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R E S E A R C H Open Access
Reference values for gait temporal and
loading symmetry of lower-limb amputees
can help in refocusing rehabilitation targets
Andrea Giovanni Cutti
*
, Gennaro Verni, Gian Luca Migliore, Amedeo Amoresano and Michele Raggi
From Second World Congress hosted by the American Orthotic & Prosthetic Association (AOPA)
Las Vegas, NV, USA. 06-09 September 2017
Abstract
Background: The literature suggests that optimal levels of gait symmetry might exist for lower-limb amputees.
Not only these optimal values are unknown, but we also don’t know typical symmetry ratios or which measures of
symmetry are essential. Focusing on the symmetries of stance, step, first peak and impulse of the ground reaction
force, the aim of this work was to answer to three methodological and three clinical questions. The methodological
questions wanted to establish a minimum set of symmetry indexes to study and if there are limitations in their
calculations. The clinical questions wanted to establish if typical levels of temporal and loading symmetry exist, and
change with the level of amputation and prosthetic components.
Methods: Sixty traumatic, K3-K4 amputees were involved in the study: 12 transfemoral mechanical knee users (TFM),
25 C-leg knee users (TFC), and 23 transtibial amputees (TT). Ninety-two percent used the Ossur Variflex foot. Ten
healthy subjects were also included. Ground reaction force from both feet were collected with the Novel Pedar-X.
Symmetry indexes were calculated and statistically compared with regression analyses and non-parametric analysis
of variance among subjects.
Results: Stance symmetry can be reported instead of step, but it cannot substitute impulse and first peak symmetry.
The first peak cannot always be detected on all amputees. Statistically significant differences exist for stance symmetry
among all groups, for impulse symmetry between TFM and TFC/TT, for first peak symmetry between transfemoral
amputees altogether and TT. Regarding impulse symmetry, 25% of TFC and 43% of TT had a higher impulse on the
prosthetic side. Regarding first peak symmetry, 59% of TF and 30% of TT loaded more the prosthetic side.
Conclusions: Typical levels of symmetry for stance, impulse and first peak change with the level of amputation and
componentry. Indications exist that C-leg and energy-storage-and-return feet can improve symmetry. Results are
suggestive of two mechanisms related to sound side knee osteoarthritis: increased impulse for TF and increased
first peak for TT. These results can be useful in clinics to set rehabilitation targets, understand the advancements
of a patient during gait retraining, compare and chose components and possibly rehabilitation programs.
Keywords: Gait, Ground reaction force, Symmetry, Rehabilitation, Amputees, Prosthesis, Osteoarthritis, C-leg,
Microprocessor controlled knees, Energy storage and return feet
* Correspondence: ag.cutti@inail.it
INAIL Prosthetic Center, Via Rabuina 14, 40054 Vigorso di Budrio, BO, Italy
© The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and
reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver
(http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
Cutti et al. Journal of NeuroEngineering and Rehabilitation 2018, 15(Suppl 1):61
https://doi.org/10.1186/s12984-018-0403-x
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Background
Lower-limb amputees tend to walk asymmetrically when
looking at gait temporal and loading parameters, with
more time spent and load exerted on the intact limb [1–9].
Temporal asymmetry is typically measured based on step
or stance duration; loading asymmetry based the magni-
tude of the first peak of the vertical ground reaction force
(GRF), and the impulse of GRF [2,3,6,10].
Temporal and loading asymmetries were associated to
several comorbidities [5]: increased falls [11], osteoarth-
ritis of the sound limb [10,12–15], osteoporosis of the
contralateral limb [15,16], back pain [17–20]. In addition,
walking in public with noticeable asymmetries attracts the
general attention [21], which can be very uncomfortable
for some prosthesis users. With this background, it is not
surprising that a common, almost unquestioned [22], goal
for rehabilitation is to regain a symmetric walking [9,23].
However, the literature does not clearly indicate that
striving for perfect symmetry is really and always the best
option. Already in 1998, Winter & Sienko [1]statedthat
“human system with major structural asymmetries in the
neuromuscular skeletal system cannot be optimal when
gait is symmetrical. Rather, a new non-symmetrical opti-
mal is probably being sought by the amputee within the
constraints of his residual system and the mechanics of
his prosthesis”. Later in 2005, Schmid and co-workers [3]
compared the center of pressure trajectories under the
sound and prosthetic foot of transfemoral amputees and
concluded that the longer stance on the sound side can be
ascribed to the greater ability of the sound leg to advance
the step and maintain balance until the prosthetic limb
can sustain the body weight. Hof et al. [4] corroborated
this explanation in the theoretical framework of the
“extrapolated center of mass”[24], concluding that stance
time asymmetry is a “sensible adaptation”of experienced
transfemoral amputees to improve stability during walking,
to overcome the missing lateral ankle strategy of prosthetic
feet. More recently, Adamczyk & Kuo [8], with a theoret-
ical and experimental approach involving transtibial ampu-
tees, concluded that “some asymmetry may be unavoidable
in cases of unilateral limb loss”due to the reduced ankle
plantar flexion of the ankle, with direct consequences on
stance duration, greater collision work at the sound side,
greater work overall, and increased peak force at loading
response [25–27]. Imposing symmetry can actually be
detrimental, as also observed by [27,28].
The evidences from the literature, therefore, indicate that
optimal symmetry ratios might exist, to obtain a com-
promise among stability, forward progression, preservation
of body structures and perception of a “normal and sym-
metric biped locomotion”[21]. Unfortunately, at present
not only optimal symmetry ratios are unknown, but we
also don’t know typical symmetry ratios or which measures
of symmetry are essential and which are redundant.
In our opinion, 3 methodological and 3 clinical questions
should be answered to clarify these open issues. The
methodological questions are:
–Q1: do all amputees show the typical M-shaped pattern
of the GRF [29], with presence and appropriate timing
of its two peaks? In case of a negative answer, the
measure of loading symmetry based on the first
peak of GRF will be restrict to patients presenting
the M-shaped pattern;
–Q2: can we limit the study of temporal symmetry to
stance, leaving out step symmetry? We will give a
positive answer if stance and step symmetries are
very strongly correlated for all amputees, with a
coefficient of determination R
2
> 0.64 [30];
–Q3: can we further limit the study of gait symmetry
to just stance symmetry, leaving out loading
symmetry, whose measure requires more
cumbersome and expensive equipment? We will give
a positive answer if stance symmetry is very strongly
correlated (R
2
> 0.64) with the symmetry of the first
peak and impulse of GRF.
The clinical questions are:
–Q4: does gait symmetry depend on the level of
amputation? In case of a positive answer, typical
ranges of symmetry should be established, which
can be used to understand how far a new patient is
from well adapted prosthesis users in terms of
percentiles;
–Q5: do advanced prosthetic components improve
temporal and loading symmetry? In particular, do
C-leg users have better results than mechanical knee
users of the same mobility level?
–Q6: is it always true that amputees overload the
sound side both in terms of first peak and impulse
of GRF, thus contributing to the development of
osteoarthritis?
Unfortunately, at present it is difficult to answer to
these questions based on the available literature, because
there are no studies that considered, at the same time 1)
both temporal and loading asymmetries, 2) both trans-
femoral and transtibial amputees treated at the same
prosthetic & rehabilitation center, 3) mechanical and
electronic knees, 4) energy-storage-and-return feet instead
of the SACH (Solid-Ankle Cushion-Heel) foot. Moreover,
the number of patients included is typically limited to 8,
both for studies on transtibial and transfemoral amputees.
Finally, no studies addressed the correlation between
temporal and loading parameters.
The aim of this study was to overcome these limita-
tions and answer to questions Q1-Q6 on three groups of
Cutti et al. Journal of NeuroEngineering and Rehabilitation 2018, 15(Suppl 1):61 Page 30 of 72
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well-adapted, traumatic, K3-K4 amputees: transfemoral
amputees using a restricted set of mechanical knees
(TFM), transfemoral amputees using the C-leg (TFC),
transtibial amputees using energy-storage-and-return feet
(TT). A additional group of healthy control subjects
(“Controls”in short), was also included to highlight
general trends.
Methods
Subjects
Sixty K3-K4 lower-limb amputees participated in the
study after signing an informed consent: 12 mechanical
knee users (TFM, 46± 10 y.o.), 25 C-leg users (TFC, 48 ±
13 y.o), 23 transtibial amputees (TT, 44 ± 14 y.o.), with no
statistically significant differences in term of age (ANOVA,
p> 0.62). Ten controls were also included (28 ± 2 y.o.). All
amputees had completed a 3-week, intense gait training
program at the same specialized prosthetic & rehabilitation
center, with the support of the same rehabilitation team.
The clinical center has ISO 9001 treatment pathways for
amputees and provides over 800 transfemoral and 1200
transtibial prostheses every year. Following training, all
patients had been successfully using their prostheses for at
least1monthatthetimeoftesting.
The components provided to patients are summarized
in Table 1. Almost 92% of patients used either the Variflex
or Variflex LP foot. Mechanical knees were selected to
match the activity level of the C-leg, and are consistent
with knees selected for comparison with the C-leg in
previous studies [31,32].
Measurements
After standing still for 10 s, subjects walked along a long
indoor hall at self-selected speed, that was noted. During
this trial, the GRF was measured on each side through
instrumented insoles (Pedar-X, Novel, D), sampling at
100 Hz [33,34].
Data processing
For each subject, GRF data were export to MATLAB.
Based on the 10 s’orthostatic posture, body weight was
calculated. Assuming a foot-floor contact threshold at
10% body weight, we detected heel-strike and toe-off
events for the two sides. We isolated the steady state
condition by considering the central 10 strides.
Calculation of temporal symmetry
For each stride, we calculated the step and stance duration.
Then, for each couple of consecutive sound-affected gait
cycles, we calculated the following indexes of symmetry:
–Step Symmetry (SPS): Step Duration
SOUND
/ Step
Duration
AFFECTED
–Stance Symmetry (SNS): Stance Duration
SOUND
/
Stance Duration
AFFECTED
For Controls, ratios were right over left side. A value
of 1 represents perfect symmetry. For each index of
symmetry, we calculated the subject’s median value
over the trial. Finally, we obtained the distribution of
the median values for the two indexes over TFM, TFC,
TT and Controls.
Calculation of loading symmetry
For each gait cycle, the integral over the stance period of
GRF was calculated, i.e. the impulse of GRF, as previously
reported by [2]. Then, for each couple of consecutive
sound-affected gait cycles, we calculated the index of
symmetry:
–Impulse Symmetry (IMS): Impulse
SOUND
/
Impulse
AFFECTED
A value of 1 represents perfect symmetry. Right over
left side was used for Controls.
Afterward, the GRF profile of each gait cycle was
checked to verify the presence of the first peak within
the 0–40% of the gait cycle, and of a second peak within
the 60–100%. Subjects reporting both peaks in more
than half of the trials formed the “Two-Peaks”subgroup.
For the subjects in Two-Peaks we operated as follows.
For each couple of consecutive sound-affected gait cycles,
we calculated the following index:
–First Peak Symmetry (P1S): First peak
SOUND
/
First peak
AFFECTED
P1S provides a measure of peak force asymmetry at
loading response, while IMS provides a measure of the
asymmetry in cyclic loading. These are two different
mechanism of osteoarthritis development [10,35–37].
For each index of symmetry, we calculated the subject’s
median value over the trial. Finally, we obtained the distri-
bution of the median values for the two indexes over
TFM, TFC, TT and Controls.
Table 1 Prosthetic components used and associated quantities
TFM TFC TT
Foot Variflex LP: 10
1C40: 2
Variflex LP: 25 Variflex: 18
Variflex LP: 2
Truestep: 1
Esprit: 1
1C40: 1
Knee TotalKnee 2100: 5
3R60: 2
Mauch: 2
C-leg: 25
Cutti et al. Journal of NeuroEngineering and Rehabilitation 2018, 15(Suppl 1):61 Page 31 of 72
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Statistical analysis
The distribution of the four indexes of symmetry (SPS,
SNS, IMS and P1S) was checked for normality within
each group (TFM, TFC, TT and Controls) and over all
subjects, both visually with the Normal Probability Plot
and with the Lilliefors test. This last failed for SPS
TFM
and P1S
TT
and there were doubts about IMS in general.
The relationship between SNS and the three indexes
SPS, IMS and P1S was evaluated with regression methods
with the MATLAB Curve Fitting Toolbox. The strength
of the relationship was primarily evaluated in terms of R
2
.
This statistical parameter, multiplied by 100, is usually
interpreted as the variance of “y”accounted for by “x”,
where in this case “y”is SPS or IMS or P1S, and “x”is
SNS. In addition, the root-mean-square error (RMSE) of
the residuals was also reported.
Distributions were reported in terms of median and
interquartile range [3], with box plots. For each symmetry
index, the Kruskal-Wallis test (α= 0.05) was adopted to
check for overall statistically significant differences among
TFM, TFC, TTand Controls. In identifying pairwise differ-
ences, the Tukey-Kramer “HSD”correction was applied
within the MATLAB “multcompare”function.
Results
Gait speed was compared among TFM (1.12 ± 0.13 m/s),
TFC (1.17 ± 0.12 m/s), TT (1.23± 0.19 m/s) and Controls
(1.41 ± 0.21 m/s). ANOVA did not show statistically
significant differences among amputees (p=0.14), but
only between Controls and amputees (p= 0.0005).
Further results are reported hereinafter based on their
relevance for questions Q1-Q6.
Question Q1
Figure 1reports the number of subjects in subgroups
Two-Peaks, which decreases from TT (20/23), to TFM
(7/12) to TFC (10/25). The number of TFC with non-
standard GRF is remarkably high (60%); these patients
report a consistent “alternative”pattern (example provided
in Fig. 1b). Based on these results, the answer to Q1 was
negative and the calculation of the symmetry index P1S
was restricted to the subjects in Two-Peaks.
Question Q2
Figure 2reports the regression analysis for SPS vs SNS
considering the whole set of patients and Controls
(“ALL”in brief ). R
2
and RMSE values for each group
Fig. 1 aNumber of subjects in subgroup Two-Peaks for TFM (transfemoral mechanical knee users), TFC (transfemoral C-leg users), TT (transtibial
amputees), and Controls: btypical alternative vertical ground reaction force pattern shown by TFC patients not included in Two-Peaks
Cutti et al. Journal of NeuroEngineering and Rehabilitation 2018, 15(Suppl 1):61 Page 32 of 72
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(TFM, TFC, TT, Controls) and ALL are reported in
Table 2.R
2
was at least 0.70 for all amputees, with
RMSE < 0.042. Therefore, the answer to Q2 was positive
and only SPS was further considered.
Question Q3
Figure 3a and breport the regression analysis for IMS vs
SNS and P1S vs SNS, respectively, for ALL. R
2
and
RMSE values for each group (TFM, TFC, TT, Controls)
and ALL are reported in Table 2. For IMS vs SNS, R
2
was lower than 0.64 for TT, with RMSE > 0.128. For P1S
vs SNS, R
2
was lower than 0.2 for all amputees. Therefore,
the answer to Q3 was negative and IMS, P1S and SPS
were separately considered in all subsequent analyses.
Questions Q4-Q6
Figures 4a,5a and 6a report the distribution of SNS,
IMS and P1S for TFC, TFC, TT and Controls. Numerical
values are reported in Table 3.
For SNS and IMS, the Kruskal-Wallis test showed
statistically significant differences among the medians
of the groups (p< 0.0001) (Figs. 4b and 5b). The pairwise
analyses for:
SNS (Fig. 4c) showed that all amputee groups are
different among each other, supporting a positive
answer for Q4 and Q5;
IMS (Fig. 5c) showed a statistically significant
difference between TFM and all other groups, with
all TFM values > 1 as opposed to TFC and TT. This
supports a partially positive answer to Q4, a positive
answer to Q5 and a negative answer to Q6.
For P1S, the Kruskal-Wallis test reported a statistically
significant difference in the medians among groups
(p= 0.0443) (Fig. 6b). The pairwise comparison did
not show differences (Fig. 6c). This is a very possible
situation for three reasons:
the Kruskal-Wallis and pairwise comparisons try to
negate different hypotheses;
we applied a quite conservative multiple comparison
strategy (HSD);
the statistical power is reduced by the decreased
number of transfemoral amputees (TF) within
Two-Peaks.
Fig. 2 Step symmetry index (SPS) vs Stance symmetry index (SNS). Each dot represents one subject. Subjects of the same group feature the same
color (see legend in the plot). The purple parabolic line is the regression line for ALL subjects together. The equation of the fitting is reported on
the right, with the fitting quality parameters R
2
(coefficient of determination) and RMSE (Root Mean Squared Error)
Table 2 Quality of fit of the regressions for step (SPS), impulse
(IMS) and first peak symmetry (P1S) indexes vs stance symmetry
index (SNS)
SPS vs SNS IMS vs SNS P1S vs SNS
R
2
RMSE R
2
RMSE R
2
RMSE
TFM 0,97 0,042 0,81 0,137 0,06 0,154
TFC 0,81 0,042 0,69 0,090 0,04 0,177
TT 0,70 0,036 0,37 0,128 0,20 0,289
CONTROLS 0,51 0,017 0,47 0,040 0,05 0,038
ALL 0,95 0,037 0,79 0,103 0,00 0,247
The coefficient of determination (R
2
), and the Root Mean Squared Error (RMSE)
are reported for every group (TFM transfemoral mechanical knee users, TFC
transfemoral C-leg users, TT transtibial amputees, Controls), and for all subjects
altogether (ALL). Bold: R
2
> 0.64, Regular: 0.36 < R
2
< 0.64, Italic: R
2
< 0.36 [30]
Cutti et al. Journal of NeuroEngineering and Rehabilitation 2018, 15(Suppl 1):61 Page 33 of 72
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For this reason, we grouped subjects per level of ampu-
tation (TFM and TFC together), and results are reported
in Fig. 7a. The Kruskal-Wallis test now shows a stronger
significance among groups (p= 0.0186) and the pairwise
analysis shows a statistically significant difference between
TF and TT. The variability in P1S is much higher in am-
putees than in Controls (Bartlett’s test for equal variances,
p= 0.001). These results support a negative answer to Q6.
Discussion
In this study, we addressed three methodological and
three clinical questions regarding the temporal and loading
symmetry of transfemoral amputees (both mechanical and
C-leg users) and transtibial amputees, to support in the
development of more targeted rehabilitation goals, that are
particularly needed [9,38].
As a general consideration, the self-selected walking
speed was not statistically different among amputees,
despite a slight increase in the median from TFM, to
TFC, to TT toward Controls. Absolute values compare
well with previously reported data [2,7,39].
For the sake of clarity, results are discussed below for
each question, in comparison with the available literature
whenever possible.
Question Q1
Question Q1 asked if all amputees show the typical
M-shaped pattern of the GRF, with presence and
Fig. 3 aImpulse symmetry index (IMS) vs Stance symmetry index (SNS) and bFirst peak symmetry index (P1S) vs SNS. Each dot represents a
subject. Subjects of the same group feature the same color (see legend in the plot). In (a), the purple parabolic line is the regression line for ALL
subjects together. The equation of the fitting is reported on the right, with the fitting quality parameters R
2
(coefficient of determination) and
RMSE (Root Mean Squared Error). No valid regression was found for P1S vs SNS. TFM: transfemoral mechanical knee users, TFC: transfemoral C-leg
users, TT: transtibial amputees
Cutti et al. Journal of NeuroEngineering and Rehabilitation 2018, 15(Suppl 1):61 Page 34 of 72
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appropriate timing of its two peaks. Results support a
negative answer.
As previously noted, this is particularly evident for TFC,
who presented a consistent “alternative”pattern: after a
steep rise (initial contact/loading response), GRF shows a
further (almost) monotonical increase (midstance), after
which it drops (terminal stance/pre-swing). TFM falling
out of Two-Peaks did not present this pattern, and were
typically not included in Two-Peaks due to a delayed P1
after 40% of the stance phase. Since no kinematic and
Fig. 4 aBox plot for the stance symmetry index (SNS) over the groups; bResults of the Kruskal-Wallis test; cPairwise comparisons:
non-overlapping lines indicate a statistically significant difference. TFM: transfemoral mechanical knee users, TFC: transfemoral C-leg
users, TT: transtibial amputees
Fig. 5 aBox plot for the impulse symmetry index (IMS) over the groups; bResults of the Kruskal-Wallis test; cPairwise comparisons: non-overlapping
lines indicate a statistically significant difference. TFM: transfemoral mechanical knee users, TFC: transfemoral C-leg users, TT: transtibial amputees
Cutti et al. Journal of NeuroEngineering and Rehabilitation 2018, 15(Suppl 1):61 Page 35 of 72
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kinetic data were collected, we can just speculate that this
TFC pattern is the combined effect of:
the Variflex behavior, with strong energy storage in
loading response [25,26];
C-leg knee flexion in loading response [40];
theconfidencegainedbythisgroupofamputees
on the capacity of the C-leg to sustain them at
heel-strike and loading response, with no need to
force extension.
The ultimate effect for this pattern is a “soft landing”
on the prosthetic side, which might increase comfort
[32]. These speculations require future experimental
confirmations, but match well with previous evidences
that only a fraction of transfemoral amputees can fully
rely on C-leg stability despite knee flexion during early
stance [32,40]. This might be the effect of a specialized
rehabilitation.
Question Q2
Question Q2 asked if we can limit the study of temporal
symmetry to stance leaving out step symmetry. Results
support a positive answer.
The regression of SPS vs SNS for each group and for
ALL was quadratic, with excellent fits.
SNS explained from 70 to 97% of the variance in SPS
data in amputees (R
2
, as reported in Table 2). Even for
Controls, who feature a very small peak-to-peak SNS
(.97 to 1.01), the explained SPS variance is 50% with a
RMSE as small as 0.017.
Fig. 6 aBox plot for the first peak symmetry index (P1S) over the groups; bResults of the Kruskal-Wallis test; cPairwise comparisons: non-
overlapping lines indicate a statistically significant difference. TFM: transfemoral mechanical knee users, TFC: transfemoral C-leg users, TT:
transtibial amputees
Table 3 Numerical values for the indexes of symmetry SNS (stance), IMS (impulse) and P1S (first peak)
SNS IMS P1S
Median 25th 75th IQR Median 25th 75th IQR Median 25th 75th IQR
TFM 1,22 1,20 1,29 0,09 1,32 1,20 1,55 0,36 0,91 0,90 1,06 0,16
TFC 1,11 1,05 1,15 0,09 1,16 1,00 1,24 0,24 0,98 0,80 1,04 0,24
TF 0,94 0,87 1,05 0,18
TT 1,03 1,00 1,06 0,06 1,02 0,96 1,13 0,17 1,07 0,98 1,24 0,26
CONTROLS 1,02 0,98 1,01 0,03 0,95 0,91 0,99 0,09 0,98 0,94 0,99 0,05
For each group, the median is reported together with the 25th, 75th and interquartile range (IQR). TFM transfemoral mechanical knee users, TFC transfemoral C-
leg users, TF transfemoral, TT transtibial amputees
Cutti et al. Journal of NeuroEngineering and Rehabilitation 2018, 15(Suppl 1):61 Page 36 of 72
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This is the first time that the SPS vs SNS regression is
reported in the literature and that a quadratic relationship
is described. The nonlinear fit is not surprising, because
SPS is non-linearly related to the interplay of 1) the
sounds and affected side stance durations and 2) the two
double support durations. The quadratic fit stresses the
importance of stance time symmetry, since it influences
step asymmetry by a factor 2.
Question Q3
Question Q3 asked if the study of gait symmetry can be
limited to just stance temporal symmetry, leaving out
loading symmetry. Results support a negative answer.
When IMS vs SNS was examined considering the full
set of subjects, a quadratic fit emerged: SNS explained
as much as 79% of the variance in IMS. This is the first
time this relationship is examined and reported. Since
IMS is the integral of GRF over the stance phase, it is
not surprising that IMS and SNS are related: a high
stance time asymmetry is a leading factor for a high
impulse asymmetry. However, GRF magnitude does
not linearly increase with time, and has a shape which
can differ between the sound and affected side. When
all these elements become part of a ratio, it is not
surprising that the relation between IMS and SNS can
be non-linear.
This conclusion is valid for TFM and TFC at group
level too, given the R
2
> 0.64. However, this is just
partially true for TT, because R
2
decreases to 0.37 and
the RMSE is high (0.128): reporting SNS and not IMS
can be misleading. This different evidence for TT can
be ascribed to two factors only:
The improvement in SNS asymmetry (1.03, IQR
0.06) compared to TFM (1.22, IQR 0.09) and TFC
(1.11, IQR 0.09) (Table 3);
A greater asymmetry in GRF magnitude between
sides. This is supported by the evidences for P1S, as
reported in Fig. 7. Further discussions are postponed
to Q6 below.
An adequate regression for P1S vs SNS was not found
for none of the groups and ALL: the two indexes must
measure different construct and therefore they must be
separately reported.
Question Q4
Question Q4 asked if gait symmetry depends on the
level of amputation. Results support a positive answer.
With reference to SNS, all amputee groups had statisti-
cally different median values. All TF spend more time on
the sound side: TFM have the highest asymmetry (median
asymmetry of 22%), which is twofold the TFC’s(11%).As
canbeseeninFig.4, this is also true for 75% of TT, which
means that ¼ of TT do spend more time on the affected
side. This was never clearly reported in the literature. The
TT asymmetry (3%) is 4 times less than TFC. Controls, in
median, have a perfect symmetry, with a IQR of just 3%.
Fig. 7 aBox plot for the first peak symmetry index (P1S) after grouping all transfemoral amputee together (TF); bResults of the Kruskal-Wallis
test; cPairwise comparisons: non-overlapping lines indicate a statistically significant difference. TF: transfemoral amputees, TT: transtibial amputees
Cutti et al. Journal of NeuroEngineering and Rehabilitation 2018, 15(Suppl 1):61 Page 37 of 72
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The SNS median for TFC (1.11) compares well with
the median SNS that can be calculated from the results
reported in [3] (1.09). Furthermore, our results can be
compared with the study of Nolan et al. [2], that involved
4 transfemoral and 4 transtibial amputees using a single
hinge knee and a SACH foot. Once appropriately converted
to our indexes, Nolan’s results are reported in Table 4.
Results, can also be compared with Bateni et al. [41], which
reported a mean stance asymmetry for TT of about 7%
(calculated as the ratio of the mean between sides).
Compared to these studies, our SNS values are lower.
In particular, 63% of TT and 20% of TFC have a SNS
lower than ±5%, which makes them unperceived by
others as “impaired”walkers with regard to temporal
symmetry [21]. This is not surprising given the different
prosthetic components used and the fact that our
patients followed a specialized rehabilitation training.
Our SNS results for TT are also in very good agree-
ment with results reported by Jarvis et al. [38]for
young veterans (median 1.04, IQR = 0.03). For TFC,
our SNS is higher (1.11 compared to 0.98) but the IQR
is much smaller (0.09 compared to 0.20). This remarks
that the training for transfemoral amputees is more
challenging.
When looking at IMS, the TFM median was statisti-
cally different from TFC and TT: TFM asymmetry is
twice that of TFC and 16 times TT’s. The comparison
with Nolan et al. [2] is striking: our TFM had an impulse
asymmetry which is half Nolan’s; for TT it is 10 times
less. This result points, again, in the direction of the
benefits of energy-storage-and-return feet and more
advanced knees. Improvement in loading asymmetry with
energy-storage-and-return feet and feet with improved
roll-over shape has been previously reported in [25,27,42],
and match well with simulation studies [8].
Finally, P1S results show statistically significant differ-
ences between TF and TT (Fig. 7). About 59% of TF have
a higher peak on the prosthetic side. Our results agree with
Castro et al., which did not report an increased peak GRF
on the sound side, but rather an increase in the GRF im-
pulse. TT clearly show an asymmetric loading with higher
values for the sound side (70% of patients), but 3 times less
than that reported by Nolan and co-workers. As previously
reported, it is reasonable to ascribe this improvement
to the use of energy-storage-and-return feet compared
to SACH [27,43].
Question Q5
Question Q5 asked if advanced prosthetic components
improve temporal and loading symmetry, and if C-leg
users have better results than mechanical knee users of
the same mobility level. Results support a positive answer.
Results have been partially discussed while addressing
Q1 and Q4 and can be summarized stating that TFC were
statistically different from TFM for SNS and IMS. Results
for IMS bring TFC to undistinguishable results to TT.
Also, the C-leg in combination with Variflex triggers a
new GRF pattern that possibly ensures an increased
comfort during walking (Question Q1). This requires fur-
ther experimental confirmations.
Petersen et al. [44] have previously reported about
SNS in C-leg users compared to TFM. However, that
study was not able to prove a statistically significant
improvement but just a trend, probably due to the
small number of subjects included (5) with different
amputation etiologies. Our results confirm that trend,
with statistically significant differences. More generally,
a considerable body of knowledge is available about
the positive effects of the C-leg on amputees’mobility
[31,45–47], gait kinematic [32–40], kinetic [39]and
step-length symmetry [32]. Our findings match well
with this general trend toward improved symmetries.
As discussed in Q4, the comparison of the literature
with our results for TT suggests a possible positive effect
of energy-storage-and-return feet in comparison with
SACH, for all the indexes of symmetry.
Question Q6
Question Q6 asked if it is always true that amputees
overload the sound side both in terms of first peak and
impulse of GRF, thus contributing to the development of
osteoarthritis. Results support a negative answer.
As previously discussed about Q4, if we focus on IMS,
100% of TFM overload the sound side. This percentage
decreases to 75% of TFC and 57% of TT. If we look at
P1S, 41% TF load more the sound side. However, this
percentage rises to 70% for TT. Based on these different
percentages of TT and TF for IMS and P1S, it could be
argued that two different mechanisms might be related
to knee osteoarthritis for the two groups: peak overload
for TT (measured by P1S), and extended duration of force
action (impulse) for TF (measured by IMS). Given the
higher prevalence of knee osteoarthritis in TF compared
to TT [5,10], it might be speculated that the second
mechanism is more detrimental than the first.
Table 4 Results from Nolan et al. [2], converted to the indexes
of symmetry used in this study. SNS (stance), IMS (impulse) and
P1S (first peak)
SNS IMS P1S
TFM 1,27 1,69 1,22
TT 1,05 1,36 1,25
CONTROLS 1,03 1,08 1,08
Having named Nthe indexes in [2], the new values follow from this
equation: New =(2+N)/(2-N)
TFM transfemoral mechanical knee users, TT transtibial amputees
Cutti et al. Journal of NeuroEngineering and Rehabilitation 2018, 15(Suppl 1):61 Page 38 of 72
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
Conclusions
In the Introduction, we posed three methodological
and three clinical questions regarding the gait temporal
and loading symmetry of lower-limb amputees. Based
on the results collected on traumatic, K3-K4, transfemoral
(mechanical knees and C-leg users) and transtibial patients
successfully fit and trained in using their prosthesis, we
can answer as follows.
The three methodological questions wanted to establish
a minimum set of symmetry indexes to study and if there
are limitations in their calculations. First, the first peak of
the vertical ground reaction force at loading response
cannot be clearly identified in all amputees, and the
calculation of its index of symmetry was limited to patients
with the typical M-Shaped pattern of the ground reaction
force. Second, the analysis of temporal symmetry can be
limited to stance, leaving out step symmetry. Third, stance,
impulse and first peak symmetries should be separately
reported.
The three clinical questions wanted to establish if “typical”
levels of temporal and loading symmetry exist and change
with the level of amputation and prosthetic components.
First, the symmetries of stance, impulse and first peak are all
influenced by the level of amputation. In particular, the time
spent on the sound side decreases significantly from transfe-
moral mechanical knee users, to C-leg users, to transtibial
patients. The impulse on the sound side decreases signifi-
cantly from mechanical knee users to C-leg and transtibial
patients. Transtibial patients have a higher first peak at load-
ing response on their sound side, while most transfemoral
patients do not. Second, advanced prosthetic component
seem to positively influence the temporal and loading sym-
metry. In particular, the C-leg in combination with the Vari-
flex foot improves stance, impulse symmetry and for about
60% of patients smooths the first peak at loading response.
About 20% of C-leg users have a stance asymmetry which is
below the level of perceived impaired gait, compared to 0%
of mechanical knee users. For transtibial patients, compari-
sons of our results with the literature point toward an
improvement of all indexes of symmetry, possibly due to the
use of energy-storage-and-return feet instead of SACH feet.
Third, it is not always true that amputees overload the
sound side. Percentagewise, transfemoral amputees tend to
overload the sound side with increased impulse, while TT
with increased peak GRF. This might be suggestive of two
separate mechanisms for the onset of knee osteoarthritis.
We think that our results can be exploited in the clinical
routine. First, clinicians can use our results to set reason-
able targets for rehabilitation. Specifically, they can
compare the level of symmetry of a new patient with
the ranges provided, and put the patient’s performance
and advancements during rehabilitation in perspective.
Moreover, technical and healthcare professionals might
use our findings to compare the effect of different
prosthetic components and potentially the effect of dif-
ferent rehabilitation programs. Second,itisoftenre-
quired by payers (e.g. insurances, public healthcare
services, or patients), to justify the use of advanced
prosthetic components. We think that our results sup-
port the use of C-leg and energy-storage-and-return feet
on K3-K4 traumatic patients: thanks to the improvement
in temporal and loading symmetry compared to mechan-
ical knees and SACH foot, these components can poten-
tially have a positive effect on the asymmetry-related
comorbidities analyzed in the Introduction and de-
crease social stigma. Further research is required to ex-
tend these results to other groups of patients, such as
K2 and non-traumatic amputees. Finally,ourresults
might suggest possible strategies to mitigate knee
osteoarthritis of the sound side. Pending further re-
search, transfemoral amputees might take advantage of
prosthetic components with an improved knee-foot co-
ordination to specifically tackle stance time asymmetry.
Transtibial patients might benefit from improved socket
construction that does not limit knee extension, and pros-
thetic feet with improved push-off, roll-over shape and
range of motion to reduce the first peak at loading
response.
Abbreviations
GRF: Vertical component of the ground reaction force; IMS: Impulse
symmetry index; P1S: Symmetry index of the first peak of the ground
reaction force; SNS: Stance duration symmetry index; SPS: Step duration
symmetry index; TF: Transfemoral amputees; TFC: Transfemoral amputees
using a C-leg knee (Ottobock, D); TFM: Transfemoral amputees using a
mechanical knee; TT: Transtibial amputees
Funding
This research was conducted with internal institutional funds of INAIL. The
publication cost of this article was funded by the American Orthotic &
Prosthetic Association (AOPA).
Availability of data and materials
All data generated or analysed during this study are included in this
published article.
About this supplement
This article has been published as part of Journal of NeuroEngineering and
Rehabilitation Volume 15 Supplement 1, 2018: Advancements in Prosthetics
and Orthotics: Selected articles from the Second World Congress hosted by
the American Orthotic & Prosthetic Association (AOPA). The full contents of the
supplement are available online at https://jneuroengrehab.biomedcentral.com/
articles/supplements/volume-15-supplement-1.
Authors’contributions
AGC, MR and GV designed the experiment. MR and AGC collected and
processed the data. All Authors contributed to data analysis and manuscript
preparation. All authors read and approved the final manuscript.
Ethics approval and consent to participate
The Centro Protesi institutional scientific committee approved the study.
Consent for publication
Not applicable.
Competing interests
The authors declare that they have no competing interests.
Cutti et al. Journal of NeuroEngineering and Rehabilitation 2018, 15(Suppl 1):61 Page 39 of 72
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Publisher’sNote
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
Published: 5 September 2018
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