Comparison of Core Muscle Activation at Different Hip Angles during Core Exercises Based on Dynamic Neuromuscular Stabilization

Article information

Asian J Kinesiol. 2025;27(1):45-52

Publication date (electronic) : 2025 January 31

doi : https://doi.org/10.15758/ajk.2025.27.1.45

Won-Chul Ahn¹, Kyoung-Bin Min², Myung-Ki Kim^2,

¹Department of Sport Medicine, Korea University, Sejong Campus, Sejong-si, Republic of Korea

²Department of Global Sport Studies, Korea University, Sejong Campus, Sejong-si, Republic of Korea

^*Correspondence: Myung-Ki Kim, Department of Global Sport Studies, Korea University Sejong Campus, 2511 Sejong-ro, Sejong-si, Republic of Korea (300190); Tel: +82-10-4457-2341; E-mail: kmk1905@korea.ac.kr

Received 2024 October 16; Accepted 2025 January 11.

Abstract

Introduction

Core exercises have become a fundamental component of contemporary fitness regimens, extending their application to the field of sports medicine. Core stabilization and strengthening are emphasized in various fitness disciplines such as Pilates, personal training, and yoga, owing to their multiple benefits, including improved functional performance, injury prevention, and alleviation of lower back pain [1].

The core musculature is conceptualized as a “box” comprising the abdominal muscles that form the anterior wall, the spinal muscles that constitute the lateral and posterior walls, the diaphragm at the top, and the pelvic floor and gluteal complex forming the base. These muscles work synergistically to maintain spinal stability, which is crucial for both daily activities and athletic performance. Effective exercise execution requires proximal stability to support distal mobility, underscoring the essential nature of core stabilization and strengthening [2].

Among the core muscles, the transverse abdominis (TrA), multifidus, pelvic floor muscles, and diaphragm are categorized as local core muscles that stabilize the spine. These muscles activate proximally to regulate intra-abdominal pressure (IAP), which plays a pivotal role in spinal stabilization [3]. Studies have identified two primary techniques to increase IAP: the abdominal draw-in maneuver and abdominal bracing. Among these, abdominal bracing has been shown to be more effective in enhancing IAP and promoting spinal stability [4].

Dynamic Neuromuscular Stabilization (DNS), one of the core stabilization methods, is grounded in the principles of Developmental Kinesiology (DK). This approach mimics the developmental movement patterns observed during the first year of life and applies them to adult exercise training. DNS emphasizes the coordinated activation of intrinsic spinal stabilizers to balance IAP and rib alignment, providing dynamic spinal stability [6]. Research suggests that maintaining a cylindrical abdominal shape and proper rib alignment through IAP regulation is critical for posture and spinal stabilization, achieved via the automatic feedforward mechanism that precedes voluntary movement [7].

Further research has demonstrated significant activation of abdominal wall muscles, including the rectus abdominis and TrA, during core exercises involving hip flexion, such as leg-raising movements. During hip flexion, primary muscles like the rectus femoris and hamstrings exhibit heightened activity, playing a critical role in supporting and stabilizing the spine [8]. Greater hip flexion angles increase the load on the spine and pelvis, necessitating more active engagement of core muscles to maintain stability [9].

Although hip flexion angles significantly influence spinal stability and core muscle activation, prior studies have predominantly focused on single angles, offering limited insights into the effects of varying hip flexion angles on muscle activation. Furthermore, while DNS emphasizes developmental movement patterns, its application to understanding hip flexion angle variations and their implications for spinal stabilization remains underexplored [10].

Hip flexion angle changes during DNS-based exercises could influence lumbar muscle activation and consequently affect IAP and spinal stability. For example, Juan et al. [11] explored the impact of hip flexion angles on abdominal muscle activation but did not incorporate DNS principles, highlighting the need for further research in this area [12].

This study aims to analyze the effects of hip flexion angle variations on abdominal muscle activation and spinal stability within the context of DNS-based exercises. By comparing muscle activation across different angles, this research seeks to provide evidence for optimizing exercise programs and enhancing their practical application in rehabilitation and athletic training.

Material and Methods

Participants

This experimental study was designed to analyze and compare the effects of Dynamic Neuromuscular Stabilization (DNS)-based core exercises on lumbar muscle activation across varying hip flexion angles. The sample size was determined based on previous studies. For instance, Crommert et al. [13] measured core muscle activation in 10 participants and successfully collected significant electromyographic (EMG) data, while Badiuk et al. [14] conducted a similar study with 8 participants, also obtaining reliable EMG results. Accordingly, this study recruited 9 participants to ensure the reliability of the findings and to facilitate comparisons with existing studies.

The participants were adult males residing in metropolitan areas, aged between 20 and 40, with no history of musculoskeletal or cardiovascular diseases. Eligibility criteria included the absence of hip or lower back pain within the last year and the ability to perform DNS-based core movements. Participants were fully informed about the purpose and procedures of the study, and only those who voluntarily agreed to participate and signed the informed consent form were included. The general characteristics of the study participants are presented in <Table 1>.

Table 1.

General characteristics of the subjects.

Study design

Prior to the experiment, the age, height, weight, and BMI of the nine participants were recorded. To establish a baseline for muscle activation in each muscle group (rectus femoris, erector spinae(Iliocostalis lumborum), rectus abdominis, and external oblique), maximum voluntary isometric contraction (MVIC) was measured <Figure 1>. Following this, a goniometer (Fabrication Enterprises Inc., Korea) was used to determine the angles for assessing muscle activation during hip flexion. Electromyography (EMG) tests were conducted five times at each angle, with a 3-minute rest period between tests. The results were then analyzed based on the data obtained from the tests <Figure 2>.

Figure 1.

MVIC measurement for each muscle.

Figure 2.

Experimental procedure.

Measurement method

Electromyography measurement

To measure lumbar muscle activation and muscle recruitment order during core-strengthening exercises at varying hip flexion angles, a wireless electromyography (EMG) system (TeleMyo DTS, Noraxon, USA) was utilized. The electrodes used in this study were Ag/AgCl electrodes. To minimize skin resistance and reduce signal errors, the electrode application area was prepared by shaving with a disposable razor, cleaning with alcohol swabs, and allowing the skin to dry completely before electrode placement.

Electrodes were placed following the guidelines of the SENIAM (Surface Electromyography for the Non-Invasive Assessment of Muscles) manual. For consistency, the inter-electrode distance was standardized at 20 mm, with electrodes aligned parallel to the muscle fibers. The electrodes were attached to key muscles of the lumbo-pelvic-hip complex, including the rectus femoris, erector spinae(iliocostalis lumborum), rectus abdominis, and external oblique, as shown in <Table 2 and Figure 3> [15].

Table 2.

Electrode placement sites for each muscle.

Figure 3.

Electrode placement for each muscle.

EMG signals were collected using eight channels, with the signals received by the receiver unit (MR–XP 1.08 Master Edition) and converted into digital signals for analysis. The sampling rate for the EMG signals was set at 1,000 Hz. A bandpass filter of 20–450 Hz and a notch filter at 60 Hz were applied to the data. The processed signals were analyzed using the root mean square (RMS) method. To standardize muscle activation data, a reference voluntary isometric contraction (RVIC) was used. Muscle activation was measured over a 5-second period, excluding the first second to ensure maximal amplitude had been reached and the final second to avoid fatigue caused by sustained maximal contraction. The mean value from three trials was used, with a rest period of 3 minutes between trials. Muscle activation was expressed as a percentage of the RVIC (%RVIC) and measured at hip flexion angles of 45°, 90°, and 130°.

Core movements based on dynamic neuromuscular stabilization

The basic posture for dynamic neuromuscular stabilization (DNS) core exercises is derived from the supine position of a 3-month-old infant. In the fundamental position, the hips, knees, and ankles are flexed at 90°, with the hips slightly abducted and externally rotated. The individual lies supine with the chest open, maintaining a neutral posture [10]. The most critical aspect of this posture is ensuring proper intra-abdominal pressure (IAP) to maintain correct alignment, with the ribs and pelvis parallel and the entire abdominal wall evenly activated [12]. Based on this fundamental 90° posture, hip angles of 45° and 130° were also assessed <Figure 4>.

Figure 4.

Hip flexion 90°, 45°, 130°.

Results

The purpose of this study was to compare and analyze differences in lumbar stabilizing muscle activation across hip flexion angles (45°, 90°, and 130°) during dynamic neuromuscular stabilization (DNS)-based core exercises in nine healthy adult males. The results of the study are as follows.

Comparison of %MVIC in the Rectus Femoris

The analysis of rectus femoris muscle activation according to hip flexion angles revealed a statistically significant difference in muscle activation for both the left (p < 0.005) and right (p < 0.001) sides. Post-hoc analysis indicated that muscle activation was highest at 130°, followed by 90° and 45°, with significant differences observed between the angles (p < 0.05) <Table 3>.

Table 3.

Comparison of %MVIC of rectus femoris.

Comparison of %MVIC in the Erector Spinae(iliocostalis lumborum)

The analysis of erector spinae muscle activation according to hip flexion angles showed a statistically significant difference in muscle activation for both the left (p < 0.005) and right (p < 0.05) sides. Post-hoc analysis indicated that for the left erector spinae, muscle activation was highest at 130°, followed by 45° and 90°. For the right erector spinae, activation was highest at 130°, followed by 90° and 45°. However, these differences were not statistically significant on either side (p > 0.05) <Table 4>.

Table 4.

Comparison of %MVIC of elector spine (Iliocostalis lumborum).

Comparison of %MVIC in the Rectus Abdominis

The analysis of rectus abdominis muscle activation according to hip flexion angles revealed a statistically significant difference for both the left (p < 0.001) and right (p < 0.005) sides. Post-hoc analysis showed that for both sides, muscle activation was highest at 45°, followed by 130° and 90°, with significant differences between the angles (p < 0.05) <Table 5>.

Table 5.

Comparison of %MVIC of rectus abdominis.

Comparison of %MVIC in the External Oblique

The analysis of external oblique muscle activation according to hip flexion angles demonstrated a statistically significant difference for both the left (p < 0.005) and right (p < 0.05) sides. Post-hoc analysis indicated that muscle activation was highest at 45°, followed by 130° and 90°, with significant differences observed between the angles (p < 0.05) <Table 6>.

Table 6.

Comparison of %MVIC of external oblique.

Discussion

Dynamic Neuromuscular Stabilization (DNS)-based core exercises utilize movement patterns observed during the first year of life to evaluate and treat motor dysfunctions. These exercises focus on improving strength, neuromuscular coordination, and posture, emphasizing intra-abdominal pressure (IAP) and muscle co-activation to maintain dynamic spinal stability [5]. DNS has been shown to effectively enhance lumbar muscle activation and is increasingly applied in posture correction and therapeutic interventions [6,8]. However, there is a lack of studies exploring the effect of hip flexion angles on muscle activation. Therefore, this study aimed to investigate the impact of varying hip flexion angles on lumbar muscle activation, and the results are discussed as follows. Previous studies have demonstrated that DNS-based abdominal strengthening exercises result in higher abdominal muscle activation compared to conventional abdominal exercises [16]. DNS optimizes neuromuscular coordination to effectively enhance the stability of the spine and pelvis. Specifically, DNS has been shown to positively influence the stabilization of the lumbopelvic complex by activating core muscles such as the transverse abdominis (TrA), internal oblique (IO), external oblique (EO), and rectus abdominis (RA) [17]. DNS stimulates neuroplasticity, strengthening core muscle coordination through integrated motor learning, including breathing and posture control. This mechanism was confirmed in a study involving stroke patients, where DNS significantly increased the activation of abdominal and lumbar muscles compared to neuro-developmental treatment (NDT) [18].

Rectus femoris activation In this study, the analysis of rectus femoris activation across varying hip flexion angles revealed that muscle activation increased with higher hip flexion angles, peaking at 130°. These findings align with previous research, which reported increased muscle tension at hip flexion angles exceeding 90° [19]. This result is consistent with the length-tension relationship, which indicates that muscle tension changes depending on joint angle during open kinetic chain movements [20]. Conversely, some studies reported higher muscle activation at 45° [21]. This discrepancy can be explained by the role of knee joint positioning. When the knee joint is flexed, the rectus femoris primarily acts as a hip flexor, whereas with an extended knee, it predominantly functions as a knee extensor [22]. The findings of this study confirm that rectus femoris activation increases with greater hip flexion angles. This suggests that DNS exercises facilitate lumbar stabilization and core muscle strengthening through the interaction between hip flexion angles and muscle activation.

Erector spinae activation Regarding the erector spinae, previous studies have shown that hip flexion angles and erector spinae activation are closely related, explained by the flexion-relaxation phenomenon (FRP) [20]. The present study found that erector spinae activation exhibited a trend of eccentric contraction with increasing hip flexion angles, aligning with the findings of Descarreaux et al. [23]. The erector spinae contribute to postural stability and assist neuromuscular coordination, with increased muscle tension observed as hip flexion angles rise [24]. Recent studies highlight the coordination between hip and spinal muscles as crucial for postural stability and biomechanics. For example, erector spinae and multifidus activation reportedly increases with workload and angle variations, supporting the findings of this study regarding the correlation between higher hip flexion angles and increased erector spinae activation [24]. In contrast, the lower activation of the erector spinae compared to abdominal muscles (e.g., rectus abdominis) is attributed to the stabilizing role of abdominal muscles during hip flexion. Ludwig et al. [25] emphasized that the balance between abdominal and lumbar muscles is essential for pelvic alignment and spinal posture, explaining why the erector spinae are more prominently activated during back extension movements. This study corroborates previous findings, demonstrating that the erector spinae( iliocostalis lumborum) increase activation through eccentric contraction to support postural stability with increasing hip flexion angles. These results suggest that the interaction between hip flexion and spinal muscles plays a critical role in postural stability and strength training.

Rectus abdominis and external oblique activation Furthermore, rectus abdominis and external oblique activation varies significantly with hip flexion angles, indicating coordinated interactions between hip angles and abdominal muscles [26]. Recent research shows that rectus abdominis and external oblique activation peaks at 45°, corresponding to the optimal length-tension relationship for maximal muscle force [27]. Muyor et al. [28] observed that at hip flexion angles of 130°, rectus femoris and iliopsoas engagement increases while rectus abdominis and external oblique activation relatively decreases, which aligns with this study’s findings. At 90°, pelvic neutrality results in minimal abdominal muscle activity [29]. Kim and Park [30] reported increased rectus abdominis and external oblique activation during hip and knee flexion at 45°, as these muscles co-activate with hip flexors (e.g., iliopsoas and rectus femoris) to enhance stability and efficiency during leg-raising movements [31]. This aligns with this study’s finding that rectus abdominis and external oblique activation was highest at 45°, likely due to synergistic interactions with iliopsoas and rectus femoris.

However, this study is limited by the sample size, diversity of participants, the range of hip flexion angles examined, and the lack of long-term observation. Future research should include larger, more diverse populations and consider long-term effects to enhance reliability and generalizability.

Conclusion

This study analyzed the electromyographic (EMG) activity of lumbar stabilizing muscles (rectus femoris, erector spinae [iliocostalis lumborum], rectus abdominis, and external oblique) during Dynamic Neuromuscular Stabilization (DNS)-based core exercises at varying hip flexion angles (45°, 90°, and 130°). The findings demonstrated significant differences in muscle activation for rectus femoris and rectus abdominis depending on the hip flexion angle, indicating that hip flexion angle significantly influences muscle activation during core stabilization exercises.

Clinical Implications: This study highlights that adjusting hip flexion angles can selectively activate specific muscles. These results can be applied in rehabilitation and exercise prescription to design individualized programs. Specifically, strategies targeting specific regions of core muscles can enhance muscle strength and functional recovery. For instance, based on the findings, setting the hip flexion angle at 45° can maximize the activation of rectus abdominis and external oblique, whereas 130° can more effectively engage muscles like rectus femoris. Such an approach provides practical guidance for clinicians and trainers to develop interventions tailored to patients requiring spinal stabilization or athletes aiming for posture correction and performance optimization.

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