Mechanical Gait Analysis of Transfemoral Amputees: SACH Foot Versus the Flex-Foot
Janet S. Dufek, PHD
John A. Mercer, MS
Cheryl Sherwood Kosta, PT
Deanna J. Fish, MS, CPO
ABSTRACT
The purpose of this study was to evaluate the metabolic efficiency of walking with and without a rigid rotational-control ankle-foot orthosis. Heart rate and Borg's rating of perceived exertion also were evaluated to investigate their relationships to physiological system responses (VO2).
Six volunteer patients walked on a treadmill in unbraced and braced conditions while VO2, heart rate and the rating of perceived exertion were evaluated. During the braced condition, subjects reduced their energy expenditure by more than 5 percent and reduced their ratings of perceived exertion. Heart rate responses also were lower while braced. High correlations were observed between VO2 and heart rate (r = 0.884) and VO2 and rating of perceived exertion
(r = 0.930), suggesting their usefulness as clinically feasible measures of exertion.
Key Words: Ankle-Foot Orthosis; Heart Rate; Metabolic Cost; Walking.
Introduction
Much controversy exists in the lower-extremity orthotic community as to the trade-offs between the added mass of a rigid lower-extremity ankle-foot orthosis (AFO) and the support and/or correction such a device can afford the patient. Conventional thermoplastic designs incorporate decreased weight and increased flexibility as primary features with more emphasis on functional deficits and less emphasis on structural alignment or stability. Mechanically, adding mass to any system (increasing weight) will require greater energy to perform work, i.e., to move the body/system. This fact is couched in the Newtonian relationship of Work = Force 2 Distance. In itself, this is a simple and straightforward argument against using a heavier support system for correction of lower-limb dysfunctions.
Opinions also seem to differ about the physiological and psychological consequences of using a heavier lower-limb orthosis. Energy-cost assessments have shown walking and running with weighted shoes and boots require greater energy than walking and running in nonweighted shoes for healthy subjects (1). Yet, anecdotally, many patients describe their feelings of confidence and ability to better ambulate with use of a heavier, more rigid lower-extremity orthosis versus conventional thermoplastic designs. Kottke and Lehmann (2) classify this feeling of confidence as an increased safety factor and further characterize this feature as desirable for proper orthotic outcome. Hodges (3) documented reduced energy cost for a single patient and his ability to ambulate farther with use of double upright AFOs versus polypropylene AFOs, which were 20 ounces lighter. These investigators (2,3) raise the issue of possible ambulation and stability advantages afforded by a heavier orthosis.
One type of AFO that is heavier than many conventional designs is a laminated, rigid, rotational-control model. Unlike many conventional designs, this type of intervention is designed to affect triplanar control of the midtarsal, subtalar and talocrual joint alignments. Conventional designs often add less mass to the human system than the rigid, rotational-control design but affect angular changes primarily in only two (sagittal and coronal) versus three planes of motion. Efficacy of design can be evaluated by evaluating work done.
To quantify mechanical work, one must consider both components of the formula. Distance can be easily observed, quantified and/or evaluated, e.g., how far can the patient walk before becoming fatigued? Force, on the other hand, is less readily observable; while force can be measured externally, the internal factors associated with its development also must be considered. System physiology is at the root of muscular force development. In a simplified form, one can consider the rate of oxygen utilization as a measure of system efficiency and ultimately the ability and/or limits of muscular force production. Thus, metabolic efficiency sets the limits of an individual's physical work capacity. In this way, one can relate the notions of physical work and internal system physiology to equate to movement potential.
To investigate the question of orthosis weight and ambulation abilities, the relationship between oxygen utilization and physical limits was embraced. The purpose of the study was to evaluate the metabolic efficiency of walking with and without a rigid rotational-control AFO for patients previously prescribed an AFO. In addition, clinically feasible measures of heart rate and Borg's rating of perceived exertion (RPE) were evaluated to investigate their relationships to physiological system responses.
Methods
Six volunteer subjects who were patients at North Lake Physical Therapy Clinic in Lake Oswego, Ore., participated in this investigation (see Table A)
. Subjects were identified by their respective physical therapists on the basis of their experience and ability to walk independently on a treadmill. Gait abnormalities and lower-extremity joint deviations exhibited by each subject were corrected through the use of a rigid, rotational-control AFO manufactured by Oregon Orthotic System Inc. and had been previously prescribed and fitted by a certified orthotist. Each patient was judged by his or her respective therapist as having obtained moderate to maximum levels of both functional and structural orthosis intervention outcome.
Prior to testing, the test protocol was explained to the subjects, and they were given the opportunity to ask questions. Also at this time, subjects were instructed in how to use the RPE 20-point scale (4). This self-report scale associates a word with a numeric value; for example, 7 = very, very light while 19 = very, very hard (see Table B)
. Additionally, each subject gave written consent to participate and was given a period of time to walk on the treadmill to acclimate prior to testing.
The test protocol consisted of each subject walking at a self-selected speed on a treadmill (Cybex Model Q31). Condition 1 (C1) required subjects to walk without the use of their AFO; condition 2 (C2) required subjects to walk while wearing their AFO. Prior to walking, each subject sat quietly while resting oxygen consumption (VO2), and heart rate data were recorded over a period of at least four minutes (see Figure 1)
. Metabolic data were measured using the TEEM 100 Metabolic Analysis System (5) and recorded every 20 seconds. All data were collected continuously throughout the duration of the test.
After the initial resting measurements were completed, each subject walked on the treadmill at a self-selected pace that could be maintained for approximately five minutes. The walk was terminated when/if the exercise was considered hard (15 on the Borg RPE 20-point scale), if there was evidence of fatigue or if the subject walked for a period of 10 minutes. After termination of the walk, the subject was allowed as much time as required to rest while recovering from the walk. The subject was considered recovered when VO2 was within 0.5 ml/kg/min and his or her heart rate was within five beats per minute (bpm) of the initial resting state for at least three minutes. Rest periods ranged from four to 10 minutes for all subjects. After the rest period, subjects were required to walk at the same pace for the next condition, which was followed by another period of rest. During the second condition, subjects were asked to walk at least as long as the first condition. Recent research has shown gait mechanics do not change with treadmill support (6); therefore, during the walks, subjects were allowed to use the handrails for balance if desired.
The authors hypothesized the unbraced walking condition would produce the slower walking speed so this condition was performed first for all subjects except S1 and S3. S3 was not able to walk without the aid of the AFO and therefore did not perform the C1 condition. S1 completed both conditions with C2 preceding C1 due to a personal time constraint. During the second walk (C1), S1 could not maintain the pace that was selected while wearing the AFO, and, therefore, speed was slower during C1 than C2. Because of the unique protocols, the data for these subjects (S1 and S3) were not included in the group analysis but were examined individually.
Data Analysis
Data initially were tabulated graphically across the entire test phase (at 20-second increments) for each subject. Areas of graphic plateaus were interpreted as physiological steady-state responses. This procedure, producing variable steady-state time periods for individual subjects, was incorporated due to the heterogeneous physiological nature of the participating subjects.
Mean data across each steady-state phase were computed for VO2, heart rate and RPE. The average value was used for descriptive and statistical comparisons. Group comparisons were conducted with repeated measures analysis of variance techniques. Single-subject statistical comparisons were conducted using the model statistic single subject technique (7). All statistical tests were conducted at the a = 0.05 level of significance.
Results and Discussion
Group results (S2, S4, S5 and S6) for heart rate and VO2 are shown in Figure 2
, and RPE data by subject are summarized in Figure 3
. With the added weight of the AFO on the lower extremity, it would seem logical from an energy perspective that the metabolic cost during walking would increase (1). However, this was not the case when the average VO2 and heart-rate responses were compared during walking with and without the aid of the AFO (see Figure 2)
. At equivalent walking speeds, the average group VO2 during C2 (braced) was 5.5 percent lower than C1 (unbraced) walking despite subjects wearing AFOs with masses between 0.7 kg and 1.8 kg. Decreased VO2, a measure of metabolic efficiency, would indicate the gait pattern was more efficient with the use of the AFO.
RPE also was significantly lower during C2 (13 vs 11). Although RPE is a self-reported measure and therefore subjective, it has been shown to be a valid and reliable scale of physical exertion (4). Therefore, lower RPE values can confidently be associated with lesser physically (and emotionally) stressful situations. Individual subject RPE responses are given in Figure 3
.
Heart rate during both conditions for the group were similar. Specifically, heart rate while wearing the AFO was equal or lower for S1-S5 but higher for S6. Unique subject responses as well as a desire to learn more about specific patient responses led to individual examination of the data obtained from each subject to determine the effect of wearing the AFO during walking on VO2, heart rate and RPE. These data are summarized in Table C
.
S1 was the only subject who walked with the AFO prior to walking without it. The speed S1 selected during C2 (with the AFO) was 0.8 mph. The subject was not able to tolerate the same speed during C1. Therefore, during C1, the speed was lowered to the subject's desired pace, 0.5 mph. Despite the increased speed during C2, average heart rate was similar during C2 and C1. VO2 was greater during C2 due to the increased cost of walking at a faster rate. RPE, however, was lower during C2.
S2 was able to tolerate only five minutes of walking during C1 but walked the maximal duration, 10 minutes, of the test protocol during C2. The first five minutes during C2 were used in the group analysis to compare both conditions. During C2, the VO2 response was 4.6 percent lower while heart rate was almost 12 percent higher than C1. However, during the last five minutes of walking with the AFO, both VO2 and heart rate were significantly lower than C1 (VO2 = 4.5 ml/kg/min; HR = 74).
S3 was not able to walk on the treadmill without the AFO. However, with the AFO, she tolerated 4:20 of walking at 0.5 mph. The VO2 and heart rate responses during C2 were similar to those of the other subjects.
S4 walked for 5:20 for both conditions. During C2, VO2 was 10 percent lower than C1. Likewise, heart rate was 7.4 percent lower during C2 than C1.
S5 exhibited results similar to S4. During C2, VO2 was 6.2 percent lower and HR 8.3 percent lower than during C1; RPE also was reduced during C2.
S6 required bilateral lower-extremity support and therefore had an extra 1.8 kg of system mass to accommodate. Despite the additional weight, the average VO2 was identical between conditions, and the RPE was lower during C2 than during C1. In contrast, heart rate was higher during C2 than during C1. The increased heart rate could possibly be caused by fatigue toward the end of the test. The functional capacity of the subject was very low, and, even though the subject met the criteria to be considered "rested" prior to the second walk, this subject could have benefited from a longer rest period.
To investigate the relationship between critical dependent variables, Pearson's product-moment correlation coefficients (8,9) were computed. Excluding VO2 data obtained from S1 (too low for the measurement unit to detect), a strong relationship (r = 0.884, accounting for 72-percent explained variance) was obtained between VO2 and heart rate during C2. In addition, a correlation coefficient of r = 0.930
(> 86-percent explained variance) was obtained between VO2 and RPE during C2. These results suggest both heart rate and RPE might be clinically feasible measures to evaluate metabolic efficiency of patient gait performance with use of an AFO.
The question of why many of the patients evaluated in this study were able to walk (at the same speed) at a level of perceived as well as actual reduced energy cost in an orthosis (versus without an orthosis) can be explained from a mechanical perspective. In addition to the stability and control the rigid AFO provides, the particular orthosis used in this study also addressed rotational control of the lower extremity. By correcting the angular deviations of the limb (structural components) as well as functional needs such as plantarflexion assist, the orthosis fosters the patient's ability to reduce out-of-plane motions that are not particularly helpful when attempting to translate (walk). Motions that occur out of the primary plane of progression, i.e., coronal and transverse deviations, are expensive metabolically and do not assist in the goal of moving the body in the sagittal plane. This structural correction may be the key to the reduced energy costs observed in the current study.
Conclusion
The metabolic changes that occur due to the use of an AFO highlight the importance of the role of the clinician in dealing with this population of subjects. An exercise prescription with the goal of increasing a patient's functional status must take into account the lower VO2 response to walking with the use of an AFO. In the current study, all subjects perceived their exertion level to be decreased during walking with the AFO compared to walking without the AFO. However, heart rate may or may not be different between conditions. Therefore, it is the responsibility of the clinician to determine the best plan of action for the patient.
The effectiveness of walking with an ankle-foot orthosis was demonstrated; the subjects evaluated in this study walked at a lower perceived effort with a corresponding lower or equivalent metabolic response compared to walking without the AFO. The results of this preliminary investigation suggest the method of prescribing exercise intensity should take into account the metabolic changes that occur as well as the mechanical changes due to the use of any AFO. Finally, both heart rate and RPE are suggested as valid clinical measures to assess metabolic efficiency of patient gait with use of an ankle-foot orthosis.
References:
- Nielsen DH, Shurr DG, Golden JC, Meier K. Comparison of energy cost and gait efficiency during ambulation in below-knee amputees using different prosthetic feet-a preliminary report. JPO 1989;1:24-31.
- Waters RL, Yakura JS. The energy expenditure of normal and pathological gait. Phys & Rehab Med 1989;1:3:183-209.
- Michael J. The energy-storing feet: a clinical comparison. Clin Pros Orth 1987;11:154-68.
- Traugh GH, Corcoran PJ, Reyes RL. Energy expenditure of ambulation in patients with above-knee amputations. Arch Phys Med & Rehab 1975;56:67-71.
- Waters RL, Perry J, Antonelli D, Hislop H. Energy cost of walking of amputees: the influence of level of amputation. JBJS 1976;58:1:42-6.
- Jorgensen A, Siler WL, Norris RA. Grasping the treadmill handles does not alter walking style. Med & Sci in Sports and Exercise 1996;28:5:S88.
- Bates BT, Dufek JS, Davis HP. The effect of trial size on statistical power. Med & Sci in Sports and Exercise 1992;24:9:1059-68.
- McCall RB. Fundamental statistics for behavioral sciences. Orlando: Harcourt Brace & Co., 1994.
- Derrick TR, Bates BT, Dufek JS. Evaluation of time-series data sets using the Pearson product-moment correlation coefficient. Med & Sci in Sports and Exercise 1994;26:7:919-28.
|
|
