking with the FF. The greatest
differences occurred on the level and incline
grades. The ANOVA summary table (Table
2)
using ranked data identified no significant
interactions between the walking conditions.
The main effects F tests, however, were significant, indicating that the responses to the
walking difficulty scale were significantly affected by the grades, the speeds and by the
type of foot being worn. The follow-up analysis (Table 3)
identifies the following significant results: FF walking is less difficult than
CF walking across all grade and speed conditions; subjects found it easiest to walk on the
level and most difficult to walk on the incline; and walking at the fast speed was significantly more difficult than walking at either the slow or medium speed.
Questionnaire Responses
Under all grade and speed conditions the
subjects overwhelmingly preferred the FF to
the CF. Out of the 63 comparisons (7 subjects x 3 grades x 3 speeds) only one response
favored the CF over the FF - all others favored the FF.
The reasons given for the FF preference
were similar for level and incline walking and
were related to the recovery of the prosthetic
limb while the decline walking apparently
affected the heelstrike of both the normal
and prosthetic limb.
Level and Incline Walking
The dominant reason given by the subjects
for preferring the FF during level and incline
walking was that the FF "helps recover itself" whereas "you have to pull up" the CF.
One subject said "it takes longer to get the
CF out" and another said he felt he had to
decrease his step length during uphill and
fast level walking with the CF because he
couldn't bend his involved knee quickly
enough following a full stride. Other comments during level walking were that the FF
allows for "more controlled, steadier" and
"more stable" walking and is "less tiring"
and "allows one to walk further and faster"
than the CF.
Decline Walking
The comments related to decline walking
suggested that the CF has a tendency to
make the subjects fall forward and downhill,
while the FF "controls speed better." The
"normal leg prefers the FF" was a common
comment with less stress placed on the normal leg at heelstrike. Three of the heavier
subjects momentarily slowed the movement
of the treadmill belt at heelstrike of the normal leg during decline walking with the CF at
the fast speed. No such braking was observed during FF walking under similar conditions.
Two subjects found the prosthesis heelstrike was cushioned with the FF and another said he perceived "shaking up through the
leg" when his CF contacted the surface. The
one subject who favored the CF over the FF
did so only for the fast decline walking speed
where he said the CF cushioned the prosthesis heelstrike more than the FF did. However, when walking at this condition during
testing, this subject rated the CF an 11 (very
easy) and the FF a 9 (very, very easy) on the
walking difficulty scale. It could be that he
did Feel more cushioning with the CF, but
the walking condition was overall easier for
him with the FF. This subject, in contrast to
all the other subjects, chose not to wear a
shoe on his FF, however, he wore a running
shoe while walking with his CF. The cushioning by the running shoe may have assisted
him while wearing the CF and contributed to
his response.
General Comments from Questionnaire
Some overall comments supporting the FF
were related to its method of suspension. All
subjects used supracondylar straps with
waist belts to hold their CF in place while the
FF has no such equipment. Two subjects experienced back problems associated with the
straps while a third had scarred areas in his
hips from the straps which had chaffed him
during a soccer match. Two subjects had
added waist belts to their FF for optional
use-one to allow him to kick a soccer ball
with full force, the other, a farmer, to allow
him to walk through mud. Each of these
subjects had previously dislodged his FF during these activities.
Discussion
The dynamic action of the FF is designed
so that the downward and forward force of
the body mass after heelstrike is absorbed by
the "heel" of the prosthetic foot. As the
body mass passes over the supporting foot,
the FF shaft compresses or bends in a dorsiflexion direction and this is referred to as
energy-storing. Later during stance, as the
prosthesis is unweighted, the fiber glass and
carbon shaft reforms or straightens thus releasing the stored energy which aids in the
recovery of the prosthesis at the beginning of
the swing motion.
These design characteristics of the FF are
intended to accommodate the needs of a
physically active person. Based on Day's Activity classification criteria (2), the mean activity score of + 30.8 that we obtained indicated that the amputees in our study were
highly active and prime candidates for the
use of this type of prosthetic foot. In this
context we hypothesized, based on subjects'
perception of walking difficulty, that our
subjects would find walking with the FF easier than walking with the CF. The findings of
our study corroborated this expectation. The
FF was associated with less difficulty in walking compared to the CF over all nine walking
conditions. The heel absorption characteristics of the FF were described by some of the
subjects during decline walking and suggested by others as the reason they could wear
the FF for longer than the CF during the day
without tiring. The reforming of the FF shaft
could explain the comparative ease with
which the subjects felt they could recover the
prosthetic foot during level and incline walking.
In conclusion, the active BK male amputee subjects in our study favored walking
with the FF over the CF. In explanation they
found that the dynamic action of the FF allowed them to walk with less difficulty over a
range of grades and speeds that could be
encountered in everyday living.
Acknowledgements
This research is supported by a grant from Flex-Foot Inc., Irvine, Calif., and was conducted in the
Cardiopulmonary Research Laboratory of the
Physical Therapy Education Program at The University of Iowa, Iowa City, Iowa. Appreciation is
extended to Jane C. Golden, LPT, Ph.D, for her
assistance during data collection.
Pamela A. Macfarlane, Ph.D., is assistant professor of physical education at Northern Illinois
University, DeKalb, Ill. 60115, and was a Ph.D.
student at The University of Iowa when this study
was conducted.
David H. Nielsen, L.P.T., Ph.D., is associate
professor of physical therapy education at the Division of Associated Medical Sciences/College of
Medicine for The University of Iowa, 2600
Steindler Building, Iowa City, Iowa 52242; (319)
335-9791.
Donald G. Shurr, L.P.T., C.O., is with American Prosthetics, Inc. in Iowa City, Iowa, and an
adjunct lecturer for physical therapy education in
the Division of Associated Medical Sciences/College of Medicine for The University of Iowa, in
Iowa City, Iowa.
Kenneth Meier, C.P., is with American Prosthetics, Inc. in Davenport, Iowa.
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