Effects of Abdominal Bracing Versus Abdominal Hollowing on Abdominal Muscle Thickness and Activation During Static and Dynamic Trunk Stabilization Tasks in Healthy Adults

Introduction

Trunk stability is crucial for spinal alignment, limb control, and efficient force transmission. It is widely recognized as a key determinant of both athletic performance and the prevention of musculoskeletal injuries [1,2]. Trunk stability is achieved through the coordinated function of passive spinal structures, active muscular components, and neural control mechanisms that collectively maintain spinal stiffness and dynamic balance during movement [3]. When this system is impaired, abnormal motion patterns and delayed muscle activation can occur, leading to decreased mechanical efficiency, fatigue, and a higher risk of injury [4,5]. In individuals with low back pain, these impairments are often associated with reduced activation and delayed recruitment of deep stabilizing muscles such as the transversus abdominis (TrA) and multifidus, resulting in diminished spinal control and persistent pain [6,7].

The abdominal muscles play a central role in the neuromuscular system responsible for trunk stabilization. Each layer contributes uniquely to postural control and intra-abdominal pressure (IAP) regulation. The rectus abdominis (RA) primarily facilitates trunk flexion and contributes to anterior stabilization of the spine during high-load activities [8]. The external oblique (EO) and internal oblique (IO) muscles assist in trunk rotation and lateral flexion while increasing IAP to enhance spinal stiffness [9]. The deepest layer, the TrA, functions as a corset-like stabilizer that encircles the abdominal cavity horizontally, compressing it during contraction to maintain segmental spinal stability [10]. Coordinated activation of these muscles increases mechanical stiffness of the trunk and provides dynamic protection for the lumbar spine. Understanding the anatomical structure and functional roles of the abdominal muscles is essential for selecting appropriate stabilization strategies, such as abdominal bracing and abdominal hollowing, in clinical and functional training contexts. Insufficient coordination among these muscle groups has been linked to movement inefficiency, spinal instability, and the recurrence of low back pain [6,10]. From a functional perspective, the transversus abdominis, internal oblique, and multifidus are considered primary contributors to segmental stabilization by controlling the neutral zone of the spine through anticipatory and tonic activation. In contrast, the rectus abdominis and external oblique primarily contribute to global torque generation and momentum management during trunk movement. Accordingly, abdominal hollowing is thought to selectively activate the local stabilizing system, whereas abdominal bracing promotes simultaneous co-contraction of the global musculature to increase spinal stiffness and load tolerance.

To improve trunk stability and restore motor control, 2 abdominal contraction strategies – abdominal bracing and abdominal hollowing – have been widely utilized in both clinical and sports settings. Although both techniques aim to enhance spinal stabilization, they differ fundamentally in muscle recruitment and intended outcomes. The abdominal bracing technique involves simultaneous co-contraction of the global abdominal muscles, including the RA, EO, IO, and TrA, to generate high intra-abdominal pressure and increase trunk stiffness [11]. This technique enhances resistance against external loads and has been shown to improve stability during dynamic or high-intensity tasks such as lifting and sudden perturbations [12,13]. Grenier and McGill [7] demonstrated that bracing produces a substantial increase in trunk stiffness and spinal stability by activating multiple abdominal muscles concurrently. Recent studies have further emphasized abdominal bracing as a global stabilization strategy that increases trunk stiffness through coordinated co-contraction, while also showing that its effectiveness can depend on task demands and neuromuscular control requirements [14].

However, excessive global co-contraction associated with abdominal bracing may be less efficient during low-load tasks, potentially limiting fine motor control and increasing unnecessary energy expenditure when precise segmental stabilization is required.

In contrast, the abdominal hollowing technique is designed to selectively activate the deep abdominal muscles, particularly the TrA and IO, by drawing the navel gently inward toward the spine [15]. This technique aims to enhance the neuromuscular control of deep stabilizers and reduce overactivation of superficial muscles. Hollowing has been proposed as a more precise strategy for re-educating local stabilizing muscles, especially in patients with lumbar instability or early-stage rehabilitation [16]. Hodges and Richardson [17] demonstrated that the TrA activates in a feed-forward manner prior to limb movement, highlighting its essential role in anticipatory postural control. Furthermore, Teyhen et al [18] and Hides et al [19] confirmed that visual feedback from ultrasound imaging can improve the ability to perform hollowing correctly, resulting in increased TrA thickness and better deep muscle coordination. However, abdominal hollowing may have limited effectiveness during high-load or highly dynamic tasks, as selective deep muscle activation alone may be insufficient to provide the global trunk stiffness required for functional stability. Recent studies have questioned the transferability of abdominal hollowing to dynamic or functional tasks, suggesting that selective deep muscle activation does not consistently translate to improved trunk stability under higher task demands [20].

Although both bracing and hollowing are effective in improving trunk stabilization, they engage the abdominal musculature through distinct neuromuscular mechanisms. Bracing tends to promote global co-contraction that increases stiffness and overall support, while hollowing enhances selective recruitment and fine motor control of the deep stabilizers. Thus, these techniques may serve complementary roles in rehabilitation and performance enhancement. However, despite their widespread use, the scientific evidence comparing their direct effects on muscle morphology and activation remains limited. Most previous studies have evaluated only a single aspect of muscle function – either muscle thickness via ultrasonography or muscle activation via electromyography(EMG) – and have few have examined these parameters simultaneously [21]. Moreover, there is little information on how task demands (eg, static vs dynamic postures) influence abdominal muscle behavior under each technique.

Therefore, this study aimed to provide a stage-specific clinical framework by elucidating the differential effects of abdominal bracing and hollowing on abdominal muscle thickness and activation under static and dynamic trunk stabilization tasks. Using a combination of ultrasound imaging and surface EMG, the present study quantitatively analyzed the RA, EO, IO, and TrA under static (hook-lying) and dynamic (supine toe-tap) conditions. We hypothesized that the hollowing technique would elicit greater selective activation and increased thickness of the deep stabilizing muscles (IO and TrA), whereas the bracing technique would induce greater activation of superficial global muscles (RA and EO), particularly under dynamic conditions. The findings of this study are expected to provide evidence-based guidance for selecting appropriate abdominal stabilization strategies in clinical and functional training contexts.

Material and Methods

PARTICIPANTS:

A total of 18 healthy adults (9 men and 9 women) aged between 20 and 34 years voluntarily participated in this study. Participants were recruited from the local university community through flyers and online notices. All participants were physically active and had no history of low back pain, spinal surgery, abdominal surgery, or neurological or musculoskeletal disorders that could affect trunk muscle function during the previous 6 months.

The inclusion criteria were: (1) age between 20 and 40 years, (2) no current pain or functional limitation in the spine or lower extremities, and (3) ability to understand and perform the required exercises according to verbal instructions. Exclusion criteria included: (1) any history of acute or chronic low back pain, (2) pregnancy, (3) recent abdominal or orthopedic surgery, (4) obesity (body mass index >30 kg/m²), or (5) participation in regular core stabilization training within the last 3 months. Participants with recent experience in core stabilization training were excluded to minimize potential training-related neuromuscular adaptations that could influence abdominal muscle activation patterns and thickness measurements. We also excluded professional or elite athletes, whose neuromuscular recruitment patterns may differ from those of the general population, and individuals with chronic respiratory diseases or abnormal breathing patterns that could influence abdominal muscle activation.

All participants provided written informed consent prior to participation after receiving a detailed explanation of the study purpose, procedures, and potential risks. To minimize learning or fatigue effects, the order of performing the 2 contraction techniques (abdominal bracing and hollowing) was randomized for each participant using a computer-generated randomization schedule. A 1-week wash-out period was implemented between the 2 sessions to reduce potential carryover effects.

The study was approved by the Institutional Review Board of Sahmyook University (approval no. SYU 2025-03-037-003) and conducted in accordance with the ethical standards of the Declaration of Helsinki. In addition, the study was prospectively registered with the Clinical Research Information Service (CRIS), Korea (registration number: KCT0011051), on September 22, 2025.

PROCEDURES:

Prior to the experimental sessions, a computer-generated random sequence was created using a random number generator to determine the order of the 2 conditions (abdominal bracing followed by hollowing, or hollowing followed by bracing). The random sequence was generated by an independent researcher who was not involved in participant recruitment, data collection, or outcome assessment.

Participants were enrolled by the study investigators. Allocation to the intervention sequence was concealed using sealed, opaque envelopes prepared in advance. At the beginning of the first experimental session, the envelope was opened to reveal the assigned sequence, which was then implemented and maintained throughout the study. All experiments were conducted in a quiet, temperature-controlled laboratory environment (22–24°C). Prior to data collection, each participant attended a familiarization session to learn and practice both the abdominal bracing and hollowing techniques under the supervision of a licensed physical therapist with more than 10 years of clinical experience in musculoskeletal rehabilitation. During this session, participants received visual and verbal feedback using real-time ultrasound imaging to ensure correct performance of each contraction technique.

During the experimental session, participants performed both techniques in a randomized order with a 1-week wash-out period between sessions. Each participant was instructed to lie in a supine position with the hips and knees flexed to approximately 90° and feet flat on the examination table. Before measurement, they practiced each contraction 3 to 5 times to confirm consistent performance without compensatory trunk or pelvic motion.

For the abdominal bracing technique, participants were instructed to “tighten your abdominal and lower back muscles as if preparing to resist an external force or impact,” without drawing in or pushing out the abdomen. This maneuver was intended to activate the global stabilizing muscles simultaneously, including the rectus abdominis (RA), external oblique (EO), internal oblique (IO), and transversus abdominis (TrA).

For the abdominal hollowing technique, participants were instructed to “gently draw the navel inward toward the spine while breathing normally,” minimizing visible upper abdominal movement or pelvic tilting. This technique primarily aimed to selectively activate the deep stabilizing muscles, particularly the IO and TrA.

Two task conditions were used to compare muscle responses under different stability demands: (1) a static supine hook-lying position (Figure 1), and (2) a dynamic supine toe-tap task (Figure 2). The static and dynamic tasks were selected to represent differing task demands on trunk stabilization, allowing examination of abdominal muscle behavior under low-load static conditions and increased motor control demands associated with dynamic limb movement.

In the supine hook-lying position, participants lay supine with the knees flexed to approximately 90° and the hips flexed to approximately 45°, maintained the abdominal contraction for 5 s, and kept the lumbar spine in a neutral posture. The contraction duration of 5 s was selected to ensure a stable measurement window for ultrasound imaging and EMG signal acquisition while minimizing fatigue and compensatory movements. In the supine toe-tap task, participants alternately flexed the hip to approximately 45° while maintaining 90° of knee flexion, lightly tapping 1 heel on the table before returning to the starting position. This alternating movement imposed a greater dynamic challenge on the trunk and pelvis, increasing lumbopelvic stabilization demands compared with the static task.

Each condition was performed 3 times, with 30-s rest intervals between repetitions to minimize fatigue. The order of techniques and tasks was counterbalanced across participants to prevent order effects. The examiner monitored trunk and pelvic alignment throughout the trials to ensure proper form and consistency. All participants completed both study periods without loss to follow-up, and all data were included in the final analysis (Figure 3).

ABDOMINAL BRACING TECHNIQUE:

The abdominal bracing technique is a trunk stabilization maneuver that involves the simultaneous co-contraction of global and local abdominal muscles to increase spinal stiffness and intra-abdominal pressure (IAP). This technique is designed to stabilize the lumbar spine by engaging the rectus abdominis (RA), external oblique (EO), internal oblique (IO), and transversus abdominis (TrA) muscles without producing visible trunk or pelvic motion.

During the bracing maneuver, participants were instructed to “tighten your abdominal and lower back muscles as if preparing to resist an external impact,” while maintaining a neutral lumbar posture and breathing normally. They were explicitly cautioned not to draw in or push out the abdomen. This cue promotes symmetrical activation of the superficial and deep abdominal muscles, resulting in enhanced trunk stiffness and increased IAP.

Physiologically, abdominal bracing contributes to trunk stabilization by elevating IAP and creating a rigid cylindrical core that protects the spine from excessive shear and compressive forces. Grenier and McGill [7] demonstrated that bracing significantly increases trunk stiffness and spinal stability by inducing coordinated co-activation of the abdominal wall muscles. Similarly, Vera-Garcia et al [11] reported that controlled levels of torso co-contraction improve spine motion control and resistance to external perturbations. McGill [8] further explained that bracing is essential for functional stability under dynamic conditions, providing a mechanical foundation for safe load transfer during daily and athletic activities.

In this study, participants maintained the bracing contraction for 5 s in each trial while avoiding visible movement of the trunk or pelvis. The maneuver was performed under static (hook-lying) and dynamic (supine toe-tap) conditions to examine how global co-contraction patterns respond to varying trunk stability demands.

ABDOMINAL HOLLOWING TECHNIQUE:

The abdominal hollowing technique is a selective activation maneuver that targets the deep stabilizing muscles of the trunk, primarily the transversus abdominis (TrA) and internal oblique (IO), to enhance local spinal stability and neuromuscular control. Unlike the abdominal bracing technique, which emphasizes global co-contraction, hollowing focuses on isolating the deep abdominal muscles responsible for segmental stabilization of the lumbar spine.

During the maneuver, participants were instructed to “gently draw the navel inward toward the spine while breathing normally,” avoiding visible movement of the upper abdominal wall, rib cage, or pelvis. They were also reminded not to perform posterior pelvic tilting or excessive breath-holding during the contraction. Real-time ultrasound imaging feedback was provided during familiarization to ensure correct performance and to minimize the compensatory activation of superficial muscles such as the rectus abdominis (RA) and external oblique (EO).

Physiologically, the hollowing maneuver facilitates preferential recruitment of the TrA and IO, increasing their thickness and tonic activation to enhance segmental spinal control. Hodges and Richardson [9] demonstrated that the TrA activates in a feed-forward manner prior to limb movement, underscoring its essential role in anticipatory postural control. Teyhen et al [13] confirmed that real-time ultrasound-guided training improves the ability to perform the abdominal hollowing maneuver accurately and increases TrA thickness. Similarly, Hides et al [19] reported that stabilization exercises emphasizing TrA recruitment effectively restore deep muscle symmetry and prevent recurrence of low back pain. Arab and Chehrehrazi [15] also observed greater selective activation of the TrA during hollowing compared with bracing, supporting its use in early-stage motor control retraining.

In this study, participants performed the hollowing maneuver under static and dynamic conditions to compare deep muscle responses across tasks. Each contraction was maintained for 5 s while preserving a neutral lumbar posture and minimizing visible trunk or pelvic movement. The selective activation pattern produced by hollowing was analyzed to determine its contribution to deep trunk stability compared with the global activation pattern of bracing.

ULTRASOUND IMAGING:

Ultrasound imaging was used to assess the muscle thickness of the rectus abdominis (RA), external oblique (EO), internal oblique (IO), and transversus abdominis (TrA) on the right side of the abdomen using a B-mode ultrasound system (XCUBE 60, Alpinion Medical Systems, Seoul, Korea) equipped with a 7.5-MHz linear array transducer. Muscle thickness measurements were obtained on the right side only to ensure consistency across participants, as bilateral comparison was not a primary objective of this study. The researchers positioned participants in a relaxed supine posture with their knees flexed to 90° and feet flat on the table. The transducer was placed transversely along the mid-axillary line midway between the lower costal margin and the iliac crest, parallel to the muscle fibers (Figure 4).

A water-soluble gel was applied to minimize acoustic impedance, and minimal probe pressure was maintained to prevent tissue deformation. The imaging depth and gain were adjusted for each participant to ensure clear visualization of fascial boundaries. For each condition (bracing and hollowing), 3 ultrasound images were acquired at the end of normal expiration, and the mean value was used for analysis. Muscle thickness was defined as the perpendicular distance between the superficial and deep fascial borders of each muscle (Figure 5).

To ensure measurement reliability, all ultrasound scans were performed by the same experienced sonographer, who was blinded to the experimental conditions. The intra-rater reliability, calculated from a pilot test involving 5 participants, demonstrated excellent consistency, with intraclass correlation coefficients (ICC) ranging from 0.92 to 0.98 for all muscles. Although trunk and pelvic alignment were consistently monitored by a single experienced examiner, inter-rater reliability was not assessed, which should be considered a methodological limitation. This imaging protocol followed the standardized procedures recommended by Teyhen et al [13] and Arab and Chehrehrazi [15].

SURFACE ELECTROMYOGRAPHY:

Muscle activation of the RA, EO, and IO was recorded using a wireless surface electromyography (EMG) system (Ultium EMG, Noraxon Inc., Scottsdale, AZ, USA) at a sampling rate of 2000 Hz. Disposable Ag/AgCl electrodes (2 cm inter-electrode distance) were positioned over the muscle bellies according to SENIAM guidelines:

RA: 3 cm lateral to the umbilicus, parallel to the muscle fibers; EO: below the costal margin along the line between the anterior superior iliac spine (ASIS) and the lower ribs; IO: 2 cm medial and inferior to the ASIS, oriented obliquely along the fiber direction.

The skin was shaved, lightly abraded, and cleaned with alcohol to reduce impedance (<5 kΩ). A ground electrode was placed over the ASIS. EMG signals were band-pass filtered at 80–250 Hz, full-wave rectified, and smoothed using a 50-ms root-mean-square (RMS) window. The selected band-pass filter range was chosen to minimize motion artifacts and high-frequency noise while preserving the physiological frequency content of surface EMG signals, in accordance with established recommendations.

To account for inter-subject variability, EMG data were normalized to the reference voluntary contraction (RVC) obtained during quiet breathing in the relaxed supine position, as recommended by Halaki and Ginn [21]. This normalization approach was selected to facilitate within-subject comparisons across tasks while minimizing excessive trunk activation; however, it may underestimate muscle activation levels during higher-load or dynamic conditions. Mean RMS values were computed over the 3-s steady contraction plateau, and the average of 3 repetitions was used for analysis.

Intra-rater reliability for EMG amplitude measurement was also verified prior to data collection. The ICC values ranged from 0.89 to 0.96 across all muscles, indicating excellent reliability. EMG data processing and analysis were performed offline by an investigator who was blinded to the contraction technique and task condition.

STATISTICAL ANALYSIS:

All statistical analyses were performed using IBM SPSS Statistics software (version 23.0; IBM Corp., Armonk, NY, USA). Prior to analysis, the normality of all continuous variables was examined using the Shapiro-Wilk test. Descriptive statistics were expressed as mean±standard deviation (SD).

Because each participant performed both contraction techniques (bracing and hollowing) under 2 task conditions (static hook-lying and dynamic supine toe-tap), a two-way repeated-measures analysis of variance (ANOVA) was used to examine the main effects of contraction technique and task condition, as well as their interaction effects on muscle thickness and EMG activity. When a significant main or interaction effect was observed, pairwise comparisons were conducted using the Bonferroni-adjusted post hoc test.

Additionally, paired t-tests were used to compare differences in muscle thickness or activation between bracing and hollowing within each task condition. Effect sizes for pairwise comparisons were calculated using Cohen’s d, interpreted as small (0.2–0.49), medium (0.5–0.79), or large (≥0.8). For the ANOVA, partial eta-squared (η²) values were reported to indicate the magnitude of the effect.

All statistical tests were two-tailed, and the level of statistical significance was set at p<0.05. A post hoc power analysis using G*Power software (version 3.1, Heinrich Heine University, Düsseldorf, Germany) confirmed that a sample size of 18 participants was sufficient to achieve a statistical power (1-β) of 0.80 with an effect size of 0.5 at an alpha level of 0.05.

Results

PARTICIPANT CHARACTERISTICS:

Eighteen healthy adults (9 men and 9 women) participated in this study. The general characteristics of the participants are shown in Table 1. The mean age was 22.50±2.07 years, mean height 167.72±10.04 cm, and mean body weight 60.28±11.51 kg. No adverse events occurred during any of the experimental sessions.

COMPARISON OF ABDOMINAL MUSCLE THICKNESS BETWEEN TECHNIQUES:

As presented in Table 2, significant differences in abdominal muscle thickness were observed between the 2 contraction techniques. The rectus abdominis (RA) was significantly thicker during the bracing technique (10.07±1.55 mm) than during the hollowing technique (9.22±1.46 mm; t=−3.151, p=0.006). The internal oblique (IO) and transversus abdominis (TrA) were significantly thicker during hollowing (IO: 8.65±3.18 mm; TrA: 4.08±1.23 mm) than during bracing (IO: 6.63±2.14 mm; TrA: 2.41±0.80 mm; t=3.761 and 9.509, p=0.002 and <0.001, respectively). The external oblique (EO) showed no significant difference between the 2 techniques (t=1.485, p=0.158).

COMPARISON OF ABDOMINAL MUSCLE ACTIVATION BETWEEN TECHNIQUES AND TASK CONDITIONS:

Surface electromyography results demonstrated distinct activation patterns depending on contraction technique and task condition (Table 3). In the hook-lying position, muscle activation was significantly higher during the hollowing technique than during bracing for the RA (20.26±16.90%RVC vs 10.60±6.62%RVC; t=−3.020, p=0.008), EO (70.63±25.39%RVC vs 39.85±18.62%RVC; t=−5.130, p<0.001), and IO (45.21±22.98%RVC vs 22.17±14.99%RVC; t=−5.440, p<0.001). In contrast, during the supine toe-tap task, there were no significant differences between the 2 techniques for any of the 3 muscles (RA: p=0.255; EO: p=0.087; IO: p=0.535).

Comparisons among muscles within each technique revealed a consistent activation hierarchy. During bracing, the EO exhibited the greatest activation, followed by the IO and RA in both the hook-lying and toe-tap tasks (p<0.05). The same order was observed during the hollowing technique, with EO>IO>RA (p<0.05).

Overall, these findings indicate that the hollowing technique induces greater selective activation of the deep abdominal muscles (IO and TrA) and produces higher EMG activity under static conditions, whereas the bracing technique mainly enhances RA thickness and global co-contraction. No significant differences were identified between the 2 techniques during the dynamic toe-tap task, suggesting that both strategies contribute to trunk stability depending on task intensity and movement demands.

The abdominis was significantly thicker during bracing (p=0.032), with a moderate effect size (d=0.58). The internal oblique and transversus abdominis showed significantly greater thickness during hollowing (p=0.011 and p<0.001), with moderate-to-large (d=0.79) and very large effects (d=1.23), respectively. In contrast, the external oblique showed no significant difference between techniques (p=0.642), and the effect size was negligible (d=0.09).

Across both contraction techniques and task conditions, a consistent muscle activation hierarchy was observed, with the external oblique showing the highest activation, followed by the internal oblique and rectus abdominis (EO>IO>RA).

Discussion

This study aimed to compare the effects of 2 abdominal contraction techniques – abdominal bracing and abdominal hollowing – on abdominal muscle thickness and activation during static and dynamic stabilization tasks in healthy adults. The main results showed that abdominal hollowing elicited significantly greater increases in the thickness and activation of the internal oblique (IO) and transversus abdominis (TrA) muscles, whereas abdominal bracing predominantly increased the thickness of the rectus abdominis (RA). In addition, significant differences in muscle activation were observed between techniques during the static hook-lying task, but not during the dynamic toe-tap task. These findings suggest that these 2 stabilization strategies induce distinct neuromuscular responses, and that task characteristics substantially influence the recruitment patterns of abdominal muscles. Interestingly, regardless of the contraction technique used, a consistent muscle activation hierarchy (external oblique >internal oblique >rectus abdominis) was observed. This finding suggests that while abdominal bracing and hollowing can modify the magnitude of activation in individual muscles, the fundamental coordination pattern for trunk stabilization is primarily determined by the mechanical demands of the task itself.

The greater activation and thickness of the IO and TrA during the hollowing maneuver are consistent with previous studies reporting that this technique facilitates selective recruitment of the deep stabilizing muscles [9,13,19]. Hodges and Richardson [9] first demonstrated that the TrA is activated in a feed-forward manner prior to limb movement, emphasizing its anticipatory role in postural control. This deep muscle activation contributes to spinal stiffness and intersegmental stability by providing pre-emptive tension to the thoracolumbar fascia. Teyhen et al [13] and Hides et al [19] later confirmed that ultrasound-guided training with the abdominal hollowing maneuver enhances deep muscle control and symmetry, which are often impaired in individuals with chronic low back pain. In this study, the hollowing maneuver selectively activated the IO and TrA without excessive involvement of the superficial muscles, suggesting that it primarily targets the local stabilizing subsystem. This finding supports that use of abdominal hollowing can be useful as a therapeutic strategy for restoring motor control and improving spinal segmental stability in early rehabilitation contexts. To further evaluate the clinical relevance of these findings beyond simple p-values, the magnitude of the differences between the 2 techniques was analyzed using effect sizes.

Beyond statistical significance, the effect sizes observed in this study provide additional insight into the functional distinctions between the 2 techniques. Hollowing demonstrated a moderate-to-large effect on IO thickness (d=0.79) and a very large effect on TrA thickness (d=1.23), indicating strong selective recruitment of the deep stabilizing musculature. In contrast, bracing produced a moderate effect on RA thickness (d=0.58), supporting its role in enhancing global abdominal co-contraction. The EO showed a negligible effect size (d=0.09), reinforcing that this muscle contributes minimally to differentiating the 2 maneuvers. Collectively, these effect-size findings indicate that hollowing is associated with greater effects on deep muscle morphology, whereas bracing appears to primarily affect the superficial abdominal musculature.

In contrast, the abdominal bracing technique induced greater thickening of the RA and overall co-activation of the abdominal wall, consistent with prior research describing bracing as a global stabilization strategy [7,8,11]. Grenier and McGill [7] quantified trunk stiffness and demonstrated that bracing generates higher intra-abdominal pressure (IAP) by co-activating both deep and superficial muscles. Vera-Garcia et al [11] found that this increased co-contraction enhances resistance to externally applied loads, while McGill [8] emphasized its importance for maintaining spinal stability during functional movements and lifting tasks. In the present study, RA thickness increased significantly during bracing, suggesting that this technique contributes to global spinal support by recruiting the superficial stabilizers. Recent scoping reviews have suggested that abdominal hollowing and abdominal bracing can elicit different neuromuscular strategies depending on task demands and clinical context, rather than demonstrating a clear superiority of one technique over the other [22]. Physiologically, this can be attributed to the elevation of IAP and improved load transfer through enhanced muscular stiffness. These findings support the concept that bracing may be more effective under conditions requiring whole-trunk stabilization or higher mechanical demand.

Interestingly, the differences between techniques disappeared under the dynamic toe-tap condition. This observation indicates that as task complexity and external load increase, the 2 techniques converge in terms of their stabilization outcomes. During dynamic limb movement, trunk stability is challenged by continuous changes in center of mass and load direction, necessitating simultaneous activation of global and local muscles regardless of the intentional strategy. Similar results were reported by Koh et al [17], who found that the discriminative activation pattern between bracing and hollowing diminishes during functional tasks. Likewise, Arab and Chehrehrazi [15] demonstrated that both maneuvers increase abdominal muscle activity, although the recruitment patterns differ depending on the level of pelvic and trunk control. This pattern implies that in high-demand tasks, the nervous system recruits deep and superficial muscles to ensure sufficient spinal control, potentially reducing the distinction between selective activation strategies. From the perspective of motor control, this convergence may reflect an automated reorganization of muscle synergies, whereby anticipatory control strategies become less consciously differentiated as task demands increase. Under dynamic conditions, the central nervous system may prioritize global stability by recruiting coordinated deep and superficial muscle activation patterns to ensure robust trunk control, regardless of the intentional contraction strategy.

The distinct responses observed in static versus dynamic conditions highlight the task-dependent nature of core muscle recruitment. In static postures, where stability is maintained primarily through tonic control, selective activation of deep stabilizers such as the IO and TrA plays a dominant role. However, in dynamic conditions involving limb motion or load transfer, synergistic activation of global muscles, including the RA and EO, becomes necessary to generate sufficient torque and stiffness. Therefore, training programs aiming to improve trunk stability should consider the interplay between muscle layers and select appropriate exercises according to movement complexity and stability requirements.

From a clinical standpoint, the results suggest that abdominal hollowing may be more suitable during the early stages of rehabilitation, where the goal is to retrain deep muscle coordination and restore motor control. As patients progress, abdominal bracing can be incorporated to enhance overall trunk stiffness and prepare for functional activities involving greater loads. This progressive approach aligns with the model proposed by McGill [8], in which selective deep muscle activation precedes integrated co-contraction training to optimize spinal stability. The complementary use of both techniques may therefore provide an efficient and safe pathway from early rehabilitation to advanced performance conditioning.

Although both techniques have distinct benefits, their application should be individualized based on the patient’s condition, pain level, and movement control ability. For instance, hollowing can be emphasized in individuals with lumbar instability or motor control deficits, while bracing may be preferable for those engaging in high-intensity physical tasks or requiring enhanced trunk stiffness, such as athletes or manual workers. Furthermore, combining both techniques within a structured stabilization program could facilitate a balanced development of local and global muscle systems, contributing to improved functional performance and injury prevention.

Several limitations should be acknowledged. The participants were healthy young adults without musculoskeletal impairment, limiting the generalizability of these findings to clinical populations. However, rather than simply determining whether differences exist, clarifying how abdominal bracing and hollowing differentially influence muscle thickness and activation under static and dynamic task demands is essential for establishing task- and stage-specific clinical guidelines. Future longitudinal studies are warranted to determine whether repeated practice of hollowing or bracing leads to adaptive changes in muscle strength, endurance, and motor control. The present study measured muscle thickness and surface EMG activity only in selected abdominal muscles, while other key stabilizers such as the multifidus, diaphragm, and pelvic floor were not included. In addition, although natural breathing was permitted to reflect functional movement conditions, uncontrolled variations in breathing patterns may have influenced trunk muscle activation, which limits a comprehensive interpretation of coordinated lumbopelvic stabilization. These muscles play crucial roles in lumbopelvic stability and should be considered in future research to better understand the integrated core stabilization mechanism. Although a post hoc power analysis was performed, a priori sample size calculation was not conducted, as this study was designed as an exploratory crossover experiment rather than a confirmatory clinical trial. Therefore, the findings should be interpreted with appropriate caution.

In summary, the findings of this study suggest that abdominal hollowing preferentially activates deep stabilizing muscles, whereas abdominal bracing is associated with enhanced global co-contraction. The effects of these techniques differ depending on the task, with greater differentiation observed in static conditions. Clinically, combining both techniques in a staged manner may offer optimal outcomes for improving trunk stability, motor control, and injury prevention in both healthy individuals and those with spinal disorders.

Conclusions

This study demonstrated that abdominal hollowing and abdominal bracing produce distinct activation patterns of the abdominal muscles during stabilization tasks. The hollowing technique selectively increased the thickness and activation of the deep stabilizing muscles the internal oblique and transversus abdominis, while the bracing technique predominantly enhanced the rectus abdominis thickness, reflecting global co-contraction of the abdominal wall. These findings confirm that hollowing is associated with segmental control and fine motor stability, while bracing enhances trunk stiffness and load transfer.

The differentiation between techniques was more apparent during static conditions, where selective deep muscle recruitment is possible, whereas under dynamic conditions both techniques exhibited comparable levels of activation, suggesting that higher stabilization demands require integrated recruitment of global and local muscles.

From a clinical perspective, these results support the application of abdominal hollowing during the early stages of rehabilitation for retraining deep stabilizing muscles and improving motor control. Abdominal bracing, in contrast, can be implemented in later stages or in functional training where greater trunk stiffness and global stability are required. Integrating both techniques into a progressive, task-specific stabilization program may therefore provide the most effective approach to improving trunk stability, preventing recurrent low back pain, and optimizing functional performance.

Future studies should investigate long-term training adaptations, explore populations with spinal or pelvic dysfunctions, and incorporate additional stabilizing muscles such as the multifidus and diaphragm to provide a more comprehensive understanding of trunk stabilization mechanisms.