







































Brunelli S, Sancesario A, Iosa M, Delussu A.S, Gentileschi N, Bonanni C, Foti C, Traballesi M. Which is the best way to perform the Physiological Cost Index 
in active individuals with unilateral trans-tibial amputation? Canadian Prosthetics & Orthotics Journal.2019;Volume2, Issue1, No.5. 
https://doi.org/10.33137/cpoj.v2i1.32953 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
 

 
 

 
 

 

 

 

 

 

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OPEN  ACCESS 

PHYSIOLOGICAL COST INDEX IN INDIVIDUALS WITH TRANS-TIBIAL AMPUTATION 

Volume 2, Issue 1, Article No.5, December 2019 

 

 

Journal Homepage: https://jps.library.utoronto.ca/index.php/cpoj/index 

 

RESEARCH ARTICLE 

 

WHICH IS THE BEST WAY TO PERFORM THE PHYSIOLOGICAL COST INDEX IN ACTIVE 
INDIVIDUALS WITH UNILATERAL TRANS-TIBIAL AMPUTATION?   

 
Brunelli S1*, Sancesario A2, Iosa M1, Delussu A.S1, Gentileschi N2, Bonanni C2, Foti C2, Traballesi M1  
 

1 Fondazione Santa Lucia, Scientific Institute for Research, Hospitalization and Health Care, Rome, Italy. 

2 Physical and Rehabilitation Medicine, Tor Vergata University of Rome, Rome, Italy. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

INTRODUCTION   

Lower-limb amputees represent a particular group of 

interest in terms of gait, like previous studies have 

highlighted, as these persons use more energy compared 

to the healthy ones.1-4 Moreover, the energy cost and the 

effort required has been shown to be related to the level 

and the cause of the amputation: the higher the 

amputation level the greater the walking energy cost;1,5,6 

further vascular disease amputees have higher walking 

energy expenditure than traumatic amputees.7,8 

 

 

Oxygen consumption measurement (VO2) with a portable 

metabolimeter is the primary choice for assessing energy 

cost of walking (ECW) in amputees and it has been widely 

used in literature.5,9 However, it is time-consuming, the 

instrumentation needed is expensive and the methodology 

requires trained personnel. VO2 is the amount of oxygen 

taken up and utilized by the body mass per minute 

(ml/kg/min). ECW is the oxygen cost of walking and is 

defined as oxygen consumption related to walking speed: 

VO2 (ml/kg/min)/walking speed (m/min).10 

ABSTRACT 

BACKGROUND: Physiological Cost Index (PCI) is a simple method used to estimate energy 

expenditure during walking. It is based on a ratio between heart rate and self-selected walking speed. 

Previous studies reported that PCI is reliable in individuals with lower limb amputation but only if there 

is an important walking impairment. No previous studies have investigated the correlation of PCI with 

the Energy Cost Walking (ECW) in active individuals with traumatic unilateral trans-tibial amputation, 

considering that this particular category of amputees has an ECW quite similar to healthy individual 

without lower limb amputation. Moreover, it is important to determine if PCI is also correlated to ECW 

in the treadmill test so as to have an alternative to over-ground test. 

OBJECTIVES: The aim of this study was to evaluate the correlation between PCI and ECW in active 

individuals with traumatic trans-tibial amputation in different walking conditions. The secondary aim 

was to evaluate if this correlation permits to determine ECW from PCI values. 

METHODOLOGY: Ninety traumatic amputees were enrolled. Metabolic data, heart rate and walking 

speed for the calculation of ECW and for PCI were computed over-ground and on a treadmill with 0% 

and 12% slopes during a 6-minute walking test. 

FINDINGS: There is a significant correlation between ECW and PCI walking over-ground (p=0.003; 

R2=0.10) and on treadmill with 12% slopes (p=0.001; R2=0.11) but there is only a poor to moderate 

correlation around the trendline. No significant correlation was found walking on treadmill with 0% 

slope. The Bland-Altman plot analysis suggests that is not possible to evaluate ECW directly from PCI. 

CONCLUSIONS: PCI is a reliable alternative measure of energy expenditure during walking in active 

individuals with trans-tibial amputation when performing over-ground or at high intensity effort on 

treadmill. PCI is therefore useful only for monitoring a within subject assessment. 

CITATION 

Brunelli S, Sancesario A, Iosa M, 

Delussu A.S, Gentileschi N, 

Bonanni C, Foti C, Traballesi M. 

Which is the best way to perform 

the Physiological Cost Index in 

active individuals with unilateral 

trans-tibial amputation? Canadian 

Prosthetics & Orthotics Journal. 

2019; Volume2, Issue1, 

No.5.https://doi.org/10.33137/cpoj.

v2i1.32953.  

 
KEYWORDS 

Oxygen Consumption, Prosthesis, 

Gait, Treadmill, Lower Limb 

Amputation, Physiological Cost 

Index, Energy Cost of Walking, 

Walking Test. 

*CORRESPONDING AUTHOR 

Dr. Stefano Brunelli, 

Fondazione Santa Lucia, IRCCS, Via Ardeatina 306, 00179 Rome, Italy. 

ORCID: https://orcid.org/0000-0002-5986-1564 

Tel. +39 0651501844; Fax +39 0651501919 

E-MAIL: s.brunelli@hsantalucia.it 

 

DOI: https://doi.org/10.33137/cpoj.v2i1.32953 

ARTICLE INFO 

Received: August 4, 2019 

Accepted: December 12, 2019 

Published: December 14, 2019 

 

https://jps.library.utoronto.ca/index.php/cpoj/index
https://doi.org/10.33137/cpoj.v2i1.32953
https://doi.org/10.33137/cpoj.v2i1.32953
https://orcid.org/0000-0002-5986-1564
mailto:s.brunelli@hsantalucia.it
https://doi.org/10.33137/cpoj.v2i1.32953


 

 

Brunelli S, Sancesario A, Iosa M, Delussu A.S, Gentileschi N, Bonanni C, Foti C, Traballesi M. Which is the best way to perform the Physiological Cost Index in active individuals 
with unilateral trans-tibial amputation? Canadian Prosthetics & Orthotics Journal. 2019; Volume2, Issue1, No.5. https://doi.org/10.33137/cpoj.v2i1.32953 

 

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Brunelli et al. 2019 

 Physiological cost index in individuals with trans-tibial amputation 

Physiological Cost Index (PCI) considers heart rate as an 

indicator of energy expenditure. MacGregor has studied 

how PCI reflects the heart function and therefore indirectly 

the O2 consumption.11The PCI is a valuable tool, that 

provides a simple, quick and inexpensive method to 

evaluate the O2 consumption during exercise12,13 and it is 

based on the linear relationship between VO2 and heart 

rate at submaximal workloads.11  

PCI has been used as an outcome measure in many 

pathologies 14-17 and in elderly persons.18 In literature, 

several authors have used the PCI as an outcome 

measure to evaluate the energy expenditure of walking in 

lower limb amputees.1-4,19-22 Hagberg et al. have reported 

the test-retest reproducibility of the PCI between lower 

limb amputees and healthy persons, however there is no 

evidence of a linear correlation between PCI and ECW 

when performing high intensity effort.3 Chin et al. observed 

a significant correlation between PCI and oxygen uptake 

in trans-femoral amputees, however, the study was 

conducted on a small group of 6 unilateral trans-femoral 

amputees.23  

About the correlation between PCI and ECW, Graham et 

al. reported that PCI scores did not correlate with VO2, 

indicating that the PCI is not a valid measure of energy 

expenditure in healthy persons. Walking at a comfortable 

pace for healthy persons provides only a minimal stress 

on the cardiovascular system and very low energy 

expenditure very near to resting values.24 Besides 

MacGregor indicated that PCI requires a submaximal 

effort.11 Considering that there should be a positive 

correlation between PCI and ECW only in the condition 

with a significant walking impairment, we hypothesize that 

active individuals with trans-tibial amputation (TTA) 

without any stump problems or clinical comorbidities were 

quite similar to healthy persons and therefore PCI could 

not be effective in the standard testing condition, i.e. during 

over-ground walking. In effect, ECW values in active TTA 

have shown small differences compared to those of 

healthy persons.25,26 

Finally, PCI has been always calculated when the 

participants were walking over-ground: the large track 

needed for testing is not practical in all clinical setting and 

the presence of too many turns may influence the walking 

speed and consequently the PCI. A treadmill evaluation of 

PCI could be easier, especially for laboratories with small 

spaces.  

No previous studies have investigated the correlation 

between PCI and ECW on active adult traumatic TTA. The 

primary aim of this study was to evaluate the reliability of 

PCI, compared to ECW, in this particular category of 

individuals during over-ground walking test (OWT), 

treadmill walking test with 0%. slope (TWT0%), and 

treadmill walking test with 12% slopes (TWT12%). This 

last test was performed with the hypothesis that when 

increasing the cardiopulmonary requests, there would be 

a stronger correlation between the two measures. The 

secondary aim was to evaluate if it is possible to determine 

ECW from PCI values of OWT. 

METHODOLOGY 

Study design: Cross-sectional study 

Setting: Research laboratory of Amputees Section of 

Operative Unit 4, Fondazione Santa Lucia, Rehabilitation 

Hospital, Rome. 

Sample: The study sample included TTA. They were 

randomly selected from those who met the inclusion 

criteria in our database.  

Inclusion criteria: 1) unilateral traumatic TTA, 2) age 20-

65 years old, 3) mass < 116 kg, 4) use of the prosthesis 

for at least 18 months and for a minimum of 4 hours per 

day, 5) a mobility level of K3 or more based on the K-

Levels27 (i.e. amputees that have the ability for ambulation 

with variable cadence, typical of community ambulatory or 

active adults), 6) absence of pathological stump condition 

that may affect prosthesis use, 7) absence of mental 

disorder, 8) absence of heart disease (except for 

hypertension well controlled by drugs) or respiratory 

disease 9) no usage of drugs that interferes with heart rate 

(i.e. Beta-blockers, digoxin), 10) the rehabilitative program 

and the prosthetic training were completed, 11) absence 

of pain or mobility deficit on sound limb. All participants 

gave their informed consent and they received no 

payment.  

All TTA were fitted with a modular prosthesis with total 

surface bearing socket, passive vacuum suspension 

system and a dynamic-response foot. 

The TTA performed a 6-minute walking test (6MWT) in 

three different conditions: one over-ground and two on a 

treadmill. The OWT was carried out in a 61-meter hallway 

with straight course and regular surface, walking back and 

forth at comfortable self-selected walking speed 

(SSWS).26  On the treadmill (RUNRACE, Technogym, 

Italy) the TWT0% and the TWT12%, were conducted with 

the speed indicator covered; each participant chose his 

SSWS without knowing the speed indicated on the 

treadmill.26 The walking tests were performed in the 

morning in three different days in a random sequence.  

During all 6MWTs cardiac and metabolic data were 

collected with a portable gas analyzer K4b2 (Cosmed, 

Italy), that allowed a breath by breath recording of VO2 

and heart rate. The time length of the 6MWT was enough 

for the TTA to reach the steady state phase of cardiac and 

metabolic data. Before each tests the TTA sat quietly for 

10 minutes in order to collect rest heart rate data, for 

calculation of PCI. The data obtained at self-selected 

walking speed were used to calculate ECW and PCI. 

The mean walking speed of the OWT was calculated as 

the ratio of distance to time in the steady state phase only. 

For the measurement of PCI and ECW, some precautions 

were taken: before the tests, enrolled TTA had to abstain 

from exhausting efforts, fatty foods, smoking and alcohol 

(the day before) and in the previous 60 minutes they were 

denied to take any kind of stimulants (tea, coffee, 

chocolate) or smoking. 

The PCI and ECW data of each patient during the three 

walking conditions were calculated. The ECW was 

calculated using the formula “oxygen consumption/speed”. 

The formula used for PCI calculation was “walking heart 

rate – resting heart rate /speed”. 

 

https://doi.org/10.33137/cpoj.v2i1.32953


 

 

Brunelli S, Sancesario A, Iosa M, Delussu A.S, Gentileschi N, Bonanni C, Foti C, Traballesi M. Which is the best way to perform the Physiological Cost Index in active individuals 
with unilateral trans-tibial amputation? Canadian Prosthetics & Orthotics Journal. 2019; Volume2, Issue1, No.5. https://doi.org/10.33137/cpoj.v2i1.32953 

 

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CPOJ 

 

Brunelli et al. 2019 

 Physiological cost index in individuals with trans-tibial amputation 

Statistical Analysis 

Pearson’s correlation coefficient (R) was computed to 

assess the association between the values of ECW and 

PCI data of each condition. The correlations between 

ECW and PCI between trials and participants within the 

same condition were computed. The possible agreement 

between PCI and VO2 was analyzed by means of Bland-

Altman plot. The alpha-level of statistical significance was 

set at 0.05 for all the analysis. SPSS 17.0 software (SPSS, 

Inc., Chicago, IL) was used for all the statistical analysis. 

RESULTS  

We evaluated 90 male participants whose characteristics 

are summarized in Table 1. None of the participants 

interrupted the test sessions due to fatigue. 

Table 1: Demographic characteristics of the sample. 

 

The mean values of OWT were: ECW=0.195±0.039 

ml/kg/m and PCI=0.380±0.183 (beats/m), p=0.003. 

(Figure 1). The chart shows that there is a positive 

correlation between the two parameters. The large inter-

participant variation means that there is only a poor to 

moderate correlation around the trendline. 

 

Figure 1: Correlation between PCI and ECW during over-ground 

walking test, (p=0.003). 

The mean values of TWT0% were ECW=0.307±0.084 

ml/kg/m and PCI=0.465±0.205 (beats/m), p=0.075 (Figure 

2).  

Last, in the third condition (TWT+12%) mean ECW was 

0.525±0.132ml/kg/m, mean PCI was 0.977±0.355 

(beats/m), p=0.001 (Figure 3). The chart shows that there 

is a positive correlation between the two parameters but 

there is only a poor to moderate correlation around the 

trendline.  

Figure 4 shows, by means of the Bland-Altman plot, how 

the differences between PCI and VO2, evaluated for over-

ground walking, vary with respect to their mean value. 

 

Figure 2: Correlation between PCI and ECW during treadmill 

walking test with 0% slope, (p=0.075). 

 

Figure 3: Correlation between PCI and ECW during treadmill 

walking test with 12% slope, (p=0.001). 

 

Figure 4: Bland-Altman plot of PCI and VO2 for over-ground 

walking. 

DISCUSSION  

PCI is easy to calculate, is low-cost and no high 

technology equipment is required, but in order to start 

using it in trials as an alternative measure of ECW, it’s 

important to know their correlation and limits. This is the 

first study that compared ECW and PCI in a large sample 

of individuals with lower limb amputation. 

The primary aim of this study was to evaluate the 

correlation between PCI and ECW in active traumatic TTA. 

Our results showed that there is not a statistically 

significant correlation between ECW and PCI during the 

TWT0% (p=0.075) while there is a significant correlation 

90 male 
trans-tibial 
amputees 

Age 
(y) 

Weight 
(kg) 

Height 
(m) 

Time since 
amputation 
(months) 

K-
LEVEL 

Mean 46.3 86.3 1.7 114.5 3.6 

SD 9.7 13.5 0.7 86.1 0.5 

https://doi.org/10.33137/cpoj.v2i1.32953


 

 

Brunelli S, Sancesario A, Iosa M, Delussu A.S, Gentileschi N, Bonanni C, Foti C, Traballesi M. Which is the best way to perform the Physiological Cost Index in active individuals 
with unilateral trans-tibial amputation? Canadian Prosthetics & Orthotics Journal. 2019; Volume2, Issue1, No.5. https://doi.org/10.33137/cpoj.v2i1.32953 

 

4 

CPOJ 

 

Brunelli et al. 2019 

 Physiological cost index in individuals with trans-tibial amputation 

between ECW and PCI during the OWT (p=0.003). This 

may be explained by the Traballesi et al. findings where 

the authors reported that the over-ground test is the one 

that better reflects real walking with prostheses.5 

Moreover, the same study demonstrated that amputees 

tested on treadmill have shown a significantly slower 

SSWS. The absence of correlation between PCI and ECW 

on treadmill with a slope of 0% might depend on a 

decrease of physiological expenditure as participants are 

walking slower. When the treadmill test was performed 

with higher intensity effort (TWT+12%), the correlation 

between ECW and PCI is statistically significant (p=0.001). 

The effort of walking with 12% inclination causes an 

increase of the heart rate. We can speculate that, on 

treadmill, the more intense the exercise, the higher the 

correlation between ECW and PCI.  

Previous studies which examined the correlation between 

PCI and VO2 in healthy persons without gait impairment 

did not report any correlations.3,24 Compared with those 

studies, that investigated the PCI in healthy persons, we 

obtain a mean over-ground PCI value of 0.38, which is 

close but higher to Hagberg (PCI mean value=0.33) and 

Graham (PCI mean value=0.32). These PCI values are 

much lower in patients with great walking impairment such 

as spinal cord injury patients walking using a gait orthosis 

(PCI=1.97),28 or patients with stabilized hemiparesis 

(PCI=0.76),29 or trans-femoral amputees (PCI=0.55).2 

PCI reproducibility in healthy individuals and lower limb 

amputees has been reported, but there is lack of evidence 

about its correlation with ECW.2 PCI is directly proportional 

to heart rate and inversely proportional to speed, so any 

situation influencing heart rate or speed could influence 

PCI. Healthy persons have a lower increase of working 

heart rate than persons with gait impairments because the 

cardiovascular stress is much lower.24 So studies that 

investigated PCI on healthy participants could not be 

indicative for participants with gait impairment. In our study 

we assessed only TTA whose amputation was due to 

traumatic injuries. This is because dysvascular amputees 

usually have comorbidities that could influence the general 

health of the patients and their heart rate. The TTA of our 

study have yet gait difficulties with respect to the healthy 

population, but they do not have other comorbidities. In 

this way we could evaluate the correlation between PCI 

and ECW having only the gait impairment as a difference 

between individuals with trans-tibial amputation and 

healthy participants. We can confirm our hypothesis that 

TTA without any other comorbidities are quite similar to 

healthy persons, so it is important to change the PCI 

treadmill testing condition towards a submaximal exercise 

to reach a significant correlation of PCI with ECW for 

clinical or evaluation purpose. 

Ultimately the Bland-Altman plot shows two biases that do 

not allow to evaluate ECW directly from PCI. Firstly, the 

fact that the mean value of the difference between PCI and 

VO2 is different from zero revealed the presence of an 

absolute systematic difference between the two 

parameters. Then, the linear relationship clearly shows 

that the increment of this difference is proportional to the 

mean. These two biases support the idea of a relationship, 

but not an agreement, between PCI and ECW. 

 

Study limitations 

In our study we have analyzed only traumatic transtibial 

amputees so these results should not be generalized to 

larger populations with amputation or to other persons with 

gait impairments due to other reasons. The participants 

were allowed to walk at different SSWS over the three 

different conditions which will invariably affect the energy 

cost of walking and the correlations between the 

parameters. The amputees, in fact, walk slower on the 

treadmill to reduce their metabolic energy expenditure. We 

did not select the over-ground SSWT for the treadmill 

speed because we have followed the methods of 

Traballesi’s paper in which the amputees chose their 

SSWS without knowing the speed indicated on the 

treadmill.26 Further studies should test PCI and ECW with 

other treadmill inclination in order to find the equation, if it 

exists, that permits to calculate the ECW from PCI data. 

Moreover, further studies are needed to confirm the 

suggestion that PCI has a higher correlation with the ECW 

when the walking effort increase. 

CONCLUSION 

Our data allow us to state that PCI value do not permit to 

calculate ECW. Anyway the PCI can be used as an 

alternative measure of energy expenditure in active 

individuals with trans-tibial amputation for a within 

participant assessment only when recorded during over-

ground walking or on treadmill with 12% slope. In reverse, 

the PCI does not correlate with ECW when recorded 

during treadmill walking with 0% slope. These findings 

have to be considered when PCI is used as a tool for 

estimate energy expenditure of walking. 

ACKNOWLEDGEMENTS 

Thanks to Livia Brunelli, Durham University, UK, for the 

English language editing. 

DECLARATION OF CONFLICTING 

INTERESTS 

The authors have no conflicts of interest to declare. 

SOURCES OF SUPPORT 

No funding for this study was provided. 

ETHICAL APPROVAL 

The study approval was obtained by local ethics 

committee. 

AUTHOR CONTRIBUTION 

• Stefano Brunelli 

conceived the idea of the work, supported the data 

analysis and led the writing of the manuscript. 

• Andrea Sancesareo 

 drafted the manuscript, managed the data files, 

interpretation of the data. 

• Marco Iosa 

conducted the statistical analyses, interpretation of 

the data. 

https://doi.org/10.33137/cpoj.v2i1.32953


 

 

Brunelli S, Sancesario A, Iosa M, Delussu A.S, Gentileschi N, Bonanni C, Foti C, Traballesi M. Which is the best way to perform the Physiological Cost Index in active individuals 
with unilateral trans-tibial amputation? Canadian Prosthetics & Orthotics Journal. 2019; Volume2, Issue1, No.5. https://doi.org/10.33137/cpoj.v2i1.32953 

 

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Brunelli et al. 2019 

 Physiological cost index in individuals with trans-tibial amputation 

• Anna Sofia Delussu 

acquisition, analysis of the data. 

• Noemi Gentileschi 

supported the writing of the manuscript. 

• Cinzia Bonanni 

supported the writing of the manuscript. 

• Calogero Foti 

revised the manuscript critically for important 

intellectual content. 

• Marco Traballesi 

revised the manuscript critically for important 

intellectual content, design of the work. 

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