https://orcid.org/0000-0002-2249-4641
Figures
Abstract
Introduction
Metabolic assessment of prosthetic gait is useful when comparing devices, interventions, or populations. However, the standard requirement to walk continuously for six minutes or more to reach steady state (SS) is difficult for many individuals with lower limb amputation. Our goal was to assess the concurrent validity of metabolic outcomes from shorter duration walking tests with those from the standard six-minute walk, in persons with transfemoral or transtibial amputation.
Methods
Thirty participants (amputation: 10 transfemoral, 10 transtibial, 10 none) performed three walking tests while data were collected with a wearable metabolic system: 1) two-minute treadmill walk plus 10-minute recovery, 2) six-minute treadmill walk, and 3) overground two-minute walk test (2MWT). Three different analyses were performed to correlate SS metabolic outcomes from minutes 5-6 of the six-minute treadmill walk with: 1) total oxygen uptake from the two-minute treadmill walk, incorporating excess post-exercise oxygen consumption (EPOC), 2) minute interval outcomes from minutes 1-4 of the six-minute treadmill walk, and 3) outcomes during minutes 1 and 2 of the 2MWT.
Results
Strong correlations were found between total oxygen uptake of the two-minute treadmill walk plus EPOC and SS oxygen uptake (Pearson r 0.86 to 0.94). Likewise, there were strong correlations between minute interval outcomes of minutes 2, 3, and 4 of the six-minute treadmill walk and SS outcomes (Pearson r 0.82 to > 0.99). Fewer significant correlations were observed when comparing 2MWT outcomes with SS outcomes (Pearson r 0.41 to 0.78).
Conclusion
Strong correlations between metabolic outcomes of shorter duration walking tests with SS outcomes suggest that treadmill walking tests as short as two minutes may be acceptable to compare energy expenditure between conditions in individuals with lower limb amputation for circumstances where longer duration tests would not be possible. Additionally, these shorter tests would be more similar to real-life activities and more accessible for those with lower limb amputation.
Citation: Ikeda AJ, Smit JA, Simon AM, Anarwala SJ, Hargrove LJ (2025) Measuring metabolic energy expenditure with short duration walking tests for individuals with lower limb amputation. PLoS ONE 20(3): e0320384. https://doi.org/10.1371/journal.pone.0320384
Editor: Anne E. Martin,, Pennsylvania State University, UNITED STATES OF AMERICA
Received: July 29, 2024; Accepted: February 17, 2025; Published: March 31, 2025
Copyright: © 2025 Ikeda et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the paper and its Supporting Information files.
Funding: Funding for this study was provided by the National Institute on Disability, Independent Living, and Rehabilitation Research (NIDILRR TEAMM RERC 90REGE0003, awarded to LJH). NIDILRR is a Center within the Administration for Community Living (ACL), Department of Health and Human Services (HHS). The contents of this article do not necessarily represent the policy of NIDILRR, ACL, or HHS, and you should not assume endorsement by the Federal Government. Funding was also provided by Eunice Kennedy Shriver National Institute of Child Health and Human Development (R01 HD079428 and R01 HD108554, awarded to LJH) and the MSL Renewed Hope Foundation (grant awarded to LJH). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Introduction
The assessment of energy expenditure is useful in clinical and research settings to establish the energetic requirements of a given task. This can be helpful when comparing devices, populations, interventions, or tracking progress over time. Measuring breath-by-breath gas exchange during a constant speed treadmill walk of six minutes or more is the most common method for assessing steady state (SS) energy expenditure of gait [1]. The person undergoing the test typically wears a mask or similar device over their nose and mouth to measure gas exchange for the duration of the test. It is assumed that a steady rate of metabolism is reached in the first approximately four minutes, and data are typically averaged over the last two or three minutes [1].
However, for many individuals with lower limb amputation the requirement to walk for six continuous minutes is difficult or even impossible to complete. Moreover, a continuous walk of six minutes without stopping or pausing is not a typical real-world activity, even for community ambulators. This was shown by a two-week study of adults with no mobility limitations which found that 93.3% of all walking bouts were less than 2 minutes in duration and 96.8% were under 3 minutes [2]. For individuals with lower limb amputation, the large majority of walking bouts were found to be of 1 or 2 minutes in duration with only 17 steps/min taken [3]. Clearly, a walking test shorter than six minutes would be more typical of real-world ambulation and would be more achievable for individuals with mobility limitations.
Although SS metabolic values may not be attained with shorter duration tests, several recent studies involving individuals with amputation have found it useful to assess metabolic outcomes from shorter ambulation activities. These activities included two minutes of sloped walking and two minutes of stair climbing [4], ascent and descent of three flights of stairs [5], and a standard two-minute walk test (2MWT) [6]. Due to the short duration of these tests, it is likely that SS was not reached, nor did it appear to be a goal of these studies.
The problem with shorter duration tests is that they may not allow a person to reach a steady rate of metabolism. However, for individuals without mobility limitations, previous investigations have estimated or predicted SS metabolic values from intervals shorter than six minutes. One group proposed utilizing a total oxygen uptake method which includes excess post-exercise oxygen consumption (EPOC) data in analysis of one and two minute walking bouts [7]. This method showed strong correlations between outcomes of one- and two-minute tests with SS outcomes in 12 young healthy adults. In another recent study of 21 young healthy adults, the slope of net power and net oxygen consumption over time were analyzed to determine that a steady rate of metabolism could be reached by the two minute mark [8]. The authors found the net cost of transport curve for walking at 8 different speeds on a treadmill was not significantly different for minutes 2-4 compared to minutes 4-7. Another analysis of metabolic cost in 10 adults without musculoskeletal or cardiopulmonary impairments, found that subjects reached 95% of SS values in an average of 2 minutes, and 9 of 10 reached 95% within 3 minutes [9]. Although all of these studies were conducted with individuals without mobility limitations, the results give reason to believe that there may be potential for using shorter duration tests for those with lower limb amputation as well.
Another obstacle in conducting metabolic tests with people with amputations is that testing is often done on a treadmill to ensure a constant speed. For many individuals with amputation, treadmill walking is not a common activity and unfamiliarity itself may influence results. Furthermore, studies have shown that individuals with amputation tend to prefer a slower walking speed on the treadmill compared to the floor, which effects metabolic results [10–12]. A recent systematic review concluded that the slow walking speed of individuals with lower limb amputation is a key factor in their elevated energy cost of walking compared to those without amputation [1]. An overground walking test is therefore desirable for this population.
However, one challenge in collecting SS metabolic data from overground walking is maintaining a constant pace. If this constant pace requirement is waived, a standard overground 2MWT is an appealing option. The 2MWT is widely accepted and commonly used in both clinical and research settings for various populations. For individuals with lower limb amputation, the 2MWT has established normative values [13], sound psychometric properties [14–17] and is highly predictive of the six minute walk test in distance walked [17,18]. In a recent study, researchers measured oxygen consumption and step count during a 2MWT for 35 individuals with unilateral or bilateral transtibial or transfemoral amputation and did not find a difference between amputation levels [6]. The authors acknowledged that participants might not have reached SS. Still, a 2MWT is an attractive option as a commonly used overground walking test, which could potentially also be used to assess metabolic energy expenditure with a short time burden in a clinical setting.
Although previous investigations have compared shorter walking test results to SS results, these studies only included participants without mobility limitations. A shorter duration walking test for individuals with amputation would be useful in situations where six minutes of continuous walking is not possible. Our goal was to assess the concurrent validity of metabolic outcomes from shorter walking tests on the treadmill and overground, with SS metabolic outcomes, for persons with transfemoral or transtibial amputation. We hypothesized that outcomes would have strong correlations when comparing short walking tests with the SS test.
Materials and methods
Participants
Individuals with transfemoral amputation (TF group), transtibial amputation (TT group), or no amputation (NoAmp group) were recruited to participate between July 14, 2022 and December 7, 2022. Inclusion criteria for the TF and TT groups were: age 18-95, unilateral TF or TT amputation, able to ambulate with a prosthesis on level ground, ramps, and stairs, and able to speak English. For the NoAmp group, inclusion criteria were: age 18-70, no lower limb amputation or injury, and able to speak English. For all groups, exclusion criteria included significant cognitive impairment or comorbidity that would interfere with safe completion of the study. All participants provided written informed consent and the study protocol was approved by the Northwestern University IRB (IRB# STU00209522).
Procedures
Participants were requested to fast from food and drink other than water starting at least 5 hours prior to their visit and avoid intense exercise for at least 24 hours prior. All study procedures were completed in a single visit. Participants in the TT and TF groups wore their own prescribed prosthesis for the full protocol.
Participants initially walked on a treadmill to select their preferred walking speed (PWS). They were instructed to choose a comfortable speed that was representative of their typical walking speed, and which they could maintain continuously for six minutes. As they walked on the treadmill, blinded to the speed display, a study team member increased or decreased the speed based on the participant’s feedback until the participant chose their PWS. Each participant’s PWS was held constant for each of their treadmill tests. A COSMED K5 wearable metabolic system (COSMED, Rome, Italy) was used to measure oxygen consumed and carbon dioxide expelled during all tests. The K5 unit was paired to a Garmin chest strap heart rate (HR) monitor. For two participants in the TF group the chest strap HR monitor did not work and instead a Garmin Forerunner 735XT watch was used to record HR.
Each participant completed three walking tests in the same order: 1) a two-minute treadmill walk plus 10-minute recovery period (2mintread), 2) a six-minute treadmill walk (6mintread), and 3) a standard overground 2MWT. Immediately before the first test, participants were given at least seven minutes of seated rest next to the treadmill. For both treadmill tests the participant walked on the treadmill as it was brought up to their pre-determined PWS; when it reached their PWS the test began. For the 2mintread test, participants walked on the treadmill for two minutes. Once the treadmill stopped, a chair was placed on the treadmill for participants to use for 10 minutes of seated recovery, during which time metabolic data were continued to be collected. After the 10-minute recovery period, the chair was removed and participants completed the 6mintread test, walking on the treadmill for six minutes. This was followed by 10 minutes of seated rest. The last test took place in a straight, unobstructed hallway where tape marks had been placed on the floor marking a 40 m distance. After walking to the hallway, participants were given at least seven minutes of seated rest. They then completed the 2MWT, walking as far as possible in two minutes in the hallway, turning around each time they reached the 40 m tape marks. Total distance walked was recorded with a measuring wheel. This was followed by 10 minutes of seated rest.
Analysis
Data were exported from the COSMED Omnia (Version 1.16.10.0) software. Three analyses were performed to compare shorter duration tests with SS: 1) total oxygen uptake analysis, 2) minute interval analysis, and 3) 2MWT analysis.
Total oxygen uptake.
A total oxygen uptake analysis was performed for the 2mintread plus 10-minute recovery according to the methods of Blokland et al. [7]. Resting O2 consumption and the end of EPOC were determined from the 10-minute recovery period. Net O2 consumption was found by subtracting the resting value. Net total oxygen uptake (net VO2tot) was calculated by integrating net O2 consumption from the start of the test to the end of EPOC, and dividing by 2 minutes. Net VO2tot was compared to net SS oxygen uptake (net VO2ss) from minutes 5 and 6 of the 6mintread test.
Minute intervals.
For the second analysis, the following metabolic measures were calculated for each minute interval of the 6mintread: O2 consumption in ml O2/kg/min, O2 cost in ml O2/kg/m, metabolic power (calculated according to the Brockway equation [19]) in W/kg, and a dimensionless cost of transport (COT) calculated by normalizing power to walking speed. For each of these measures, SS values were calculated from minutes 5 and 6 of the 6mintread test and compared to values from minutes 1, 2, 3, and 4 of the 6mintread test.
2MWT.
For the third analysis, metabolic outcomes were compared between the gross SS values from minutes 5 and 6 of the 6mintread test and gross values of minutes 1 and 2 of the 2MWT.
Statistical analysis.
For each of the above analyses, a Pearson correlation coefficient (r) was calculated to assess concurrent validity of metabolic results from each test with the SS result for each group and all groups combined. The Pearson r indicates the strength of the linear relationship between two variables. The p-value of the Pearson r was used to test the null hypothesis that there was no correlation. Statistical significance was set to p < 0.05.
Results
Participants
A total of 30 participants completed the protocol (Table 1). One participant in the TT group and two in the TF group did not fully follow the fasting guidelines; all others abstained from food and drink for at least five hours prior to testing. No participants reported strenuous exercise in the previous 24 hours. All participants in the TT and TF groups had amputations due to non-dysvascular causes including trauma, sarcoma, infection, and surgery complications.
VO2 during the 6mintread was analyzed to assess whether steady state, indicated by VO2 variability between minutes 5 and 6 of < 10%, had been reached. Analysis showed that for one participant in the TF group VO2 varied by 13.7% and for one participant in the NoAmp group VO2 varied by 10.7%. All other participants showed < 10% variability between minutes 5 and 6, indicating they had reached steady state. However, to take the most conservative approach without introducing bias, and because steady state is commonly collected during a preset time interval, we did not exclude any participants’ data from analyses. The S1 Data file indicates the two participants who did not reach the steady state criterion. From here forward, all results presented include the full dataset.
Total oxygen uptake analysis
Each participant’s treadmill PWS was kept constant for the 2mintread test and 6mintread test. Group averages were: TF 0.83 ± 0.30 m/s (mean ± SD), TT 0.84 ± 0.25 m/s, and NoAmp 1.21 ± 0.16 m/s. Statistically significant correlations between net VO2tot and netVO2ss were found for each group and all participants combined (Table 2, Fig 1).
a) Representative net VO2 data from one participant in the TF group. Raw (dotted) and filtered (solid) VO2 with baseline subtracted to obtain net values and determine the end of EPOC. b) Correlation of net VO2tot with net VO2ss for all participants combined.
Minute interval analysis
For the minute interval analysis, correlations of SS with nearly all metabolic measures and all minute intervals (1-4) were strong and statistically significant (Table 3a, Fig 2).
a) Pearson r values for O2 cost correlations between minutes 1-4 and SS, for each group and all participants combined, b) O2 cost correlation between minute 2 and SS, for all participants combined.
2MWT analysis
All participants completed the 2MWT without stopping. Distances walked were: TF group 153.4 ± 38.5 m, TT group 188.1 ± 33.6 m, and NoAmp group 226.0 ± 15.6 m. Analysis of 2MWT metabolic outcomes resulted in fewer significant correlations with SS outcomes, which were also weaker, as compared to the total oxygen uptake or minute interval analyses (Table 3b, Fig 3).
Gross O2 cost correlation between minute 2 and SS, for all participants combined.
Discussion
The goal of this study was to assess the concurrent validity of metabolic outcomes from shorter walking tests with standard SS metabolic outcomes for people with lower limb amputation. We hypothesized that there would be strong correlations between results from shorter tests and SS results. A total oxygen uptake analysis of the 2mintread test plus recovery had very strong correlations with SS, as did a minute interval analysis of minutes 2, 3, and 4 of the 6mintread test, for all groups. However, 2MWT metabolic results were less highly correlated, or not significantly correlated, with SS.
The total oxygen uptake analysis of the 2mintread test including EPOC data produced results similar to Blokland et al. [7] who found that this method was highly correlative with SS in individuals with no mobility limitations. They reported correlations of r2 = 0.87 and 0.88. For the present study, the Pearson correlation coefficient for the NoAmp group was r = 0.94, or r2 = 0.88. These values were slightly lower for the TF (r = 0.90, r2 = 0.82) and TT (r = 0.86, r2 = 0.74) groups, but still considered strong correlations. The use of this method with individuals with lower limb amputation is promising and warrants further investigation.
Results of the minute interval analysis suggest that a constant speed treadmill walk of 4, 3, or even 2 minutes may be potential alternatives to a 6-minute walk. Strong correlations were observed in all groups between minutes 4, 3, and 2 of the 6mintread test and SS for O2 consumption, O2 cost, and HR. The use of HR as an outcome measure also has the advantage of not requiring the participant to wear a restrictive mask. SS values for O2 consumption and O2 cost were within the ranges of recent meta-analyses of studies of individuals with TF, TT, or no amputation [1,20]. These results are encouraging for clinicians and researchers who work with individuals with lower limb amputation who may not be able to walk six continuous minutes.
The range of PWSs for the TF group was quite large (0.31 – 1.25 m/sec) and likely affected the results. This is highlighted by the large standard deviations in O2 cost, since O2 cost is derived by dividing O2 volume by walking speed. Given that the metabolic cost of walking is known to be U-shaped with a minimum near one’s natural self-selected walking speed [21,22], participants who selected a very slow PWS on the treadmill likely walked at a speed much slower than their natural self-selected speed. Therefore, some participants’ PWSs were probably slower than the speed that would result in a minimum metabolic cost, while others’ PWSs may have been near this point. Unfamiliarity with treadmill walking and uncertainty in their ability to maintain a set pace for six minutes likely contributed to the choice of a very slow PWS for some. Previous studies have shown the energy cost of walking for individuals with TF amputation to be higher for treadmill walking as compared to overground, but the walking speeds in those studies were different between treadmill and overground [10,11]. One study which analyzed the same speeds on treadmill and overground found that the walking surface itself did not influence energy expenditure for either control subjects or those with TF amputations [12]. The fact that individuals with lower limb amputation choose to walk slower on a treadmill however does influence energy expenditure.
The limited number of significant correlations between metabolic results of the 2MWT and SS may be in part explained by the effect of different levels of exertion between the two tests. Although the two tests were meant to be completed at different speeds, the amount of change in level of exertion between PWS on a treadmill and walking as far as you can overground likely varied considerably by participant. Because this exertion change is difficult to accurately quantify or control from person to person, metabolic results from a standard 2MWT are more difficult to contextualize. While the 2MWT may appear to be an attractive option since it is a standardized test, commonly used clinically, and includes overground (as opposed to treadmill) walking, metabolic measurements from this test were generally not highly correlated with SS.
This study did not aim to predict SS values, as it is likely that a steady rate of metabolism is not reached in durations as short as two minutes. In a study which analyzed a range of speeds and multiple intervals within a 7-minute walk, a minimum of 4 minutes was required to obtain a net COT curve that was not statistically different than the SS curve [8]. Use of the total oxygen uptake method with a two minute walk has been shown to overestimate SS values in a population with no mobility limitations [7], as well as the present study. Although correlations of many measures with SS were high, outcomes from shorter tests should not be directly compared to SS outcomes. Nevertheless, a highly correlative outcome from a two-minute test may be a good alternative for comparing relative differences for individuals who cannot walk a continuous six minutes.
There were several limitations of this study related to the selection of participants and methodology. First, there was a limited number of subjects in each group (n = 10 per group). Also, participants in each group were not matched by age, gender, or body mass. Participants in the TT and TF groups were not representative of the population of persons with amputation in the US at large [23,24], limiting external validity. Only 2 participants were over age 65 and all causes of amputation were non-dysvascular. However, the target population who would most benefit from a shorter walking test would not have been able to complete the current protocol, and therefore this first step in evaluating these methods with a more active population was necessary. This study included only one constant treadmill speed for each participant, and repeating tests at additional speeds may have been beneficial as metabolic response is known to be speed dependent.
Continued investigation into measurement of energy expenditure for individuals with lower limb amputation could lead to the development of more appropriate protocols and analysis techniques. Improving the ecological validity of metabolic assessments is necessary to obtain clinically meaningful outcomes [20,25], and shortening the duration of the test can play an important role. Further validation of short duration constant speed treadmill walking tests would be beneficial, as the very strong correlations with SS in this study show these tests’ potential. Although the overground 2MWT was not highly correlative with SS in this study, future research incorporating overground walking at a comfortable self-selected constant speed is still warranted. Walking overground as opposed to on a treadmill would be more typical of real-life activities and would improve the ecological validity of the test.
Conclusion
The strong correlations of O2 consumption, O2 cost, and HR from minutes 2, 3, and 4 of the minute interval analysis of the 6mintread test with SS indicate that a treadmill walking test of only 2 minutes may be suitable for assessing relative differences between devices or interventions. The incorporation of EPOC data for the total oxygen uptake analysis of two minutes of walking also showed strong correlations, making it a viable alternative analysis as well. Although they produce results highly correlative with SS, results from these shorter duration tests should not be directly compared with longer duration SS tests. Demonstrating very high concurrent validity with SS outcomes, two-minute walking tests could be a good option for assessing energy expenditure for individuals with lower limb amputation when six minutes of continuous walking may not be achievable. Utilization of shorter duration tests would lessen the burden of metabolic assessments and make them more feasible for individuals with amputation.
Supporting information
S1 Data. Metabolic outcomes for all participants for all analysis methods.
https://doi.org/10.1371/journal.pone.0320384.s001
(XLSX)
References
- 1. Ettema S, Kal E, Houdijk H. General estimates of the energy cost of walking in people with different levels and causes of lower-limb amputation: a systematic review and meta-analysis. Prosthet Orthot Int. 2021;45(5):417–27. pmid:34538817
- 2. Orendurff MS, Schoen JA, Bernatz GC, Segal AD, Klute GK. How humans walk: bout duration, steps per bout, and rest duration. J Rehabil Res Dev. 2008;45(7):1077–89. pmid:19165696
- 3. Klute GK, Berge JS, Orendurff MS, Williams RM, Czerniecki JM. Prosthetic intervention effects on activity of lower-extremity amputees. Arch Phys Med Rehabil. 2006;87(5):717–22. pmid:16635636
- 4. Lathouwers E, Ampe T, Díaz MA, Meeusen R, De Pauw K. Evaluation of an articulated passive ankle-foot prosthesis. Biomed Eng Online. 2022;21(1):28. pmid:35477464
- 5. Ledoux ED, Goldfarb M. Control and evaluation of a powered transfemoral prosthesis for stair ascent. IEEE Trans Neural Syst Rehabil Eng. 2017;25(7):917–24. pmid:28113346
- 6. Smith JD, Guerra G. Quantifying step count and oxygen consumption with portable technology during the 2-min walk test in people with lower limb amputation. Sensors (Basel). 2021;21(6):2080. pmid:33809581
- 7. Blokland IJ, de Koning JJ, van Kan T, van Bennekom CAM, van Dieen JH, Houdijk H. Estimation of metabolic energy expenditure during short walking bouts. Int J Sports Med. 2021;42(12):1098–104. pmid:33862639
- 8. Adeyeri B, Thomas S, Arellano C. A simple method reveals minimum time required to quantify steady-rate metabolism and net cost of transport for human walking. J Exp Biol. 2022;225(15):jeb244471. https://doi.org/10.1242/jeb.244471 PMID: 35796105
- 9. Selinger JC, Donelan JM. Estimating instantaneous energetic cost during non-steady-state gait. J Appl Physiol (1985). 2014;117(11):1406–15. pmid:25257873
- 10. Delussu AS, Brunelli S, Paradisi F, Iosa M, Pellegrini R, Zenardi D, et al. Assessment of the effects of carbon fiber and bionic foot during overground and treadmill walking in transtibial amputees. Gait Posture. 2013;38(4):876–82. pmid:23702342
- 11. Traballesi M, Porcacchia P, Averna T, Brunelli S. Energy cost of walking measurements in subjects with lower limb amputations: a comparison study between floor and treadmill test. Gait Posture. 2008;27(1):70–5. pmid:17360186
- 12. Starholm IM, Mirtaheri P, Kapetanovic N, Versto T, Skyttemyr G, Westby FT, et al. Energy expenditure of transfemoral amputees during floor and treadmill walking with different speeds. Prosthet Orthot Int. 2016;40(3):336–42. pmid:26450911
- 13. Gaunaurd I, Kristal A, Horn A, Krueger C, Muro O, Rosenberg A, et al. The utility of the 2-minute walk test as a measure of mobility in people with lower limb amputation. Arch Phys Med Rehabil. 2020;101(7):1183–9. pmid:32272105
- 14. Balk EM, Gazula A, Markozannes G, Kimmel HJ, Saldanha IJ, Trikalinos TA, et al. Psychometric properties of functional, ambulatory, and quality of life instruments in lower limb amputees: a systematic review. Arch Phys Med Rehabil. 2019;100(12):2354–70. pmid:30986410
- 15. Brooks D, Hunter JP, Parsons J, Livsey E, Quirt J, Devlin M. Reliability of the two-minute walk test in individuals with transtibial amputation. Arch Phys Med Rehabil. 2002;83(11):1562–5. pmid:12422326
- 16. Brooks D, Parsons J, Hunter JP, Devlin M, Walker J. The 2-minute walk test as a measure of functional improvement in persons with lower limb amputation. Arch Phys Med Rehabil. 2001;82(10):1478–83. pmid:11588757
- 17. Reid L, Thomson P, Besemann M, Dudek N. Going places: Does the two-minute walk test predict the six-minute walk test in lower extremity amputees? J Rehabil Med. 2015;47(3):256–61. pmid:25588644
- 18. Hawkins EJ, Riddick W. Reliability, validity, and responsiveness of clinical performance-based outcome measures of walking for individuals with lower limb amputations: a systematic review. Phys Ther. 2018;98(12):1037–45. pmid:30184132
- 19. Brockway JM. Derivation of formulae used to calculate energy expenditure in man. Hum Nutr Clin Nutr. 1987;41(6):463–71. pmid:3429265
- 20. van Schaik L, Geertzen JHB, Dijkstra PU, Dekker R. Metabolic costs of activities of daily living in persons with a lower limb amputation: a systematic review and meta-analysis. PLoS One. 2019;14(3):e0213256. pmid:30893346
- 21. Ralston HJ. Energy-speed relation and optimal speed during level walking. Int Z Angew Physiol. 1958;17(4):277–83. pmid:13610523
- 22. Genin JJ, Bastien GJ, Franck B, Detrembleur C, Willems PA. Effect of speed on the energy cost of walking in unilateral traumatic lower limb amputees. Eur J Appl Physiol. 2008;103(6):655–63. pmid:18478251
- 23. Ziegler-Graham K, MacKenzie EJ, Ephraim PL, Travison TG, Brookmeyer R. Estimating the prevalence of limb loss in the United States: 2005 to 2050. Arch Phys Med Rehabil. 2008;89(3):422–9. pmid:18295618
- 24.
Practice analysis of certified practitioners in the disciplines of orthotics and prosthetics. American Board for Certification in Orthotics, Prosthetics & Pedorthics; 2022.
- 25. Czerniecki JM, Morgenroth DC. Metabolic energy expenditure of ambulation in lower extremity amputees: What have we learned and what are the next steps? Disabil Rehabil. 2017;39(2):143–51. pmid:26458225