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Effects of Isometric Upper Limb Contraction on Trunk and Leg Muscles During Sit-to-stand Activity in Healthy Elderly Females

J Korean Soc Phys Med, 2017; 12(1): 61-66
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Research Article Open Access
Effects of Isometric Upper Limb Contraction on Trunk and Leg Muscles During
Sit-to-stand Activity in Healthy Elderly Females
Eun-Mi Jang, PhD, PT
Jae-Seop Oh, PhD, PT
Mi-Hyun Kim, PhD, PT
Dept. of Rehabilitation Science, Graduate School, Inje University
Dept. of Physical Therapy, Inje University
Received: October 21, 2016 / Revised: October 31, 2016 / Accepted: November 26, 2016
2017 J Korean Soc Phys Med
| Abstract |1)
The purpose of this study was to investigate
the effects of isometric upper limb contraction on the trunk
and lower extremity muscles during the sit-to-stand activity
in elderly females.
Eighteen healthy elderly females performed
three directional isometric upper extremity contractions
(flexion, extension, and horizontal abduction movements)
using an elastic band during sit-to-stand activity. Electromy-
ography signals were collected from the internal oblique,
erector spinae, rectus femoris, and biceps femoris muscles.
Internal oblique activity was greater in
bilateral shoulder flexion and bilateral shoulder horizontal
abduction than in neutral position (p<.05). Erector spinae and
rectus femoris muscle activities in bilateral shoulder flexion
was greater than in neutral position and bilateral shoulder
extension (p<.05). Biceps femoris activity was significantly
greater in bilateral shoulder flexion than in bilateral shoulder
extension and horizontal abduction, and in neutral position
compared to bilateral shoulder extension (p<.05).
†Corresponding Author :
This is an Open Access article distributed under the terms of
the Creative Commons Attribution Non-Commercial License
( which permits
unrestricted non-commercial use, distribution, and reproduction
in any medium, provided the original work is properly cited.
These results suggest that incorporating
isometric upper limb contraction may be beneficial for
enhancing the contribution of trunk and lower extremity muscle
activities to trunk stabilization during sit-to-stand activity.
Therefore, isometric upper limb contraction during sit-to-stand
tasks, especially in flexion, may be used to elicit contraction of
the lumbopelvic region muscles within a tolerable range, for
developing endurance and strength in the elderly.
Key Words:
Isometric contraction, Leg muscle, Sit-to-stand,
Trunk muscle, Upper extremity
. Introduction
The ability to rise from a seated position, otherwise
known as sit-to-stand activity (STS), is essential for postural
maintenance and controlled and coordinated movement
(Park, 2002). Safe execution of STS is a prerequisite for
activities such as walking and stair climbing (Papa and
Cappozzo, 2000).
Age-related declines in lower limb strength and balance
and sensory systems have independently been shown to
impair safe and efficient execution of STS in older adults
(Lord et al., 2002; Schenkman et al., 1996). In elderly
women in particular, decreased muscle strength of the lower
extremities occurs more quickly than in elderly men,
62 | J Korean Soc Phys Med Vol. 12, No. 1
reducing balance and gait ability (Park and Yoon, 2009).
Standing up require greater muscle strength, range of
motion and a peak joint moment than walking and stair
climbing. It also requires the balance ability to translate
the body mass forward and upward from a relatively stable
sitting position to the feet. Because of these mechanical
demands, elderly people may experience difficulty when
standing from a chair (Carr and Shepherd, 2011; Yoshioka
et al., 2009). It was found that older adults exhibited a
longer execution time than younger adults for executing
STS possibly due to the poorer leg muscle strength in
elderly adults (Yamada and Demura, 2009).
Thus the balance training and muscle strengthening
exercise may be required to successful STS performance
in elderly people (Kim et al., 2010). Previous studies have
been suggested that trunk and hip muscles may be affected
by loading induced from upper- and lower-limb movement
(Arokoski et al., 2001; Park and Yoon, 2009; Tarnanen
et al., 2008). Arokoski et al. (2001) found that the activation
patterns of the abdominal muscles were affected by the
direction of restricted upper-limb movements. Other
researchers have shown that the direction of resistance on
the upper limb could affect the activation patterns of the
abdominal muscles (Lee et al., 2012). However, these
exercises were performed in a static posture only, such
as a standing or sitting position. There are no studies on
effects of the resistance direction of upper extremity on
trunk and leg muscles during a dynamic movement such
as STS. Therefore, the purpose of this study was to quantify
and compare how isometric contractions of upper extremity
affect trunk and lower extremity electromyography activity
during STS in healthy old adults.
. Methods
1. Subjects
We recruited 18 healthy elderly women volunteers from
a community dwelling in Gimhae city. The mean age of
the subjects was 68 ± 1.97 years (mean ± SD), mean height
was 154.72 ± 4.21 cm, and mean body weigh was 54.61
± 6.84 kg. The Inje University Faculty of Health Science
Human Ethics Committee granted approval for this study,
and all subjects provided written informed consent prior
to participation.
2. Instrumentation
Surface EMG signals were recorded using four
pre-amplified (gain: 1,000) active surface electrodes (Model
DE-2.3, Delsys Inc., Wellesley, MA, USA) with surface
electrodes fixed at an inter-electrode distance of 10 mm.
The sampling rate was 1,000 Hz; the band-pass filter was
450 Hz. Raw data for the four muscles were processed
into root-means square (RMS) data.
The electrodes were positioned on four sites on the right
side of the subjects’ bodies: internal oblique (IO), erector
spinae (ES), rectus femoris (RF), and biceps femoris (BF)
muscles. For normalization of the EMG signal, subjects
were asked to perform a maximum voluntary isometric
contraction (MVIC) maneuver for 5s. The EMG data
expressed the entire STS task as a percentage of maximum
voluntary isometric contraction (%MVIC). One
infrared-retro reflective marker was placed on the superior
aspects of the scapular acromion process to define the start
and end positions of the STS (Vicon 512, Oxford Metrics
Group, UK).
3. Procedures
Subjects were instructed to sit with an upright trunk
posture on a height-adjustable bed without back support
and with feet flat on the floor. The hip, knee, and ankle
joints were positioned at 90
flexion, and the feet were
positioned shoulder-width apart with the toes pointing
directly forward. The poles were placed directly in front
of (extension condition) or behind (flexion condition) the
trunk bilaterally at a distance of 1m, and the subject’s
shoulder height was measured to determine the fixed axis
for the elastic band. A blue elastic therapeutic band
Effects of Isometric Upper Limb Contraction on Trunk and Leg Muscles During Sit-to-stand Activity in Healthy Elderly Females
Muscles Condition p-values
IO 35.76±13.47a38.03±14.34 41.76±14.0143.15±15.13.002*
ES 50.64±15.54 44.28±14.93 47.50±13.86 56.99±14.64,.000*
RF 36.93±15.27 38.39±15.09 38.47±15.55 42.67±14.64,.037*
BF 36.86±18.52 31.62±17.9333.14±18.93 38.99±20.23,‡‡ .002*
Abbreviations: MVIC, maximum voluntary isometric contraction; N, neutral; Ext, extension; H-Abd, horizontal abduction;
Flex, flexion; IO, internal oblique; ES, erector spinae; RF, rectus femoris; BF, biceps femoris.
* p<. 05.
Significantly different compared to the N
Significantly different compared to the Ext
‡‡ Significantly different compared to the H-Abd
Table 1. The differences of muscle activities (%MVIC) during STS with isometric upper limb contraction (N=18)
(TheraBand, Hygenic Corporation, USA) was used to
regulate resistance in the upper limb. All subjects grasped
the elastic band 50cm from the fixed axis with both arms
so that same resistance was applied to the upper limb. A
goniometer was used to determine the shoulder movement
range (from neutral position to 40° flexion, neutral position
to 40° extension, and from 90° flexion to 90° horizontal
abduction), and then a target bar was placed in front (flexion
condition) or behind (extension condition) of subjects to
control shoulder movement (Lee at al., 2012).
Each subject was instructed to extend or flex both
shoulders with full extension of the elbow joint during
the entire STS motion and then to hold a standing position
for 3s. Horizontal abduction started at 90
flexion of the
shoulder. Subjects were asked to perform horizontal
abduction with both arms with full extension of the elbow
joint during the entire STS task and then to hold a standing
position for 3s. EMG data for the four muscles were
collected for each subject during the entire STS task.
Four types of STS experimental tasks were performed
in random order: 1) Neutral STS (N), 2) STS with isometric
upper limb extension (Ext), 3) STS with isometric upper
limb horizontal abduction (H-Abd), and 4) STS with
isometric upper limb flexion (Flex). To ensure sufficient
practice and adherence to correct patterns, these strategies
were practiced repeatedly until the physician qualified the
subject before entering the test. STS was performed at a
self-paced comfortable speed.
4. Statistical analysis
Significant differences among the four conditions (neutral,
extension, horizontal abduction, and flexion) were examined
using a repeated-measures analysis of variance (ANOVA).
If a significant difference was found, post hoc Fisher’s Least
Significant Difference (LSD) correction was performed to
determine differences in pair-wise comparison. Data were
analyzed using the SPSS software (ver. 18.0, Chicago, IL,
USA), and the significance level was set at p<.05.
. Results
The mean and standard deviations of %MVIC for each
condition are presented in Table 1. EMG activity of IO
was significantly greater for the Flex condition and the
H-Abd condition compared to the N condition, and for
Flex condition compared to the Ext condition (p<.05,
respectively). EMG activity of ES was significantly greater
for the Flex condition compared to N, Ext, H-Abd conditions
(p<.05), and N condition compared to Ext condition (p<.05).
64 | J Korean Soc Phys Med Vol. 12, No. 1
EMG activity of RF was significantly greater for the Flex
condition compared to N and Ext conditions (p<.05), and
EMG activity of BF was significantly greater for the Flex
condition compared to Ext and H-Abd condition, and for
N condition compared to Ext condition (p<.05, respectively).
. Discussion
The goal of this study was to investigate
electromyography activities of trunk and lower limb
muscles during STS with isometric upper limb contraction.
Overall, the result of this study was increased muscle
activities of the IO, ES, RF, and BF in Flex condition
compared to N, Ext, and H-Abd conditions. Our study was
the first to examine trunk and lower extremity muscle
activation on isometric upper limb contractions during an
STS activity.
Large movements are usually produced in both the hip
joint and the knee joint during STS, and thus any problems
with muscle weakness or postural instability will lead to
the use of alternative strategies (Corcos et al., 1996; Inkster
and Eng, 2004). Using momentum in any transfer technique
is a valuable option to any person who exhibits muscle
weakness or other problems in standing up (Nuzik et al.,
1986). This is important, as these interventions are targeted
toward older adults and those with pathologies.
According to previous findings (Arokoski et al., 2001;
Park and Yoon, 2009; Tarnanen et al., 2008) that suggested
that trunk and hip muscles may be affected by loading
induced from upper and lower limb movement, we used
the extra loading provided by applying isometric upper
limb contractions such as arm extension, horizontal
abduction, and flexion resistance during STS movement.
In this study, IO and RF muscles were activated more
during the STS with isometric upper limb contraction than
during STS alone. These results support that loading
resistance induced from limb movement requires an
additional stabilization effort to keep trunk and hip stability
at an optimal level (Park and Yoon, 2009).
In this study, we also investigated the effects of
resistance direction using an elastic band on trunk and hip
muscle activities during STS with isometric upper limb
contraction. The results indicate that the direction of
resistance on the upper limb appeared to affect the
activation patterns of the IO, ES, RF, and BF muscles.
When a limb is moved, the body reacts with forces that
are equal in magnitude, but in the opposite direction to
the limb movement. Tarnanen et al. (2008) reported that
bilateral shoulder extension produced the greatest activity
in the abdominal muscles. During bilateral shoulder
extension, a long lever arm and the upper-body muscles
are able to produce sufficient torque to load the trunk
muscles. Arokoski et al. (2001) found that simultaneous
resisted upper extremity extension while standing caused
high abdominal muscle activity, whereas resisted upper
extremity flexion while standing caused high back muscle
activity. As in previous studies, the current study revealed
that EMG activities of back extensor muscle such as ES
and BF muscle were significantly higher in Flex than in
Ext and H-Abd during the STS movement. However, STS
with the Flex condition led to increased activation of IO
and RF muscle as well as ES and BF muscles to control
against the force of gravity and provide more momentum
for balance and stability than the Ext condition.
These results could be affected by the experimental
posture. The previous studies were performed in a static
position, such as standing or sitting, whereas in this study
the STS movement required forward and upward movement
of the body mass from a base of support provided by the
chair and the feet to the feet only, while maintaining balance
(Shepherd and Gentile, 1994). A therapeutic band used
in this experiment was able to resist a counter force in
the opposite direction. During STS with Ext and H-Abd
conditions, the therapeutic band was fixed to the front side
of the body, and therefore the band tension was exerted
Effects of Isometric Upper Limb Contraction on Trunk and Leg Muscles During Sit-to-stand Activity in Healthy Elderly Females
in the same direction toward the body mass transfer that
might act to support the STS activity. However, during
STS with the Flex condition, the fixed band was at the
back of the body to provide resistance against moving the
body mass forward and to possibly induce co-activation
of the IO, RF, ES, and BF muscles It has been suggested
that co-contraction could be a control mechanism to
maintain postural stability, or to recover from perturbations
or the decrease in force and torque production capability
that result from aging (Benjuya et al., 2004; Cenciarini
et al., 2010). Previous researchers suggested that the elderly
tend to increase stability by stiffening the trunk or legs
to compensate for impaired neuromotor functions and
decreased muscle strength. In the elderly, this phenomenon
elevates antagonist muscle activity, which in turn increases
muscle coactivity (Izquierdo et al., 1999). Elevated muscle
co-activation increases joint stiffness; hence, joint stability
is also increased in the lower extremities (Baratta et al.,
1988). During the STS with isometric arm Flex condition,
most subjects appeared to respond with strong co-
contractions of trunk and lower extremity muscles.
Our results therefore suggest that including isometric
upper limb contraction may be beneficial for enhancing
the contribution of trunk and hip muscle activity to trunk
stabilization during STS movements. Our findings also
support previous studies that have indicated that isometric
upper limb contraction are beneficial for reinforcing the
activity of the trunk and hip muscles (Saliba et al., 2010).
Therefore, it can be supposed that isometric upper limb
contraction during STS activity, especially the Flex condition,
may be used to elicit a contraction of the lumbopelvic
region muscles within a tolerable range for developing
endurance and strength in the elderly for successful STS
activity. However, it should be taken into account that
muscle activation may be influenced by the resistance
intensity as well as the resistance direction and posture.
This study was limited in several respects. First, the
present study did not measure arm muscles that act directly
on the movement. Second, we did not control of resistance
amount by applying in the individually. Further studies
are needed to measure the strength of the arm muscles
by an objective method and to investigate whether these
results have implications for patient rehabilitation.
. Conclusion
This study investigated the effect of the resistance
direction of upper limb on trunk and lower extremity muscle
activity during STS. With isometric upper limb Flex
condition, most subjects appeared to respond during STS
with strong co-contractions of trunk and lower extremity
muscles. The STS activity with isometric upper limb Flex
condition may have better prepared the subjects for higher
skilled muscular responses to destabilizing forces when
performed independently from STS activity. These results
will be helpful in developing strategies for successful STS
task performance in training programs for older women
with muscle weakness. Further studies are needed to
measure the control of resistance amount by applying in
the individually. Measuring strength of the upper and lower
limb muscles by an objective method and investigating
whether these results have implications for the rehabilitation
of patients is also a topic for further research.
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Full-text available
The transversus abdominis (TrA) is a spine stabilizer frequently targeted during rehabilitation exercises for individuals with low back pain (LBP). Performance of exercises on unstable surfaces is thought to increase muscle activation, however no research has investigated differences in TrA activation when stable or unstable surfaces are used. The purpose of this study was to investigate whether TrA activation in individuals with LBP is greater when performing bridging exercises on an unstable surface versus a stable surface. Fifty one adults (mean ± SD, age 23.1 ± 6.0 years, height 173.60 ± 10.5 cm, mass 74.7 ± 14.5 kg) with stabilization classification of LBP were randomly assigned to either exercise progression utilizing a sling bridge device or a traditional bridging exercise progression, each with 4 levels of increasing difficulty. TrA activation ratio (TrA contracted thickness/TrA resting thickness) was measured during each exercise using ultrasound imaging. The dependent variable was the TrA activation ratio. The first 3 levels of the sling-based and traditional bridging exercise progression were not significantly different. There was a significant increase in the TrA activation ratio in the sling-based exercise group when bridging was performed with abduction of the hip (1.48 ± .38) compared to the traditional bridge with abduction of the hip (1.22 ± .38; p<.05). Both types of exercise result in activation of the TrA, however, the sling based exercise when combined with dynamic movement resulted in a significantly higher activation of the local stabilizers of the spine compared to traditional bridging exercise. This may have implications for rehabilitation of individuals with LBP.
Full-text available
Slowness of movement is a factor that may cause a decrease of quality of daily life. Mobility in the elderly and people with movement impairments may be improved by increasing the quickness of fundamental locomotor tasks. Because it has not been revealed how much muscle strength is required to improve quickness, the purpose of this study was to reveal the relation between movement time and the required muscle strength in a sit to stand (STS) task. Previous research found that the sum of the peak hip and knee joint moments was relatively invariant throughout a range of movement patterns (Yoshioka et al., 2007, Biomedical Engineering Online 6:26). The sum of the peak hip and knee joint moment is an appropriate index to evaluate the muscle strength required for an STS task, since the effect of the movement pattern variation can be reduced, that is, the results can be evaluated purely from the viewpoint of the movement times. Therefore, the sum of the peak hip and knee joint moment was used as the index to indicate the required muscle strength. Experimental kinematics data were collected from 11 subjects. The time at which the vertical position of the right shoulder fell outside three standard deviations of the vertical positions during the static initial posture was regarded as the start time. The time at which the vertical position fell within three standard deviations of the vertical positions during static upright standing posture was regarded as the finish time. Each movement time of the experimental movements was linearly lengthened and shortened through post-processing. Combining the experimental procedure and the post-processing, movements having various movement patterns and a wide range of movement times were obtained. The joint moment and the static and inertial components of the joint moment were calculated with an inverse dynamics method. The static component reflects the gravitational and/or external forces, while the inertial component reflects the acceleration of the body. The quantitative relation between the movement time and the sum of the peak hip and knee joint moments were obtained. As the STS movement time increased, the joint moments decreased exponentially and converged to the static component (1.51 approximately 1.54 N.m/kg). When the movement time was the longest (movement time: 7.0 seconds), the joint moments (1.57 N.m/kg) closely corresponded to the minimum of 1.53 N.m/kg as reported by Yoshioka et al.. The key findings of this study are as follows. (1) The minimum required joint moment for an STS task is essentially equivalent to the static component of the joint moment. (2) For fast and moderate speed movements (less than 2.5 seconds), joint moments increased exponentially as the movement speed increased. (3) For slow movements greater than 2.5 seconds, the joint moments were relatively constant. The results of this STS research has practical applications, especially in rehabilitations and exercise prescription where improved movement time is an intended target, since the required muscle strength can be quantitatively estimated.
Full-text available
Upright balance is believed to be maintained through active and passive mechanisms, both of which have been shown to be impacted by aging. A compensatory balance response often observed in older adults is increased co-contraction, which is generally assumed to enhance stability by increasing joint stiffness. We investigated the effect of aging on standing balance by fitting body sway data to a previously developed postural control model that includes active and passive stiffness and damping parameters. Ten young (24 +/- 3 years) and seven older (75 +/- 5 years) adults were exposed during eyes-closed stance to perturbations consisting of lateral pseudorandom floor tilts. A least-square fit of the measured body sway data to the postural control model found significantly larger active stiffness and damping model parameters in the older adults. These differences remained significant even after normalizing to account for different body sizes between the young and older adult groups. An age effect was also found for the normalized passive stiffness, but not for the normalized passive damping parameter. This concurrent increase in active stiffness and damping was shown to be more stabilizing than an increase in stiffness alone, as assessed by oscillations in the postural control model impulse response.
Full-text available
A visual model of the sit-to-stand movement pattern was developed from the film data of 38 women and 17 men as they assumed standing from a seated position. We used the data from these film records to identify a representative initial starting position and displacements of body segments for each of 20 equal intervals throughout the movement cycle. Trajectories of data points on the head, acromion, midiliac crest, hip, and knee also were plotted. These diagrams demonstrate the time-space relationships of various body parts during the task. This normalized model may be used by physical therapists as a standard to which they can compare the movement pattern of a patient.
[Purpose] We investigated the effects of changing the resistance direction using an elastic tubing band on abdominal muscle activities during isometric upper limb exercises in a seated position. [Subjects] Twenty ablebodied volunteers (10 males, 10 females) were recruited for the study. [Methods] All subjects performed isometric upper limb exercises with an elastic tubing band involving three different shoulder movements (extension, flexion, and horizontal abduction). Surface electromyography (EMG) signals were recorded from the rectus abdominis (RA), external oblique (EO), and internal oblique (IO) bilaterally during isometric upper limb exercises. [Results] There were significant differences in EMG activity of the bilateral RA during shoulder extension, shoulder horizontal abduction, and shoulder flexion. The EMG activities of the bilateral EO and IO were significantly higher in shoulder extension and horizontal abduction than in shoulder flexion when the subjects performed the arm exercise in the seated position. There was no significant difference between shoulder extension and horizontal abduction. [Conclusions] We suggest that upper limb extension and horizontal abduction using an elastic tubing band could be effective at improving abdominal muscle activities without trunk movement during isometric upper limb exercises.
Purpose : The purpose of this study is to investigate the effects of falls prevention exercise program(center of gravity control training, multiple sensory training, strategic posture training, ambulation training, muscle strengthening training) to balance and muscular strength in the elderly females. Methods : A total of 30 elderly womens participated in this study. All subjects have participated in exercise program on three times a week for eight weeks. Before and after of exercise program, They have measured about Berg Balance Scale (BBS), Performance Oriented Mobility Assessment (POMA), Time Up & Go (TUG), Sit to stand, Fall Efficacy Scale (FES), Quality of Life (QOL). Results : The results of this study were as follows ; 1) There were statistically significant difference in the BBS, POMA, FES, QOL test on within-subject. 2) There were not significant difference in the TUG, Sit to stand test on within-subject. 3) The BBS was correlated with POMA and QOL. The POMA was also correlated with QOL. Conclusion : The result of this study shows that falls prevention exercise program was meaningful increasing of balance ability and quality of life on elderly women
The behaviour of linked body segments during sit-to-stand was the subject of this study which investigated the relationship between the trunk and lower limb segments by varying the initial position of the trunk. Six subjects were videotaped as they stood up with feet on a forceplate from three initial positions: erect sitting, trunk flexed forward 30 deg, and 60 deg. When subjects actively flexed the trunk in the pre-extension phase, the order in which lower limb joints extended was knee, hip, ankle. However, when there was no active flexion, the order of onsets changed, the hip extending first followed by the knee and ankle. An extensor support moment (SM), a summation of extensor moments at hip, knee and ankle, occurred throughout the extension phase. The mean peak value of SM remained invariant in all three conditions despite variability in individual hip, knee and ankle moments. When active trunk flexion was absent, the duration of the extension phase was longer and a high value of SM was sustained for a longer proportion of the phase, indicating that more muscle force was required. The findings support the view that biomechanical characteristics emerge naturally from a functional coupling between segments, according to the demands of the action.
Objectives: To assess the paraspinal and abdominal muscle activities during different therapeutic exercises and to study how load increment produced by varying limb movements and trunk positions could affect these muscle activities. Design: A cross-sectional study comparing muscle activities between men and women. Setting: Rehabilitation clinic in university hospital. Participants: Twenty-four healthy volunteers (14 women, 10 men) aged 21 to 39 years. Interventions: Subjects performed 16 different therapeutic exercises commonly used to treat low back pain. Main outcome measures: Surface electromyography was recorded from the paraspinal (T9, L5) and abdominal (rectus abdominis, obliquus externus) muscles during these exercises. Average electromyographic amplitudes obtained during the exercises were normalized to the amplitude in maximal voluntary contraction (% MVC) to produce interindividually comparable muscle activity assessments. Results: Mean average normalized electromyographic amplitudes (% MVC) of the exercises were below 50% MVC. At L5 level, the multifidus muscle activities were significantly higher (p <.05) in women than in men, whereas no significant difference was found at T9 level. Similarly, rectus abdominis and obliquus externus activities were significantly higher (p <.001, p <.05) in women than in men. Load increment in hands or unbalanced trunk and limb movements produced higher paraspinal and abdominal muscle activities (p <.05). Conclusions: Simple therapeutic exercises are effective in activating both abdominal and paraspinal muscles. By changing limb and trunk positions or unbalancing trunk movements, it is possible to increase trunk muscle activities. Women were better able to activate their stabilizing trunk muscles than men; but it is also possible that men, having a much higher degree of strength on maximal contraction, only need to activate a smaller amount of that maximum to perform a similar activity.
The objective of this study was to quantify the coacti vation patterns of the knee flexor and extensor muscles as part of continued efforts to identify the role of the antagonist muscles in maintaining joint stability. The simultaneous EMG from the flexor and extensor muscles of the knee were recorded during maximal effort, slow isokinetic contractions (15 deg/sec) on the plane parallel to the ground to eliminate the effect of gravity. The processed EMG from the antagonist mus cle was normalized with respect to its EMG as agonist at maximal effort for each joint angle. The plots of normalized antagonist EMG versus joint angle for each muscle group were shown to relate inversely to their moment arm variations over the joint range of motion. Additional calculations demonstrated that the antago nist exerts nearly constant opposing torque throughout joint range of motion. Comparison of data recorded from normal healthy subjects with that of high perform ance athletes with hypertrophied quadriceps demon strated strong inhibitory effects on the hamstrings coac tivations. Athletes who routinely exercise their ham strings, however, had a coactivation response similar to that of normal subjects. We concluded that coactivation of the antagonist is necessary to aid the ligaments in maintaining joint stability, equalizing the articular surface pressure dis tribution, and regulating the joint's mechanical imped ance. The reduced coactivation pattern of the unexer cised antagonist to a hypertrophied muscle increases the risk of ligamentous damage, as well as demon strates the adaptive properties of the antagonist muscle in response to exercise. It was also concluded that reduced risk of knee injuries in high performance ath letes with muscular imbalance could result from com plementary resistive exercise of the antagonist muscle.