Full text of DOI:10.1007/s004210100509

Eur J Appl Physiol 72001) 85: 527±532
DOI 10.1007/s004210100509
ORIGINAL ARTICLE
Katsuo Fujiwara á Kenji Kunita á Hiroshi Toyama
Akiyoshi Miyaguchi
Saccadic reaction times during isometric voluntary contraction of the
shoulder girdle elevators and vibration stimulation to the trapezius
Accepted: 26 June 2001 / Published online: 22 August 2001
Ó Springer-Verlag 2001
Abstract We investigated the e€ect of increases in the lack of an additive e€ect and factors limiting
muscle a€erent information from the shoulder girdle shorteningof reaction times.
elevators on saccadic reaction time. Saccadic reaction
Keywords Saccadic eye movement á Reaction time á
Vibration á A€erent information á Shoulder girdle
elevators
time was measured under conditions of isometric vol-
untary contraction of the shoulder girdle elevators and
vibratory stimulation of the trapezius. Saccadic reaction
time was de®ned as the latency until the beginning of eye
movement toward the lateral target, which was moved at
random time-intervals in jumps of 20°amplitude. Eye
movement was measured usingthe electro-oculorgam
technique. Muscle contraction force was set in 10%
increments from 0% to 30% of maximal voluntary
contraction 7MVC), and vibration frequency was
maintained at 100 Hz. Under voluntary contraction, the
saccadic reaction time gradually shortened up to 30%
MVC. Under vibration stimulation at 0% MVC, the
reaction time shortened to the same degree as that under
voluntary contraction at 30% MVC. Under conditions
of combined vibratory stimulation and voluntary con-
traction, the reaction time was essentially identical to
these values; namely, no additive e€ect in shorteningof
the reaction time was recognized. The results demon-
strated that saccadic reaction times were remarkably
shortened by increases in muscle a€erent information
from the neck extensors. We have discussed reasons for
Introduction
A basic dynamic posture 7Howorth 1946) in which the
foot, knee, hip and trunk are slightly ¯exed is consis-
tently shown in individuals at the sudden initiation of
movement and in those pursuinga movingvisual target.
Saccadic eye movement is used to catch a visual target
quickly and carefully in many di€erent sports and work-
related activities. We found that maintainingthis basic
dynamic posture led to a shorteningof the reaction time
of saccadic eye movement 7saccadic reaction time;
Fujiwara 1994). Thereafter, a similar shorteningwas
observed while maintainingthe neck ¯exion position, in
which the neck extensors contracted 7Fujiwara et al.
2000). The saccadic reaction time also shortened in
association with isometric contraction of these muscles
duringshoulder girdle elevation 7Kunita and Fujiwara
1996; Fujiwara et al. 2000).
A review of the literature 7Magoun 1963; Klemm
1990; Nara and Kasai 1991; Vallar et al. 1995) suggests
that one common factor may explain these phenomena,
namely the activation of the part of the brain associated
with increases in muscle a€erent information from neck
extensors. Many previous studies have reported that
under isometric voluntary muscle contraction, the
amount of muscle sensory information increases in pro-
portion to muscle activity 7Vallbo 1970, 1974; Edin and
Vallbo 1990; Wilson et al. 1997). Vibratory stimulation
of muscles also allows for the recognition of increases in
muscle a€erent information 7Hagbarth and Vallbo 1968;
Burke et al. 1976a, b; Roll et al. 1989). Muscle sensory
information from the neck muscles projects to the neural
pathway controllingeye movement, directly and/or via
K. Fujiwara 7&) á H. Toyama
Department of Human Movement and Health,
Graduate School of Medical Science,
Kanazawa University, 13-1 Takara-machi,
Kanazawa, 920-8640 Japan
E-mail: fujikatu@med.m.kanazawa-u.ac.jp
Tel.: +81-76-265-2225
Fax: +81-76-234-4219
K. Kunita
Institute of Health Science and Physical Education,
Osaka City University, 3-3-138 Sugimoto,
Sumiyoshi-ku, Osaka, 558-8585 Japan
A. Miyaguchi
Kanazawa College of Economics,
10 Ushi Gosho-machi, Kanazawa, 920-8620 Japan

528
belts, one around the chest and the other around the hips.
The subjects kept their knees ¯exed at an approximate 90° angle
and rested their feet on a low table. Neck ¯exion angle was de®ned
as the rotational angle of tragus around the shoulder 7acromion)
in the sagittal plane. The ¯exion angle was the same as that during
a quiet sittingposture, usingan anuglar detector in which the
centre point was set at the acromion and the distance between
the acromion and the tragus was regulated. The rotational angle of
the head was also the same as that duringthe sittingposture, the
purpose beingto maintain constancy in the sensory stimulus
from the vestibular organ. An angular detector using the principle
of a pendulum was placed on the temple and was used to determine
this angle. A stand was used to support the subject's head in order
to relax the neck extensor muscles as much as possible. In addition,
a vertical board was placed against the occipital region of the head
in order to prevent extensional rotation of the head and neck.
To monitor the electromyogram 7EMG) of the trapezius muscle,
the signal from bipolar surface electrodes placed over the upper
trapezius muscle was ampli®ed 7´2,000) and band-pass ®ltered
71.6 Hz±1.5 kHz) usingan EMG ampli®er 7NEC-Sanei, BIOTOP-
6R12). The electrode input-impedance was reduced to less than
2 kW. The isometric contraction strength of the shoulder girdle
elevator muscles was measured using a strain-gauge transducer
7KYOWA, LU-100KSB34D) placed on a steel frame, which
was then connected to leather pads on the shoulders by a leather
belt. The two leather pads were connected by a leather belt that
prevented them from sliding laterally. The strain-gauge transducer
was located under the axillary cavity at the height of the anterior
superior iliac spine. Calibration of the electrical signal from
the brainstem reticular formation 7Dubrovsky and Bar-
bas 1977; Hinoki 1985; Role et al. 1991). Biguer et al.
71988) also reported that vibration of the posterior
muscles of the neck produced the illusion of displacement
and the motion of a visual target when the target was
presented with no visual background. This ®nding sug-
gests the in¯uence of sensory information from neck
muscles on gaze. Taylor and McCloskey 71991) found
that alteration of proprioceptive stimulus from the neck
by vibration led to the illusion of displacement of the
visual target as well as the changed perception of head
position, and that the illusion of displacement of the vi-
sual target was greater than that of head rotation. These
®ndings con®rm that sensory information from the neck
muscles strongly a€ects visual perception and motion.
We hypothesized that the neural pathway controlling
eye movement would be activated accordingto increase
in muscle a€erent information duringapplication of
vibratory stimulation of the neck extensors, and subse-
quently that the saccadic reaction time would shorten.
The present study compares saccadic reaction times
under conditions of isometric voluntary contraction of
the shoulder girdle elevators and vibratory stimulation
to the trapezius.
the transducer was set at 0.196 NámV±1
.
To vibrate the trapezius, the vibrator 7HEIWA DENSHI TMT-
18), in which a small projection contacted the skin surface
72´4 cm2), was applied to the skin over the upper trapezius muscle.
The site of application was approximately the centre region of the
upper trapezius muscle. The vibration frequency was set at 100 Hz.
Pressure on the trapezius was detected as the signal from the strain-
gauge transducer 7KYOWA, LU-5KSB34D) connected to the
vibrator. Prior to the trial, the pressure on the trapezius was
measured when the trapezius muscle was manually extended for as
longas possible under the condition of no vibration. Under the
vibration stimulation, an approximate 70% of maximal pressure
was applied to the trapezius muscle so as not to extend it maxi-
mally. To monitor EMG of the trapezius, contraction strength and
pressure on the trapezius, each signal was directed to a digital
oscilloscope 7Iwatsu, DS-6612).
A visual stimulator 7NIHON KOHDEN, SLE-5100) was used
to induce saccadic eye movement. Visual stimuli from the left and
right luminous emission diodes 7LED) were alternately lit for
random durations of 2±4 s usinga personal computer 7NEC,
PC9801CV21) with a D-A converter 7I/0-DATA, PIO9035). Two
LED were placed at the height of the nose root, and the distance
between the central point of the two LED and the nose root was set
at 50 cm. The visual angle was set at 20° and thus the LED were
located 10° left and right of the central point. Horizontal eye
movement was measured usingthe electro-oculorgam 7EOG)
technique. Surface electrodes were placed at the outer canthus of
each eye with a reference electrode at the centre of the forehead.
Electrode-input impedance was reduced to less than 10 kW. The
signal from the electrodes was ampli®ed 7´2,000) usinga DC am-
pli®er 7NIHON KOHDEN, AN-601G). To obtain a steady EOG,
the time interval from the point of securingthe electrodes to the
data recordingwas set at over 20 min.
Methods
Subjects
The subjects were nine men and three women, ranging in age from
20 to 47 years [mean 7SD) 26.2 79.0)]. All subjects appeared to be
free of any neurological or orthopaedic impairment and all gave
informed consent to the experiment protocol.
Apparatus and data recording
The outline of the experiment setup is shown in Fig. 1 of the
present study and that of the previous study 7Fujiwara et al. 2000).
The subjects sat on a steel-frame chair, with their backs resting
against a vertical wall and their trunks secured with two acrylic
The signals from the strain-gauge transducer, the electrodes and
D-A converter were stored in a digital tape recorder 7TEAC, RD-
130TE) to be analysed subsequently.
Procedure
Fig. 1 Experiment setup: A acrylic belt, B bar set at an angle of Prior to the start of data recording, in order to relax the trapezius
neck ¯exion, C angular detector placed on the temple, D stand, E muscle, contraction and relaxation of the shoulder girdle elevator
vertical board, F leather pad, G leather belt, H vibrator, I strain- muscles were alternately repeated several times and a deep breath
gauge transducer
was taken. The experimenter verbally instructed the subjects to

529
relax the trapezius muscle, and relaxation of that muscle was Statistical analysis
monitored by EMG. Furthermore, a trial series followingthe al-
ternately lit LED was carried out for 20 s. Next, right and left force Two-way analysis of variance was used to study the e€ect of muscle
of maximal voluntary contraction 7MVC) of the shoulder girdle force and any individual di€erences in saccadic reaction time
elevator muscles were separately measured under conditions amongsubjects. A multiple comparison analysis usingFisher's
whereby the neck ¯exion angle was maintained at 0° with the chin protected least signi®cant di€erence was performed to examine any
not restingon the stand. The measurement was carried out twice
di€erences suggested by the analysis of variance. A paired Stu-
with a 3 min rest between measurements. Thereafter, the saccadic dent's t test was used to compare the voluntary contraction con-
reaction time was measured for 20 s per trial under conditions of dition and the vibration condition. Signi®cance was accepted at the
combined bilateral voluntary contraction with vibration of the P<0.05 level. All statistical analyses were performed usingExcel
upper trapezius muscles, with the chin restingon the stand. We
de®ned 0% MVC with no vibratory stimulation as the no-load
condition. Target forces were set at 0%, 10%, 20% and 30% MVC.
The measurement sequence amongtarget forces was randomly set
for each subject. Prior to the trial under the vibration stimulation,
we checked whether extension of the head 7tonic vibration re¯ex)
was caused by vibratory stimulation. Three trials were undertaken
with a 3 min rest between each trial.
version 98 7Microsoft Corp.) with Statcel 7OMS Co. Ltd.).
Results
The mean value for maximal force of the shoulder girdle
elevators was 608.5 7151.7) N for the right side, and
684.9 7195.0) N for the left side. The di€erence between
these two values was not signi®cant. The mean and
standard deviation values of exerted relative muscle
force under each target force are shown in Fig. 3. The
relative muscle force exerted was essentially the same as
the set point in all conditions [voluntary condition:
10% MVC 11.7 72.0)%, 20% MVC 21.1 72.5)%, 30%
MVC 31.3 74.3)%; vibration condition: 10% MVC
12.1 71.5)%, 20% MVC 21.2 73.7)%, 30% MVC
29.4 75.2)%]. No signi®cant di€erences in the muscle
force exerted were observed between the voluntary
contraction and vibration condition at any target force.
The mean and standard deviation of pressure on the
trapezius under each target force of vibration are shown
in Fig. 4. A signi®cant e€ect of relative muscle force on
pressure was observed 7F3,47=16.18, P<0.01), and the
pressure gradually increased up to 30% MVC [0% MVC
23.7 76.1) N, 10% MVC 27.3 74.2) N, 20% MVC
28.2 75.1) N, 30% MVC 29.7 74.5) N]. The pressures at
target forces of 10%±30% MVC were signi®cantly
larger than that at 0% MVC 7P<0.01). A signi®cant
di€erence in the pressure between 10% MVC and 30%
MVC was also observed 7P<0.02). The means and
standard deviations of saccadic reaction times under
each condition are shown in Fig. 5. The saccadic reac-
tion time under the no-load condition was
184.4 720.6) ms. Under the voluntary contraction, a
signi®cant e€ect of relative muscle force on saccadic
reaction time was observed 7F3,47=11.10, P<0.01), and
the reaction times gradually decreased up to 30% MVC.
The saccadic reaction times at a target force of
10%±30% MVC were signi®cantly shorter than the re-
action time under the no-load condition 7P<0.01). The
time di€erence between 30% MVC and the no-load
condition was 14.7 711.7) ms. Under the vibration con-
dition, the saccadic reaction time at 0% MVC was sig-
ni®cantly shorter than that under the no-load condition.
The shorteningof reaction time was 17.0 710.0) ms, and
this value was roughly the same as the value at 30%
MVC under the voluntary condition. The saccadic
reaction times under the combined condition of volun-
tary contraction and vibratory stimulation were essen-
tially the same as that of the vibratory stimulation at 0%
MVC, and no signi®cant di€erence was observed at any
Data analysis
The electrical signal from the strain-gauge transducer in the mea-
surement of maximal muscle force was recorded usinga digital
oscilloscope with a resolution of 0.784 N. The larger of two mea-
surement values was used for analysis.
The data from the transducer, EOG and the visual stimulus in
the measurement of saccadic reaction time were sent to a computer
7NEC, PC-9821V233) via an A-D converter 7Canopus, ADJ-98) at
1,000 Hz with a 12 bit resolution. The muscle force exerted by the
shoulder girdle elevator was assessed by the relative ratio of the
maximal muscle force. Pressure on the trapezius in the vibration
condition was assessed by the mean value in each trial. In all
conditions, measurement of saccadic reaction time could be iden-
ti®ed by visual inspection. The saccadic reaction time was de®ned
as the latency to the beginning of eye movement following the onset
of target movement 7Fig. 2). The processing of the muscle force of
the shoulder girdle elevators, pressure on the trapezius and reaction
times was performed usingBIMUTAS-E version E2.20 7Kissei
Comtec Co., Ltd.).
Fig. 2 Analysis of saccadic reaction time 7SRT) in two sample
records 7No load and 30% maximal voluntary contraction, MVC,
with vibration)

530
Fig. 3 Relative muscle force as a function of relative target force
exerted by the shoulder girdle elevators under conditions of
voluntary contraction and vibration. Values are mean and SD for
12 subjects
Fig. 5 Saccadic reaction time as a function of relative target force
exerted by the shoulder girdle elevators under conditions of
voluntary contraction and vibration. Values are mean and SD for
12 subjects. *Signi®cant di€erence of the mean value between
conditions of voluntary contraction and vibration 7P<0.05)
the present study [14.7 711.7)] and our previous study. In
the present study, shorteningof the reaction time under
vibratory stimulation alone was approximately the same
as that under the voluntary contraction at 30% MVC.
As mentioned previously, under isometric voluntary
muscle contraction, it has been reported that the amount
of muscle a€erent information increases in proportion to
muscle activity 7Vallbo 1970, 1974; Edin and Vallbo
1990; Wilson et al. 1997). In addition, in response to
vibratory stimulation of the muscle, the increase in in-
formation in muscle a€erent nerves has been recognized
7Hagbarth and Vallbo 1968; Burke et al. 1976a, b; Roll
et al. 1989). It was believed that the increase in muscle
Fig. 4 Pressure on the trapezius as a function of relative target a€erent information with vibratory stimulation of the
force exerted by the shoulder girdle elevators under vibration
condition. Values are mean and SD for 12 subjects. *Signi®cant
activated the neural system associated with visual per-
di€erence from the value at 0% maximal voluntary contraction
neck extensors and voluntary contraction of the muscles
ception and eye movement, which in turn led to a
7P<0.05)
shorteningof the saccadic reaction time.
Saccadic eye movement in this study was referred to
force. Signi®cant di€erences in reaction time between the
voluntary contraction condition and vibration condition
were observed at target forces of 10% and 20% MVC
710% MVC t11=2.18, P<0.05; 20% MVC t11=1.88,
P<0.05). However, the di€erence between the two
conditions at 30% MVC was not signi®cant.
as the visual-guided saccade, in which the neural path-
way is composed of the lateral genicular body, occipital
cortex, posterior parietal cortex, parietal eye ®eld,
frontal eye ®eld, superior colliculus and reticular for-
mations. We consider that there are two possible mech-
anisms by which a€erent information from neck
extensors in¯uences the activation of this neural path-
way. First, a€erent information from the neck extensors
indirectly a€ects the neural pathway, via the brainstem
reticular formation. Changing the position of the upper
Discussion
Under voluntary contraction of the shoulder girdle ele- limbs, one of the commonly used proprioceptive neu-
vators, the saccadic reaction time decreased up to 30% romuscular facilitation techniques, has been shown to
MVC in a similar pattern as observed in the previous shorten limb reaction time 7Taniguchi et al. 1980; Nara
study 7Kunita and Fujiwara 1996). The di€erence in and Kasai 1991). This phenomenon is believed to be
reaction time between 30% MVC and the no-load con- attributable to an enhancement of the level of activation,
dition in the previous study was 16.2 76.1) ms, and no in tandem with an increase in a€erent information, via
signi®cant di€erence in reaction time was found between an activation system primarily present in the brainstem.

531
Additionally, the unilateral visual neglect in patients a€erent information may have acted as a limitation.
with right hemisphere lesions is improved by a€erent Thirdly, the increase in the transmission speed of
information from the neck extensors 7Karnath 1995; information in the neural pathway controllingeye
Vallar et al. 1995). Vallar et al. 71995) reasoned that the movement had reached the upper limit. The amount of
factor contributingto the improvement was a non-spe- muscle a€erent information under the combined condi-
ci®c activation in the right hemisphere associated with tion of vibratory stimulation and voluntary contraction
a€erent information from neck extensors. Second, has been reported to be essentially the same as that
a€erent information from neck extensors directly a€ects under vibratory stimulation alone 7Hagbarth and Vallbo
the neural pathway of the visually-guided saccade. 1968). This appears to support the ®rst possibility.
Hinoki et al. 71975) reported that a€erent information Accordingto the report of Roll et al. 71991), vibration of
from the neck extensors directly projected to the nervous the inferior rectus and the sterno-cleido-mastoidus
system to elicit optokinetic nystagmus. Kasai and Zee a€ects the neural pathway controllingvisual perception
71978) focused their attention on the neural pathway that and gives rise to visual illusions in the same direction.
involved a€erent information from the neck extensors However, no signi®cant di€erence in the magnitude of
projected to the vestibular nuclei relatingto the cervico- the illusion was found between stimulation of both
ocular re¯ex generated during gaze, and illustrated muscles and each muscle individually. The illusion
mechanisms to stabilize the gaze. Further, vibration intensity might increase with respect to the total amount
stimulation to neck muscles has been shown to induce of muscle a€erent information. The equivalent intensity
visual illusion 7Biguer et al. 1988; Taylor and McCloskey of illusion under the di€erent stimulus conditions, as in
1991). In the visual illusion, Roll et al. 71991) attached the above, may support the second possibility. The third
great importance to the report of Dubrovsky and Barbas possibility is supported by our previous study in which
71977), which stated that the a€erent information from we investigated diurnal ¯uctuations in the saccadic
the neck muscles projected to the frontal eye ®eld.
reaction times duringthe positions of neck rest and neck
Grantyn and Berthoz 71987) and Berthoz 71996), ¯exion 7Kunita and Fujiwara 1998). The results of the
however, both proposed that e€erent signals from the above study showed that the reaction time with neck
higher nervous system synergistically regulate the activity ¯exion was shorter at all times than that in the neck rest
of neck muscles and eye movement. Therefore, at this position. However, the time di€erence between the two
time, we cannot rule out a facilitative e€ect of e€erent conditions was not signi®cant in the daytime. In all other
signals that accompanies neck extensor activity as a fac- measurements of the saccadic reaction time examined by
tor that induces shorteningof the saccadic reaction time. us to date, a shorteningof the reaction time greater than
Under the vibration condition, special care was taken that observed in the present study has not been seen. We
to control the pressure of vibration of the trapezius in propose that all three factors are possibly involved, and
order not to extend maximally its length. Eklund and we are at present unable to specify which factor causes
Hagbarth 71966) reported that the strength of tonic vi- this result.
bration re¯ex was hardly dependent upon muscle length
The present results determined that saccadic reaction
of less than maximum when the subject isometrically times were remarkably shortened by an increase in
contracted the muscle. Under the vibration condition in muscle a€erent information from neck extensors. We
the present study, the saccadic reaction time in response have discussed the reason for the lack of an additive
to vibratory stimulation with voluntary contraction was e€ect and the factors limitingthe shorteningof the
probably almost completely una€ected by the length of reaction times. Further research will be required to
the trapezius. The relative muscle force was essentially manipulate the amount of muscle a€erent information
the same as the set point under all conditions. Pressure by changing the frequency of vibratory stimulation. In
on the trapezius under vibratory stimulation with vol- addition, we intend to investigate the activation region
untary contraction were from 27.3 N to 29.7 N, and this of the central nervous system and its task speci®cation.
value became approximately 4% MVC. The component
force in the perpendicular direction must have been less
than 4% MVC. Hence, the net muscle force of shoulder
girdle elevators under the vibration condition was
somewhat greater than the set point. This supports the
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