Balance Measures for Discriminating between
Functionally Unstable and Stable Ankles
2
2
2
SCOTT E. ROSS', KEVIN M. GUSKIEWICZ , MICHAEL T. GROSS , and BING YU
'Virginia Commonwealth University, Richmond, VA; and 2 Universitv of North Carolina, Chapel Hill, NC
ABSTRACT
ROSS, S. E., K. M. GUSKIEWICZ, M. T. GROSS, and B. YU. Balance Measures for Discriminating between Functionally Unstable
and Stable Ankles. Med. Sci. Sports Exerc., Vol. 41, No. 2, pp. 399-407, 2009. Purpose: To identify force plate measures that
discriminate between ankles with functional instability and stable ankles and to determine the most accurate force plate measure for
enabling this distinction. Methods: Twenty-two subjects (177 + 10 cm, 77 ± 16 kg, 21 ± 2 yr) without a history of ankle injury and 22
subjects (177 + 10 cm, 77 + 16 kg, 20 ± 2 yr) with functional ankle instability (FAI) performed a single-leg static balance test and a
single-leg jump-landing dynamic balance test. Static force plate measures analyzed in both anterior/posterior (A/P) and medial/lateral
(M/L) directions included the following: ground reaction force (GRF) SD; center-of-pressure (COP) SD; mean, maximum, and total
COP excursion; and mean and maximum COP velocity. COP area was also analyzed for static balance. A/P and M/L time to
stabilization quantified dynamic balance. Greater values of force plate measures indicated impaired balance. A stepwise discriminant
function analysis examined group differences, group classification, and accuracy of force plate measures for discriminating between
ankle groups. Results: The FAI group had greater values than the stable ankle group for A/P GRF SD (P 0.027), M/L GRF SD
(P = 0.006), M/L COP SD (P = 0.046), A/P mean COP velocity (P = 0.015), M/L mean COP velocity (P 0.016), A/P maximum
COP velocity (P = 0.037), M/L mean COP excursion (P = 0.014), M/L total COP excursion (P = 0.016), A/P time to stabilization
(P = 0.011), and M/L time to stabilization (P = 0.040). M/L GRF SD and A/P time to stabilization had the greatest accuracy scores of
0.73 and 0.72, respectively. Conclusion: Although 10 measures identified group differences, M/L GRF SD and A/P time to stabilization were the most accurate in discriminating between ankle groups. These results provide evidence for choosing these GRF
measures for evaluating static and dynamic balance deficits associated with FAI. Key Words: CHRONIC INSTABILITY, CENTER
OF PRESSURE, FORCE PLATE, FUNCTIONAL INSTABILITY, POSTURE, STABILITY
Freeman et al. (10,11) originally proposed ligament
deafferentation as a causal factor of FAL. The damaged
ankle ligament sensory receptors were thought to disrupt
sensorimotor function by diminishing proprioceptive messages related to joint movement and position to afferent
pathways. The resulting proprioceptive deficits could also
lead to diminished postural reflex responses and consequently contribute to balance deficits. Thus, sensorimotor
deficits were one potential contributing factor to recurrent
ankle sprains (10,11).
Although Freeman et al. (10,11) used single-leg Romberg
tests to identify balance impairments, other researchers have
used instrumented force plates to identify balance deficits
associated with ankle instability (2,6,7,12,14,16,18,20,21,
23-26,35,37,38). Center-of-pressure (COP) force plate
measures and ground reaction force (GRF) measures that
have been used to quantify balance in the ankle instability
literature included the following: COP SD; mean, maximum, and total COP excursion; COP velocity; COP area;
and ground reaction force (GRF) SD (2,6,7,12,14,16,18,
20,21,23-26,35,37,38). In our review of literature, we
found that 55% of static force plate measures used in research studies detected balance deficits in subjects with
ankle instability (2,6,7,12,14,16,18,20,21,23-26,35,37,38).
among physically active individuals (13,33). Approxinjuries
theoflateral
ligaments
of the ankle
are common
mately to
25%
all injuries
in athletics
are ankle
sprains,
and 85% of these injuries involve lateral ligament sprains
(13). A functional instability at the ankle joint might persist
after initial injury, and anywhere from 30% to 78% of injured individuals may suffer recurrent sprains (1,3,10,11,
31,34,36). Although the exact casual factor is unknown,
functional ankle instability (FAI) is believed to occur from a
combination of mechanical instability, ankle strength deficits, and ligament deafferentation (10,11,21).
Address for correspondence: Scott E. Ross, Ph.D., ATC, Department of
Health and Human Performance, Virginia Commonwealth University,
PO Box 842020, 1015 W. Main St., Richmond, VA 23284-2020; E-mail:
seross@vcu.edu.
Submitted for publication January 2008.
Accepted for publication July 2008.
0195-9131/09/4102-0399/0
MEDICINE & SCIENCE IN SPORTS & EXERCISE8
Copyright © 2009 by the American College of Sports Medicine
DOI: 10.1249/MSS.0b013e3181872d89
399
Some researchers have reported greater COP excursions
and greater GRF SD in ankles with functional instability
compared with stable ankles,(14,18,21,23,37,38), and
others have reported contrary results (2,14,18,20,24,26).
Thus, this discrepancy suggests that selected force plate
measures may be more sensitive than others at discriminating between ankles with functional instability and stable
ankles.
Although balance deficits can exist with single-leg stance,
the functionality of this test has been questioned (26). Dynamic single-leg hopping tests that challenge postural control greater than single-leg stance have been recommended
as alternative assessment techniques to single-leg static balance tests (26). Anterior/posterior (A/P) and medial/lateral
(M/L) time to stabilization force plate measures have been
used to evaluate single-leg jump-landing dynamic balance
of subjects with FAI (4,26-28,39). Subjects with FAI take
longer to stabilize than subjects with stable ankles after a
single-leg jump landing (4,26-28,39). This stabilization measure has been theorized to have greater sensitivity than static
measures for discriminating between ankles with functional
instability and stable ankles (26,27).
The sensitivity of static and dynamic force plate measures
for discriminating between ankles with functional instability
and stable ankles has not been established. Determining force
plate measures that accurately discriminate between ankles
with functional instability and stable ankles might assist
researchers in identifying balance impairments that might
otherwise go undetected with less accurate measures. Therefore, the objectives of this study were 1) to identify force plate
measures that discriminate between ankles with functional
instability and stable ankles and 2) to determine the most accurate force plate measure for enabling this distinction between ankles with functional instability and stable ankles. The
identification of these force plate measures might assist researchers in detecting individuals at risk for sprains or help
researchers determine the efficacy of an injury prevention
program.
signs and symptoms of injury included pain, loss of function,
mild point tenderness, swelling, and abnormal range of
motion. Subjects with stable ankles with a history of ankle
sprain injury were excluded from this study.
Informed written consent was obtained from all subjects
before participation in this study. The Committee for the
Protection of the Rights of Human Subjects granted approval
for this project. All subjects completed the Ankle Joint
Functional Assessment Tool to assess ankle stability (30).
Possible scores on this instrument range from 0 to 48, and
greater scores indicate greater functional instability. A
certified athletic trainer (SER) tested the subjects' ankle
laxity by performing the anterior drawer and talar tilt
orthopedic tests before balance testing.
Single-leg balance test. Subjects were instructed to
remain as motionless as possible while standing with their
test leg on a force plate. Subjects kept their eyes open, their
hands on their hips, and their non-weight-bearing leg slightly
flexed at the hip and knee. The weight-bearing leg was
slightly flexed at the knee, and the foot was in a neutral toe in/
out position with the tips of their shoes pointed straight ahead.
All subjects wore athletic shoes during testing; however, we
did not control for the type of athletic shoes. Our rationale for
testing subjects while wearing athletic shoes was to keep
contact surfaces consistent between static and dynamic tests.
Additionally, this single-leg balance protocol has been used in
previous ankle instability studies (28,29). Subjects performed
one 10-s practice trial, followed by three 20-s testing trials.
Subjects rested 20 s between each trial. Trials were repeated
TABLE 1. Charateristics of subjects with and without FAI
Dominant test limb, n
Nondominant test limb, n
Height (cm), mean (SD)
Weight (kg), mean (SD)
Age (yr), mean (SD)
Sprains in the past year
Stable Ankle Group
(10 males, 12 females)
FAIGroup
(10 males, 12 females)
13
9
177 (10)
77 (16)
21 (2)
Number ofSubjects
No subjects reported a
history of ankle sprain
injury.
13
9
177 (10)
77 (16)
21 (2)
Number of Number of
Sprains
Subjects
2
8
3
4
4
3
5
1
6
1
7
2
1
9
10
2
Number of Number of
Give Ways
Subjects
2
1
3
1
4
4
5
2
7
3
10
2
15
5
20
4
Number of Give Ways
Drawer
Talar Tilt
11
12
11
10
33.05 (4.02)
METHODS
Subjects. Subject characteristics are reported in Table 1.
Twenty-two control subjects with stable ankles were matched
by height, mass, age, sex, and leg tested to 22 subjects with
clinically diagnosed FAI. Subjects with stable ankles were
tested on the same leg (dominant or nondominant) as their
match. Dominance was defined as the preferred leg used to
kick a ball. Inclusion criteria for FAI included 1) self-report of
a history of one sprain followed by at least 3 d of immobilization and 2) self-report of at least two ankle sprains and at
least two episodes of "giving way" sensations during physical
activity within the year before the subjects' enrollment in this
study. Potential subjects were excluded if they displayed acute
signs and symptoms of injury or reported an ankle sprain
within 6 wk before their participation in the study. Acute
400
Official Journal of the American College of Sports Medicine
Giving way sensalions
the past year
Special Test
Positive
Positive
AJFAT score, mean (SD)
Number of Subjects
No subjects reports
reported a history of giving
way sensations.
Number of Subjects
Drawer
Talar Tilt
4
4
18
18
22.60 (1 22)
AJFAT, Ankle Joint Functionsal Assessment Tool,
http://www.acsm-msse.org
TABLE 2. Definitions of force plate measures.
Measure
Definition
GRF SD
Overall SD of the GRF in a given direction in a given
time for a given number of trials
Overall SD of the COP in a given direction in a given
time for a given number of trials
The absolute averaged distance between the
instantaneous COP and the average COP position
in a given direction during a given time
The absolute maximum distance between the
instantaneous COP position and the average COP
position during a given period
The absolute length of the path movements over the
testing period
The absolute mean value of the instantaneous
velocity of the COP in a given direction during a
given period
The absolute maximum value of the instantaneous
velocity of the COP in a given direction during a
given period
A rectangular area defined by the maximum anterior,
posterior, medial, and lateral sways during a
given time
Time needed to reduce the variation of a smoothed
GRF to the range of vibration of the corresponding
component of the GRF in a stabilized single-leg
stance of individuals with stable ankles
COP SD
Mean COP excursion
Maximum COP excursion
Total COP excursion
Mean COP velocity
Maximum COP velocity
COP area
Time to stabilization
GRF, ground reaction force; COP, center of pressure.
if subjects hopped on the weight-bearing leg or touched
down with the non-weight-bearing leg.
Single-leg jump landing. Methods for performing
single-leg jump-landing tests have been described in previous
reports (26-28). First, subjects performed maximum standing
reach while standing flat footed directly underneath a Vertec
(Sports Imports, Columbus, OH) by flexing their shoulders
to 180' and touching the highest plastic rod that they could
reach with the distal end of their fingers. The Vertec has
adjustable plastic rods set at different heights that permit
maximum reach and jump height assessments. Subjects were
then assessed for maximum vertical jump height as they
stood on the floor 70 cm away from a Vertec. Three maximum vertical jump heights were assessed while subjects
jumped using a bilateral foot takeoff technique. Subjects
were allowed to swing their arms while they jumped off of
the floor. Then, subjects flexed their shoulders to 180' and
fully extended their elbows, touched the highest plastic rod
that they could reach with the distal end of their fingers, and
then landed on both feet on the floor. The greatest jump
height of the three vertical jumps for each subject was recorded and then subtracted from their respective maximum
standing reach. Thus, the difference between maximum vertical jump height and maximum standing reach was recorded
as the true maximum jump. Next, the plastic rods on the
Vertec were placed in line with the center of a force plate,
and the plastic rods were then set between 50% and 55% of
subjects' true maximum jump. Subjects stood on the floor
70 cm away from the center of the force plate and performed
a jump using a bilateral foot takeoff technique. Again, subjects were allowed to swing their arms while they jumped off
of the floor. However, subjects were required to reach with
their shoulder flexed at 180' and elbow fully extended after
take off. The reaching arm was the ipsilateral arm of the
ankle with functional instability or the matched testing ankle
of subjects with stable ankles. As subjects jumped vertically,
they touched the plastic rods between 50% and 55% of their
maximum jump with the distal end of their fingers, and then
they landed on a single leg (their test leg) on a force plate.
Subjects were instructed to stabilize as quickly as possible and
remain as motionless as possible in single-leg stance for 20 s.
Three practice trials and seven testing trials were performed
with 30 s of rest between trials. Trials were repeated if subjects
failed to jump within the 50-55% range, hopped on the test
leg, or touched down with the non-weight-bearing leg.
Data collection and reduction. A Bertec force plate
model number 4060-08A (Bertec Corp., Columbus, OH)
collected GRF data at a sampling rate of 180 Hz (4,26-28).
Analog signals were amplified with a Bertec amplifier (AM6701). Signals were then passed through a BNC adapter
chassis that was interfaced with an analog-to-digital board
within a personal computer. MotionSoft Balance Assessment
TABLE 3. Mean + SO and ES for force plate measure analyses.
Force Plate
A/P GRF SO
M/L GRF SD
A/P COP SD (cm)
M/L COP SD (cm)
A/P mean COP excursion (cm)
M/L mean COP excursion (cm)
A/P maximum COP excursion (cm)
M/L maximum COP excursion (cm)
A/P total COP excursion (cm)
M/L total COP excursion (cm)
A/P mean COP velocity (cm.s-1)
M/L mean COP velocity (cm.s-1)
A/P maximum COP velocity (cm.s-1)
M/L maximum COP velocity (cm.s-1)
2
COP area (cm )
A/P time to stabilization (s)
M/L time to stabilization (s)
FAl
0.45
0.54
1.01
0.73
0.80
0.60
3.01
2.02
63.61
64.63
3.17
3.23
24.99
21.53
0.21
1.86
2.09
±
±
±
±
±
±
±
±
±
±
±
±
+
±
0.18a
0.22a
0.38
0.19
0.30
0.15
1.32
0.52
20.03
15.89
1.00
0.80
15.81
7.93
±0.14
± 0.67
±
0.80
Stable Ankle
ES
0.35
0.39
0.85
0.64
0.67
0.51
2.41
0.72
0.92
0.54
0.60
0.57
0.71
0.60
0.46
0.58
0.75
0.76
0.75
0.65
0.54
0.60
0.80
0.62
+ 0.08a
± 0.07'
± 0.18
+ 0.10
± 0.12
+ 0.10
± 0.52
1.82 ± 0.33
54.16 ± 11.00
54.88 ± 9.03
2.59 ± 0.39
2.74 ± 0.45
17.48 ± 4.24
18.07 ± 3.47
0.15 + 0.01
1.44 ± 0.33
1.70 ± 0.38
.A, F(1 , 42), Pvalue
A = 0.90,
A = 0.84,
A = 0.93,
A = 0.91,
A = 0.92,
A = 0.87,
A = 0.92,
A = 0.95,
A = 0.92,
A = 0.87,
A = 0.87,
A = 0.87,
A = 0.90,
A = 0.92,
A = 0.92,
A = 0.86,
A = 0.90,
F(1,42)= 5.24,
F(1,42)= 8.23,
F(1 ,42 ) = 3.26,
F(1,42 ) = 4.24,
F(1,42) = 3.72,
F(1,421 = 6.51,
F(1,42)= 3.92,
F(I.42) = 2.23,
F(I 42 ) = 3.76,
F(1 . 42 ) = 6.27,
F(1,42)= 6.45,
F(1.42 ) = 6.29,
F(1.42 ) = 4.62,
F(1i42)= 3.53,
F(1,42)= 3.82,
P = 0.027*
P = 0.006*
P = 0.078
P = 0.046*
P = 0.061
P = 0.014*
P = 0.054
P = 0.143
P= 0.060
P = 0.016*
P = 0.015*
P = 0.016*
P = 0.037*
P = 0.067
P = 0.057
F(142) = 7.03, P= 0.011*
F(1,42i= 4.49, P = 0.040'
GRF SD for A/P and M/L measures were divided by body weight (N) and multiplied by 100.
Statistical significance (P< 0.05).
AJP,anterior/posterior; M/L, medial/lateral; COP, center of pressure; GRF, ground reaction force: ES, effect size; A, Wilks' Lambda.
DISCRIMINATING UNSTABLE AND STABLE ANKLES
Medicine & Science in Sports & Exercisee
401
GRF Standard Deviation
COP Standard Deviation
10.9
0.9
0.8-
-7
0.8-
0.7 -
0.7-
S0.6-
0.6
0.5.
0.5
~0.4-
040.30.2
7/
0.10
0.3-
-AllP
7
-
-
-
-
0.20.1-
Di.g
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1-Specificity
Diagonal
--
--
0
1
AIP
-
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1-Specificity
1
FIGURE 1-ROC curves for ground reaction force (GRF) SD.
Anterior/posterior (A/P) GRF SD and medial/lateral (M/L) GRF SD
discriminated between ankle groups. However, M/L GRF SD had
greater accuracy than A/P GRF SD in discriminating between ankle
groups. The diagonal dotted black line divides the area of the graphs
into the upper and lower 50%.
FIGURE 2-ROC curves for center-of-pressure (COP) SD. Neither
anterior/posterior (AIP) COP SD nor medial/lateral (M/L) COP SD
were accurate in discriminating between ankle groups. The diagonal
dotted black line divides the area of the graphs into the upper and
lower 50%.
computer software package version 2.0 (MotionSoft Inc.,
Chapel Hill, NC) converted digital data to GRF vectors.
Data were filtered using a second-order recursive low-pass
Butterworth digital filter with an estimated optimum cutoff
frequency of 12.53 Hz (4,26-28).
Table 2 presents definitions of static and dynamic force
plate measures calculated to evaluate balance. Static force
plate measures analyzed in both anterior/posterior (A/P) and
medial/lateral (MWL) directions included the following: GRF
SD; COP SD; mean, maximum, and total COP excursion;
and mean and maximum COP velocity. COP area was also
analyzed for static balance. A/P and M/L time to stabilization
examined dynamic balance and were calculated using the
vibration magnitude curve fitting technique (27,28). Greater
magnitudes of force plate measures were indicative of impaired balance.
Statistical analysis. A stepwise discriminant function
analysis determined 1) the force plate measure differences
between groups with univariate F-tests; 2) the percentage
of subjects with FAI and subjects with stable ankles that
were correctly classified as having FAI or stable ankles,
respectively; and 3) the force plate measures that accurately
discriminated between ankle groups. Effect size (ES) values
were calculated using Cohen's (5) ES d to indicate the degree of differences between ankle groups on univariate
F-tests. Cohen (5) defines low, medium, and high ES as
0.30, 0.50, and 0.80, respectively. The stepwise discriminant analysis regressed force plate measures on group membership. In other words, this analysis determined force plate
measures that predicted group membership. Thus, force
plate measures that predicted group membership were accurate at distinguishing between ankle groups. Additionally,
the percentage of individuals classified correctly as having
ankles with functional instability or stable ankles was identified with the discriminant analysis. Accuracy values for
discriminating between groups were determined by calculating the area under the curve (AUC) for receiver operating
characteristic (ROC) curves. ROC curves plot sensitivity
(true-positive rate) versus 1-speficity (false-positive rate),
describing how sensitivity and false-positive rate vary
together. The full area under the ROC curve was used as
an index to classify the accuracy of force plate measures.
Perfect accuracy is indicated by an AUC of 1.0, whereas a
no apparent accuracy is indicated by an AUC of less than or
equal to 0.50 (17,22). Thus, force plate measures were more
accurate at discriminating between ankle groups as AUC
values approached 1.0. A traditional academic point scale
was used to classify the accuracy of the AUC for discriminating between ankle groups with force plate measures
(0.90-1.00 = "excellent"; 0.80-0.89 = "good"; 0.70-0.79 "fair"; 0.60-0.69 = "poor"; and 0.50-0.59 = "fail") (17,22).
SPSS version 13.0 (SPSS, Inc., Chicago, IL) was used
for statistical analyses. The alpha and the asymptotic
402
Official Journal of the American College of Sports Medicine
Mean COP Excursion
1
0.9
0.8
0.7
06
0.5
0.4
0.3
0.2
--- -
0.1
A/P
Diagonal
_MtL
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
1-Specificity
FIGURE 3-ROC curves for mean center-of-pressure (COP) excursion. Medial/lateral (M/L) mean COP excursion accurately discriminated between ankle groups, whereas anterior/posterior (A/P) mean
COP excursion did not. M/L mean COP excursion had greater
accuracy than A/P mean COP excursion in discriminating between
ankle groups. The diagonal dotted black line divides the area of the
graphs into the upper and lower 50%.
http://www.acsm-msse.org
Maximum COP Excursion
Mean COP Velocity
1,-
I1-
0.9-
7
0.9.
0.8-
0.80.7-
0.7, 0.60.5-
0.4
0.3
7
7
7
S0.6-
7
0.5-
"00.4S0.3-
AIP
02///0.11
-
-
-
-
0.2-
Diagonal
0
,
7/
Z
AI/P
Diagonal
0.1-
-MIL
,
U
//
MI L
i
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
0
1-Specificity
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
1-Specificity
FIGURE 4-ROC curves for maximum center-of-pressure (COP)
excursion. Neither anterior/posterior (A/P) maximum COP excursion
nor medial/lateral (M/L) maximum COP excursion were accurate in
discriminating between ankle groups. The diagonal dotted black line
divides the area of the graphs into the upper and lower 50%.
levels were set a priori at P < 0.05 to indicate statistical
significance.
FIGURE 6-ROC curves for mean center-of-pressure (COP) velocity.
Anterior/posterior (A/P) mean COP velocity and medial/lateral (M/L)
mean COP velocity discriminated between ankle groups. However,
M/L mean COP velocity had greater accuracy than A/P mean COP
velocity in discriminating between ankle groups. The diagonal dotted
black line divides the area of the graphs into the upper and lower 50%.
The ROC curves for static and dynamic force plate measures are presented in Figure 1 (GRF SD), Figure 2 (COP
SD), Figure 3 (mean COP excursion), Figure 4 (maximum
RESULTS
ferences were found for A/P COP SD, A/P mean COP
COP excursion), Figure 5 (total COP excursion), Figure 6
(mean COP velocity), Figure 7 (maximum COP velocity),
Figure 8 (COP area), and Figure 9 (time to stabilization).
Table 4 reports the percentage of subjects correctly classified into their respective groups as well as the AUC and the
asymptotic significance for force plates measures. The AUC
values had accuracy ratings of "fair" for A/P GRF SD, MIL
GRF SD, M/L total COP excursion, M/L mean COP velocity, and A/P time to stabilization. The AUC values had ac-
excursion, A/P maximum COP excursion, M/L maximum
COP excursion, A/P total COP excursion, M/L maximum
curacy ratings of "poor" for A/P COP SD, M/L COP SD, A/P
mean COP excursion, MiL mean COP excursion, A/P max-
COP velocity, or COP area.
imum COP excursion, A/P total COP excursion, A/P mean
Table 3 reports means + SD and ES for static and dynamic
force plate measures. The FAI group had greater force plate
values than the stable ankle group for A/P GRF SD, MIL
GRF SD, M/L COP SD, M/L mean COP excursion, M/L
total COP excursion, A/P mean COP velocity, M/L mean
COP velocity, A/P maximum COP velocity, A/P time to
stabilization, and M/L time to stabilization. No group dif-
r
r
Q
L
IF
Total COP Excursion
rr
L
Maximum COP Velocity
0.9
0
"1l
0.8
0.7-
0.7
0.61
.•0.6
7/
0.5
M/L
0.4
m 0.4
S0.3
0.3-
02
020.1-
----
Diagonal
0O
v
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1-Specificity
,J
1
FIGURE 5-ROC curves for total center-of-pressure (COP) excursion.
Medial/lateral (M/L) total COP excursion accurately discriminated
between ankle groups, whereas anterior/posterior (A/P) total COP
excursion did not. M/L total COP excursion had greater accuracy than
A/P total COP excursion in discriminating between ankle groups. The
diagonal dotted black line divides the area of the graphs into the upper
and lower 50%.
DISCRIMINATING UNSTABLE AND STABLE ANKLES
0.1
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1-Specificity
FIGURE 7-ROC curves for maximum center-of-pressure (COP)
velocity. Anterior/posterior (A/P) maximum COP velocity accurately
discriminated between ankle groups, whereas medial/lateral (M/L)
maximum COP velocity did not. A/P maximum COP velocity had
greater accuracy than M/L maximum COP velocity in discriminating
between ankle groups. The diagonal dotted black line divides the area
of the graphs into the upper and lower 50%.
Medicine & Science in Sports & Exercisea
403
COP Area
10.90.8-
/7
/////7
0.7-
7/
7
0.60.5-
0.4-
7/
7
0.2
0.1
0
0
Area
0.1
-/
DiegonI
."
_1-_
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
I-Specificity
FIGURE 8-ROC curve for center-of-pressure (COP) area. COP area
was not accurate in discriminating between ankle groups. The diagonal
dotted black line divides the area of the graphs into the upper and
lower 50%.
COP velocity, A/P maximum COP velocity, M/L maximum
COP velocity, COP area, and M/L time to stabilization. The
MJL maximum COP excursion failed to discriminate between
ankle groups.
Accuracy scores indicated that five force plate measures
had ratings of "fair." The stepwise discriminant analysis
indicated, however, that the M/L GRF SD and the A/P time
to stabilization were the most accurate force plate measures
for discriminating between ankle groups (Wilks' lambda
(A) = 0.76, F(2 ,41) = 6.45, P = 0.004). Furthermore, the stepwise discriminant analysis indicated that the M/L GRF SD
slightly out performed A/P time to stabilization for discriminating between ankle groups (Wilks' lambda (A)
0.84, F(1,42) = 8.23, P = 0.006).
DISCUSSION
Although 10 force plate measures identified group differences, the M/L GRF SD and the A/P time to stabilization were
Time -To-Stabilization
1
0.9,
0.8.
0.70.6 -
0.5
0.4
0.3
A/P
0.2
0.2-
I-Specificity
FIGURE 9-ROC curves for time to stabilization. Anterior/posterior
(A/P) time to stabilization accurately discriminated between ankle
groups, whereas medial/lateral (M/L) time to stabilization did not. A/P
time to stabilization had greater accuracy than M/L time to
stabilization in discriminating between ankle groups. The diagonal
dotted black line divides the area of the graphs into the upper and
lower 50%.
404
Official Journal of the American College of Sports Medicine
TABLE 4. Discriminant accuracy for force plate measures.
% Correctly
Force Plate
Classified
AUC
A/P GRF SO
M/L GRF SO
A/P COP SO
M/L COP SD
A/P mean COP excursion
M/L mean COP excursion
A/P maximum COP excursion
M/L maximum COP excursion
A/P total COP excursion
M/L total COP excursion
N/P mean COP velocity
M/L mean COP velocity
A/P maximum COP velocity
M/L maximum COP velocity
COP area
A/P time to stabilization
M/L time to stabilization
65.9
682
59.1
59.1
56.8
56.8
54.5
47.7
63.6
65.9
61.4
65.9
61.4
56.8
56.8
61.4
54.5
0.70
0.73
0.62
0.64
0.65
0.69
0.60
0.57
0.63
0.70
0.68
0.70
0.67
0.63
0.60
0.72
0.64
Asymptotic
Significance
0.021
0.011*
0.185
0.110
0.080
0.034*
0.755
0.439
0.133
0.024*
0.038*
0.024*
0.050*
0.156
0.296
0.012*
0.121
Statistical significance (P _<0.05).
A/P, anterior/posterior; M/L, medial/lateral; COP, center of pressure; GRF, ground
reaction force.
the most accurate force plate measures for discriminating
between ankle groups. However, accuracy scores indicated
that M/L GRF SD performed slightly better than A/P time to
stabilization at discriminating between ankle groups. These
results provide evidence for choosing M/L GRF SD and A/P
time to stabilization as primary force plate measures for
evaluating static and dynamic balance deficits associated with
FAI, respectively. These static and dynamic force plate measures might assist researchers in detecting balance impairments that might otherwise go undetected during static or
dynamic single-leg balance tests with less accurate measures.
Additionally, M/L GRF SD and A/P time to stabilization
measures might help researchers in detecting individuals at
risk for sprains or help researchers determine the efficacy of
an injury prevention program.
Although less accurate than M/L GRF SD, A/P GRF SD
and several COP measures detected static balance impairments associated with FAL. Our significant COP force plate
measures for detecting ankle group differences indicate that
subjects with FAI swayed excessively and quickly compared with subjects with stable ankles. Swaying quickly is
associated with greater accelerations of the center of mass,
which was quantified in the GRF SD force plate measures.
Thus, greater A/P GRF SD and M/L GRF SD resulting from
greater GRF indicates that subjects with FAI had greater
center of mass accelerations than subjects with stable ankles.
We speculate that sensorimotor deficits associated with FAI
might have impaired subjects' ability to control or to detect
changes in center of mass positions. Damaged articular, musculotendinous, and cutaneous receptors associated with FAT
have been purported to disrupt sensorimotor function by diminishing messages related to ankle joint movement and position to afferent pathways (10,11,19). These deficits could
lead to reduced postural reflex responses and consequently
diminish stabilizing moments that are important in maintaining balance. Additionally, sensorimotor deficits have been
implicated in causing subjects with FAI to balance closer to
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their limits of stability (18). Given this significant infonnation,
we speculate that sensorimotor deficits associated with FAI
might have caused subjects' center of mass to accelerate to
their limits of stability without adequate stabilizing moments.
Furthermore, sensorimotor impairments and strength deficits associated with FAI might have also contributed to
increased time to stabilization values in subjects with FAT.
Evidence indicates that subjects with recurrent ankle sprains
have impaired sensorimotor reflexes during dynamic balance (25). Sufficient sensorimotor reflexes are essential for
controlling balance. Additionally, eccentric plantarflexion
strength deficits of the foot have been associated with FAI
(9). Eccentric strength of the plantarflexors is important in
energy absorption after landing; however, plantarflexors of
the foot have been underused in producing deceleration
moments after landing (32). On the basis of this information, we hypothesize that subjects with FAI might have
underused potentially weak plantarflexors of the foot after
landing, impacting their ability to develop adequate deceleration moments to control their center of mass after landing. Additionally, a delay in sensorimotor reflexes might
have decreased the time subjects with FAT had to develop
deceleration moments to stabilize the center of mass.
Consequently, muscle force requirements might have
increased to produce deceleration moments in very short
periods of time for subjects with FAT. Subjects with stable
ankles likely had adequate muscle force to develop appropriate deceleration moments to stabilize their center of
mass and lower extremity very quickly. Subjects with FAI,
however, might have gone longer without adequate deceleration moments, consequently increasing stabilization
times. We did not directly measure moment generation
capabilities of ankle muscles in this study, and future research should explore relationships between ankle moment
production, muscle activation, and time to stabilization to
confirm our contention.
We recommend that researchers use MJL GRF SD for
static single-leg balance and A/P time to stabilization for
dynamic single-leg balance to discriminate ankles with
functional instability and stable ankles. This recommendation is based on our results that indicate the M/L GRF
SD and the A/P time to stabilization had the greatest accuracy scores of 0.73 and 0.72, respectively. Interestingly,
ES were greatest for the univariate F-tests of M/L GRF SD
(0.92) and A/P time to stabilization (0.80). Thus, these high
ES indicate that these measures distinguish between
ankle groups. These findings are not surprising, as Goldie
et al. (15) reported that the MIL GRF SD was a good
predictor of single-leg balance deficits, and M/L GRF SD
was greater in individuals with a history of inversion ankle
sprains than individuals with stable ankles (16). Additionally, A/P time to stabilization has consistently detected
differences between ankles with functional instability and
stable ankles (4,26-28,39). Using these two force plate
measures might help researchers detect balance impairments
associated with FAT.
DISCRIMINATING UNSTABLE AND STABLE ANKLES
Additional static force plate measures with slightly less
accuracy than M/L GRF SD might also be used to discriminate between ankle groups. Fifteen percent (2/13)
and 23% (3/13) of our traditional COP sway measures had
AUC values classified as "fair" (AUC values >0.70) and
"poor" (AUC between 0.67 and 0.69) with statistically
significant asymptotic values, respectively. Although these
significant COP measures were not better than M/L GRF
SD at discriminating between ankle groups, the following static force plates measures could be additional
measures included in single-leg balance assessments to
discriminate between ankles with functional instability and
stable ankles: 1) "fair" measures: A/P GRF SD, M/L total
COP excursion, and MIL mean COP velocity; and 2) "poor"
measures: M/L mean COP excursion, A/P mean COP
velocity, and A/P maximum COP velocity. The remaining
62% of our static force plate measures with AUC values
less than or equal to 0.66 and asymptotic values greater than
0.05 should not be used to discriminate between ankle
groups.
Reasons for certain COP measures being inaccurate for
discriminating between ankle groups than other COP measures during static balance are not currently known. We
speculate that the dimensions of the foot may have negated
the effect of sensorimotor deficits associated with FAT on
static balance. The long A/P dimension of the base of support allowed subjects in both ankle groups to use long
excursion paths to maintain stability in the A/P direction.
Additionally, our data indicate that subjects in both groups
might have swayed excessively and very quickly in the
short M/L base of support at some point during single-leg
balance, causing maximum excursions and maximum
velocity measures to lack sensitivity in discriminating
between ankle groups.
A/P time to stabilization might have been more accurate
than M/L time to stabilization at discriminating between
ankle groups as a result of the anterior jump protocol
perturbing sagittal plane stability. Wikstrom et al. (40) recently reported that a lateral jump protocol increased M/L
dynamic postural stability scores over an anterior jump
protocol in subjects with stable ankles. A lateral jump protocol might tax frontal plane postural control more than
an anterior jump protocol. We speculate, therefore, that a
lateral jump protocol might increase the accuracy of M/L
time to stabilization in discriminating between ankle
groups. Future research should explore the accuracy of
A/P and M/L time to stabilization measures in distinguishing between ankle groups with lateral jump protocols.
We did not control for the type of athletic shoes worn in
our study. Subjects wore running and cross-trainer athletic
shoes. Differences in shoe support or soles could have
affected balance and introduced variability into our data.
Future research should explore the impact of different
athletic shoes on our static and dynamic balance tasks. An
additional limitation to our study was subject spectrum bias.
Balance deficits associated with FAT could be similar to
Medicine & Science in Sports & Exerciseg,
405
balance impairments associated with other ankle injuries. In
the first 3 wk after an ankle sprain injury, for example,
balance impairments have been present in subjects with
acute ankle sprains (8). Future research should examine the
accuracy of force plate measures in discriminating between
ankles with functional instability, stable ankles, and other
ankle pathologies.
CONCLUSION
The M/L GRF SD and the A/P time to stabilization
accurately discriminated between ankle groups. Our results
provide evidence for choosing M/L GRF SD and A/P time
to stabilization as primary force plate measures for
evaluating static and dynamic balance deficits associated
with FAI, respectively. Other noteworthy static force plate
measures that did not perform as well as M/L GRF SD and
A/P time to stabilization but still accurately discriminated
between ankle groups were as follows: A/P GRF SD, M/L
mean COP excursion, M/L total COP excursion, A/P mean
COP velocity, M/L mean COP velocity, and A/P maximum
COP velocity. Traditional COP and GRF force plate
measures were examined in our current study, and we
recommend that future research examine the ability of other
nontraditional nonlinear and spatiotemporal single-leg
stance force plate measures (e.g., time to boundary,
approximate entropy) or dynamic force plate measures
(e.g., dynamic postural stability index) to discriminate
between ankles with functional instability and stable ankles.
Finally, researchers should identify ankles at risk for sprains
or determine the efficacy of an injury prevention intervention program with force plate measures that discriminate
between ankles with functional instability and stable ankles.
The authors thank Carol Giuliani, P.T., Ph.D., and Richard G.
Mynark, Ph.D., for their roles as committee members on Dr. Ross'
dissertation. Manuscript preparation was supported by the Department of Health and Human Performance at Virginia Commonwealth
University, Richmond, VA. Data collection for this research was
conducted as part of Dr. Ross' doctoral dissertation at the University
of North Carolina at Chapel Hill. Data were collected in the Sports
Medicine Research Laboratory in the Department of Exercise and
Sport Science at the University of North Carolina at Chapel Hill. The
results of the present study do not constitute endorsement by the
American College of Sports Medicine.
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TITLE: Balance Measures for Discriminating between
Functionally Unstable and Stable Ankles
SOURCE: Med Sci Sports Exercise 41 no2 F 2009
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