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Figure 15–12. Stimulus paradigm for the MCT. The support surface is translated backward (or forward) while
the position of the upper body remains initially station-
Note that the amplitude of the translation is exag-
ary.
gerated for purposes of illustration. From Handbook of
Balance Function Testing, Jacobson et al., 1997.
compensatory postural movements are termed automatic postural responses. This is because their latencies
are too long to be mediated by the segmental stretch
reflex pathways but too short to be initiated under voluntary control of the subject (Nashner & Cordo, 1981).
The MCT protocol summarized in Table 15–2 analyzes the automatic postural response system over a
range of velocities and directions using forward and
backward translations varying in magnitude (threshold, intermediate, and saturating) and timing (random
intervals between stimuli). Translations of the same
direction and size are always imposed in groups of
three. Results from the three trials are then averaged
to obtain a stable characterization of the active force
responses.
Responses to forward and backward translations
are analyzed separately, because the flexor and extensor pathways mediating automatic postural responses
are anatomically separate and may be affected differently by a disease process. Responses of the left and
right legs are analyzed separately, because the pathways mediating responses on the two sides can also
be selectively affected by disease. The random intervals between trials minimize the likelihood that the
patient can anticipate the onset of displacements (see
Table 15–2).
Protocol for the Motor Control Test
In the standard MCT protocol, the support surface
translates at a constant velocity over a fixed interval
Table 15–2. Protocol for the Motor Control Test
Stimulus
Backward translation 1.25 cm in 250 ms (height scaled) 3
Backward translation 3.15 cm in 300 ms (height scaled) 3 Active force responses: latency,
Backward translation 5.7 cm in 400 ms (height scaled) 3
Toes-up rotation 4.0° in 500 ms 5 Sway energy
Forward translation 1.25 cm in 250 ms (height scaled) 3
Forward translation 3.15 cm in 300 ms (height scaled) 3 Active force responses: latency,
Forward translation 5.7 cm in 400 ms (height scaled) 3
Toes-up rotation 4.0° in 500 ms 5 Sway energy
Number
of Trials Measurements
strength symmetry
strength symmetry
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of time to achieve a predetermined velocity and amplitude of COG displacement. Using a 180-cm (6 feet)-tall
patient as the example, a small translation of 1.25 cm
over a 250 ms interval rotates the COG about the ankle
joints at 2.8°/s for a total distance of 0.7°. This is approximately the threshold displacement required to elicit an
automatic postural response. A medium translation of
3.15 cm over a 300 ms interval rotates the COG about
the ankle joints at 6°/s for a total distance of 1.8°. Finally,
a large translation of 5.70 cm over a 400 ms interval
rotates the COG about the ankle joints at 8°/s for a total
distance of 3.2°. This stimulus produces an approximately maximum amplitude automatic response in the
healthy subject (Horak, Diener, & Nashner, 1989).
For individuals shorter or taller than 180 cm, the
translation amplitude is normalized by reducing and
increasing the translation velocity and distance, respectively, to produce the same velocity and amplitude
of COG sway. For individuals shorter or taller than
180 cm, the distance the surface translates is proportionately smaller or larger, respectively, while the durations of the translations are the same.
Recording Active Force Responses
Automatic postural responses to support surface translations are recorded using separate forceplates for each
foot, each of which measures the active compensatory
force exerted by the foot against the support surface.
When each foot is properly positioned on the forceplate, the forceplate measures the portion of total body
weight carried by the foot and the position of the center of vertical force exerted by the foot relative to the
ankle joint position. The weight and position quantities are used to calculate the active torque exerted by
the musculature of leg about the ankle joint (Nashner,
1977; Nashner et al., 1979).
Presentation of MCT Results
Real-Time MCT Display
The computer displays a real-time summary of the raw
COG sway data and the resulting active force latency
scores during administration of the MCT. The latency
plot on the left side of the display indicates the current movement direction, amplitude, and trial number,
as well as plotting the latency scores of all completed
trials. In the system printouts, green bars show latencies falling within the normal range, and red bars show
latencies falling above the normal limits established for
the age-matched clinically normal sample. Numbers on
each bar indicate the confidence level of the automatic
scoring algorithm. If the operator chooses to repeat a
previously executed and scored trial, the replacement
bar is striped in the EquiTest system.
The right section of the display charts the AP and
lateral positions of the COG in real time. This display
accurately indicates COG position relative to the center of the base of support when the patient’s feet are
properly placed on the support surface. Because accurate scoring of MCT latencies and strengths depends
on weight bearing within the normal range, alignment
of the COG (lateral, forward, or backward) should be
corrected if possible with the following procedure. The
positions of the patient’s feet should be checked first to
ensure that inaccurate foot placement is not the cause.
Each foot should not be positioned away from the
proper position on the forceplate as the means to correct the COG misalignment. Instead, the patient should
be encouraged by the operator to shift the upper body
to center the COG.
Weight Symmetry
The graphic and raw data components of a typical MCT
result are shown for one movement direction in Figure 15–13. In the complete MCT printout, an identical
series of plots are included for backward and forward
movement responses on the left and right sides of the
page, respectively. For each movement direction, separate plots show weight symmetry, active force response
latency, and active force response strength scores. In all
of the plots, open areas encompass the range of scores
considered to be within the clinically normal limits
based on results from the asymptomatic sample.
A weight symmetry plot for each movement direction indicates the percentage of total body weight borne
by each leg during the automatic postural response.
Weight symmetry is displayed as a nondimensional
quantity, with a score of 100 indicating that weight is
borne equally by the two legs. The weight symmetry
score decreases to zero or increases to 200 when all the
weight is borne by the left or right leg, respectively.
Weight symmetry scores are important, because the
onset latencies and the strengths of automatic postural
responses in each leg are influenced by the fraction of
total body weight supported by the leg.
Active Force Latency
The automatic postural response elicited by a support surface translation compensates for COG sway
by exerting active torque about the ankle joints. As
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figure 15–13. Graphical and raw summaries of MCT results of a typical clinically
normal adult for backward support surface translations.
sway, horizontal shear force, and left and right center of C
averaged over three trials each for the three sizes of perturbations.
show onset of surface translations. Arrows on the C
the active force responses. Plots on the right show the relative bearing of weight
between the two legs (100 equals perfect symmetry), the active force response
latencies for left and right legs and the two larger sized translations, and the active
force response strengths for the left (x) and right (+) legs. Shaded areas show scores
falling outside the clinically normal limits, based on the normal population study.
From Handbook of Balance Function Testing, Jacobson et al., 1997.
the patient’s COG moves forward (or backward) in
response to horizontal surface translation, the initial
resistance due to the inherent stiffness of the ankle
joints is small and insufficient to stabilize the COG
sway (Gurfinkel, Lipshits, & Popov, 1974; Nashner,
1976). The myotatic reflexes are either too weak or
inhibited during standing to have a significant overall impact on the active force response (El’ner, Popov,
& Gurfinkel, 1972; Gurfinkel et al., 1974). Active force
increases abruptly within 30 to 40 ms following onset
of the automatic component of the postural response
at 90 to 100 ms latencies as recorded by electromyography (EMG) (Dichgans & Diener, 1985; Nashner, 1977;
Nashner et al., 1979). This is indicated by an abrupt and
rapid increase in force resistance as measured by the
forceplates. This sequence of events is illustrated in the
raw data components of Figure 15–13.
Traces on the left show COG
OF (torque) responses
Vertical lines
OF traces show onset times of
The active force response latencies are calculated
automatically for each leg and for each direction and
magnitude of displacement, along with a quality factor indicating the reliability of the latency score. To
produce each latency score, four separate slope detection algorithms are used to identify the active force
“takeoff point.” The four results are then compared. If
results of none of the algorithms agree within a 10 ms
tolerance, the longest of the four latency scores is used,
and a quality factor of 1 is indicated. If two or more
scores are within 10 ms, the longest of these scores is
used, and a quality factor of 2 to 4 indicates the number
scores within the tolerance. Thus, a quality factor of
4 indicates the highest level of consistency, while a 1
indicates the least consistency. Latency scores for each
leg and translation size are presented as separate bars
in the latency plot.
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The second page of the MCT printout shows the
active force response traces of each leg and the COG
sway averaged over the three trials. The latency takeoff
points identified by the computer for the left and right
leg active force responses are marked with arrows as
shown in Figure 15–13. If the operator disagrees with
the placement of one or more arrows, a magnified view
of the force traces in question can be displayed on the
monitor and the takeoff points manually marked by the
operator. In these cases the quality factor is displayed
as an M on the graphic printout page.
Because the active force response elicited by a small
translation is near the automatic response threshold, the
associated latency is quite variable and in some instances
impossible to identify. Medium and large translations,
by contrast, elicit vigorous active force responses with
a high degree of latency symmetry between left and
right legs, and between forward and backward translations of the same magnitude. Thus, results from latency
of medium and large translations can be of significant
value in identifying motor system abnormalities in
patients with imbalance or postural instability.
Active Force Strength
Adjusting the size of support surface translations for
variations in patient height ensures that the velocity and
amplitude of COG sway are the same for all patients.
In physical terms, this means that a given size translation imparts the same sway momentum to all patients,
regardless of individual variations in body height and
weight. To counteract the sway momentum produced
by the translation, the patient must exert a restoring
force that imparts approximately twice the momentum
in the opposite direction. Half of the restoring momentum is required to stop the COG sway; the other half,
to return the body to the original equilibrium position.
For this reason, the magnitude (strength) of the active
force response is best characterized by the momentum
generated by the active force.
Following onset of the active force response, the
support surface continues to translate, displace the COG,
and stretch the ankle joint muscles. This requires the continued exertion of active force by the two legs to regain
balance. The magnitude (strength) of this active force is
quantified relative to the rate of increase of ankle torque
exerted by each leg over a 150 ms interval following
the latency onset by dividing the raw scores by patient
height and weight, which compensates for differences
in patient stature and produces strength scores with the
appropriate units of (sway) angular momentum.
For each movement direction, active force strength
is plotted as a function of translation size for each leg
and translation size. In the clinically normal individual, strength scores for equivalent-sized translations
in the two directions are similar. In addition, lateral
strength symmetry is demonstrated when the active
force responses for a given size translation are similar
in the two legs. When differences in strength between
the two legs exceed the normal range, the data points
in question are enclosed by a box.
MCT Results in a Clinically Normal Sample
Selected results from the asymptomatic normal sample
of individuals are described in Appendix 15–A. Latency
scores in normal children are like those of adults (Hassan & Azzam, 2012). For all age ranges, the average
weight symmetry scores are near 100, indicating that
weight is borne equally by the two legs. Because scores
both significantly above and below 100 (weight bearing
to the right and left of center, respectively) are considered abnormal, the clinically normal limits for symmetry are set at ±2 SD from the average.
In all age groups and in both movement directions, onset latencies are slightly longer for the medium
than for the large translations. Furthermore, the
response latencies show the expected slight increases
with age. Other research studies have also shown subtle variations in EMG latencies that are dependent on
the velocity and magnitude of the evoking translation
(Diener et al., 1984; Nardone et al., 1990). These latencies have been shown to increase with age (Peterka &
Black, 1990b; Wolfson et al., 1992). Only those latency
scores significantly longer than the averages of the
asymptomatic normal sample are considered clinically
abnormal. Thus, the upper limits for clinically normal
latencies are set at 1.67 SD above the sample averages.
In a recent side-by-side comparison study of normal subjects on the EquiTest and Balance Advantage
systems, all MCT scores were statistically the same
(Trueblood et al., 2018).
In all age groups, the height and weight compensated response strengths are symmetrical for the two
legs and movement directions. For all translation magnitudes, the average strength scores are sufficient to
return the COG to the centered position without excessive undershoot or overshoot. Specifically, the angular
momenta imparted to the body by the small, medium,
and large translations are 2.8°/s, 6.0°/s, and 8.0°/s,
respectively. To return the COG to center, the total
angular momentum generated by the two legs must
be twice that produced by the translation. One half of
the response momentum is needed to arrest the sway,
the other half to return the body COG to the center
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position. This is accomplished when each leg generates
angular momentum equal to that of the translation.
Figure 15–14 shows that, on average, asymptomatic
normal individuals produce the amount of active force
necessary to accurately return the body COG to the original centered position. Response strengths significantly
weaker or stronger than the averages of the asymptomatic normal sample are considered clinically abnormal.
Thus, the normal limits of the strengths for each age
group are set at ±2 SD from the sample averages.
AUTOMATIC RESPONSE:
THE ADAPTATION TEST (ADT)
Slow Support Surface Rotations
The adaptation protocol exposes the patient to a series
of five identical toes-up or toes-down rotations of the
support surface. The rotations are imposed at a relatively small amplitude of 8º reproducing COF movements similar to those generated by the large support
surface translations during the MCT. Because these
stimulus velocities do not saturate the automatic posture control system, it is possible to measure adaptive
changes in the patient’s responses over the course of
the five rotations.
A series of five identical rotations is typically
interposed between sets of support surface translations. When the first unexpected surface rotation is
imposed after a set of surface translations, the automatic posture control system is initially prepared to
actively resist ankle joint rotation as a means to stabilize COG sway. Because resisting the ankle joint
rotation is destabilizing when the surface rotates, the
COG sway is frequently increased during the recovery
period following the first surface rotation. Asymptomatic subjects, however, do not typically lose their balance. By the fourth and fifth rotations of the series, the
automatic system attenuates the ankle joint resistance.
This adaptive change enhances the subject’s stability
during the recovery period (Nashner, 1976).
The adaptive mechanisms responsible for attenuating ankle joint resistance and reducing COG
sway during the recovery period following support
surface rotation are complex and probably involve
both (1) reduction in background ankle stiffness and
the amplitude of the stretch-evoked (destabilizing)
responses and (2) enhancement of the later stabilizing responses. Nevertheless, when the patient makes
these adaptive changes, COG sway is reduced during
the recovery periods following the last few rotational
trials of the series.
Figure 15–14. Active force strengths in a clinically normal sample of persons 20 to 59 years of age as functions of translation size and direction. The heavy line
shows the response strengths needed to exactly recenter the Cog. Vertical units of measurement refer to
angular momentum per unit of body mass (deg/s/kg).
Horizontal notations refer to amplitude of displacement
(S = small; M = medium; L = large). From Handbook of
Balance Function Testing, Jacobson et al., 1997.
Presentation of Adaptation Results
Sway energy is used as a nonspecific but quantitative measure of the magnitude of COG sway during the recovery period following a support surface
rotation. Sway energy is a weighted sum of the root
mean square COG sway velocity and sway acceleration measured over the 2-second interval immediately
following rotation. While this measure does not reflect
changes within the individual response components
described previously, it is an accurate measure of the
overall functional effect of these adaptive changes.
Adaptation results are presented by plotting a
separate sway energy score for each of the five rotations. The display of results of toes-up rotation, which
elicits a backward COG displacement, is displayed
under the backward movement response results of
the MCT. Similarly, the results of toes-down rotations
are displayed under the forward movement results of
the MCT.
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Adaptation Results in the
Clinically
Results of ADT differ somewhat between the EquiTest
and Balance Advantage systems. In EquiTest, asymptomatic normal individuals show progressive reductions in sway energy over the course of five repeated
exposures to toes-up or toes-down rotations, as shown
in Figure 15–15. Adaptation scores are similar in chil-
Normal Sample
dren (Hassan & Azzam, 2012). Even though the sway
energy scores of younger and older subjects are similar
during the first toes-up and toes-down trials, improvements in stability are significantly greater in the
younger subjects by the fourth and fifth trials. Poorer
adaptation in the elderly may be caused by a slowing
of central adaptation, musculoskeletal factors which
increase mechanical stiffness and/or reduce muscle
strength at the ankles, or a combination of these factors
(Horak, Shupert, & Mirka, 1989; Wolfson et al., 1992).
The upper limits for clinically normal sway energies
are set at 1.67 SD above the sample averages. This is
because only significantly increased sway energies are
considered abnormal.
In the Balance Advantage system, little if any
adaptive change is noted from the first to the fifth toesup and toes-down trials. This is not due to differences
in forceplates or platform rotation characteristics. The
most likely reason for the difference is the more challenging virtual reality environment that, by providing
less useful visual orientation information, forces individuals to remain more reliant on support surface inputs.
Figure 15–15. Graphical summary of the adaptation
results of a typical clinically normal subject. The vertical
axes show the sway energy expended during corrective responses. The horizontal axes show trial numbers.
Shaded areas show scores falling outside the clinically
normal limits, based on the normal population study.
From Handbook of Balance Function Testing, Jacobson
et al., 1997.
CASE EXAMPLES FOR MCT INTERPRETATION
Case 3
A 31-year-old male presented with onset of head
motion–provoked vertigo with constant imbalance
when standing and walking. He denied any vestibular crisis event or auditory complaints. His symptoms
were more concentrated in sagittal plane movement
and when rolling left or right from a supine position.
These symptoms had been ongoing for several years
with intervals when the vertigo was resolved and the
imbalance was reduced but not absent. He reported an
MRI from several years prior to this evaluation that
was normal with a cervical MRI positive for mild disk
abnormalities. Audiologic examination was normal.
Other than the development of mild paresthesia of
the right hand and arm over the last year, he had no
other neurologic complaints and his past medical history was noncontributory. His direct office examination
was remarkable for anterior semicircular canal BPPV
canalithiasis and inability to maintain quiet stance on
foam with his eyes closed during the Clinical Test for
Sensory Interaction on Balance (CTSIB, see Chapter 9).
The remainder of the examination was normal. He was
treated in the office with a canalith repositioning procedure and referred for a formal vestibular balance rehabilitation therapy (VBRT) program. Secondary to the
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length of time of the symptoms and the complaints of
persistent imbalance (although this is a common report
with BPPV), vestibular and balance function testing
was requested. The laboratory studies continued to
show anterior canal BPPV with no other indications
of peripheral vestibular system involvement. Pursuit
tracking tests were normal, but saccade testing was
positive for mild right internuclear ophthalmoplegia
(INO). Postural control abnormalities were collectively
consistent with those seen in demyelinating disorders
and increased latency on the MCT test. The MCT test
abnormality, although nonspecific to the cause of his
symptoms, was completely unexpected and difficult to
explain on the basis of BPPV alone. However, given the
ocular motor results, the prolonged latencies to active
recovery fit with the overall suspicion of central nervous system involvement. His SOT results showed an
increase in sway under conditions 5 and 6 without fall
reactions. The SOT results are nonspecific to the disorder underlying the condition but reflective of his functional ability to maintain quiet stance when challenged.
The SOT results simply reflect the fact that he was hav-
ing difficulty using vestibular system information when
visual and proprioceptive/somatosensory cues were
absent or disrupted. Secondary to these findings and
his report of paresthesia starting in the left foot, a new
MRI was obtained that showed multiple hyperintense
spots throughout the brainstem region. He was referred
on to neurology and is being followed with a diagnosis
of probable multiple sclerosis with BPPV.
Case 4
A 44-year-old female presented with spontaneous
spells of lightheadedness and imbalance lasting hours.
Frequency of occurrence was one per every eight
weeks but increasing. Typically, she would return to
her normal baseline between events. The core studies
involving ENG and ocular motor tests were normal.
Her CTSIB was normal, but as her history was that of
unsteadiness, episodic without vertigo, full SOT and
MCT were indicated. The results of these evaluations
are given in Figure 15–16. This figure demonstrates
figure 15–16. SOT and MCT results, respectively, for the patient in Case 4. Refer to Figure
15–7 for color coding. (The figure is based on data provided by Shelly Massingale, PT and
Bertec Corporation).
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normal SOT with significantly prolonged latencies on
the MCT. The abnormally long MCT latencies for both
limbs is suggestive of possible central nervous system
involvement above the level of the spinal cord. Therefore, this patient’s care was directed to neurology from
otolaryngology, and she has been followed for possible
demyelinating disease as a result of the test findings
guiding the management.
Case 5
A 50-year-old-male with a known right medullary
infarct secondary to a posterior inferior cerebellar artery
(PICA) distribution stroke was seen for a full vestibular system evaluation. His symptoms were that of persistent unsteadiness when standing and walking that
resolved completely when sitting or lying down. His
office examination and laboratory studies were without any indications of peripheral vestibular system
involvement but did show indications for right ocular
lateral pulsion (eyes deviate to the right when visual
fixation is removed), a finding consistent with a dorsal lateral medullary infarct. His SOT and MCT results
for the pre-therapy testing are given in Figure 15–17,
top half. While his SOT results show an inconsistent
pattern, it was highly repeatable and his reduction in
volitional limits of sway was 28% with his COG alignment within normal limits. So these findings for the
SOT could be a result of motor compromise from his
stroke, and yet his normal MCT results did not support that interpretation. Therefore, it was assumed that
he had problems interpreting his sensory inputs. He
was placed in a vestibular and balance rehabilitation
program that worked on his reaction to different sensory inputs. His post-therapy results are given in the
bottom half of Figure 15–17, showing now normal SOT
results along with the normal MCT findings, and his
complaint of constant unsteadiness was resolved.
SUMMARY
Computerized dynamic posturography uses a multiplicity of independent test protocols to objectively
assess the major sensory and motor components of
balance. All protocols are based on documented physiologic principles of human balance. Some protocols
provide information relative to the patient’s functional
capacity within a variety of daily life tasks. Others provide information that can help localize the cause of a
balance system disorder. The ability to identify inconsistencies among independent test results is an additional
advantage in identifying causes of unsteadiness such as
anxiety. Experimental studies with various populations
of clinically normal individuals indicate that CDP can
provide reliable and repeatable test results.
figure 15–17. Case 5 results. A. SOT pre-therapy. B. MCT pre-therapy. C. SOT post-therapy.
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D. MCT post-therapy. Refer to Figure 15–7 for color coding. (The figure is based on data
provided by Shelly Massingale, PT and Bertec Corporation.)
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