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Transfemoral Amputee Physiological Requirements: Comparisons Between SACH Foot Walking and Flex-Foot Walking

Pamela A. Macfarlane, PhD
David H. Nielson, PhD, PT
Donald G. Shurr, MA, PT, CPO
Kenneth G. Meier, CPO
Rex Clark, MPT
Janelle Kerns, MPT
Michele Moreno, MPT
Beth Ryan, MPT

ABSTRACT

This study compared exercise intensity, oxygen uptake and gait efficiency when active traumatic transfemoral amputees used a SACH foot or a Flex-Foot" attached below a hydraulic knee joint. The five male subjects completed two test sessions, one with each foot, one week apart. They walked overground at five controlled speeds ranging from 1.5 mph to 3.5 mph (40.2 m/sec to 93.9 m/sec).

For each condition, when subjects had reached physiological steady state, heart rate was recorded by telemetry, and expired gas was collected in a Douglas bag and analyzed in the laboratory. There was no statistical interaction between foot type and speed allowing foot-type comparisons to be made across speed. Results of the measures analysis of variance (ANOVA) were recorded.

The analyses indicate Flex-Foot walking was associated with significantly lower exercise intensity, less energy expenditure and improved gait efficiency compared to the SACH foot. The results of this study indicate the Flex-Foot is physiologically beneficial for active traumatic transfemoral amputees walking overground across a functional range of walking speeds. While the differences are relatively small, they may be clinically important given the high energy cost of transfemoral amputee walking.

Key Words: Energy Expenditure; Above-Knee Amputee; Transfemoral Amputee; Gait Efficiency; Flex-FootŪ; SACH Foot.

Introduction

Effective walking for a person with a transfemoral amputation is characterized by the individual's ability to walk at a reasonable speed within his or her physiological limits using as unimpaired a gait as possible. To accomplish this, the prosthetic limb must facilitate a relatively symmetrical gait with minimal energy expenditure.

Subjects with lower-limb amputations have been found to walk more slowly, using a less-efficient, asymmetrical gait, than individuals without amputation (1,2). More proximal amputations are shown to exacerbate these physiological and mechanical complications (2). Walking dynamics in subjects with transtibial amputation and transfemoral amputation compared to normal subjects previously have been studied, with a considerable amount of research emphasis on the effects of different prosthetic lower limbs. Recently, a focus of research has been to compare dynamic elastic response feet (3), sometimes referred to as energy-storing feet, to conventional prosthetic feet.

Walking gait mechanics affect physiological needs and vice versa. For example, gait limitations may necessitate a very slow, self-selected walking velocity, which is associated with a low gait efficiency (1). Having a limited energy capacity contributes to a lower self-selected walking velocity (4,5). Amputees tend to reduce their walking velocity in an attempt to keep their rate of energy expenditure within normal limits (5). It therefore is suggested that physiological and mechanical variables both be included in any comprehensive study of impaired gait.

This article presents the physiological responses of a group of active subjects with traumatic transfemoral amputations walking overground across a functional range of walking speeds alternatively using a SACH foot^a or a Flex-FootŪ^b. The mechanical gait analysis from the same study is presented in a separate article immediately following this article. (6). It was hypothesized that the relative exercise intensity, energy cost and gait efficiency of transfemoral amputees would be better with the Flex-Foot compared to the SACH foot; regardless of foot type, it was hypothesized these factors would deviate from normal in a similar manner as seen in transtibial amputee gait but to a greater extent.

Review of the Literature

The solid-ankle, cushioned heel (SACH) foot historically has been the most common prosthetic foot used in the United States (7). It is composed of a rigid longitudinal keel with a solid ankle. A wedge of polyurethane foam provides cushioning in the heel section, with hyperextension of the rubber toe section possible during late stance (7). Dynamic elastic response feet are designed to deform during heel contact and midstance and rebound during late stance to simulate the "pushoff" characteristics of a normal ankle (8). Anticipated benefits of the dynamic design compared to a conventional foot for transfemoral and transtibial amputees include a reduced exercise intensity and energy cost, improved walking efficiency and a more symmetrical gait at matched speeds (8,9).

The Flex-Foot is a lightweight dynamic elastic response foot that has a relatively long pylon (>23 cm from floor to socket) bolted to a knee unit and running continuously through to the distal toe section. It is designed for very active unilateral or bilateral transtibial amputees to foster springy walking, running and jumping (10) but may be used by all lower-limb amputees (11). Active transtibial amputees preferred the Flex-Foot to the conventional prosthesis while walking on level ground, inclines and declines over a functional range of speeds (12) as well as walking over uneven ground, ascending and descending stairs, and running (13). Overall, compared to the non-energy-storing designs, dynamic elastic response foot designs do appear to be beneficial for active transtibial amputees. It is not known if transfemoral amputees might similarly benefit from dynamic elastic response foot designs.

The energy requirement of walking with a transfemoral prosthesis far exceeds that of normal ambulation (4). The asymmetry of the transfemoral amputee gait increases the excursion of the trunk during each gait cycle, thereby increasing the cost of ambulation (14). Mechanical work increases with an increase in vertical displacement of the center of mass of the body (15). In transtibial amputee gait the vertical trunk displacement correlated highly (r > 0.94) with oxygen uptake (16). In this study excessive lowering of the trunk during weight acceptance by the uninvolved leg was identified as the cause of the increased trunk displacement. Constant knee extension during stance also would increase the vertical trunk displacement. Due to the absence of quadriceps contribution in knee control of the involved leg, transfemoral amputees are forced to maintain extension or hyperextension during stance; otherwise, the artificial knee joint would collapse into flexion when weight was accepted (17). These gait abnormalities could help explain the increased oxygen uptake found in transfemoral amputee walking compared to normal walking.

Compared to normal subjects walking at their own self-selected walking velocity, healthy, traumatic transfemoral and transtibial amputees tend to use about the same amounts of energy but walk at a slower speed (18). As disability limitations increase, energy efficiency decreases (18). Using data combined from a number of studies focusing on transtibial amputees, through-knee amputees, transfemoral amputees, hip disarticulation patients and hemipelvectomy subjects, Waters and Yakura (2) found that as the level of amputation moves proximally, the subjects' average self-selected walking velocity decreases while the rate of oxygen consumption remains at about the value for normal subjects (12.04 mlO₂?kg^-1?min^-1). The slower, self-selected walking velocity is a measure of efficiency because the energy expended per minute results in less work accomplished. Thus, the oxygen uptake per meter traveled is a more sensitive measure than the oxygen uptake per unit time and is the best way to compare gait efficiency at different speeds and between populations (5).

Gait efficiency values, expressed as mlO₂?kg^-1?m^-1 at self-selected walking velocity have been reported as 0.15 for normal subjects, 0.21 for traumatic transtibial amputees and 0.27 for traumatic transfemoral amputees (1,5,14, 19). Note an increased mlO₂?kg^-1?m^-1 value means more energy is needed to accomplish the same work, indicating a less efficient gait. In comparison to normal walking at self-selected velocity, the gait efficiency values were higher in unilateral transtibial amputees (9 percent), unilateral transfemoral amputees (49 percent) and bilateral transfemoral amputees (280 percent) (20). Jaegers et al. (21) found that at the most efficient walking speeds, normal subjects (at 78 m?min^-1) and transfemoral amputees (at 57 m?min^-1) had similar energy needs resulting in a 25-percent higher energy expenditure per meter traveled for the transfemoral amputee. According to Jaegers et al., the self-selected walking velocity also was the most efficient speed for the normal subjects, but the transfemoral group's self-selected walking velocity (50 m?sec^-1) was slower than their most efficient speed.

Saunders et al. (22) identified the following gait characteristics that minimize energy expenditure in normal gait: pelvic rotation, pelvic tilt, knee flexion during stance to minimize the vertical displacement of the trunk; a smooth knee and ankle interaction during stance that minimizes ground reaction forces; and a limited lateral shift of the trunk during the gait cycle. Compared to normal subjects, transfemoral amputee gait appears to compromise each of these determinants.

Methods

Design

The independent variables used in this study were type of prosthetic foot and walking velocity. A repeated measures design was used to compare physiological responses during SACH foot or Flex-Foot walking over a functional range of speeds, which included 40.2, 53.6, 67.1, 80.5 and 93.9 m?min^-1 (1.5, 2.0, 2.5, 3.0 and 3.5 mph, respectively).

The physiological dependent variables were relative exercise intensity, energy cost and gait efficiency measured while the subjects were walking at physiological steady state at each of the five selected speeds. Relative exercise intensity was measured as the percent of age-predicted maximum heart rate (% APMHR) using (220 - age) to predict maximum heart rate (MHR). Energy cost was measured as the steady state oxygen uptake per kilogram body weight per minute (mlO₂?kg^-1?min^-1), and gait efficiency was measured as the energy cost divided by walking speed (mlO₂?kg^-1?min^-1).

Subjects

The five male subjects in this study were paid volunteers who had midfemoral unilateral traumatic amputations and were considered by a certified prosthetist to be good walkers with both the SACH foot and the Flex-Foot. Only data from subjects who had midfemoral or more distal amputations were included for analysis in this study because subjects with more proximal amputations adopted different basic gait mechanics compared to those with more distal amputations. They shifted their upper body more laterally over the prosthetic leg during prosthetic stance and appeared to depend a lot more on upper-body movements for balance and prosthetic leg swing than subjects with more distal amputations. Table A shows descriptive statistics of the subjects who participated in the study.

The same certified prosthetist fitted all subjects with a prosthesis designed to accept both the SACH foot and the Flex-Foot below the SNS (swing and stance) knee. This procedure eliminated "standard" prosthesis differences in socket fit and prosthetic knee joint design. The subject pool was limited to active subjects who could walk continuously for at least four minutes across a functional range of speeds.

Since testing was conducted in a relatively rural area and subjects were required to commute to the testing sessions, a maximum driving distance of 150 miles was adopted when identifying possible subjects; this limited the subject pool. Women were not systematically excluded from the study; however, no female who fit the subject selection criteria was found. Men experience a higher frequency of accidents resulting in amputation, which generally contributes to a limited pool of female traumatic transfemoral amputees (23).

Heart rate was monitored by ECG radiotelemetry. The system consisted of three small chest electrodes placed in a modified VM5 pattern, a miniature radio transmitter worn on a belt around the subject's waist, an FM receiver and a standard single-channel electrocardiograph recorder connected to a digital cardiotachometer. Oxygen uptake was measured using the open circuit methods previously described (24). This method involves the timed volume collection of air expired into a Douglas bag with subsequent analysis using a semi-automated, on-line computer system.

Procedures

The preliminary session and the first testing session were completed on the first day in the laboratory. The second testing session was held one week later. Testing order relative to foot type was balanced across subjects, with two wearing the SACH foot first and three wearing the Flex-Foot first. To ensure acclimation to the prosthetic foot for testing purposes, subjects wore that prosthesis continuously for the week prior to testing. Subjects were instructed to wear the same footwear and to keep the same hydraulic unit settings throughout the testing period.

During the preliminary session, subjects signed informed written consent forms in compliance with the Human Subjects Review Committee of the College of Medicine. Subjects practiced walking at the testing speeds under simulated, but shorter, test conditions. They then were required to remain quietly in the laboratory for one hour prior to the first testing session.

During the two testing sessions, data were collected at the five walking speeds in order of increasing speed while the subject walked around a 50-m indoor course. The course consisted of straight sides and semicircular ends, which allowed the subjects to maintain their walking speed. Walking speed was regulated by an investigator who carried a calibrated speedometer cane while walking next to the subject (25). At each speed, subjects walked continuously for three minutes to achieve physiological steady state then continued walking for between one and three minutes until a timed volume of about 30 liters of their expired air was collected in a Douglas bag carried by the accompanying investigator.

Heart rate was recorded at the beginning and end of the gas collection to verify physiological steady state had been achieved and maintained. The volume and gas concentrations (O2 and CO2) of the exhaled air were measured for oxygen uptake determinations immediately following the collection period for each walking speed. Subjects were allowed to take a short break between walking speeds if necessary. At the completion of each testing session, the subject's prosthesis (including footwear) was weighed on a calibrated autopsy scale.

Data Analysis

Descriptive statistics were calculated on the dependent variables. A two-way repeated measures analysis of variance (ANOVA) was used to test for main effects on walking speed and type of prosthetic foot as well as for any interaction between speed and foot type. To decrease the chance of type I errors associated with the testing of multiple dependent variables, a Bonferroni adjustment was made to an alpha level of 0.05, resulting in the adoption of a p value of < 0.017 to determine statistical significance (26).

Results

One subject could not walk at 3.5 mph using his SACH foot; so this speed for this subject was excluded from the comparisons. Figure 1 , Figure 2 , and Figure 3 represent the means and standard errors at each walking speed for the exercise intensity, energy cost and gait efficiency variables, respectively. These figures include results from related studies from the literature on transtibial amputee and normal subject walking for comparative purposes only.

As indicated in Figure 1 , the exercise intensity increased systematically with speed, with the Flex-Foot condition associated with a lower mean exercise intensity for all speeds compared to the SACH foot condition. At matched speeds the transfemoral amputees' relative exercise intensity was about 10 percent higher than that of transtibial amputees and about 20 percent higher than that of normal subjects.

The energy cost results in Figure 2 follow the trend for relative exercise intensity as would be expected due to the established linear relationship between oxygen uptake and heart rate during submaximal aerobic exercise. The Flex-Foot condition showed a lower oxygen demand than the SACH foot condition. Compared to transtibial amputees and normal subjects, transfemoral amputees use more energy to walk at similar speeds regardless of foot type.

Figure 3 indicates the least efficient gait with both types of prosthetic foot was at the slowest speed; the most efficient gait occurred at the mid-speeds. At each speed the mean walking efficiency was better (lower value) with the Flex-Foot than with the SACH foot. Regardless of foot type, transfemoral amputee gait efficiency in this study was associated with a less-efficient gait compared with the transtibial amputees and normal subjects from the literature.

There was no statistically significant interaction between foot type and walking speed for any of the variables; this condition allowed foot-type comparisons to be made across all speeds. The ANOVA results for the foot-type comparisons for the three physiological variables are shown in Table B , with the three variables following similar trends. Across the five speeds the main effect of foot type was statistically significant for all three variables. Follow-up analyses showed the Flex-Foot was associated with a lower exercise intensity, less energy expenditure and a more efficient gait compared to the SACH foot.

Discussion

Subjects walking with a transfemoral prosthesis require a greater exercise intensity and more energy and are less efficient than subjects walking with a transtibial prosthesis or subjects without amputation. The large physiological demands of transfemoral amputee walking compared to transtibial amputee or normal subject walking have been attributed to the adoption of an asymmetrical gait with mechanical gait parameters including self-selected walking velocity, stride length and duration of the gait cycle two standard deviations below normal subject walking and one standard deviation below transtibial amputee walking (14). With such impairment any small improvement in the walking ability could be important.

In the present study, walking with a Flex-Foot attached below the artificial knee resulted in a lower exercise intensity, lower energy costs and a more efficient gait across a functional range of speeds compared to using a SACH foot. While the differences were small, they were statistically significant. Previous research had shown active transtibial amputees benefit physiologically (1) from using a Flex-Foot instead of a conventional foot; the present study suggests the Flex-Foot similarly benefits transfemoral amputees.

In transtibial amputee walking, gait asymmetry was found to increase the energy cost of ambulation (14), and the Flex-Foot has been shown to increase gait symmetry compared to the conventional prosthetic foot (9), suggesting that in transtibial amputees the physiological benefits of the Flex-Foot may be related to gait symmetry improvement. To explain the physiological benefits to the transfemoral amputees in this study when using a Flex-Foot instead of a SACH foot, a gait analysis comparing Flex-Foot to SACH foot walking mechanics was conducted and is presented in the next article in this issue (6).

Acknowledgements

This research study was partially supported by a grant from Flex-Foot Inc. in Irvine, Calif., and the United States Manufacturing Co. in Pasadena, Calif. The research was conducted in the Cardiopulmonary Research Laboratory of the Physical Therapy Graduate Program at the University of Iowa, Iowa City, Iowa. The authors would like to thank the subjects for volunteering for this study and for participating with such willing enthusiasm.

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