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VOLUME 5, ISSUE 1 

 2022 

 

RESEARCH ARTICLE 

John S, Orlowski K, Mrkor K.U, Edelmann-Nusser J, Witte K. Differences in hip muscle strength and static balance in patients with transfemoral amputations 

classified at different K-levels: A preliminary cross-sectional study. Canadian Prosthetics & Orthotics Journal. 2022; Volume 5, Issue 1, No.5. 

https://doi.org/10.33137/cpoj.v5i1.37456 

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https://doi.org/10.33137/cpoj.v5i1.37456


 

1 

John S, Orlowski K, Mrkor K.U, Edelmann-Nusser J, Witte K. Differences in hip muscle strength and static balance in patients with transfemoral amputations 
classified at different K-levels: A preliminary cross-sectional study. Canadian Prosthetics & Orthotics Journal. 2022; Volume 5, Issue 1, No.5. 
https://doi.org/10.33137/cpoj.v5i1.37456 

 

 

 
RESEARCH ARTICLE 

 

DIFFERENCES IN HIP MUSCLE STRENGTH AND STATIC BALANCE IN PATIENTS WITH 

TRANSFEMORAL AMPUTATIONS CLASSIFIED AT DIFFERENT K-LEVELS: A PRELIMINARY 

CROSS-SECTIONAL STUDY 

John S1*, Orlowski K 2, Mrkor K.U 2, Edelmann-Nusser J 1, Witte K 1  
 
1 Department of Sports Science, Faculty of Humanities, Otto von Guericke University, Magdeburg, Germany. 
2 Department of Computer Science and Media, Brandenburg University of Applied Sciences, Brandenburg an der Havel, Germany.  

 
 

 

 

 

  

 

 

 

 

 

 

 

 
 

 

 

INTRODUCTION   

Amputations of the lower limbs, especially transfemoral (TF) 

or transtibial (TT) amputations have a severe impact on the 

patient’s life. The irreversible loss of an extremity affects 

physical integrity and leads to social and psychological 

burdens.1 Following  amputation,  rehabilitation  programs  

 

 

 

 

and proper prosthetic fitting, as well as prosthetic usage, are 

important factors for improving the quality of life of the 

patients.2 The use of a prosthesis has been associated with 

higher physical function, gain in independence and 

increased self-esteem.3  

The prescription of the type of prosthesis and its specified 

components as well as the financial coverage by health 

insurance are based on the expected functional mobility of 

the patient.4 To identify functional mobility, several 

classification systems exist to assign patients with lower 

 
OPEN  ACCESS Volume 5, Issue 1, Article No.5. 2022 

 

 

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

 

ABSTRACT 

BACKGROUND: Following amputation, patients with lower limb amputations (LLA) are classified into 

different functional mobility levels (K-levels) ranging from K0 (lowest) to K4 (highest). However, K-level 

classification is often based on subjective criteria. Objective measures that are able to differentiate 

between K-levels can help to enhance the objectivity of K-level classification. 

OBJECTIVE(S): The goal of this preliminary cross-sectional study was to investigate whether 

differences in hip muscle strength and balance parameters exist among patients with transfemoral 

amputations (TFA) assigned to different K-levels. 

METHODOLOGY: Twenty-two participants with unilateral TFA were recruited for this study, with four 

participants assigned to K1 or K2, six assigned to K3 and twelve assigned to K4. Maximum isometric 

hip strength of the residual limb was assessed in hip flexion, abduction, extension, and adduction using 

a custom-made diagnostic device. Static balance was investigated in the bipedal stance on a force plate 

in eyes open (EO) and eyes closed (EC) conditions. Kruskal-Wallis tests were used to evaluate 

differences between K-level groups.  

FINDINGS: Statistical analyses revealed no significant differences in the parameters between the three 

K-level groups (p>0.05). Descriptive analysis showed that all hip strength parameters differed among 

K-level groups showing an increase in maximum hip torque from K1/2-classified participants to those 

classified as K4. Group differences were also present in all balance parameters. Increased sway was 

observed in the K1/2 group compared to the K4 group, especially for the EC condition.  

CONCLUSION: Although not statistically significant, the magnitude of the differences indicates a 

distinction between K-level groups. These results suggest that residual limb strength and balance 

parameters may have the potential to be used as objective measures to assist K-level assignment for 

patients with TFA. This potential needs to be confirmed in future studies with a larger number of 

participants. 

ARTICLE INFO 

Received: September 17, 2021 

Accepted: January 5, 2022 

Published: January 12, 2022 

CITATION 

John S, Orlowski K, Mrkor K.U, 

Edelmann-Nusser J, Witte K. 

Differences in hip muscle strength 

and static balance in patients with 

transfemoral amputations classified 

at different K-levels: A preliminary 

cross-sectional study. Canadian 

Prosthetics & Orthotics Journal. 

2022; Volume 5, Issue 1, No.5. 

https://doi.org/10.33137/cpoj.v5i1.3

7456 

KEYWORDS 

Amputation, K-level, Balance, 

Transfemoral Amputation, Muscle 

Strength, Residual Limb, Static 

Balance 

 

 

* CORRESPONDING AUTHOR 

Stefanie John, 
Department of Sports Science, Faculty of Humanities, Otto von 
Guericke University, Magdeburg, Germany. 

Email: Stefanie.John@ovgu.de 
ORCID ID: https://orcid.org/0000-0001-6722-7195 

https://doi.org/10.33137/cpoj.v5i1.37456
https://jps.library.utoronto.ca/index.php/cpoj/index
https://doi.org/10.33137/cpoj.v5i1.37456
https://doi.org/10.33137/cpoj.v5i1.37456
mailto:Stefanie.John@ovgu.de
https://orcid.org/0000-0001-6722-7195


 

2 

John S, Orlowski K, Mrkor K.U, Edelmann-Nusser J, Witte K. Differences in hip muscle strength and static balance in patients with transfemoral amputations 
classified at different K-levels: A preliminary cross-sectional study. Canadian Prosthetics & Orthotics Journal. 2022; Volume 5, Issue 1, No.5. 
https://doi.org/10.33137/cpoj.v5i1.37456 

ISSN: 2561-987X HIP MUSCLE STRENGTH AND STATIC BALANCE IN PATIENTS WITH TRANSFEMORAL AMPUTATIONS  

John et al., 2022 CPOJ 

 
limb amputations (LLA) to different mobility levels.4,5 In the 

United States, the Medicare’s Functional Classification 

Level (MFCL) distinguishes patients with LLA into five 

functional levels ranging from K0 (lowest) to K4 (highest). 

This classification, which is intended to reflect the 

individual’s abilities to ambulate with the prosthesis, 

strongly influences the selection and assignment of the 

different prosthetic components. Patients with LLA 

classified as K2 will not have the possibility to receive high 

functioning prosthesis components as patients classified at 

K4. In Germany, a similar classification system is used with 

the same categories as the MFCL system. The assignment 

into the different levels is based on the so-called profile 

survey sheet, in which doctors or orthopedic technicians 

subjectively evaluate abilities concerning functional 

mobility.6 As objective parameters, only the range of motion 

(ROM) of joints of the lower extremities are documented. 

Further objective evaluation criteria are missing. Due to the 

relevance of K-level assignment for the patients with LLA, 

the lack of objectivity has been recognized and 75% of 

orthopedic technicians would support additional objective 

measures to improve the subjectivity of K-level 

classification.7 In a recent study, Sions et al. emphasized 

the necessity of reliable and valid objective measures to 

differentiate between K-level classifications.8 

Addressing the subjectivity of the existing K-level 

classification, Gailey et al. were the first to develop a clinical 

tool, the Amputee Mobility Predictor (AMP),4 to objectively 

assess the patient’s functional abilities. The AMP consists 

of 21 ambulation and balance tasks with and without 

prostheses, which are individually rated by an examiner 

using a point system. The AMP was shown to have the 

potential to distinguish between K-levels.4,9 In two recent 

papers, physical performance tests were performed and 

tested if they are suitable measures to improve the 

objectivity of K-level assignment.8,10 Differences between 

patients with LLA classified as K3 and K4 were seen in the 

Timed Up and Go test and the 6-minute walk test.8 In the 

study of Beisheim et al., functional strength and dynamic 

balance tests were performed with patients with LLA. K4-

classified patients showed higher functional strength and 

better dynamic balance when compared to participants 

classified as K3.10 These studies show that walking tests, 

as well as functional tests, may help to assign patients with 

LLA to the different K-levels.  

Performance in functional tests and walking tests are often 

associated with lower limb strength. Several studies have 

demonstrated that patients with transfemoral amputation 

(TFA) have significantly reduced strength in the residual 

limb compared to the sound leg as well as to controls.11,12 

The muscles surrounding the hip are important to stabilize 

the pelvis during standing and locomotion. Weak hip 

abductors are one cause of the compensatory trunk shifting 

over the prosthetic side13 and poor balance performance.14 

In a recent review, Hewson et al. concluded that muscle 

strength deficits exist in lower limb prosthesis users and 

contribute to balance and mobility impairments.12  In 

patients with TFA, these strength deficits are particularly 

pronounced in the hip of the residual limb.12 

However, no study has included standardized hip strength 

tests of the residual limb as possible measures for assisting 

to objectify K-level classification. Objective evaluation 

methods must be tested for their suitability in assisting in  

K-level assignment, particularly methods that evaluate 

lower limb strength and balance. Therefore, the goal of this 

preliminary study was to investigate whether there are 

differences in hip muscle strength of the residual limb as 

well as differences in static balance parameters among 

patients with TFA assigned to different K-levels. The 

authors hypothesized that participants classified at higher 

K-levels would demonstrate higher performance on the 

strength and balance tests than participants that were 

assigned to lower K-levels.  

METHODOLOGY 

Participants 

Participants were recruited from January 2018 to 

September 2019 through calls and articles in official 

journals of amputee organizations as well as in a local 

newspaper. Inclusion criteria were a unilateral transfemoral 

amputation with a post-amputation time of at least one year, 

an age ≥ 18 years and the current use of the prosthesis. 

Due to the measurement setup, one further inclusion 

criterion was a minimum residual limb length of 15 cm. 

Participants were excluded if the amputation was caused by 

diabetes mellitus, or if they had open wounds, edema, or 

acute pain in the residual limb. All participants gave written 

consent to participate in this study after being informed 

about the procedure and its purpose. The study was 

approved by the local ethics committee of the Otto von 

Guericke University Magdeburg and carried out in line with 

the Declaration of Helsinki (no. of vote: 31/18 on March 19, 

2018).   

Measurement protocol 

For this cross-sectional study, the participants attended a 

single testing session, in which all measurements were 

conducted. Measurements were performed in two 

institutions, University of Magdeburg and University of 

Applied Sciences Brandenburg, which were equipped with 

the same measurement systems. Prior to physical 

performance tests, demographic and anthropometric data 

were collected and participants were asked to answer 

amputation related questions (e.g. type of prosthesis, years 

of using the prosthesis, and K-level assignment). The  

K-level assignment was obtained from medical records in 

https://doi.org/10.33137/cpoj.v5i1.37456


 

3 

John S, Orlowski K, Mrkor K.U, Edelmann-Nusser J, Witte K. Differences in hip muscle strength and static balance in patients with transfemoral amputations 
classified at different K-levels: A preliminary cross-sectional study. Canadian Prosthetics & Orthotics Journal. 2022; Volume 5, Issue 1, No.5. 
https://doi.org/10.33137/cpoj.v5i1.37456 

ISSN: 2561-987X HIP MUSCLE STRENGTH AND STATIC BALANCE IN PATIENTS WITH TRANSFEMORAL AMPUTATIONS  

John et al., 2022 CPOJ 

 
collaboration with the respective orthopedic technician. The 

physical performance tests included isometric strength tests 

of the hip muscles of the residual limb as well as 

examinations of static balance. The strength assessment of 

the hip muscles was performed without the prosthesis 

whereas the static balance tests were performed with the 

prosthesis.  

Maximum isometric hip strength analysis 

The measurement of the maximum isometric strength of the 

hip muscles of the residual limb was performed in a custom-

made diagnostic device (Figure 1). This diagnostic device 

was built specifically for patients with LLA. An individually 

adjustable pelvic support provides stability and safety 

during the measurements. The 270° rotatable base plate 

enables hip muscle strength diagnostics in different 

directions (hip flexion, extension, abduction and adduction) 

while participants do not need to change position within the 

device. An additional resting chair, which can be slid under 

the participants, is integrated into the device to provide relief 

of the standing leg between examinations.  

 

Figure 1: The sensor-based diagnostic device in the overall 

display. 

For the strength measurement, participants were standing 

in an upright position supported by the pelvis support 

without the prosthesis. A neoprene brace was placed 

around the residual limb (Figure 2). This brace served as an 

attachment possibility for the cuff of the hauling rope. A 

force transducer (Hottinger Baldwin Messtechnik GmbH, 

Darmstadt, Germany) integrated into the hauling rope was 

used to measure the isometric strength at a sampling rate 

of 1000 Hz for hip flexion, extension, abduction, and 

adduction in the neutral hip position (vertical position of the 

thigh perpendicular to the pelvis). Before the 

measurements, participants were asked to familiarize 

themselves with the setup. For each motion direction, one 

submaximal test (pretest) and three maximum tests (main 

tests) were performed with one minute of rest between 

trials. Participants were instructed to successively build up 

strength and pull maximally without an abrupt push. They 

could follow their current measured strength values live on 

the screen during the measurement. The maximum 

achieved strength value from the pretest was visualized on 

the screen as a threshold value and participants were 

verbally encouraged to exceed this in the main tests. The 

threshold value was readjusted after exceeding the 

previous threshold value to ensure that the maximum 

possible force value was reached within the three main 

tests. The distance between the greater trochanter and the 

point of applied force (center of the cuff) served 

approximately as the lever arm (Figure 2).  

 

 

Figure 2: Setup for measuring strength in hip abduction in the 

neutral hip position. 

Data were further processed in Matlab (Version 2018b, The 

MathWorks Inc., Natick, MA) and filtered with a 4th order 

Butterworth low-pass filter (5 Hz). Torques for each motion 

direction were calculated from the force and the lever arm 

Force plate 270° rotatable 

baseplate 

Pelvis support 
Resting chair 

Feedback 

Pulling direction 
Lever arm 

https://doi.org/10.33137/cpoj.v5i1.37456


 

4 

John S, Orlowski K, Mrkor K.U, Edelmann-Nusser J, Witte K. Differences in hip muscle strength and static balance in patients with transfemoral amputations 
classified at different K-levels: A preliminary cross-sectional study. Canadian Prosthetics & Orthotics Journal. 2022; Volume 5, Issue 1, No.5. 
https://doi.org/10.33137/cpoj.v5i1.37456 

ISSN: 2561-987X HIP MUSCLE STRENGTH AND STATIC BALANCE IN PATIENTS WITH TRANSFEMORAL AMPUTATIONS  

John et al., 2022 CPOJ 

 
and normalized to the body mass of the participants. Out of 

the three main trials for each motion direction, the trial with 

the highest torque was used for further analyses. 

The reliability of the isometric hip strength measurement of 

the diagnostic device had been investigated in a test-retest 

design before the examinations. The calculated intraclass 

correlation coefficients (ICCs) showed values ranging from 

0.85 to 0.95 for the isometric hip strength measurement (hip 

flexion, extension, abduction, and adduction). According to 

Koo and Li, these ICCs indicate good to excellent 

reliability.15 These results suggest that the custom-made 

diagnostic device provides an environment to reliably 

quantify maximum isometric hip strength.  

Balance assessment 

Static balance was assessed in two different conditions: 

bipedal stance with eyes open (EO) and bipedal stance with 

eyes closed (EC). Before generating balance data, the 

prosthetic socket comfort was determined with the 

Prosthetic Socket Fit Comfort Score16 as poor socket fit 

might influence static balance parameters. Socket score 

has been deemed a valid and reliable outcome 

measurement.16 The participants were asked to rate the 

comfort of their socket on a scale from 0 to 10 with 0 being 

the most uncomfortable and 10 being the most comfortable 

socket imaginable. Mean comfort scores were between 7 

and 8 points for all K-level groups.  

For the bipedal stance, subjects were instructed to stand 

hip-width apart on a 45x45 cm force plate (PLUX-Wireless 

Biosignals S.A, Lisbon, Portugal) with the arms hanging 

down at the sides and to remain as still as possible. For the 

EO conditions, participants were asked to focus on a fixed 

point at eye-level on the wall in front of them whereas for 

the EC condition the participants closed their eyes. Prior to 

collecting data, participants practiced both poses for a few 

seconds. For safety reasons, an examiner stood near the 

participants during the entire familiarization and 

measurement period. Once the familiarization period was 

over, two trials with a duration of twenty seconds were 

recorded for both conditions with a sampling frequency of 

250 Hz. Balance data were further processed using Matlab 

and filtered applying a 4th order Butterworth low-pass filter 

with a 10 Hz cut-off frequency. The total length of the center 

of pressure (COP) during the two standing conditions was 

computed as well as the maximum and mean deviations in 

mediolateral (ML) and anteroposterior (AP) directions. 

These COP-based measures have been used in most 

studies examining static balance in participants with LLA.17  

Data analysis 

Statistical analyses were performed using IBM SPSS 

Statistics 26 (IBM SPSS, Armonk, NY). Based on the K-

level assignment, participants were divided into groups. For 

each K-level group, descriptive statistics were determined 

for all anthropometric, demographic and measurement 

variables. Due to the small sample size of participants and 

the unequal distribution of participants across K-levels, 

variables were described using the median and interquartile 

range (IQR: 25th percentile, 75th percentile). To detect 

differences between K-level groups, Kruskal-Wallis tests 

were performed for each variable. Pairwise posthoc 

comparisons with Bonferroni correction for multiple testing 

followed where appropriate. The significance level was set 

at p< 0.05.  

RESULTS 

Participants  

Twenty-two participants fulfilled the inclusion criteria and 

were considered for the study. All participants were able to 

complete all tests and were included in the data analysis. In 

Table 1, participants’ anthropometric and demographic data 

are represented according to the K-level assignment. Due 

to the small numbers of patients classified as K1 or K2, they 

were combined as one group. The Kruskal-Wallis test 

showed that age differed significantly between K-level 

groups. Posthoc tests revealed that participants of the K4 

group were significantly younger than the ones of the K1/2 

group (p=0.03). The K4 group was not only younger on 

average, but also had a longer residual limb length and the 

amputation had not occurred as long ago as for K1/2 and 

K3-classified participants. 

Table 1: Anthropometric and demographic data of the participants 

presented as median and IQR (25th percentile, 75th percentile).  

 
K1/2 
(n=4) 

K3 
(n=6) 

K4 
(n=12) 

p-value 
(Kruskal-
Wallis) 

Age [yrs.] 
75.0a 

(53.8, 
80.5) 

61.0 
(51.0, 
76.8) 

51.5a 
(36.3, 
60.8) 

0.04* 

Sex (m=male, 
f=female) 

4 m, 
0 f 

4 m, 
2 f 

12 m, 
0 f 

- 

BMI [kg/m2] 
28.8 

(23.9, 
30.1) 

25.8 
(23.8, 
26.7) 

26.6 
(24.8, 
30.5) 

0.40 

Residual limb 
length [m] 

0.22 
(0.22, 
0.39) 

0.32 
(0.26, 
0.35) 

0.38 
(0.28, 
0.43) 

0.18 

Years since 
amputation 
[yrs.]  

28.5 
(3.0, 67.5) 

19.0 
(7.3, 35.5) 

8.5 
(4.3, 24.3) 

0.63 

 
*Significant across groups (p<0.05) 
a Significant difference between the K1/2 and the K4 group 
 

Isometric hip strength of the residual limb 

The results of the hip muscle strength test of the residual 

limb are shown in Table 2. A significant difference across  

K-level groups was only detected for hip flexion (p=0.04). 

The posthoc tests did not reveal significant differences 

https://doi.org/10.33137/cpoj.v5i1.37456


 

5 

John S, Orlowski K, Mrkor K.U, Edelmann-Nusser J, Witte K. Differences in hip muscle strength and static balance in patients with transfemoral amputations 
classified at different K-levels: A preliminary cross-sectional study. Canadian Prosthetics & Orthotics Journal. 2022; Volume 5, Issue 1, No.5. 
https://doi.org/10.33137/cpoj.v5i1.37456 

ISSN: 2561-987X HIP MUSCLE STRENGTH AND STATIC BALANCE IN PATIENTS WITH TRANSFEMORAL AMPUTATIONS  

John et al., 2022 CPOJ 

 
between the individual groups (p>0.05). If the medians of 

the parameters are considered, an increase in maximum hip 

torque can be observed from participants classified as K1/2 

to those classified as K4. Differences were especially visible 

between the K3 and K4 groups with a mean difference of 

0.97 Nm/kg for hip flexion, 0.51 Nm/kg for hip abduction, 

0.45 Nm/kg for hip extension and 0.44 Nm/kg for hip 

adduction. A graphical representation of the data in form of 

boxplots is shown in Figure 3.  

Table 2: Maximum hip torque presented as median and IQR (25th 

percentile, 75th percentile).  

 

*Significant across groups (p<0.05) 

Static balance  

The parameters from the examination of the static balance 

are presented in Table 3. For neither the EO nor the EC 

condition, the Kruskal-Wallis tests revealed significant 

differences in the parameters across the three K-level 

groups (p>0.05). The descriptive analysis showed that 

differences in the medians were especially seen between 

K1/2 classified participants and the ones assigned to K4. 

For the EO condition, the mean and maximum deviation in 

ML and AP directions decreased from K1/2 to the K4 group. 

For the maximum deviation in ML and AP directions, a mean 

difference of 8 mm and 6 mm was determined between K1/2 

and K4. For the EC condition, differences between groups 

became more evident. For the COP length, the mean 

difference between the K1/2 group and K4 group was 340 

mm, and the mean differences of the maximum excursions 

in ML and AP directions were 23 mm and 20 mm, 

respectively. Figure 4 shows an example of sway paths from 

one participant of the K1/2 group and one K4-classified 

participant in both EO and EC conditions. An increase in 

sway from the EO condition to the EC condition is visible for 

both K-groups, although the increase is considerably more 

pronounced for the K1/2-participant than for that of the 

participant of the K4 group. 

Hip Torque 
K1/2 

 
K3 

 
K4 

 

p-value 
(Kruskal-

Wallis) 

Hip flexion 
[Nm/kg] 

1.06 
(0.53, 
1.68) 

1.21 
(0.97, 
1.53) 

2.18 
(1.39, 
2.42) 

0.04* 

Hip abduction 
[Nm/kg] 

0.96 
(0.41, 
1.67) 

1.00 
(0.85, 
1.09) 

1.51 
(0.91, 
1.80) 

0.32 

Hip extension 
[Nm/kg] 

0.73 
(0.28, 
1.13) 

0.98 
(0.93, 
1.39) 

1.43 
(0.85, 
1.63) 

0.09 

Hip adduction 
[Nm/kg] 

0.98 
(0.49, 
1.38) 

1.05 
(0.74, 
1.24) 

1.49 
(1.09, 
1.92) 

0.07 

 

Figure 3: Maximum hip torque for the four movement directions of the three K-level groups. 

 

 

 

 

Hip Abduction 
Hip Flexion 

Hip Extension Hip Adduction 

T
o
rq

u
e
 [
N

m
/k

g
] 

T
o
rq

u
e
 [
N

m
/k

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] 

T
o
rq

u
e
 [
N

m
/k

g
] 

T
o
rq

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e
 [
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/k

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K 1 / 2                           K3                             K4     K 1 / 2                           K3                             K4     

K 1 / 2                           K3                             K4     K 1 / 2                           K3                             K4     

https://doi.org/10.33137/cpoj.v5i1.37456


 

6 

John S, Orlowski K, Mrkor K.U, Edelmann-Nusser J, Witte K. Differences in hip muscle strength and static balance in patients with transfemoral amputations 
classified at different K-levels: A preliminary cross-sectional study. Canadian Prosthetics & Orthotics Journal. 2022; Volume 5, Issue 1, No.5. 
https://doi.org/10.33137/cpoj.v5i1.37456 

ISSN: 2561-987X HIP MUSCLE STRENGTH AND STATIC BALANCE IN PATIENTS WITH TRANSFEMORAL AMPUTATIONS  

John et al., 2022 CPOJ 

 
Table 3: COP parameters of the bipedal stance presented as 

median and IQR (25th percentile, 75th percentile).  

Eyes 
open (EO) 

K1/2 K3 K4 
p-value 

(Kruskal-
Wallis) 

COP 
length 
[mm] 

292.7 
(243.3, 
684.4) 

244.3 
(192.7, 
456.5) 

270.5 
(217.1, 
343.2) 

0.54 

Mean Dev. 
ML [mm] 

4.2 
(2.2, 5.9) 

3.2 
(2.7, 4.8) 

3.2 
(1.8, 4.2) 

0.79 

Mean Dev. 
AP [mm]  

4.7 
(3.5. 6.6) 

4.0 
(3.5, 4.7) 

3.4 
(2.4, 5.3) 

0.50 

Max. Dev. 
ML [mm] 

21.0 
(11.8, 29.3) 

14.7 
(11.8, 19.0) 

12.8 
(10.4, 20.1) 

0.59 

Max. Dev. 
AP [mm] 

24.7 
(19.7, 29.7) 

17.8 
(16.2, 23.9) 

18.8 
(12.8, 23.0) 

0.23 

Eyes 
closed 
(EC) 

    

COP 
length 
[mm] 

925.0 
(488.9, 
1386.7) 

624.5 
(442.5, 
993.8) 

583.9 
(387.7, 
873.4) 

0.50 

Mean Dev. 
ML [mm] 

8.0 
(5.3, 10.2) 

3.6 
(2.9, 5.3) 

3.4 
(2.3, 7.6) 

0.19 

Mean Dev. 
AP [mm]  

12.0 
(6.5, 16.6) 

9.1 
(5.5, 9.9) 

8.1 
(5.7, 9.5) 

0.41 

Max. Dev. 
ML [mm] 

38.0 
(27.3, 47.2) 

17.9 
(15.4, 30.9) 

15.1 
(11.9, 39.9) 

0.27 

Max. Dev. 
AP [mm] 

58.1 
(35.0, 76.7) 

45.1 
(31.4, 48.7) 

38.1 
(28.1, 48. 7) 

0.39 

 

DISCUSSION 

In this study, examinations of hip muscle strength of the 

residual limb and examinations of static balance were 

performed in patients with TFA. The goal was to investigate 

whether these objective measures could differentiate 

between patients who were classified at different K-levels. 

As hypothesized, participants classified at higher K-levels 

performed better on the strength and balance tests than 

participants assigned to lower K-levels. However, statistical 

analyses revealed no significant differences in the 

parameters between the three K-level groups.  

While previous studies showed that participants with TFA 

suffer from a strength deficit of the residual limb, this is the 

first study that included an isometric muscle strength 

assessment of the hip muscles on the affected side for 

potential K-level distinction. Strength differences were 

particularly visible between the K3 and the K4 group as well 

as between the K1/2 and the K4 group. For maximum 

torque, mean group differences ranged from 0.4 Nm/kg to 

0.9 Nm/kg for hip flexion, hip abduction, hip extension and 

hip adduction. Although not statistically significant, the 

magnitude of these values implies a distinction between  

K-level groups. 

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Eyes Open Eyes Closed 

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Figure 4: Examples of COP sway of one K1/2-participant compared to one classified as K4 in the eyes open and eyes closed condition. 

https://doi.org/10.33137/cpoj.v5i1.37456


 

7 

John S, Orlowski K, Mrkor K.U, Edelmann-Nusser J, Witte K. Differences in hip muscle strength and static balance in patients with transfemoral amputations 
classified at different K-levels: A preliminary cross-sectional study. Canadian Prosthetics & Orthotics Journal. 2022; Volume 5, Issue 1, No.5. 
https://doi.org/10.33137/cpoj.v5i1.37456 

ISSN: 2561-987X HIP MUSCLE STRENGTH AND STATIC BALANCE IN PATIENTS WITH TRANSFEMORAL AMPUTATIONS  

John et al., 2022 CPOJ 

 
Heitzmann et al. investigated maximum hip torque in a 

similar measurement setup and the hip torque differences 

between participants with TFA (K-level 2 to 4, no 

differentiation) and healthy participants were in the same 

numerical range (0.5-0.7 Nm/kg) and statistically 

significant.11 Reasons for the lack of statistical significance 

in our study may include the small number of participants in 

the K1/2 and K3 groups, the uneven distribution of 

participants among K-level groups and the individuality of 

each participant. Beisheim et al. examined lower extremity 

strength differences between K3 and K4 classified 

participants with TFA applying the functional 5-Times Sit-to-

Stand Test and found a significant difference.10 Functional 

strength tests were shown to have the potential to 

differentiate between K-levels. However, they could not 

explain the reasons why participants of the K3 group 

performed worse than those assigned to K4. Muscle 

strength tests of isolated muscle groups of the residual limb, 

as performed in this study, have the advantage to identify 

specific muscle weaknesses. Knowing the individual 

strength deficits are especially important for patients with 

LLA as hip abductor strength is associated with gait 

deviations18 and hip extension strength has been reported 

as the greatest predictor of performance on the 6-minute 

walk test.19  

As lower extremity strength of lower limb prosthesis users 

is linked to postural control, parameters of static balance 

were also investigated to find potential outcome measures 

that may be able to distinguish between K-levels. The 

statistical analyses, however, did not reveal significant 

differences in the parameters across the three K-level 

groups. Concerning the medians of the parameters, 

participants classified as K1/2 showed greater COP length 

as well as greater mean and maximum sway deviations in 

ML and AP directions in the bipedal stance than the 

participants of the K3 and K4 groups. For all groups greater 

sway was observed in the AP than in the ML direction, which 

has been observed in previous studies.17 This may be 

explained by the missing ankle plantar and dorsiflexor 

muscles on the amputated leg which are relevant for 

stability control in the AP direction.20  

In the eyes-closed condition, COP parameters increased 

and the differences in COP between K-level groups became 

larger. Group differences of maximum excursions in AP and 

ML directions were observed up to 14 mm as well as a mean 

difference in COP length of 240 mm. Increased COP sway 

due to the absence of visual input is in line with previous 

studies investigating patients with LLA during quiet 

standing.14,17 The eyes-closed condition has a great effect 

on patients with LLA as vision is especially relevant to 

compensate for balance impairments due to missing 

somatosensory feedback from the prosthetic leg.21 

Although not proven in this study, using a closed-eye 

condition in quiet standing might be a sensitive method for 

distinguishing between different K-levels as balance control 

mechanisms differ in relation to functional abilities. 

Measures of static balance have been criticized that they 

cannot reflect postural demands in daily life.22 However, 

they may be helpful to identify weaknesses in postural 

control differentiated in AP or ML directions, which can be 

relevant for patients with LLA. Static balance tests could be 

performed in addition to dynamic balance tests, which have 

been shown to be suitable for K-level distinction among 

participants with TFA.10  

Several limitations have to be addressed. The major 

limitation is the small number of participants in each K-level 

group as well as the heterogeneity between groups, which 

may be the reason that no significant differences were 

detected. The groups differed in age, residual limb length 

and in post-amputation time, which may have affected the 

results of physical performance tests. Therefore, 

generalization of the data is not possible and studies with 

larger and more homogeneous samples need to confirm the 

presented results. Further, the cause of amputation was not 

recorded in this study except that patients who experienced 

LLA due to diabetes mellitus were excluded. The 

amputation etiology may impact physical performance and 

should be recorded in future studies. In balance 

examinations, prosthetic alignment and different types of 

prosthetic components (socket, prosthetic knee and foot) 

may also affect performance and should be controlled in 

future studies. However, this is the first study that included 

participants with TFA classified as K1/K2 to find objective 

measures for supporting K-level classification. Future 

studies should not only focus on differentiating between 

participants with LLA classified as K3 and K4 but should 

also include participants classified as K2.   

CONCLUSION 

This study was the first one to perform hip strength tests of 

the residual limb and static balance tests with participants 

with TFA classified at different K-levels to find parameters 

that may be suitable to enhance objective K-level 

classification. Statistical analyses could not reveal any 

significant group differences but the value of the magnitude 

of the group differences detected may be relevant to 

differentiate between K-level groups. The results of the 

study suggest that residual limb strength and balance 

parameters may have the potential to serve as objective 

measures to support K-level classification but this potential 

needs to be confirmed by future studies with a larger 

number of participants. 

ACKNOWLEDGEMENTS 

The authors are grateful for all participants who volunteered to 

participate in this study. 

https://doi.org/10.33137/cpoj.v5i1.37456


 

8 

John S, Orlowski K, Mrkor K.U, Edelmann-Nusser J, Witte K. Differences in hip muscle strength and static balance in patients with transfemoral amputations 
classified at different K-levels: A preliminary cross-sectional study. Canadian Prosthetics & Orthotics Journal. 2022; Volume 5, Issue 1, No.5. 
https://doi.org/10.33137/cpoj.v5i1.37456 

ISSN: 2561-987X HIP MUSCLE STRENGTH AND STATIC BALANCE IN PATIENTS WITH TRANSFEMORAL AMPUTATIONS  

John et al., 2022 CPOJ 

 
DECLARATION OF CONFLICTING 

INTERESTS 

The authors declare that they have no competing interests. 

AUTHOR CONTRIBUTION 

Stefanie John: Contributed to the study concept and design, 

participated in data gathering, analyzed, and interpreted data, 

contributed to the drafting of the manuscript, read, and 

approved the final manuscript. 

Katja Orlowski: Contributed to the study concept and design, 

participated in data gathering, contributed to the drafting of the 

manuscript, read, and approved the final manuscript. 

Kai-Uwe Mrkor: Participated in data gathering, contributed to the 

drafting of the manuscript, read, and approved the final 

manuscript. 

Jürgen Edelmann-Nusser: Contributed to the study concept and 

design, contributed to the drafting of the manuscript, read, and 

approved the final manuscript. 

Kerstin Witte: Contributed to the study concept and design, 

analyzed, and interpreted data, contributed to the drafting of the 

manuscript, read and approved the final manuscript. 

SOURCES OF SUPPORT 

German Central Innovation Program for small and medium-sized 

enterprises for the project ‘Multifunctional diagnostic device for 

patients of lower limb amputations’ (ZF4096303TS6). 

ETHICAL APPROVAL 

The study was approved by the local ethics committee of the Otto 

von Guericke University Magdeburg and carried out in line with the 

Declaration of Helsinki (no. of vote: 31/18 on March 19, 2018).  

Signed informed consent was obtained from all participants. 

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classified at different K-levels: A preliminary cross-sectional study. Canadian Prosthetics & Orthotics Journal. 2022; Volume 5, Issue 1, No.5. 
https://doi.org/10.33137/cpoj.v5i1.37456 

ISSN: 2561-987X HIP MUSCLE STRENGTH AND STATIC BALANCE IN PATIENTS WITH TRANSFEMORAL AMPUTATIONS  

John et al., 2022 CPOJ 

 
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