Single-Leg Assessment of Postural Stability After Anterior Cruciate Ligament Injury: a Systematic Review and Meta-Analysis
Lehmann et al. Sports Medicine - Open
Single-Leg Assessment of Postural Stability After Anterior Cruciate Ligament Injury: a Systematic Review and Meta-Analysis
Tim Lehmann 0 1
Linda Paschen 1 2
Jochen Baumeister 0 1
0 Exercise Neuroscience & Health Lab, Institute of Health, Nutrition and Sport Sciences, University of Flensburg , Campusallee 2, 24943 Flensburg , Germany
1 O E , e s i a r
2 Exercise Science, Department of Exercise & Health, Faculty of Science, Paderborn University , Paderborn , Germany
Background: Previous reports of single-leg assessment demonstrated functional deficits in postural stability following anterior cruciate ligament (ACL) injury. However, quantified measures describing postural stability vary among investigations and results seem not to be clear. The first aim of this systematic review was to quantify postural deficits in eyes open single-leg stance in patients after ACL injury. Moreover, the second aim was to examine the potential of traditional center of pressure (CoP) measures in order to distinguish postural stability between ACL patients and healthy controls. Methods: A systematic literature search in the databases PubMed and Scopus was conducted from their inception to December 2016 to identify relevant articles. Eligibility criteria were limited to controlled trials of eyes open static single-leg stance on a force or pressure plate recording CoP measures in patients after ACL injury. Results: Eleven studies were included, involving a total of 329 ACL-injured and 265 control subjects. Randomeffects meta-analysis showed significantly increased sway magnitudes (SMDwm = 0.94, p = 0.003) and velocities (SMDwm = 0.66, p = 0.0002) in the ACL group compared to the healthy controls. Sway magnitude in anteroposterior (SMDwm = 0.58, p = 0.02) and mediolateral (SMDwm = 1.15, p = 0.02) direction were significantly increased in ACL patients. No differences were found for the non-injured side. Similarly, no differences have been observed among ACL patients between the injured and non-injured side for sway velocity, while sway magnitude significantly differed (SMDwm = 0.58, p = 0.05). Conclusions: The findings of this systematic review and meta-analysis demonstrated decreased postural stability in individuals with ACL injury. Sway magnitude and velocity were significantly increased in the ACL group compared to the healthy controls. Although the included research still exhibited considerable heterogeneity, it may be proposed that fundamental CoP measures are suitable to differentiate patients after ACL injury and healthy controls with respect to postural stability in eyes open single-leg stance.
This is the first systematic review and meta-analysis
examining gold standard center of pressure
measures in patients with injury to the anterior cruciate
ligament, irrespective of clinical treatment.
Injury to the anterior cruciate ligament is associated
with increased sway magnitudes and velocities during
a standardized single-leg stance in the injured leg.
Gold standard center of pressure measures allow for
a functional distinction between subjects with and
without injury to anterior cruciate ligament in terms
of postural stability.
Injuries to the anterior cruciate ligament (ACL) are the
most frequent knee injuries in sports and cause immediate
disability for athletes followed by long-term consequences
in terms of functional deficits in motor coordination
3, 17, 25, 55
]. Generally, functional stability of the
knee joint during voluntary movement is predominantly
regulated by the sensorimotor system. As a dynamic
system, it contributes to the transmission and integration
of somatosensory, vestibular, and visual information to the
central nervous system, in order to provide adaptability to
the environment [
8, 31, 56
]. Alterations of afferent
sensory information, potentially caused by
mechanoreceptor damage, may subsequently contribute to
disturbances of sensorimotor control [
1, 25, 59
on this rationale, research has shown altered
sensorimotor control after ACL injury [
4–6, 10, 15, 19, 44
whereas contradictory findings indicate that these
alterations may not necessarily be correlated with
postural control of standing balance [
15, 23, 34
Postural control is defined as the ability to monitor body
position and alignment in space, involving multimodal
interactions of musculoskeletal and neural systems [
]. It is
comprised of two components: postural orientation and
postural stability. While postural orientation describes the
visually and vestibular-guided ability of monitoring the
interrelationship between body segments relative to the
environment, postural stability predominantly
incorporates somatosensory information to control the center of
mass (CoM) in relationship to the base of support [
To date, center of pressure (CoP) trajectories, as the
vector of total force applied to the center of the supporting
], are measured by laboratory-based force or
pressure-sensitive platforms in order to assess postural
12, 30, 47, 48
]. After ACL injury, measures
describing postural stability vary among investigations and
results seem not to be clear [
Postural stability as a crucial determinant for
functional movement reflects a multimodal interaction of the
sensorimotor system [
]; however, there is no gold
standard to assess postural stability in patients after ACL
injury. Therefore, the purpose of the present systematic
review and meta-analysis was to investigate postural
stability after ACL injury. The second aim was to examine
the potential of CoP measures to distinguish postural
stability between ACL patients and healthy controls.
Given that there is no clear consensus about the feasibility
of these measures in ACL research, this meta-analysis on
postural stability measures may further the development
of valid tools to examine functional outcomes after
rehabilitation or reconstruction in ACL patients.
Conducting this meta-analysis, the Preferred Reporting
Items for Systematic Reviews and Meta-Analyses
(PRISMA) provided by Moher et al. [
] were followed
and adapted to the current data properties.
A systematic literature search in the databases PubMed
and Scopus was conducted from their inception to
December 2016 to capture all pertinent articles
investigating postural stability in ACL patients. The search strategy
included the key terms: (postural control OR postural
balance OR vestibular OR posture OR balance) AND
(“ACL” OR “anterior cruciate ligament”). Since there is no
universal definition of postural control and balance, this
search strategy comprised a widespread spectrum in order
to cover all potentially relevant studies. Search limitations
were imposed to full access articles in English language
and studies investigating human species. Additionally,
reference lists of articles found were inspected, and relevant
review articles [
20, 28, 41
] were scrutinized to identify
The inclusion criteria for this meta-analysis were as
follows: (1) controlled trials of post-injury postural stability
in patients after ACL injury, (2) static postural stability
tests in single-leg stance utilizing force or pressure
plates, (3) subjects of all ages and sexes without any
neurological or psychological diseases or history of lower
limb musculoskeletal surgery, and (4) investigations
reporting at least one primary outcome measure of static postural
stability based on the CoP. Due to standardization
demands, the testing protocol was limited to ordinary joint
loading tasks that allow for functional assessment of the
ACL-injured and ACL-non-injured limbs. Therefore, any
papers not meeting these criteria, solely investigating
dynamic tasks, double leg stance or eyes closed, just as effect
or interventional studies were not eligible for inclusion.
Based on the predetermined inclusion criteria, records
were identified and screened through database searching.
Records of both databases were then merged, and
duplicates were removed using Mendeley Desktop (v.1.17,
Mendeley Ltd., London, UK). Two independent
reviewers (TL and LP) conducted the study selection. If
the included studies did not report means, standard
deviations, or F values, the corresponding authors were
contacted. In two of three cases, the authors responded
] and the respective study was included, while the
remainder  was excluded from this meta-analysis.
To assess methodological quality of the studies, a
modified Quality Assessment Tool for Observational
Cohort and Cross-Sectional Studies [
independently applied to each included article in order to assess
the internal validity and risk of selection-, information-,
or measurement bias. The tool is composed of 14
criteria inspecting the objectives, population, participation,
exposures, and outcomes of the particular investigations.
In case of this meta-analysis, three criteria (3, 10, 13)
were not applicable in relation to the research objectives
pursued and therefore excepted from the assessment.
The remaining 11 criteria were evaluated on the scale
“yes,” “no,” “not applicable,” “not reported,” or “cannot
determine,” with any response other than “yes” posing a
certain risk of bias. A total score was generated counting
all “yes” responses for each study. On the basis of
recommendations provided by Aderem and Louw [
scores below 50% were considered as “poor,” total scores
between 50 and 75% as “fair,” and total scores above 75%
as “good” methodological quality. Additionally, funnel plots
of the effect size and the standard error were generated for
the included trials in order to assess publication bias.
The outcome measures considered in this review
correspond to basic descriptions of CoP trajectories with
regard to magnitude, direction, and velocity of the
1) The sway amplitude is the mean of all data points
collected for one or more trials.
2) Path line length further represents the total distance
traveled by the CoP over the course of a trial.
3) Area of sway describes the total area covered by the
CoP in both anteroposterior (AP) and mediolateral
4) CoP mean velocity is determined as the total
distance traveled by the CoP divided by time.
5) The maximum speed is calculated as the peak
velocity reached by CoP dislocation across trials.
For each study meeting the inclusion criteria, descriptive
information related to the country of origin, subject
characteristics, sample size, time from injury/surgery to
testing, the research protocol, and associated injuries
were summarized using a customized Excel (Microsoft,
Redmond Washington, USA) spreadsheet. Measures for
these data were means and standard deviations. The
primary outcome measures for the present meta-analysis
were categorized to sway magnitude (sway amplitude,
sway area, path length) and sway velocity (mean velocity,
maximum speed) in total, AP, and ML direction. In two
cases, the data for classified groups of functional recovery
], just as males and females [
], were matched
together by means, since no differentiation was intended for
these subgroups of patients in the current meta-analysis.
If repeated measures or different conditions were
reported, the first or baseline measurement was considered
The main statistical analyses were executed for leg
(injured, non-injured, matched) and direction (total, AP, ML)
for the parameters sway magnitude and sway velocity.
Based on sample size, means/F values, and standard
deviation, the particular effect sizes were calculated as the
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standardized mean difference (SMD) for each CoP
measure and study [
] in order to examine statistical
differences between patients after ACL injury and healthy
controls. The SMDs of all studies were then weighted with
respect to the magnitude of their standard error (SMDwm).
Positive effect sizes indicate better postural stability in the
control group or leg, while negative effect sizes favor the
Using meta-analyses, eight hypotheses were tested for
differences in ACL and healthy subjects with regard to
sway magnitude, sway velocity, injured vs. matched leg,
non-injured vs. matched leg, injured vs. non-injured leg,
anteroposterior sway, and mediolateral sway. All
comparisons were computed with a random-effects model
using Review Manager (v.5.3.5, The Nordic Cochrane
Centre, The Cochrane Collaboration, Copenhagen, DK)
to calculate the overall standardized mean difference of
the respective outcome measures. Further, the 95%
confidence interval (CI) was computed for the individual
and overall effect. Based on recommendations provided
by Cohen [
], effect sizes were interpreted as follows:
0.00 to 0.49 indicate a small effect, 0.50 to 0.79 were
considered a medium, and values greater than 0.80
indicate a large effect. Heterogeneity between trials was
tested and interpreted using I2 percentages. Hereof, the
impact of potential heterogeneity on the results of the
meta-analysis was estimated referring to suggestions
from Higgins [
]: I2 values from 30 to 50% indicate
moderate heterogeneity, values greater than 50% display
a substantial heterogeneity, and values of greater than
75% may be interpreted as considerable heterogeneity.
The literature screening revealed a total of 535 records
through database searching (Fig. 1). Additionally, five
27, 37, 42, 52, 61
] were identified from the
reference lists of eligible articles. Two of them [
qualified for inclusion. Ultimately, 11 studies published
from 1996 to 2016 were included in the meta-analysis.
These investigations enrolled 594 subjects: 329
patients after ACL injury and 265 healthy controls (Table
1). Mean sample sizes were 29.90 ± 20.03 for the patient
group and 24.09 ± 10.38 for the controls. The ACL
group comprised 19.89 ± 7.47 males and 10.44 ± 9.54
females on average (ratio 66%/34%). Similarly, the
control group was composed of 19.50 ± 8.05 males and
8.00 ± 5.13 females (ratio 70%/30%). Both the ACL and
control subjects were physically active for 1–3 days per
] or were involved in team sports like soccer
and basketball [
33, 38, 40, 62
According to the type of treatment after injury, 224
patients underwent surgery and 105 patients were treated
conservatively. Three studies [
13, 27, 53
] reported a mean
time from injury to surgery of 44.74 ± 54.47 weeks, while
the range for all studies varied from 7 days to 7 years.
No study outlined muscular or neurological damages,
but three studies [
13, 38, 53
] depicted a total of 59
patients (26.3%) with associated meniscal tears. None
of the patients in either group had a history of
previous ACL injury. The trial length varied among studies
from 10 to 30 s (20.28 ± 9.14) and was measured in
one to five sets (2.8 ± 1.08). Total recording time,
summing all trials per study, ranged from 30 to 90 s
(50.84 ± 23.87).
The quality assessment tool (Table 2) has shown an
overall methodological appraisal score of 58%,
indicating fair methodological quality according to the
predetermined classification criteria. Most studies (9/11)
reached a level of fair quality, whereas two studies
were classified as poor quality. Furthermore, no
evidence for publication bias was found through funnel
Quantitative Data Synthesis
Eight investigations in ACL patients [
13, 16, 18, 27, 33,
52, 53, 57
] compared sway magnitude of the injured leg
to the matched leg of healthy controls, indicating a
significantly increased total sway magnitude in the ACL
group (Fig. 2, SMDwm = 0.94, p = 0.003, CI = 0.32,
1.56, I2 = 88%). A subsequent analysis also revealed
large effects for increased sway magnitude in the
anteroposterior direction (Fig. 3, SMDwm = 0.58,
p = 0.02, CI = 0.10, 1.06, I2 = 62%) as well as the
mediolateral direction (Fig. 4, SMDwm = 1.15, p = 0.02, CI = 0.18,
2.12, I2 = 89%) for the injured knee [
16, 18, 38, 52
Comparisons of sway velocity in the injured and matched
limb were conducted in seven studies [
13, 18, 38, 40, 52, 57,
]. Collectively, the meta-analysis yielded a medium effect
for sway velocity (SMDwm = 0.66, p = 0.0002, CI = 0.31,
1.00, I2 = 56%), indicating a significant increase in the
ACLinjured leg compared to the control group (Fig. 5).
Four trials assessed total sway magnitude [
16, 27, 33,
] and sway velocity [
38, 40, 52, 57, 62
] for the
ACL-non-injured leg. Among these studies, no
significant difference has been found for comparisons of the
non-injured and controls for neither total sway
magnitude (Fig. 6, SMDwm = −0.12, p = 0.72, CI = −0.74, 0.51,
I2 = 78%) nor sway velocity (Fig. 7, SMDwm = 0.11,
p = 0.42, CI = −0.15, 0.37, I2 < 0.001%).
Statistically increased sway magnitude (Fig. 8,
SMDwm = 0.58, p = 0.05, CI = 0.01, 1.15, I2 = 71%) was
found for the comparison between the ACL-injured
and ACL-non-injured leg [
16, 27, 33, 52, 57
], but no
difference (Fig. 9, SMDwm = 0.42, p = 0.10, CI = −0.08, 0.92,
I2 = 55%) was detected for sway velocity [
38, 52, 57, 62
The objective of this meta-analysis was to quantify
postural stability during single-leg stance in patients after
ACL injury compared to healthy controls. The
comprehensive analysis revealed that postural stability was
decreased in patients after ACL injury. During eyes open
single-leg stance, patients showed significantly increased
sway magnitudes and velocities in the injured limb.
Additionally, postural sway was significantly increased in
AP and ML direction. However, the non-injured side
demonstrated no differences in sway magnitude or
velocity compared to matched controls. Similarly, no
difference in sway velocity has been observed among ACL
patients between the injured and non-injured side, but
sway magnitude significantly differed.
Following the model of Kapreli and Athanasopoulos
], mechanoreceptor damage may lead to a
disturbance of sensory transmission, contributing to alterations
of afferent feedback and stabilizing reflexes that may
implicate increased body sway [
1, 8, 25, 59
]. In line with
Howells et al.  and Negahban et al. [
], the present
findings support that postural sway is altered in patients
after ACL injury. Medium to large effects were found for
increased total sway magnitudes [
13, 16, 18, 52, 53, 57
as well as increased anteroposterior and mediolateral
sway in the injured leg [
16, 18, 38, 52
]. On the other
hand, two studies [
] found decreased postural sway
in ACL patients compared to healthy controls. Differences
in post-injury and post-surgical rehabilitation may explain
the inconsistency of these results. While rehabilitation
protocols commonly include balance training to positively
influence clinical status and postural stability of ACL-injured
patients , healthy controls may not be trained
comparably for specific balance tasks and finally achieving worse
results. Furthermore, the increases in AP and ML direction
are consistent with previous reports [
34, 37, 49, 59
demonstrating postural impairments along these two axes.
Since postural adjustments are limited in the knee joint,
ankle and hip strategies may compensate for modified
conditions to control the center of mass in AP and ML
direction in relationship to the base of support [
46, 50, 54
Although compensational motor strategies may take part,
ACL patients exhibit deficits in postural stability,
supporting the supposition of a systematic change in sensorimotor
The present meta-analysis found no differences of
postural sway between the non-injured and matched control
leg. Previous systematic reviews [
] have shown the
non-injured leg to be affected by ACL injury. Similar to
other reports [
], one study [
] in this meta-analysis
showed less postural sway in the non-injured limb
compared to healthy controls. Nevertheless, other studies
indicated a bilateral deficit of postural stability in
ACLinjured patients [
40, 52, 57
]. Thus, higher-level
sensorimotor control may be affected in addition to sensory
afferent transmission. In fact, Baumeister et al. [
increased cortical processing in the brain related to ACL
injury, also demonstrating significantly higher frontal
brain activity in both the injured and non-injured leg.
However, in contradiction to earlier reports [
], within-group differences of sway magnitudes
were found between the ACL-injured and
ACL-noninjured leg in the present meta-analysis. Future studies
should apply neurophysiological measures to investigate
the underlying mechanisms of sensorimotor processing
after ACL injury.
Parameters of sway velocity were investigated in seven
13, 18, 38, 40, 52, 57, 62
] revealing significant
differences with medium to large effects for the
comparison between the injured and matched leg. The
mean sway velocity is arithmetically related to total path
length of the CoP trajectory. It is usually calculated by
total path length divided by trial duration [
an increase in sway velocity may naturally be
accompanied by increased sway magnitude, as demonstrated in
this meta-analysis. With respect to the mathematical
formula for mean sway velocity, the trial duration chosen
for the assessment of postural stability may therefore
crucially affect the outcomes. Moreover, other
confounding variables may relate to differences in limb-matching
procedures applied in the included studies. Howells et
al.  suggested considering leg dominance as an
influential factor for the comparison of ACL and healthy
subjects. They found greater impairments in postural
stability of the ACL group when compared to the
dominant leg of healthy control. However, solely two of 11
included studies reported leg dominance. When
evaluating a potential influence to the outcomes, future studies
may explicitly provide detailed information about leg
Some limitations associated with the current review need
to be considered. A major limitation was the heterogeneity
of variance between studies. Except of one (Fig. 7), all
other comparisons exceeded the recommended level of
50% heterogeneity [
]. Although studies were selected
for highly specific criteria, the included research still
exhibited variability. First, there are restraining factors
inherent to the subject populations. One aim of this
metaanalysis was to examine CoP measures in order to
distinguish between a general population of ACL-injured
patients and healthy individuals. Nevertheless, group
differences may have affected the results of this systematic
review. Further, influencing group factors may relate to
sex distribution [
], age [
], or physical activity [
the experimental and control groups. Second, time frames
for time after injury may influence the outcomes of ACL
]. However, there is no evidence
classifying the length of time after ACL injury and the related
effects on postural stability. Future studies should
investigate the time frames of ACL rehabilitation in more depth.
Third, the research protocols considered in this systematic
review differed in terms of methodological properties. In
fact, none of the included examinations matched recent
recommendations of Salavati et al. [
], to conduct five
trials with duration of 60 s in assessments of postural
control. Future studies may contemplate these methodological
suggestions ensuring recordings to be mostly valid and
Fourth, matching of the injured to a matched limb of
the control group was not consistently implemented
throughout investigations. Since leg dominance was
deemed to influence postural stability in ACL patients
], conflicting methods for matching may distort the
results of this meta-analysis. Information about the ratio
of injuries to the dominant or non-dominant leg in ACL
patients was solely provided in two studies [
study reported the injured leg being matched with the
dominant leg of the control group . When
considering that leg dominance may differ between matched
samples, divergent conclusions may result for postural
stability. Collectively, the abovementioned methodological
confounders may compromise the consistency of the
findings and may therefore have contributed to the high levels
The present meta-analysis indicates that postural
stability in a standardized single-leg stance is impaired in
patients after ACL injury. Furthermore, CoP measures
appear to be suitable to differentiate ACL patients and
healthy controls with respect to postural stability. Thus,
the proposed measurement procedure may help
physicians and physiotherapists to identify patients at greater
risk for suffering a subsequent ACL injury and
consequently allow adjusting their treatment or return to play
strategies. Nevertheless, caution should be exercised
when using the non-injured leg as a reference measure.
However, the potential of these measures to provide
further insights into underlying mechanisms of altered
postural control is limited to theoretical considerations.
While current investigations mainly describe motor
responses to multimodal sensory feedback, further
etiological approaches may assess neurophysiological
mechanisms underlying functional deficits in ACL patients,
providing valuable indications for diagnostics, rehabilitative
treatment, or return to play assessment.
The authors acknowledge financial support by Land Schleswig-Holstein
within the funding program “Open Access Publikationsfonds.” No other
sources of funding were used to assist in the preparation of this systematic
review and meta-analysis.
All authors were involved in the conception and design of this work. TL
implemented the systematic search strategy, extracted and analyzed the
data, and wrote the first draft. LP independently applied the study selection,
reviewed the critical appraisal of selected articles, and assisted with the
compilation of the systematic review. The entire process was supervised by
JB. All authors contributed to the process of writing and approved the final
Tim Lehmann, Linda Paschen, and Jochen Baumeister declare that they have
no conflicts of interest relevant to the content of this review.
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