The small relative muscle activity levels reported in the
present and the latter studies were not necessarily related
to a nonstabilizing capacity of this muscle. Only small
activity levels seem to be necessary to ensure sufficient stability
in a neutral spine posture in non-weightbearing
positions [40]. Generally for most tasks of daily living very
modest levels of abdominal wall co-contraction are sufficient
[2]. Cholewicki et al. [40] highlighted the importance
of motor control to coordinate muscle recruitment
between so-called global and local muscles during functional
activities to ensure mechanical stability is maintained.
Under such conditions they suggested that
intersegmental muscle activity as low as 1 to 3 % MVC
may be sufficient to ensure dynamic stability [40]. Furthermore,
biomechanical modelling is needed to draw
conclusions about stability contributions as stability is
also proportional to the square of the muscle's moment
arm.
The objective for the use of the ratio of IO/RA activity in
the present study, was to enhance the understanding of
the co-activation of both local and global muscles during
this kind of stabilization exercises. Other researchers
stated that analysis of the IO/RA ratio is important to verify
if the activity of the RA is minimal in comparison with
all other muscles of the lumbopelvic region to fulfil the
requirement for a good stabilization exercise [17]. In our
opinion, respecting adequate activation levels depending
on the demands during different tasks is essential, rather
than aiming at minimal activity of certain muscles. Ratios
assist in providing further insight in the co-operation of
the different muscles during various tasks.
During the ball bridge exercise, the EO showed significantly
higher relative EMG than the IO. Consequently, the
IO/EO ratio was low (< 1) during this exercise. In accordance
with these results, McGill [1] assumed that the EO
may have a greater potential in stabilizing the trunk than
the local abdominal muscles. Vera-Garcia et al. [29] found
that when performing curl-ups on a gymnastic ball, there
was much more co-contraction of the EO muscle with the
RA muscle when compared to other tasks because of the
greatest possibility of rolling laterally off the ball. In order
to enhance this stability, it appears that the motor control
system selects to increase EO activity more than the other
abdominal muscles. However, recent research evaluating
bridging exercises showed no significant differences in relative
EO and RA activity between performance on firm or
ball surfaces [24,25]. Debate exists on increased [24] or
unchanged IO activity [25] during ball bridge excises.
However, the ball bridge exercises described in the latter
studies were performed with the feet flat on the ball, in
contrast to the calf position on the ball in the present
study. Although only the calfs were positioned on the ball,
the global torque producing EO might be activated more
than the local segmental stabilizing IO to prevent the
limbs from rolling of the ball and jepardizing the trunk
stability. Analysis of the relative EMG activity levels
showed a greater increase in EO activity compared to IO
activity between the single bridging and the ball bridge
exercise. This could explain the small ratio of the IO to EO
during the ball bridge exercise in the current study.
During the unilateral bridging exercise, the ipsilateral IO
showed significantly higher EMG than the ipsilateral EO
and the contralateral IO demonstrated significantly lower
activity than the contralateral EO. Consequently, the contralateral
IO/EO ratio was low (< 1) and the ipsilateral IO/
EO ratio was higher than 2 during this exercise 3. Kavcic
et al. [15] reported that during single and unilateral bridging
exercises the IO and EO seem to demonstrate consistently
a large impact on induced increasing and decreasing
stability. Both so-called local and global oblique muscles
seem to work together and may have an important role in
The small relative muscle activity levels reported in thepresent and the latter studies were not necessarily relatedto a nonstabilizing capacity of this muscle. Only smallactivity levels seem to be necessary to ensure sufficient stabilityin a neutral spine posture in non-weightbearingpositions [40]. Generally for most tasks of daily living verymodest levels of abdominal wall co-contraction are sufficient[2]. Cholewicki et al. [40] highlighted the importanceof motor control to coordinate muscle recruitmentbetween so-called global and local muscles during functionalactivities to ensure mechanical stability is maintained.Under such conditions they suggested thatintersegmental muscle activity as low as 1 to 3 % MVCmay be sufficient to ensure dynamic stability [40]. Furthermore,biomechanical modelling is needed to drawconclusions about stability contributions as stability isalso proportional to the square of the muscle's momentarm.The objective for the use of the ratio of IO/RA activity inthe present study, was to enhance the understanding ofthe co-activation of both local and global muscles duringthis kind of stabilization exercises. Other researchersstated that analysis of the IO/RA ratio is important to verifyif the activity of the RA is minimal in comparison withall other muscles of the lumbopelvic region to fulfil therequirement for a good stabilization exercise [17]. In ouropinion, respecting adequate activation levels dependingon the demands during different tasks is essential, ratherthan aiming at minimal activity of certain muscles. Ratiosassist in providing further insight in the co-operation ofthe different muscles during various tasks.During the ball bridge exercise, the EO showed significantlyhigher relative EMG than the IO. Consequently, theIO/EO ratio was low (< 1) during this exercise. In accordancewith these results, McGill [1] assumed that the EOmay have a greater potential in stabilizing the trunk thanthe local abdominal muscles. Vera-Garcia et al. [29] foundthat when performing curl-ups on a gymnastic ball, therewas much more co-contraction of the EO muscle with theRA muscle when compared to other tasks because of thegreatest possibility of rolling laterally off the ball. In orderto enhance this stability, it appears that the motor controlsystem selects to increase EO activity more than the otherabdominal muscles. However, recent research evaluatingbridging exercises showed no significant differences in relativeEO and RA activity between performance on firm orball surfaces [24,25]. Debate exists on increased [24] orunchanged IO activity [25] during ball bridge excises.However, the ball bridge exercises described in the latterstudies were performed with the feet flat on the ball, incontrast to the calf position on the ball in the presentstudy. Although only the calfs were positioned on the ball,the global torque producing EO might be activated more
than the local segmental stabilizing IO to prevent the
limbs from rolling of the ball and jepardizing the trunk
stability. Analysis of the relative EMG activity levels
showed a greater increase in EO activity compared to IO
activity between the single bridging and the ball bridge
exercise. This could explain the small ratio of the IO to EO
during the ball bridge exercise in the current study.
During the unilateral bridging exercise, the ipsilateral IO
showed significantly higher EMG than the ipsilateral EO
and the contralateral IO demonstrated significantly lower
activity than the contralateral EO. Consequently, the contralateral
IO/EO ratio was low (< 1) and the ipsilateral IO/
EO ratio was higher than 2 during this exercise 3. Kavcic
et al. [15] reported that during single and unilateral bridging
exercises the IO and EO seem to demonstrate consistently
a large impact on induced increasing and decreasing
stability. Both so-called local and global oblique muscles
seem to work together and may have an important role in
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