Background and objectives: The purpose of this study was to investigate the effect of
hip joint abduction an-gle change on trunk muscle activities during plank
Materials and methods: This study was conducted with 34 subjects but 4 subjects
dropped out during the measurement and 30 subjects completed the study. Muscle
activities were measured while performing plank exercise with hip joint abduction
angles at 0, 15, 30, or 45 degrees. Electrodes were placed on four muscles:
rectus abdominis, external oblique, internal oblique, and erector spinae muscles.
Two-way repeated measures ANOVA was used to analyze the interaction between hip
joint abduction angle and trunk muscle activities. One-way repeated measures
ANOVA was performed to determine trunk muscle activities of each muscle at each
hip joint abduction angle. Eta-squared (
The Muscles play a variety of roles in our bodies. Most obviously, their contractions allow movement, but they also have many other functions , which include supporting the body, maintaining correct posture, protecting internal organs, and allowing blood to circulate properly [1, 2]. As such, muscle weakening can cause health issues.
Due to science and technology developments, people mainly spend more time sitting, and as a result, body activity is reduced and muscles are weakened or muscle mass even lost, and often results in musculoskeletal disease [1, 3]. In particular, trunk muscles that support the body are often weakened , and trunk stability decreases. Trunk stability, which is also referred to as body stability, enables the maintenance of correct body posture and greater trunk stability reduces the risk of injury [5, 6]. The stability of the trunk is also referred to as core stability, and it plays an important role in the stability of the lumbopelvic-hip complex, which is a complex including the lumbar spine and pelvic and hip joints . The lumbopelvic-hip complex transmits force from the lower extremities to the pelvis and spine during athletic functions and activities, and the instability during movement increases due to this transmitted force, increasing the risk of injury. The stability of the trunk helps to stabilize the pelvis during activity, reducing this instability and risk and preventing injuries .
The muscles that affect trunk stability include the rectus abdominis, internal obliques, external obliques, and the erector spinae muscles, and trunk stability requires that these muscles cooperate properly during body movements. When these muscles are activated, they can also maintain abdominal pressure against the weight of the gravitational force applied to the body and maintain the correct posture of the body .
Several exercise methods have been devised to strengthen trunk muscles [9, 10, 11, 12], and of these exercises, the plank exercise method is widely used by many people . This exercise is performed without any tools in a prone position with both elbows placed on the floor and supported by forearms to keep upper arms perpendicular to the ground . Previous studies on this exercise have reported that it has good effects on trunk muscles and that when the posture is maintained trunk muscle activities increase. Others have shown that regular plank exercise strengthens trunk muscles [10, 14]. Interestingly, it has also been reported that if the exercise posture is altered exercise difficulty and trunk muscle activations are changed [15, 16]. For this reason, studies on the effect of posture change during plank exercise are being actively conducted.
However, few research studies have addressed the effects of posture changes on trunk muscle activity during plank exercise [15, 16]. Therefore, in this study, we investigated the effect of hip joint abduction angle change on trunk muscle activities during plank exercise.
Initially, 40 men in their twenties residing in P city (Korea) were considered
for the study, but six refused to provide consent, and thus, 34 subjects were
enrolled in the study. The selection criteria were as follows: the absence of any
disease that might affect the study, no visual or auditory impairment, no nervous
system or vestibular organ condition, and the ability to understand the
requirements of the experiment. Four dropped out during the study, and thus, the
final study cohort was composed of 30 men. Mean subject age was 22.90
|Age (year)||22.90 |
|Height (cm)||171.04 |
|Weight (kg)||69.43 |
|SD, standard deviation.|
Muscle activities were measured while performing plank exercise with hip joint
abduction angles at 0, 15, 30, or 45 degrees. Each posture was maintained for 10
seconds, and muscle activity data collected during the 4th, 5th, 6th, and 7th
seconds were subjected to analysis. Subjects took a two-minute rest after each
exercise to prevent fatigue. To reduce bias, the order of hip joint abduction
movements was randomized for each subject. All measurements were performed 3
times, and results are expressed as means
Subjects were required to maintain a straight line from feet to head while holding both elbows on the floor in a prone position and supporting them with forearms while keeping upper arms perpendicular to the ground. Hip joint abduction was added according to verbal feedback during the exercise session, and trunk muscle activities were measured in four postures at spreading angles of 0, 15, 30, and 45 degrees.
To measure trunk muscle activities, we used the FreeEMG 1000 (BTS, Milano, Italy) wireless system and Ag-Ag/Cl electrodes (diameter 2 cm), which were attached to 4 locations to measure activities of rectus abdominis (RA), external oblique (EO), internal oblique (IO), and erector spinae (ES) muscles. The electrode attachment site of the rectus abdominis muscle was attached to the side about 2 cm from the navel, so that the electrode was parallel to the muscle fiber direction, and the electrodes were attached 3 cm apart from each other. The electrode attachment site of the abdominal internal oblique muscle was attached to the upper part of the anterior superior iliac spine (ASIS) and to the side of the rectus abdominis muscle, and the electrodes were attached so that they were 2 cm apart from each other .
The attachment method was made to be parallel to the muscle fiber direction, and was attached after finding the middle part of the muscle belly using a manual test. To minimize skin resistance, after shaving the electrode attachment site, wipe it with alcohol, and after the electrode attachment area was completely dry, two electrodes coated with electrolyte gel were attached to the skin.
The EMG signals were collected at a sampling rate of 1000 Hz and processed by full-wave rectification. Data storage was performed by bandpass filtering at 30~500 Hz using BTS EMG-Analyzer (BTS, Milano, Italy) software and notch filtering at 60 Hz to remove noise.
Each subject had a different skin impedance, so a standardized value was created for each individual in order to generalize the measured values through a standardization process. In order to standardize trunk muscle measured activities, raw data were converted into RMS (Root Mean Square) values . The analysis was performed using the average trunk muscle activities of three measurements. In addition, MVIC (maximal voluntary isotonic contraction) was measured for each muscle and these values were used to standardize muscle activities. MVICs were obtained by performing maximal activity tests using the anti-gravity position of manual muscle testing positions.
SPSS (SPSS, Inc., Chicago, IL, USA) for Windows (version 22.0) was used for the
data analysis. Two-way repeated measures ANOVA was used to analyze the
interaction between hip joint abduction angle and trunk muscle activities.
One-way repeated measures ANOVA was performed to determine trunk muscle
activities of each muscle at each hip joint abduction angle. Post hoc analysis
was conducted using Fisher’s LSD. Eta-squared (
A significant interaction was found between hip joint abduction angle and muscle
|(muscle * hip abduction)|
Muscle activity according to hip abduction. RA, rectus abdominis; EO, external oblique; IO, in-ternal oblique; ES, erector spinae.
|Muscle||0°||15°||30°||45°||F||p||effect size (|
In this study, we investigated rectus abdominis, external oblique, internal oblique, and erector spinae trunk muscle activities at different hip joint abduction angles during plank exercise. In previous studies that examined trunk muscle activities during Plank exercise in young, healthy subjects, as was performed during the present study, reported non-significantly higher or lower levels than we obtained.
Kim et al.  in a study on 30.44
On the other hand, others have reported results similar to those we obtained. Cortell-Tormo et al.  in a study on adults in their twenties, reported RA, IO, and EO muscle activities were slightly higher than we found, though ES activity (4.74% MVIC) was almost identical. Byrne et al.  reported an RA activity of 22% MVIC, which was similar to our result, and Lehman et al.  reported RA and ES MVICs of 26.6% and 4.98%, respectively which agreed with our results. Analysis of the results of these previous studies showed they are comparable with previously reported results for young adults.
When we analyzed trunk muscle activity results at hip joint abduction angles of
0, 15, 30, and 45 degrees, a significant interaction was found between abduction
angles and muscle types. Only RA and ES muscles showed significant trunk muscle
activity differences at different abduction angles. In particular, muscle
activities of RA muscles gradually decreased on increasing hip joint abduction
angle. On the other hand, no significant difference was observed between the
activities of ES muscles at 0
Previous studies have shown that increased instability of the supporting surface increases trunk muscle activities. Calatayud et al.  reported muscle activities of RA and ES increased during plank exercise when the support surface was unstable and increased when an unstable posture was adopted, although this study was performed in the supine position. Chanthapetch et al.  also reported that when the support surface became un-stable trunk muscle activities also increased. Imai et al. , Lehman et al. , and Escamilla et al.  compared trunk muscle activities after performing exercise on stable and unstable surfaces, and also found activities increased when same movements were performed on unstable sides. As mentioned above, our analysis of the results of previous studies suggested that even in the same exercise posture, trunk muscle activities decrease when the stability of the supporting surface body increases. This supports our result that trunk muscle activities decrease when hip joints were abducted in the Plank exercise posture. When the abduction of the hip joint is increased in this posture, the support area of the body gradually increases, which increases postural stability, and it is considered that as postural stability increases, load on trunk muscles decreases, and as a result, trunk muscle activities decrease.
In addition, we found no significant change in EO or IO muscle activities when hip joint abduction angle was increased. We attribute this finding to the fact that the EO and IO muscles are no larger than the RA, and that plank exercise does not involve rotation, which is a function of EO and IO muscles. Furthermore, stability against rotational movement of lumbar bone is controlled by EO, IO, and TA (Transversus Abdominis), which are attached horizontally or diagonally, whereas the RA is attached vertically .
Some studies have reported that oblique muscles showed less change than the RA. Escamilla et al. , compared muscle activity during plank exercise conducted on stable and unstable support surfaces, and found changes in EO and IO activities were less than that of the RA. Fong et al.  examined changes in muscle activities in various exercise postures, and also found EO changes were smaller than RA changes. These findings show that when the hip joint is abducted during Plank exercise in healthy young adults, the muscle activities of RA (rectus abdominis) and ES (erector spinae) are inversely pro-portional to hip joint abduction angles and that those of the EO (external oblique) and IO (internal oblique) are not significantly affected by hip joint abduction angle changes.
The limitations of this study are that our findings do not represent all adults because all subjects were in their twenties and that we only measured trunk muscle activities. We recommend that future studies be conducted on subjects of different ages and various body parts in different postures.
In conclusion, when the hip joint is abducted during Plank exercise in healthy young adults, the muscle activities of RA (rectus abdominis) and ES (erector spinae) are inversely proportional to hip joint abduction angles and that those of the EO (external oblique) and IO (internal oblique) are not significantly affected by hip joint abduction angle changes.
Based on these findings, we suggest hip joint abduction could be added to reduce the level of plank exercise difficulty level for subjects with insufficient muscle strength. In addition, hip joint abduction angle changes could be used to reduce RA activity during plank exercise and to strengthen abdominal oblique muscles (EO and IO).
EMG, electromyography; ES, erector spinae; EO, external oblique; IO, internal oblique; MVIC, maximal voluntary isotonic contraction; RA , rectus abdominis; RMS, Root Mean Square; TA, Transversus Abdominis.
Conceptualization—SGK; methodology—MKK and SGK; software—MKK; validation—MKK and SGK; formal analysis—SGK; investigation—MKK and SGK; resources—MKK and SGK; data curation—MKK; writing—Original draft preparation—MKK and SGK; writing—Review and editing—SGK; visualization—MKK and SGK; supervision—SGK. All authors have read and agreed to the published version of the manuscript.
The study was conducted according to the guidelines of the Declaration of Helsinki, and ap-proved by the Institutional Review Board of Daegu Catholic University (No. CUIRB-2021-0013). Ethical issues regarding plagiarism, informed consent, misconduct, data fabrication and/or falsification, double publication and/or submission, and redundancy have been completely observed by the authors.
The authors gratefully thank all subjects for participating in this study.
This research received no external funding.
The authors declare no conflict of interest.