Chapter 15 - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles
The Energy Expenditure of Amputee Gait
Robert L. Waters, M.D.
Although there is a considerable body of literature on the physiologic energy expenditure of amputee gait, a direct comparison of the results of the different studies is difficult for the following reasons. First, young (usually traumatic) amputees are not consistently distinguished from older (usually vascular) amputees, and there are significant differences between these two groups with respect to gait performance. Second, there is often no distinction between amputees who use upper-limb assistive devices and those who do not. Third, the adequacy of prosthetic fit and prosthetic gait experience is not always specified. Therefore the majority of the data presented in this review are based on investigations that utilized consistent procedures conducted in the Pathokinesiology Laboratory of Rancho Los Amigos Medical Center.
After several minutes of exercise at a constant sub-maximal work load, the rate of oxygen consumption reaches a level sufficient to meet the energy demands of the tissues, and a "steady-state" condition is achieved that reflects the energy expended during the activity.
The functional unit of energy for muscle contraction is adenosine triphosphate (ATP). In aerobic oxidation (citric acid cycle), carbohydrates and fats are oxidized through a series of enzymatic reactions leading to the production of ATP. The net equation for the aerobic metabolism of glucose is as follows:
Glucose + 36ADP + 36Pi + 36H+ + 602—> 6C02 + 36ATP + 42H20,
where ADP is adenosine diphosphate.
A second type of oxidative reaction is available that does not require oxygen. The net equation for the glycolytic metabolism of glucose is as follows:
Glucose + 2Pi + 2ADP —> 2 Lactate + 2ATP.
The lactate is buffered in the blood by bicarbonate, and this leads to the formation of C02, which is exhaled in the expired air; this can be summarized by the following reactions:
Lactate + NaHC03 —> Na lactate + H20 + C02(gas).
Aerobic vs. Anaerobic Metabolism
During continuous exercise there is an interplay between the aerobic and anaerobic metabolic pathways that depends on the exercise work load. During mild or moderate exercise the oxygen supply to the cell and the capacity of aerobic energy-producing mechanisms are usually sufficient to satisfy ATP requirements. During more strenuous exercise both anaerobic and aerobic oxidation processes occur. The serum lactate and expired C02 levels rise, thus reflecting the additional anaerobic activity. The respiratory exchange ratio (RER) is defined as the ratio of expired carbon dioxide (C02) to inspired oxygen (02). When exercise is performed at more strenuous work rates above the anaerobic threshold, the RER rises and reflects the contribution of anaerobic energy production required to meet the additional ATP demands.
The amount of energy that can be produced by anaerobic means is limited. As reflected in the above equations, approximately 19 times more energy is produced by the aerobic oxidation of carbohydrates than by anaerobic oxidation. Anaerobic oxidation is also limited by the individual's tolerance to acidosis resulting from the accumulation of lactate. From a practical standpoint, the anaerobic pathway provides muscle with an immediate supply of energy for sudden and short-term strenuous activity.
If exercise is performed at a constant rate at which the aerobic processes can supply the necessary ATP production, an individual can sustain exercise for a prolonged time without an easily definable point of exhaustion.
Walking, Power and Work Units
The terms power and work are utilized to describe energy expenditure. The power requirement (rate of 02 consumption) is the milliliters of 02 consumed per kilogram body weight per minute (mL/kg-min).
Physiologic work is the amount of energy required to perform a task. Physiologic work (02 cost) during level walking is the amount of oxygen consumed per kilogram body weight per unit distance traveled (mL/ kg-m). The 02 cost is determined by dividing the power requirement (rate of energy expenditure) by the speed of walking. By comparing the energy cost of pathologic gait to the corresponding value for normal walking, it is possible to determine the gait efficiency.
The rate of 02 consumption relates to the level of physical effort, and the 02 cost determines the total energy required to perform the task of walking. In the interpretation of data, it is extremely important to recognize that the velocity is in the denominator and the rate of 02 consumption is in the numerator of the energy cost calculation. Commonly there is a misinterpretation of the clinical data concerning the 02 cost and 02 rate. The 02 cost may be elevated, thus indicating a physiologically inefficient gait, but the rate of 02 uptake may be normal, and the subject will therefore not experience fatigue during customary walking activities but, however, will be limited by the slow speed.
Maximal Aerobic Capacity
The maximal aerobic capacity (V02max) is the highest oxygen uptake an individual can attain during physical work while breathing air at sea level. It is the single best indicator of physical fitness. Generally an individual is able to reach his V02max within 2 to 3 minutes of exhausting work.
Age influences the V02max. Up to approximately 20 years of age, the maximum oxygen uptake increases. Thereafter, the maximum oxygen uptake declines primarily due to a decrease in both maximum heart rate and stroke volume and generally due to a more sedentary life-style. Differences in body composition and hemoglobin content are factors that account for a difference in the V02max between the sexes.
The maximal aerobic capacity also depends on the type of exercise performed. The oxygen demand is directly related to the muscle mass involved; therefore, the V02max during upper-limb exercise is lower than with the lower limbs. For any given work load, however, heart rate and intra-arterial blood pressure are higher in upper-limb exercise than lower-limb exercise.
In the absence of cardiac disease, there is a linear relation between the rate of 02 uptake and heart rate.
When performing exercise at the same rate of 02 uptake, higher heart rates are associated with leg exercise than with arm exercise. The ratio of the rate of 02 uptake to heart rate is the oxygen pulse. The oxygen pulse is higher during arm exercise than during leg exercise.Deconditioning due to sedentary activity or any disease process that impairs the delivery of oxygen to the cells also decreases the oxygen pulse.
A physical-conditioning program can increase the aerobic capacity by several processes: improving cardiac output, increasing the capacity of the cells to extract oxygen from the blood, increasing the level of hemoglobin, and increasing the muscular mass (hypertrophy). As a result, endurance is increased.
A sedentary life-style has the opposite effect on maximum oxygen consumption. Not only does atrophy of peripheral musculoskeletal structures occur, but there is also a central decline in stroke volume and cardiac output as a result of inactivity. Any disease process of the respiratory, cardiovascular, muscular, or metabolic systems that restricts the supply of oxygen to the cell will also decrease the V02max. Bed rest for 3 weeks can result in a 27% decrease in the V02max by decreasing cardiac output, stroke volume, and other factors.
A special problem confronting most older vascular amputees is their limited exercise ability. Physical work capacity and V02max are reduced not only due to the effects of aging but also due to commonly associated diseases in dysvascular amputees such as arteriosclerotic heart disease and peripheral vascular disease. Diabetes decreases capillary permeability and therefore oxygen supply to the muscle due to basement membrane thickening.
The status of physical fitness can be assessed by examining the oxygen pulse, which is the ratio of the mean rate of 02 uptake and heart rate. In different amputee groups walking without crutches, the 02 pulse is lower than normal, which suggests that despite successful prosthetic use, the average amputee leads a less active life-style resulting in a lower level of physical conditioning. This conclusion is supported by the result of V02max measurements during one-legged and two-legged exercise in normal subjects and transfemoral amputees.
Energy Expenditure During Normal Walking
At the chosen walking speed (CWS), the rate of oxygen consumption for young adults aged 20 to 59 years and senior subjects between 60 and 80 years of age does not significantly differ and averages 12.1 and 12.0 mL/ kg-min. Expressed as a percentage of the V02max, the rate of oxygen consumption at the CWS requires approximately 32% of the V02max of an untrained normal subject 20 to 30 years of age and nearly 48% of the V02max of a senior subject 75 years of age. The RER is less than 0.85 for normal subjects of all ages at their CWS, thus indicating anaerobic metabolism is not required.
Senior subjects have a slightly lower rate of oxygen consumption and average CWS than do young adults, and this may be a purposeful effort to keep the exercise within the aerobic range. The fact that walking taxes less than 50% of the V02max in normal subjects in all age groups and does not require anaerobic activity accounts for the perception that walking requires little effort in healthy individuals. It is significant that with advancing years older persons progressively have smaller aerobic reserves to accommodate to the added physiologic penalties imposed by amputation.
Loading the body with weights increases the rate of energy expenditure depending on the location of the loads. Loads placed peripherally on the foot have a much greater effect than do loads placed over the trunk. Placement of a 20-kg load on the trunk of a male subject did not result in a measurable increase in the rate of energy expenditure. On the other hand, a 2-kg load placed on each foot increased the rate of oxygen uptake 30%. This finding is predictible since forward foot acceleration is much greater than trunk acceleration and, therefore, greater effort is required. These findings are of clinical significance for patients requiring lower-limb prostheses and indicate the importance of minimizing weight.
Prosthesis vs. Crutches
Lower-limb amputation with or without prosthetic replacement imposes energy penalties for ambulation. The patient must choose between walking without a prosthesis and using crutches or walking with a prosthesis and utilizing the remaining muscles to substitute for lost function and control the additional mass of the prosthesis.
Crutch walking without a prosthesis with a three-point gait pattern in unilateral amputees may be a primary or secondary means of transportation when an adequate prosthesis is unavailable or inadequate. Swing-through crutch locomotion requires a high rate of physical effort in comparison to normal walking. The arms and shoulder girdle musculature must lift and then swing the entire body weight forward with each step. Swing-through crutch-assisted gait is required in amputees walking without a prosthesis.
A direct comparison of walking in unilateral amputees with and without a prosthesis utilizing a three-point crutch-assisted gait pattern revealed that all, with the single exception of vascular transfemoral amputees, had a lower rate of energy expenditure, heart rate, and 02 cost when using a prosthesis. This difference was insignificant in the vascular transfemoral amputation group and probably relates to the fact that even with a prosthesis, most of these patients relied on crutches for some support, thus increasing the energy demand and heart rate.
It may be concluded that a well-fitted prosthesis that results in a satisfactory gait not requiring crutches significantly reduces the physiologic energy demand. Since crutch walking requires more exertion than walking with a prosthesis does, crutch walking without a prosthesis should not be considered an absolute requirement for prosthetic prescription and training.
Unilateral Prosthetic Ambulation
The combined results of two studies in which patients were tested at their CWSs under similar conditions illustrate the importance of the level of amputation. In the first study, energy expenditure in unilateral amputees was measured at the transtibial, knee disarticulation, and transfemoral levels following amputation secondary to trauma. Patients had also worn their prostheses at least 6 months and did not use upper-limb aids (with the exception of some transfemoral amputees in the vascular group). In the second study, healthy hip disarticulation and transpelvic (hemipel-vectomy) amputees were tested at their CWS by utilizing a similar methodology. These surgical amputees met the following criteria: were young and healthy at the time of testing, had not received radiation or chemotherapy for at least 6 months prior to testing, had no evidence of tumor recurrence, had worn their prosthesis for at least 6 months, and did not utilize crutches.
In the groups of traumatic and surgical amputees described above, the 02 cost progressively increased at each higher amputation level ranging from the transtibial to the transpelvic levels (Fig 15-1.). Patients with higher-level amputations had a less efficent gait and higher 02 cost than did those with lower-level amputations.
The average rate of oxygen consumption for different-level amputees was not dependent on level and was approximately the same as the value for normal subjects (Fig 15-2.) (Table 15-1.).
The CWS depended on the level of amputation and declined at each higher amputation level, progressing from the transtibial, knee disarticulation, transfemoral, hip disarticulation, and transpelvic levels in the traumatic and surgical amputee groups. These values averaged 71, 61, 52, 47, and 40 m/min (Fig 15-3.). These findings indicate that amputees slow their CWS to keep the rate of 02 consumption from rising above normal limits. The reduced speeds at higher amputation levels are inversely proportional to the increased 02 cost.
Other investigators who have tested amputees at their CWS have also reported that the rate of 02 uptake was approximately the same as for normal subjects at their CWS. Clearly as more joints and muscles of the leg are lost due to higher-level amputations, the greater the loss of the normal locomotor mechanisms; therefore, the greater energy cost and slower speed.
Relationship of Mechanical and Physiologic Energy Expenditure
Human locomotion involves smooth advancement of the body through space. While the goal of walking is progression in the forward direction, limb motion is based on the need to maintain a symmetrical, low-amplitude displacement of the center of gravity of the head, arms, and trunk (HAT) in the vertical and lateral directions. This conserves both kinetic and potential energy and is the principle of biological "conservation of energy."
Saunders et al. described six determinants of normal gait that minimize energy expenditure. Ankle motion during stance serves to improve shock absorption and smooth out the points of inflection of vertical rise and fall of the HAT and the consequent vertical ground reaction force. Therefore, as long as ankle stability is provided, the loss of ankle and foot motion has a small effect on mechanical and physiologic energy expenditure. It is not surprising that the physiologic cost of walking with a well-fitted Syme ankle disarticulation or transtib-ial prosthesis increased the energy cost minimally.
On the other hand, knee motion plays a more important role in minimizing the vertical rise and fall of the HAT, and consequently amputation at the transfemoral level substantially increases the energy cost and lowers the speed to keep the rate of energy expenditure from rising above normal limits. Since hip motion also plays an important role in minimizing vertical rise and fall of the HAT, the hip disarticulation or transpelvic-level amputation further increases energy cost and reduces speed.
Dysvascular amputees walking with a prosthesis also selectively adjust their CWS to keep the 02 rate from rising above normal limits. As with traumatic amputees, 02 cost progressively increases, and the CWS progressively slows at higher amputation levels.
The CWS and rate of 02 uptake were significantly higher for the traumatic transtibial and knee disarticulation amputees than for the dysvascular transtibial amputees and ankle disarticulation amputees. (There is no difference in the 02 rate at the transfemoral level due to the fact that some of the dysvascular transfemoral amputees required crutches involving significant upper-limb exercise reflected by the higher mean heart rate.) It is logical to conclude that the higher exercise capacity of the typical younger traumatic amputee enables selection of a higher 02 rate and CWS at any given amputation level than that selected by his older dysvascular counterpart.
Most older patients who have transfemoral amputations for vascular disease are not successful long-term prosthetic ambulators. Only a small percentage of these patients are functional ambulators. If able to walk, most have a very slow gait velocity and an elevated heart rate if crutch assistance is required. In contrast, traumatic transfemoral amputees have an adequate gait. It may be concluded that every effort must be made to protect dysvascular limbs early so that transfemoral amputation does not become necessary. If amputation is required, every effort should be made to amputate below the knee.
Length of the Residual Limb
Gonzales et al. evaluated transtibial amputees with stumps ranging from 14 to 19 cm in length. Patients wore a patellar tendon-bearing prosthesis except for one who had a prosthesis with a thigh corset. No significant differences were noted in speed or energy expenditure between groups. Of particular clinical importance, a stump as short as 9 cm will result in acceptable transtibial performance that is superior to performance at reported values at the knee disarticulation and transfemoral levels.
Few energy expenditure studies have been performed on bilateral amputees. Interpretation of the data on bilateral traumatic amputees must be made with caution since relatively few subjects have been studied. Table 15-2 summarizes data on both traumatic and vascular patients with bilateral amputation. This limited information indicates that the bilateral amputee expends greater effort than the unilateral amputee does. Of interest, vascular patients with the Syme ankle disarticulation/Syme ankle disarticulation combination walked faster and had a lower 02 cost than did vascular patients with the transtibial/transtibial combination. This parallels the findings among the unilateral amputees demonstrating that performance relates to amputation level. Traumatic transtibial/transtibial amputees walked faster and at a lower energy cost than did their vascular transtibial/transtibial counterparts (Table 15-2.).
Gonzales et al. pointed out that in view of the fact that approximately 24% to 35% of diabetic amputees lose the remaining leg within 3 years, it is important to preserve the knee joint even if the stump is short since, should a unilateral transtibial amputee undergo another transtibial amputation, he would still expend 24% less energy than would a patient with a unilateral transfemoral amputation. Bilateral vascular amputees rarely achieve a functional ambulation status if one amputation is at the transfemoral level.
Finally, of special interest, Wainapel et al. measured energy expenditure in a 21-year-old bilateral knee dis-articulation/knee disarticulation patient who walked on stubby prostheses with a walker. The patient walked faster at a slightly greater rate of oxygen consumption than with conventional prostheses and crutches. While walking on stubbies is cosmetically unacceptable for most patients (except for gait training or limited walking in the home), the data from this single patient illustrates that it can result in a functional gait.
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Chapter 15 - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles