Determining Consistency of Elbow
Joint Threshold Angle in Elbow
Flexor Muscles With Spastic
Hypertonia
Background and Purpose. Threshold angle, the point in passive range
of motion where a muscle response or torque change is elicited, may
be a potentially valid measure of hypertonus. Because the relationship
of initial muscle length to threshold angle has not been addressed
previously, this preliminary study examined whether starting elbow
joint position and speed of stretch to elbow flexor muscles affect
threshold angle. Subjects. Five subjects with stroke-induced hypertonia
of the elbow flexor muscles participated. Methods. Two starting angles
and two designated stretch speeds were applied randomly by a torque
motor at each of three testing sessions. Results. Starting angle, subject,
and session affected threshold angle. A 90-degree starting angle at a
stretch speed of approximately 1.0 radian/s produced the most
consistent threshold angles between sessions within subjects, and
threshold angle was relatively consistent for some subjects, irrespective
of speed. Conclusion and Discussion. If future research indicates that
these data can be generalized, the use of threshold angle as a
consistent measure of hypertonia will require comparison within
individuals, use of a consistent starting angle, and a movement
condition of a 90-degree starting angle and an approximate movement
speed of 1.0 radian/s across sessions. [Wolf SI,, Segal RL, Catlin PA,
et al. Determining consistency of elbow joint threshold angle in elbow
flexor muscles with spastic hypertonia. Phys Ther. 1996;76:586-600.1
Key Words: Elbow; Joints; Musde spasticity; Muscle tonus; Upper extremity.
Steven L Wolf
Richard L Segal
Pamela A Catlin
Julie Tschorn
Tina Raleigh
Heather Kon tos
Patricia Pate
Physical Therapy . Volume 76 . Number 6 . June 1996
ypertonia is a symptom associated with many
central nervous system disorders that frequently contributes to impaired motion or
compromised functional independence.l.2
Physical therapists use a variety of treatment techniques
to contrlol hypertonia so that greater range of motion
(ROM), strength, and functional ability can be achieved.
Assessing the severity of muscle hypertonia, however,
presents a challenge. The most common assessment
used in the clinic is a manual determination of the
amount of resistance perceived during a passive "quick
stretch." This technique, however, requires clinicians to
make judgments about the response, and the method
may be prone to errors because of variability in the
applied stretch. Measurements indicative of volitional
movement, such as the Ashworth scale, provide valuable
information but do not specifically assess hypertonia.'
Improved measurement techniques are needed to accurately quantify the degree of hypertonia. Better quantification will allow therapists to draw correlations or
inferential relationships with measures relating to physical therapy interventions and, ultimately, to changes in
disability.
Spastic hypertonia is observed in motor disorders that
are characterized by velocity-dependent increases in
SL Wolf, PhD, PT, FAPTA, is Professor and Director of Research, Department of Rehabilitation Medicine,
and Associate Professor, Department of Anatomy and Cell Biology, Emory University School of Medicine,
1441 Clifto'n Rd NE, Atlanta, GA 30322 (USA) ([email protected]).
Address all correspondence to Dr Wolf:
RL Segal, PhD, PT, is Associate Professor, Department of Rehabilitation Medicine, Division
of Physical Therapy, and Assistant Professor Department of Anatomy and Cell Biology,
Emory University School of Medicine.
PA (htlin, EdD, PT, is Professor and Director, Division of Physical Therapy, Department
of Rehabilitation Medicine, Emory University School of Medicine.
J Tschorn, PT, is Staff Physical Therapist, Grady Memorial Hospital, 80 Butler St SE,
Atlanta. GA 30335.
T Raleigh, PT, is Staff Physical Therapist, Promina Gwinnette Health Systems, Lawrenceville, GA 30245.
H Kontos, PT, is Staff Physical Therapist, Tampa General Rehabilitation Hospital, Tampa. FL 33601.
P Pate, PT, is Staff' Physical Therapist, Rancho Los Arnigos Hospital, Downey, CA 90242
Ms Tschorn, Ms Raleigh, Ms Konto5, and Ms Pate were graduate students, Division of Physical Therapy,
Department of Rehabilitation Medicine, Emory University, during this study, which was undertaken
in partial fulfillment of thr requirements for their Master of Physical Therapy degree.
This stndy was supported in part by NIH Research Grant No. NS28784, US Public Health Service,
Department of Health and Human Services, and was approved by the Emory University School
of Medicine Human Investigation Committee.
Thzs nrlzclv iunr ruhmztt~dMarch 4, 1995, and iuns arc~/)tpdF~brunry20, 1996.
Physical Therapy . Volume 76 . Number 6 . June 1996
Wolf et al . 587
not be construed as suggesting that slower passive
tonic stretch reflexes with exaggerated tendon jerks,
resulting from hyperexcitability of the stretch r e f l e ~ . ~ stretches cannot also yield exaggerated muscle reflex
responses.
When a muscle is stretched, spindle afferent neurons
initiate a short-latency motor response (the stretch
The starting angle may be an indirect index of the point
reflex) to correct the perturbation and return the musto which a muscle may be passively lengthened before
cle toward its resting length. Motoneurons with memeliciting an EMG response (threshold). Speed of passive
brane potentials that are nearer to threshold of activastretch may affect the magnitude of the hyperactive
tion may be more readily excited by spindle afferent
stretch reflex and the threshold angle. At present, the
nerve input during sudden muscle lengthening. This
effects of starting position (muscle length) and speed of
situation, along with reduced inhibitory influences from
passive stretch on the threshold angle of a joint acted
interneurons, constitute one possible mechanism for
upon by a hypertonic muscle are not well-documented.
spastic h y p e r t ~ n i aThis
. ~ enhanced motoneuron sensitivDevelopment of an effective measurement technique to
ity may result in shorter latency of muscle response to a
detect onset of muscle response to passive stretch may
phasic length change and may be manifest as a heighthelp quantify hypertonia.
ened muscle responsiveness to these imposed length
changes. The threshold angle is the joint angle during a
This study, therefore, examined the effect of starting
passive stretch at which electromyographic (EMG) activangle and speed on the threshold angle in subjects
ity increases above a resting l e ~ e l . l ,Threshold
~-~
angle is
diagnosed with stroke-induced hypertonia. We see this
presumably a measure of the relative state of motoneuimpairment-based evaluative approach as a necessary
ron membrane depolarization and has been proposed as
precursor to gathering information about the impact of
a quantitative measure of h y p e r t ~ n i a . ~ , ~
interventions on sensitivity to muscle-length changes.
Indeed, reliable intrasubject findings about the relationMechanical influences from muscle and connective tisship ofjoint starting angle, speed of passive motion, and
The initial
sue alter the manifestations of hypert~nia.'-~
position of a joint (starting angle) affects the length of
reflex responses pave the way to ultimately correlating,
the associated muscles; consequently, the degree of
in a quantitative fashion, the relationships between a
specified impairment (hypertonicity), interventions, and
depolarization among motoneurons is influenced by the
functional outcomes.
spatial and temporal summation of cutaneous and proprioceptive inputs onto them at any given starting angle.
Muscle response to a passive stretch and the threshold
angle, therefore, would be affected. If threshold angle
remains the same for each respective starting angle, then
Design
the threshold angle may be a valid measure of hyperFive subjects were randomly assigned to 1 of 24 possible
tonia regardless of mechanical influences, such as
sequences of movement at each of three sessions. No
changes in connective tissue viscoelastic properties, or
sequence was repeated. Each sequence included four
the initial joint position. If threshold angle changes for
conditions of either 70 or 90 degrees of starting elbow
flexion and a target speed of either 0.5 or 1.0 radian/s of
each respective starting angle, then threshold angle may
be a valid measure of hypertonia only within the respecmovement. Data were collected for each of three
stretches during each condition.
tive starting angle. The measurement of threshold angle
may require specification of joint position for which a
reproducible threshold angle can be seen prior to
Sample
initiating any intervention. The measurement of threshSubjects were adults 31 years of age or older with
old angle attempts to objectively reflect the variability of
hypertonicity in the upper extremity secondary to stroke
hypertonia seen clinically.
occurring more than 4 months prior to data collection.
Presence of hypertonicity was determined by detection
The speed of stretch may affect muscle tension or torque
of resistance to a manual passive stretch of the elbow
development, altering the presentation of hyperflexors of the involved limb. Each subject's involved limb
t ~ n i a . ~ , " ~ - I Vmagnitude
he
of stretch reflexes is observcould be passively moved through the entire elbow ROM
ably altered by speeds greater than 1.0 radian/s (1 radineeded for the data-collection procedure, and no elbow
joint contractures were present. Subjects were selected
an=57.3"),yet the mechanism by which speed alters the
from a registry of patients with stroke, and participation
stretch reflex is controversial. Faster speeds could alter
the stretch reflex by decreasing the threshold of
was voluntary. Prior to participation, each subject signed
a consent form approved by the Emory University School
motoneuronal excitability, by increasing muscle stiffness
secondary to speed-dependent viscoelastic properties, by
of Medicine Human Investigations Committee. Physician consent for subject participation was obtained. The
altering muscle spindle sensitivity, or by any combinarights and privacy of the subjects were protected.
tion of these variables4 This situation, however, should
588 . Wolf et al
Physical Therapy . Volume 76 . Number 6 . June 1996
Table 1.
Characteristics of Subiects
Characteristic
Age (Y)
Subject No.
1
2
3
4
5
58
42
42
41
31
Gender
Female
Female
Male
Female
Male
Site of lesion
Bilateral lacunar
lesions in basal
ganglia,
thalamus, and
periventricular
white matter
Right frontal
lobe and
internal
capsule
Right middle
cerebral artery
infarct at right
basal ganglion
Basilar artery
thrombosis at
brain stem
Left circle of
Willis,
intracranial
Hand dominance
Right
Right
Right
Right
Right
Time since lesion
(mo)
Recovery stage'
4
4
18
72
7
3
2-3
3
4
3
Left
Right
Right
Side of
hypertonia
Right
Left
The selection criteria included (1) presence of resistance to muscle stretch," (2) elbowjoint passive ROM of
10 to 90 degrees of flexion, (3) ability to achieve an
appropriate sitting position for testing, (4) ability to
follow verbal commands, (5) appropriate cognition,14
(6) stage 2 moving into 3 to 5 of recovery according to
the Brunnstrom stages of re~overy,~"nd (7) EMG
activity above a resting baseline level upon passive
stretch.
Measurements
The starting angle was defined as the position of the
elbow joint of the affected extremity measured just
before an induced passive stretch using standard goniometric technique.l"tarting
angles corresponding to
functional length-tension relationships of the elbow
flexor nluscles were chosen. These angles corresponded
to slightly more than 50% of normal passive ROM. A
series of three extension stretches were performed at a
starting angle of 70 or 90 degrees of elbow flexion.
Movemlent speed was defined as the speed of the extension stretch applied by the torque motor in servo mode.
The mean of the baseline levels of elbow flexor muscle
activity during passive stretch in four persons without
hypertonia was obtained. These baseline values were
typically around 4 pV (SD=3) for the biceps brachii
muscle and 6 pV (SD=4.5) for the brachioradialis
muscle. The threshold angle was then defined as the
elbow joint position during stretch at which a subject's
muscle activity exceeded two and a half standard deviations above this mean. The resultant criterion values
required that biceps brachii muscle activity be greater
than 10 pV and that brachioradialis muscle activity be
greater than 15 pV. The EMG signal was amplified
Physical Therapy. Volume 76 . Number 6 . June 1996
(X1,OOO) and filtered between 20 and 500 Hz. The
actual threshold value was determined from filtered
EMG activity. The EMG activity was measured with
surface electrodes placed o n the skin over the bellies of
the biceps brachii and brachioradialis muscles. The
EMG activity and joint position were recorded during a
stretch of a 1-radian (57.3") arc. The computer sampled
the following measures at 1 kHz: joint angle, stretch
speed, EMG activity of the biceps brachii and brachioradialis muscles, and torque. Electromyographic activity
above the set criterion also was determined by the
computer.
Joint position versus EMG activity was graphed from
computer data. Graphic representation of the threshold
angle was used to visually confirm the computer criterion for designating the threshold angle.
Subject Selection
Subject characteristics and selection criteria were measured during a n initial screening session (Tab. l ) . Fortunately, enlistment of individuals with discretely different lesion sites was possible, allowing examination of
consistency of responses. Information regarding age,
gender, site of lesion, time since lesion, medical history,
diagnosis, medical treatment received, past physical therapy treatment, aerobic exercise, and current medications was obtained by review of the medical record or
subject interview prior to the initial screening session.
During the initial screening session, side of hand dominance, upper-extremity recovery stage,]'. elbow passive
ROM, ability to follow simple verbal commands, ability
to achieve a comfortable sitting position, appropriate
cognition, and EMG responses were determined by
standard clinical tests. Cognition was considered normal
Wolf et al . 589
if the subject responded correctly to all items on the
modified Folstein Mini-Mental State Exam.14 Each subject answered questions concerning feelings of general
health, level of relaxation, and amount of caffeine intake
prior to both the initial screening session and each of
two data-collection sessions. A variety of lesion sites are
represented.
Subject and Electrode Positioning
Subject sitting position was controlled during each datacollection session. Position was determined at each
session by measuring the following variables using standard goniometric technique: (1) knee flexion between
80 and 100 degrees, (2) hip flexion between 80 and 100
degrees, (3) shoulder abduction between 40 and 60
degrees, (4) shoulder flexion at 20 degrees, (5) elbow
flexion at 70 or 90 degrees, (6) forearm in 10 to 20
degrees of pronation, and (7) wrist neutral.16 Subjects
faced forward with feet shoulder width apart and the
uninvolved upper extremity placed comfortably on their
lap.
Two silver-silver chloride electrodes with a Qmm recording surface were placed over each muscle at the beginning of the screening session. Electrode placement was
determined by isolating the bellies of the biceps brachii
and brachioradialis muscles during isometric contractions." Measurement of the anatomical point of widest
girth of the biceps brachii or brachioradialis muscle
during an isometric contraction of that muscle was used
to isolate each muscle belly. The vertical and horizontal
midpoint of the each muscle belly at the point of widest
girth was marked with a felt-tip pen. The electrode pair
was placed on either side of the marking, oriented
parallel to the muscle-fiber direction." Electrode pairs
were spaced at 1.7 cm center-to-center to improve specificity of the EMG signal. Isometric contractions also
were used to verify correct electrode placements. All
EMG activity was monitored on an oscilloscope to detect
artifact during actual movements or prior to stretch.
Impedance between the skin and the overlying electrode
was reduced by abrading the skin with sandpaper and
cleansing the skin with alcohol. Skin impedance, measured with an ohmmeter, between the ground electrode
and each electrode was less than 5 kfl. The resistance
between an electrode pair on each muscle belly was less
than 2 kfl.
After initial electrode placement was determined during
the screening session, an anatomical skin map was made.
The skin map measured electrode placement from
nearby landmarks such as anatomical structures, scars,
or prominent freckles. The distance from the acromion
to the biceps brachii muscle electrode and the distance
from the styloid process of the radius to the biceps
590 . Wolf et al
brachii and brachioradialis muscle electrodes were measured. The skin map was used a t each session to ensure
consistent electrode placement.
Reliability
Intrarater and interrater reliability of all measurements
was maintained at 100% agreement of values, obtained
by repeated measurements by the same researcher or by
a second researcher. Total agreement was observed for
defining age, gender, hand dominance, site of lesion,
time since onset, side of hypertonicity, upper-extremity
stage of synergy involvement,'%nd ability of the subject
to achieve testing position. Total agreement also was
achieved for identifying medical history, physical therapy treatments, exercise, medications, general health,
level of relaxation, and caffeine intake. There was total
intrarater and interrater reliability in defining subject
cognition and onset of EMG responses. Starting angle
was equal within 2 1 degree, subject position was equal
or within ?3 degrees, skin map placement was equal
within k0.5 mm, and skin impedance equaled exact
agreement. Agreement on threshold angle was determined by comparing computer calculation with graphic
displays, as described previously, and was always maintained at 100% when performed in this manner. Calibration of all equipment was maintained throughout
data collection.
Limb Movements
Slow acceleration and deceleration of the forearm by the
torque motor produced a smooth, passive stretch. The
set speed produced by the torque motor was not instantaneous. The acceleration period needed by the torque
motor to reach the set speed was affected by the inertia
of the limb and the inherent delay in the acceleration
phase of the motor. To ensure consistency of speed,
responses were examined within and across sessions.
Figure 1 shows such consistency for the 70-degree starting angle at 0.5 radian/s within a subject (A) and across
sessions (B). T h e across-sessions graph exemplifies the
superimposition of the first stretch. Periodically, threshold angle was reached before the torque motor achieved
the preset speed (ie, the limb was still accelerating). The
actual speed at the occurrence of threshold was determined for every stretch using the slope of time versus
joint angle. Intrarater and interrater determinations of
actual speed agreed 100%. All calculations regarding
threshold angles were made at the actual speed at which
they occurred. Thus, in calculating data by conditions,
although the true speeds were used, we refer to conditions in the sense that the torque motor was programmed to move at either 0.5 or 1.0 radian/s. Data in
Figures 2 and 3, for example, depict the actual speed at
which the threshold angle was achieved, and computations made in Table 2 were based on actual speeds.
Physical Therapy . Volume 7 6 . Number 6 . June 1996
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TIME (s)
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Plot of tirr~eversus ioint angle from typical biceps brachii muscle stretches showing approximately linear speed. Speed was calculated as the slope
of the graph at the threshold angle. (A) Superimposition of three stretches from one session. (B)Superimposition of first stretch at each of three sessions.
Actual excursion is approximately 0.93 radian; actual speed is 0.46 radian/s. Y axis is identical in A and B.
Proced~rre
If a subject met all screening criteria, the first datacollection session was begun immediately. Prior to the
screening session and each data-collection session, the
subject was asked a set of pretrial questions to assess his
or her emotional and physical state at the time of data
collection. Electrode placement sites over the biceps
brachii and brachioradialis muscles were determined at
the screening session. The subject's skin was prepared,
and the electrodes were placed on the electrode sites.
Skin impedance was measured. The subject was positioned in the chair, the involved forearm was placed in
the prefabricated arm splint, and the elbow was aligned
to the axis of the 7.2-hp torque motor (Fig. 4).* The
electrodes were connected to the amplification system
and to the oscilloscope. Electromyographic activity was
preamplified at the source ( X 10) and subsequently
amplified ( X 100) prior to analog-to-digital processing.
The subject was asked to contract and relax the upperextremity muscles to check for EMG activity o n the
oscilloscope and then was positioned correctly with the
involved elbow placed at the selected starting angle.
The speed was set using a computer servo-mode program. Subjects were instructed to maintain the testing
positioin without movement prior to testing. The muscles
under the electrodes were determined to be passive
prior to testing by recording a baseline of EMG activity
100 milliseconds prior to stretch while the subject
remained relaxed. Three stretches were performed during each of the four conditions. All stretches were
separated by at least a 60-second interval. The procedure
was repeated for two additional sessions, separated by 1
to 7 days.
Each of the remaining data-collection sessions began by
positioning the subject at the torque motor in the servo
mode. The skin map was used to locate the electrode
placement sites. The subject received a different random
sequence of the four conditions at each of the three
data-collection sessions.
Data Analysis
A computer graphics program plotted the raw data on a
graph, with joint position on the X axis and EMG activity
of the biceps brachii or brachioradialis muscle o n the Y
axis. Typical changes in the biceps brachii muscle during
stretch are depicted by the arrows in Figures 5A and 5B.
The occurrence of threshold angle can vary. Figure 5A
demonstrates a n abrupt EMG response to stretch, and
Figure 5B demonstrates a more gradual response for the
same muscle but from a different subject.
Threshold angle was determined for the biceps brachii
and brachioradialis muscles during each stretch. A mean
threshold angle also was calculated for each muscle
"'PMIMotion Technologies, 49 Mall Dr, Commack, NY 11725.
Physical Therapy . Volume 76
. Number 6 . June 1996
Wolf et al . 591
1
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during each condition per session. The EMG activity was
monitored during each stretch for torque motor noise,
and data for a stretch were discarded if the baseline
showed harmonics of 60-cycle activity. Trials that were
discarded for this reason were repeated. Graphic representation of muscle activity during each stretch was
examined to detect movement artifact. For 12 of 360
data points, information was irretrievable from disks,
and a program for maximum likelihood estimation that
assumes compound symmetry was used to estimate
stretches. Using the Statistical Analysis System (SAS)
program,t riormality of distribution of threshold angle
per muscle was assured prior to statistical analysis, based
on assessment of the skewness and kurtosis of the data.
The difference in threshold angles within conditions,
within subjects, behveen testing sessions, and per muscle
was determined using a one-way repeated-measures
analysis of variance (ANOVA). The Tukey pair-wise
procedure was performed post hoc when a statistically
significant effect was found. The difference in threshold
angles behveen conditions, within subjects, and per
muscle was determined using a two-way (starting
angle X speed) ANOVA. A Bonferroni post hoc multiplecomparison procedure was performed when a statistically significant effect was found.lHFor all tests, P<.05
was considered significant. A power analysis to determine the applicability of the results to a larger population could not be undertaken due to the complexity of
the design and of the data analysis. The SAS program
was used to calculate the ANOVAs.lWata analysis across
subjects was not performed, as it was determined that
these analyses would be irrelevant due to expected
differences in muscle response to passive stretch across
individual^.^,"^^
0.60 -
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0
-
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9
0.30 0.20 0.10
0.w
-
0
10
20
30
40
50
60
70
80
90
THRESHOLD ANGLE (")
Figure 2.
Composite threshold angles at specified speeds for (A) biceps brachii
muscle and (B) brachioradialis muscle. There is wide variation of
threshold angles across all subiects, demonstrating inconsistency in
threshold angles between subjects.
The complete distribution of threshold angle determination for all subjects is shown in Table 3. Figures 2A
and 2B present, for the biceps brachii and brachioradialis muscles, respectively, the actual threshold angles for
all stretches among all subjects and demonstrate the
wide variability across subjects per condition. If the
threshold angle would have been consistent, a vertical
orientation of threshold angles across subjects would be
expected for each condition.
The effect of session on threshold angle was variable and
dependent on muscle and condition (Tabs. 4 and 5 ) .
Table 4 shows Fstatistic values for significant session
effects seen at all conditions except at a 90-degree
starting angle and a stretch speed of 1.0 radian/s.
For the condition of 90-degree starting angle and 1.0-
'SAS Institute Inc, PO Box 8000, Cary, NC 2751 1
592 . Wolf et al
Physical Therapy
. Volume 76 . Number 6 . June 1996
radian/s stretch, no effects were found in the brachioradialis muscle and only subject 4 showed a n effect for the
biceps brachii muscle. For the condition of 70-degree
starting angle and 0.5-radian/s stretch, three subjects (1,
2, and 4) showed a difference in threshold angle for the
biceps brachii muscle and only subject 4 showed a
difference for both muscles.
Three subjects (2, 3, and 4) showed a difference in
threshold angle in the biceps brachii muscle as well
as the brachioradialis muscle for the condition of 90degree :starting angle and 0.5-radian/s stretch. Two
subjects showed a difference in threshold angle in the
biceps brachii muscle as well as the brachioradialis
muscle fix the condition of 70-degree starting angle and
1.0-radian/s stretch. Only subject 1 showed a difference
in threshold angle for both muscles. An examination of
the data1 presented in Table 5 reveals no consistent
pattern of session effect of the mean threshold angles.
Figures 3A and 3B demonstrate the magnitude of consistency between mean threshold angles per session per
subject iin the biceps brachii and brachioradialis muscles.
In both muscles, this consistency is most observable
in the condition of 90-degree starting angle and 1.0radian/s stretch. Each data point represents mean
threshold angle per subject per session. The symbols
were most clustered in a vertical pattern for each subject
at the 90-degree and 1.0-radian/s condition. The more
vertically superimposed the symbols representing a subject were, the more consistent was the response. Only the
biceps brachii muscle response of subject 4 in the
90-degree and 1.0-radian/s condition showed substantial
variance (Fig. 3A). For most subjects, the vertical distribution of mean session responses for the brachioradialis
muscle showed tight vertical clustering in the 90-degree
and 1.O-radian/s condition (Fig. 3B).
Threshold angle was not affected by speed for the biceps
brachii or brachioradialis milscle among individual subjects (Tab. 2). In an effort to determine the relationship
between the true speed of movement and the threshold
angle, a one-way repeated-measures analysis of covariance was performed, substituting the actual speeds of
movement for all five subjects over three sessions for
each muscle. These results also showed n o relationship
between threshold angle and speed. Threshold angle
responses, however, were affected by the starting angle.
The starting angle had an effect on threshold angle for
the biceps brachii muscle for subjects 1, 3, and 5 and
for the brachioradialis muscle for subjects 1, 3, and 4.
Table 6 shows the relationship between the starting
angles ;at which an excursion was found. Clearly, a
stretch from a 90-degree starting angle consistently
showed a greater amount of limb excursion prior to
occurrence of threshold angle than did a stretch from a
70-degree starting angle.
Physical Therapy . Volume 76
. Number 6 . June 1996
The actual threshold angle also was recorded among all
subjects at select speeds and starting angles. This record
provides a surrogate measure for muscle length at the
time an elevated response in muscle activity was
observed. Despite the greater excursion seen at a starting
angle of 90 degrees, the threshold angles for some
subjects were remarkably similar to those seen at a
starting angle of 70 degrees. We calculated actual threshold angles averaged across trials among subjects who had
complete data sets (subjects 1, 3, and 5 for the biceps
brachii muscle and subjects 1 and 3 for the brachioradialis muscle) and who also showed differences in excursion, as depicted in Table 6. Among these differences,
subject 1, for example, exhibited only a 2-degree difference in the actual value of threshold angle for the biceps
brachii muscle between 70 and 90 degrees (Fig. 3A) arid
a 1-degree difference for the brachioradialis muscle
(Fig. 3B) for threshold angles recorded at 0.5 radian/s.
The most extreme averaged differences in actual threshold angle with different starting angles were measured in
the biceps brachii muscle for subject 3 at 0.5 radian/s
(15", compare top left and right panels, Fig. 3A).
Discussion
Muscle responsiveness to passive lengthening using
threshold angle as a measurement has been presented
by Rymer and c ~ l l e a g u e s l ,as
~ ,a~justifiable
~~
technique
for assessing hypertonia. Extraction of information from
their studies suggests that threshold angle can be influenced by factors such as time of testing, individual
attributes, viscoelastic properties of the muscle, starting
muscle length, and speed of stretch. Of these factors, the
resting length of a muscle exhibiting hypertonia and the
speeds at which the muscle is moved have not been
presented in terms of their consistency to produce a
threshold angle response within subjects with hypertonia. Our data shed light on these issues.
Within any subject or condition, no pattern of session
effect could be determined (Tab. 5). Results from previous studies4.6 suggest a change in threshold angle
between sessions. The variability of the threshold angle
may be explained by the pseudorandom reflex proposed
by Dietz and Berger," who found that motoneuronal
firing may be altered by a functional movement and may
change the occurrence of threshold. Dietz and Berger
suggest that the strength of the hypertonic reflex
response is decreased because the motoneuronal firing
is dampened by a functional position. A 90-degree
starting angle and a specified 1.0-radian/s speed had the
most reproducibility across sessions, with only one session effect for the biceps brachii muscle for subject 4
(Tab. 5). The condition of a 90-degree starting angle
and an approximate speed of 1.0 radian/s may produce
the most consistent threshold arigle response within
each subject due to a combination of the shorter muscle
Wolf et al . 593
A
BICEPS BRACHll
MUSCLE
0
CONDITION 70' & 0.5 radianls
1.0
1.0
-
0.9
-
0.8
-
0.7
-
$
0.6
-
6
0.5
--_
0.4
-
0.9 0.8 0.7
c
.-
0.8
-
0.4
V)
A
-
0.5
B
E
.u
-
B
-
V)
--
fi
Y
.
A
-
0.3
-
0.2
-
0.2
-
0.1
-
0.1
-
0.0
0
-
-
0.0
0
10
20
30
40
50
60
70
0
THRESHOLD ANGLE (O)
-
1.0
-
0.9
-
0.9
-
0.8
-
0.8
-
0.7
-
0.7
-
0.6
-
0.6
-
0.5
-
0.4
-
A
f
.-m
u
0.5
-
0
w
0.4
V)
0
2
0
3
0
4
0
5
0
6
0
7
0
8
0
9
0
CONDITION go0 & 1.o radianls
1.0
.u
1
THRESHOLD ANGLE (O)
1.o radianls
CONDITION 700
6
radianls
0.3
- -
-$
SUBJECT 4
SUBJECT 5
CONDITION 90' & 0.5
-
A
+
A
SUBJECT 1
SUBJECT 2
B
-
+
+4
V)
0.3
-
0.3
-
0.2
-
0.2
-
0.1
-
0.1
-
0.0
0
0.0
1 I I II I I 1
II ,I,
0
I0
20
p
30
40
THRESHOLD ANGLE ( O )
igure 3.
50
60
70
0
1
0
2
0
3
0
4
0
5
0
6
THRESHOLD ANGLE (O)
0
7
0
8
0
9
0
-
~resholdangle determinations shown for each condition for [A) biceps brachii muscle and [B) brachioradialis muscle. Each symbol represents the
ean for threshold angles for one session for each subiect. Note that the 90degree and 1 .O-radian/s condition shows the most vertical clustering
means within subiects, demonstrating more consistent threshold angles than the other conditions. Some data points are missing for subject 4 due
artifact when eliciting threshold angle responses at 9 0 degrees and 0.5 radian/s and at 9 0 degrees and 1.0 radian/s.
594 . Wolf et al
Physical Therapy . Volume 76 . Number 6 . June 1996
BRACHlORADlALlS
MUSCLE
B
radianls
COND~T~ON
70' & 0.5
-$
.-m
CONDITION go0 & 0.5 radianls
1.0
-
1.0
-
0.9
-
0.9
-
0.8
-
0.8
-
0.7
-
0.7
-
0.6
-
0.5
-
.r?
c
m
.u
u
g
9
f
A
SUBJECT 2
I +SUBJECT
SUBJECT 4
5
0.4
0.8-
g
0.5-
g
0.4-
9
-
V)
V)
0.3
-
03
-
0.2
-
0.2
-
0.1
-
0.1
-
0.0
-
0
1
0
M
3
0
4
0
5
0
6
0
7
0
0
1
0
2
THRESHOLD ANGLE (O)
CONDITION 700 & 1.o radianls
-$
.-
1.0
-
0.9
-
0.8
-
0.7
-
0.6
-
n
0.5
+A
0
4
+
-$
.-m
Oco
-
0
4
0
5
0
6
0
7
0
8
0
9
0
-
1.0
-7
0.9
-
0.8
-
0.7
-
0.6
-
0.5
-
0.4
-
A
A%o
u
*
W
0.4
3
CONDITION go0 & 1 .o radianls
u
g
0
THRESHOLDANGLE (O)
g
n
w
V)
V)
0.3
-
0.3
-
0.2
-
0.2
-
0.1
-
0.1
-
0.0
-
0.0
-
0
1
0
2
0
3
0
4
0
5
0
6
0
7
0
THRESHOLD ANGLE ( O )
length prior to stretch and the specified speed of stretch.
The act.ual speed of movement when threshold was
reached was close to 1.0 radian/s at the starting position
of 90 degrees for most stretches. The greater reproducibility in. this condition may be due to the actual speed
being more consistently generated than at the condition
of 70 degrees and 1.0 radian/s.
Physical Therapy. Volume 76 . Number 6 . June 1996
0
1
0
2
0
3
0
4
0
5
0
6
0
7
0
8
0
9
0
THRESHOLD ANGLE ( O )
Pretrial questions revealed consistency within all subjects
for caffeine intake, medications, level of relaxation, and
perception of general health. All subjects also were
tested at the same time for each testing session. These
factors, apparently, did not contribute an explanation
for the variability in session effect.
Wolf et al . 595
Table 2.
Two-Way Analysis of Variance Testing for a Condition Effect for Threshold Angles Within Subjects
Speed
Subject No.
F
Starting Angle
P
F
Interaction
P
F
P
Biceps brachii
muscle
1
2
3
4
5
Brachioradialis
muscle
1
2
3
4
5
The lack of an effect for speed within subjects on
threshold angle does not support the clinical evidence
The
that hypertonia is speeddependent (Tab. 2)
fastest speed used in this study, 1.0 radian/s, may not be
as fast as the "quick stretch" performed in clinical
settings to assess hypertonia. Given et al" recently found
that speed of movement did not strongly affect stretch
reflex behavior within four subjects who had sustained a
stroke, using speeds ranging from 0.1 to 2.0 radian/s.
Their subject with the greatest hypertonia assessed clinically, however, required a lower stretch speed to reach
threshold than did a subject with mild hypertonia.
Previous studies4."~'"have found an effect of speed on
reflex threshold in some subjects, with a decrease in
threshold angle occurring as a function of an increase in
speed. These studies used stretch speeds ranging from
0.25 to 3.0 radian/s. The large range may explain the
reported impact of speed on threshold. Our study used
only speeds approximating 0.5 and 1.0 radian/s, which
may not have been fast enough to create a speed effect
on threshold angle. These speeds were chosen, however,
to allow accurate comparison of results with those of
previous studies that used similar speeds during data
collection.
.43"9-12
A correlation between decreased limb excursion prior to
reaching the threshold angle and greater sensitivity to
stretch was reported by Powers et al,H who did not
consider the relationship of the starting position before
the stretch occurred. In our study, an effect of starting
angle on the magnitude of joint excursion to reach
threshold angle was noted for both the biceps brachii
and brachioradialis muscles in three subjects (Tab. 2).
These effects accounted for 60% of total threshold angle
values. The smaller excursion to reach the threshold
596 . Wolf et al
angle occurring frorn a 70-degree starting angle or the
larger excursion to reach threshold angle occurring
from a 90-degree starting angle may be a result of
increased stretch reflex sensitivity at the 70-degree position. Table 6 shows that the 90-degree starting angle
permitted a greater limb excursion prior to reaching the
threshold angle than did the 70-degree starting angle.
Stretch reflex sensitivity is increased at 70 degrees, when
stretch begins with the muscle in a more lengthened
position. Muscle spindle sensitivity may be greater at a
70-degree starting angle, or motoneurons may be closer
to threshold level, as evidenced by less excursion of the
limb prior to the occurrence of threshold (Tab. 2). With
a 90-degree starting angle, the limb goes through greater
excursion prior to threshold, indicating either decreased
stretch reflex sensitivity or a greater magnitude of "slack"
in the muscle prior to activating sufficient spindle input
to further depolarize motoneurons to threshold.
Elevated stretch reflex sensitivity is thought to be due, in
large part, to supraspinally mediated increased background levels of depolarization of motoneurons prior to
stretch, manifest in a greater threshold angle.3 At a
70-degree starting angle, the motoneuron is closer to
depolarization and therefore threshold angle occurs
earlier. Powers et alRplaced their subject's biceps brachii
muscle in a shortened position, at 120 degrees of elbow
flexion, creating an even greater amount of slack than at
90 degrees. Threshold occurred broadly, at between 117
and 63 degrees of flexion.# This observation suggests
that joint angles may have an important influence on
threshold angle. If analysis was possible between the
starting angles used in both studies, a 120-degree starting angle would probably allow greater limb excursion
prior to reaching threshold angle than both the 70- and
Physical Therapy . Volume 76 . Number 6 .June 1996
Fi ure 4.
%
Su ject sealed at the torque motor with elbow positioned at 9 0 degrees
of flexion in a prefabricated splint. Note electrode placements on biceps
brachii, brachioradialis, and triceps brachii muscles.
90-degree starting angles. Unfortunately, whether Powers et alHconsidered the effect that startingjoint position
may have on the threshold angle is not clear.
Our observations support the possibility that, among
the three subjects who showed greater joint excursion
to threshold angle for either the biceps brachii or
brachiora.dialis muscle when the starting angle began at
90 degrees, the threshold angle was determined by
muscle spindle sensitivity and not by muscle slack. The
fact that our calculations within individual subjects yielded
actual threshold angles that were similar in most instances,
irrespective of starting angle or speed (0.5 or 1.0 radian/s) ,
suggests that each subject's stretch sensitivity was consistent
under these circumstances. Close examination of the relationship of actual threshold angle to condition within
subjects (X axis, Figs. 3A and 3B) sustains the notion that
the joint angles at threshold were remarkably similar at
conditions specified by speed within subjects 1,3, and 5 for
the biceps brachii muscle and subjects 1 and 5 for the
brachioradialis muscle. The fact that this close relationship
between the occurrence of threshold angle and condition
does not exist within all subjects highlights the reality of
intersubject variability in spindle sensitivity across the spectrum of patients with stroke.
limitations
The small sample limits the generalization of the findings. This study, however, was not intended to explore a
sizable group effect but rather to determine whether
starting angle can affect the threshold angle response in
subjects with biceps brachii and brachioradialis muscle
hypertonia. Our results are limited by using only two
starting angles and two speeds to determine the effect of
starting angle and speed on threshold angle and may be
applicable only to the two speeds used. A limited number of repeated measurements of threshold angle were
Figure 5.
(A) Biceps brachii muscle electromyographic (EMG] response, with a sharp rise in activity at occurrence of threshold angle, which i s shown by the
arrow (72"); (B] biceps brachii muscle EMG response from another subiect, with a gradual rise in activity prior to threshold angle (40.5"). Note
slightly different scaling in Y axes.
Physical Therapy . Volume 7 6 . Number 6 . June 1996
Wolf et al . 597
Table 3.
Mean and Standard Deviation Values of Threshold Angles for All Subiects at Every Condition
Condition (Starting AngleISpeed)
70'10.5
radianls
-
Subject No.
70'1 1.o
radianls
90'10.5
radian/s
-
SD
SD
X
Session
X
1
1
2
3
67.3
67.3
57.0
1.5
1.5
1.5
67.0
63.8
64.0
2.2
1.2
1.3
69.6
58.7
67.7
2
1
2
3
57.8
53.1
65.4
3.5
2.4
6.4
60.1
70.8
68.1
2.3
2.6
4.8
3
1
2
3
64.8
67.8
68.8
4.5
1.3
0.8
80.4
80.4
86.9
4
1
2
3
38.4
60.6
61.3
3.7
4.6
4.8
5
1
2
3
61.4
62.0
54.3
1
1
2
3
2
90'1 1.o
radian/s
-
X
SD
0.3
1.8
1.7
71 .O
67.9
75.6
1.3
3.7
10.6
67.3
67.2
55.3
0.6
2.1
3.2
75.3
72.4
79.1
4.7
1 .O
3.7
0.5
0.5
1.2
67.2
64.7
66.1
1.6
5.2
4.7
82.1
83.4
81.9
6.2
4.7
5.8
0
73.3
73.7
0
4.1
5.5
66.1
61.5
51.2
3.5
8.2
15.6
22.8
57.6
75.9
19.8
2.5
13.2
5.1
4.8
12.9
0
75.9
58.0
0
8.7
8.7
65.7
62.7
63.4
1.2
2.6
3.4
73.8
70.0
68.5
3.8
5.9
8.0
65.8
62.8
60.0
2.0
0
5.1
66.3
63.6
62.0
2.5
0.9
4.2
67.8
58.7
68.8
0.5
1.8
0.6
70.0
69.2
68.9
0.9
2.0
3.2
1
2
3
57.6
54.5
58.8
4.6
1.9
7.8
56.7
71.7
68.5
2.8
3.3
1 .O
67.3
65.5
57.5
0.6
3.3
6.9
74.7
74.4
71.2
6.3
2.4
3.3
3
1
2
3
65.5
67.5
64.4
3.4
1.2
2.6
68.5
81.2
82.3
3.7
1.7
3.5
63.5
68.1
67.5
8.8
0.1
2.0
85.0
80.5
78.3
3.7
1.6
6.0
4
1
2
3
25.3
56.9
33.9
1.8
3.3
4.4
0
56.6
0
0
0.6
0
60.9
43.2
49.5
7.4
5.9
16.0
0
0
0
5
1
2
3
55.8
48.9
21.6
4.2
8.6
37.5
62.8
71.9
58.0
2.7
11.3
8.1
59.5
48.3
40.2
0.3
1.1
2.8
69.7
64.7
53.3
X
SD
Biceps brachii
muscle
Brachioradialis
muscle
recorded. A greater number of measurements may be
necessary to yield a more accurate measurement of
threshold angle. The speed generated by the torque
motor at different starting angles is an inherent feature
of the motor and the mass being moved. As long as
displacement is linear and reproducible and the approximate speed of the stretch is known, as was the case in
this study, the motor itself should not be a deterrent to
the generalization of interpretation of data.
Future Investigations
Suggestions for future research include examining the
consistent use of a 90-degree starting angle and a
1.0-radian/s stretch to yield a reproducible threshold
angle in elbow joint muscles within a subject over
multiple testing sessions. The effect of consistency in
598 . Wolf et al
0
0
0
0.8
7.8
16.9
threshold angle for this condition should be assessed in
a larger sample of subjects who have sustained a stroke
and preferably those with similar, verifiable lesion sites.
Research 011 the effect of starting position on threshold
angle also should include evaluating a greater range of
starting angles. These types of studies may well produce
a more demonstrable relationship between starting
angle and threshold angle and also will increase the
ability to generalize our results to a larger population. In
addition, use of torque motors that provide nearly
instantaneous and accurately specified speeds would
reduce the need to approximate the true speed at the
time threshold angle is achieved.
Clinicians who treat patients with hypertonia usually
measure the amount of resistance to phasic stretch to
Physical Therapy . Volume 76 . Number 6 . June 1996
Table 4.
Significant0 Results of One-Way Repeated-Measures Analysis of Variance (d\ = 1 ) of Session Effect on Threshold Angle
Condition (Stafiing Angle/Speed)
Subject No.
Biceps brachii muscle
1
2
3
4
5
70'/0.5 radian/s
F=46.45
F=5.89
90'10.5 mdian/s
F=26.29
F=7.85
F=64.34
F=953.28
F=71.91
F=28.86
F=18.81
F=28.66
Brachioradialis muscle
1
3
--
90'/ 1.0 radian/s
F=48.83
F=28.64
F= 1 1.47
F=68.47
2
4
5
70"/1.0 radian/s
Fz96.44
Table 5.
Tukey Pair-\~iseAnalysis of Significanto Session Effects
Condition (Stadng Angle/Speed)
Subject No.
Biceps brachii muscle
1
2
3
4
5
70"/0.5 mdian/s
1 or 2>3
3>2
90'/0.5 mdian/s
2or3>1
2> 1
3>1 and 2
2>1 and 3
2>1 or 3
2 or 3>1
2 or 3>1
2>1 or 3
70"/1.0 mdian/s
90"/1.0 radian/s
1 or 3>2
1 or 213
2 or 3>1
Brachiorcrdiolis muscle
1
2
3
4
5
1 or 3>2
1 >2>3
Table 6.
Bonferroni Significanta Post Hoc Multiple Comparison of Starting Angle Effects Within Subiects
Subject No.
0.5 radian/s
1.0 mdian/s
Biceps brachii muscle
1
2
3
4
5
Brachioradialis muscle
1
2
3
4
5
-
Physical Therapy . Volume 76 . Number 6 . June 1996
Wolf et al . 599
determine the severity of hypertonia. There is also a
need to examine a change in the severity of hypertonia
resulting from an intenfention. This study illustrates the
importance of controlling the starting position of the
joint and the speed of the stretch when evaluating
hypertonia from session to session to determine whether
the hypertonia has really changed o r whether the conditions have caused a change in the presentation of the
hypertonia. The importance of starting position of the
joint controlled by a hypertonic muscle prior to stretch
has been demonstrated. A difference in starting position
from one assessment to the next may cause inaccurate
and unreliable results. The need for further research on
the development of an objective and reliable measurement of hypertonia, for the use of both researchers and
clinicians, also has been emphasized.
Conclusions
If the analyses of data from these five subjects who
sustained strokes can be generalized, a consistent starting angle prior to stretch should be used to determine a
consistent threshold angle. The condition of a 90-degree
starting angle and speeds approaching 1.0 radian/s
within subjects and across sessions appears to yield a
more reproducible threshold angle than the other three
conditions. There is also a need to examine a greater
range of starting angles a n d speeds and the related
changes in threshold angle over time to specified
interventions.
Acknowledgments
We express our appreciation to the following individuals
who gave so much of their time and effort in helping to
see this project to completion: Mr James Hudson, without whose resources, equipment modifications, and biomedical engineering background this project could not
have been done; Mrs Jane Hudson, for her guidance and
suggestions during the data collection procedure; and
Mr John Kontos, for his assistance with data collection
and counsel during data analysis.
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Abstract.
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Physical Therapy . Volume 7 6 . Number 6 . June 1996