Dynamic versus fixed pillow height systems and cervical muscle function: a randomized crossover trial

Abstract

BACKGROUND

Pillow height and mechanical support influence cervical alignment and neuromuscular load. However, the physiological effects of dynamically adjustable pillow systems remain poorly defined.

AIM

To compare the effects of fixed and dynamically adjustable pillow height inserts on cervical muscle oxygenation, neuromuscular modulation, and comfort in healthy young adults.

DESIGN

Randomized crossover trial.

SETTING

Controlled laboratory setting simulating sleep-related postures.

POPULATION

Thirty healthy adults (18-45 years) without cervical or musculoskeletal disorders.

METHODS

Participants underwent six pillow conditions: no pillow (NP), non-adjustable polyester pillow (NA), three fixed inserts (memory foam [MF], latex [LT], polyester fiber [PE]), and a dynamically adjustable air-based (DA). Muscle oxygen saturation (SmO₂) of the sternocleidomastoid (SCM) and upper trapezius (UT) was assessed using near-infrared spectroscopy. Surface electromyography (EMG) was analyzed via root mean square (RMS) and zero-crossing (ZC) slopes, representing adaptive motor unit recruitment and firing modulation.

RESULTS

Repeated-measures ANOVA revealed a significant pillow effect for SmO₂ (SCM: η²P=0.24; UT: η²P=0.14, P<0.05). DA and PE inserts showed superior SmO₂ across phases, with DA yielding the highest levels and fastest reoxygenation. Both DA and PE conditions exhibited steeper RMS and more negative ZC slopes during activity, indicating more efficient neuromuscular adaptation. Comfort ratings were highest for DA (VAS 75.52±16.94 mm).

CONCLUSIONS

DA pillows may enhance cervical oxygenation, adaptive neuromuscular control, and comfort compared with fixed-height systems.

CLINICAL REHABILITATION IMPACT

DA pillows may provide individualized ergonomic support to improve cervical alignment and comfort.

Introduction

Sleep quality and spinal health are strongly connected, with pillow design playing a key role in preserving proper cervical alignment and reducing musculoskeletal strain. The cervical spine is particularly vulnerable to mechanical stress from prolonged static sleep postures, which can lead to muscle fatigue and postural adaptation over time. Prior studies emphasize the impact of pillow height, material properties, and structural design on cervical muscle activity, spinal posture, and perceived sleep quality.^1-3 Supporting the natural cervical curve is essential for reducing strain on the neck and upper back, while inadequate support has been linked to discomfort and dysfunction.^{4, 5} Addressing these ergonomic factors improves comfort and biomechanics and contributes to preventive musculoskeletal care, an emerging priority in public health.

Pillow height directly influences cervical mechanics and neuromuscular responses. Both overly high and low pillow heights can misalign the cervical spine, increasing neuromuscular load, and promoting chronic discomfort.^{1, 5} Improper height may tilt the neck forward or backward, disrupting curvature⁶ and redistributing mechanical load across the neck and shoulder regions.⁵ Electromyography (EMG) studies show that poorly fitted pillows elevate activity in the sternocleidomastoid (SCM) and upper trapezius (UT), indicating increased neuromuscular effort.^{7, 8} In contrast, a height that matches the individual’s body structure supports spinal neutrality and reduces muscular stress. Jiao et al. (2024)⁹ demonstrated that aligning pillow height to individual shoulder width minimizes cervical muscle activation, reinforcing the importance of personalized support, a principle central to dynamically adjustable pillow systems.

To accommodate diverse sleep postures, modern pillows employ modular inserts made of memory foam, latex, or polyester fiber. However, these static configurations often fail to adapt to frequent positional changes, as adults typically shift over 20 times per night, resulting in compromised alignment.¹⁰ Dynamically adjustable air-based (DA) provide a promising alternative by allowing real-time manual regulation of height and firmness without electronic components. Although such designs have been patented,^{11, 12} little is known about their physiological effects on cervical muscle performance or circulation. Direct comparisons between DA and conventional fixed-height systems remain scarce, despite their contrasting principles of support and adaptability.

Most existing sleep ergonomics research emphasizes mechanical alignment or EMG-based analyses, while physiological responses such as muscle oxygenation (SmO₂) and their integration with EMG slope analysis remain largely unreported in pillow studies. SmO₂ reflects local perfusion and muscular demand, making it a valuable indicator of ergonomic efficiency.¹³ Perceived comfort is similarly multifactorial and depends on cervical support, pressure relief, and muscle relaxation, all of which impact sleep quality.^{2, 3}

Near-infrared spectroscopy (NIRS) offers a non-invasive, continuous measure of local tissue oxygenation and hemodynamics, providing insight into muscle perfusion during postural activity.¹⁴ When combined with EMG, which quantifies electrical muscle activation, this dual-modality approach enables simultaneous assessment of metabolic and neuromuscular aspects of function. Geršak and Geršak¹⁴ reported that concurrent NIRS and EMG recordings were able to detect the onset of voluntary isometric contraction and associated changes in SmO₂, supporting the potential complementary use of these techniques under controlled conditions. More recently, Song et al.¹⁵ demonstrated that progressive alterations in EMG parameters and NIRS-derived tissue oxygenation indices occur during repetitive submaximal contractions, with significant interrelationships between electrophysiological and metabolic responses during fatigue development, further supporting the integrative value of combining these modalities.

The present study applies this integrative approach to evaluate the effects of pillow design on cervical muscle physiology. Six configurations, including fixed inserts (memory foam [MF], latex [LT], polyester fiber [PE]), DA, non-adjustable (NA) pillow control, and no-pillow (NP) control, were assessed using near-infrared spectroscopy (NIRS) and EMG slope analysis in a randomized crossover design. We hypothesized that DA systems would enhance cervical SmO₂, promote adaptive neuromuscular modulation characterized by EMG slope analysis, and improve subjective comfort compared with fixed-height systems. By integrating physiological and perceptual outcomes, this study provides a comprehensive framework for evidence-based ergonomic pillow design, potentially informing personalized rehabilitation and sleep-health interventions.

Materials and methods

Ethical approval and clinical trial registration

All procedures involving human participants were conducted in accordance with the ethical standards of the institutional research committee and the 1964 Declaration of Helsinki, as revised and amended. The study was approved by the institutional ethics committee prior to the start of the experiment and conducted in compliance with the principles outlined in the Declaration of Helsinki. Ethical approval was granted by the Human Ethics Committee of Burapha University (Protocol No. HS078/2567(C2); Approval No. IRB1-116/2567; approval date: October 15^th, 2024; Chairperson: Assoc. Prof. Dr. Witawat Jangiam). The trial was prospectively registered with the Thai Clinical Trials Registry (TCTR20250130005) on 30^th January 2025, in accordance with ICMJE guidelines. Written informed consent was obtained from all individual participants prior to their inclusion in the study.

Study design

This randomized crossover study utilized a within-subject, repeated-measures experimental design to assess the impact of different pillow height adjustment mechanisms on cervical SmO₂, neuromuscular activation, and subjective comfort. The study design, analysis, and reporting followed CONSORT 2010 guidelines and the CONSORT extension for crossover trials, with detailed documentation of randomization, washout periods, and sequence control to minimize carryover and period effects.¹⁶ All participants completed six pillow conditions presented in a randomized order using concealed block randomization. The random allocation sequence was generated by an independent researcher using computer-generated block allocation (block size = 6), with assignments concealed in sequentially numbered opaque envelopes opened before each session. The six configurations included NP, NA, and four hybrid pillows with insert-based adjustments: LT, MF, PE, and DA. Participants were enrolled by a study coordinator blinded to allocation, and assignment was handled by a separate investigator uninvolved in assessments. Outcome assessors were blinded to pillow type, as all inserts, including air-based and fixed pads, were fully enclosed within a double-shell main pillow, there by concealing their identities.

Participants

Thirty healthy adults (aged 18-45 years) with no history of neck pain or musculoskeletal disorders were enrolled. Inclusion criteria included: absence of neck pain, office syndrome, or frozen shoulder in the past 3 months; no history of cervical spondylosis; ability to lie supine for 30 minutes without discomfort; BMI between 18.00-24.99 kg/m²; and willingness to complete all study procedures. Exclusion criteria included: cervical trauma, spinal infection, surgery, congenital spinal deformities, blood disorders affecting SmO₂, use of heart rate-altering medications, tobacco or vape use, and allergy to tape or latex. Participants abstained from vigorous exercise and alcohol for 24 hours, and caffeine for 6 hours, prior to testing. All participants provided written informed consent. This protocol was limited to healthy adults to isolate device effects.

Sample size estimation was based on pilot data comparing SCM muscle SmO₂ across two pillow conditions, yielding mean±SD values of 52.12±17.67 and 88.62±19.12. Using an effect size of 0.426, α=0.05, and power = 0.80, the required sample size was calculated to be 25. To account for potential data loss or incomplete recordings, 30 participants were recruited.

Pillow conditions

All test configurations used a standardized hybrid pillow (OrthoCore^™, mr.big^®) featuring a dual-shell structure filled with Elasta Fiber^™ polyester and enclosed in a 100% cotton cover. The pillow measured 19 × 29 inches with a total unloaded height of 6 inches (approximately 3 inches per shell). Each inner shell contained a modular pocket designed for insert placement to provide adjustable support. The two shells were joined by an external zip closure, forming an identical double-layer enclosure across all conditions to ensure consistent tactile and visual characteristics (Figure 1). The manufacturer provided the pillow but was not involved in study design, execution, or analysis.

Figure 1.

—Schematic of pillow configurations and insert pad placement.

Mechanical properties were characterized by quantifying pillow firmness as the compressed height under a 12 N static load applied centrally using a flat circular acrylic plate (diameter = 12 cm) for 3 min. Each condition was tested three times, and the mean value was recorded as an indicator of relative mechanical compliance.

The average compressed heights (measured for one insert pad within the hybrid shell) were as follows (mean±SD): NA (no insert pad), 4.7±0.2 in; MF, 5.5±0.2 in; LT, 5.5±0.1 in; PE, 5.7±0.2 in. The DA insert provided a variable height range of 5.0-8.5 in, depending on the participant’s selected inflation level.

All configurations shared the same hybrid shell and external materials to eliminate visual and tactile bias during testing.

Insert pad conditions

Three insert-based conditions used LT, MF, or PE pads, each ~1.5 inch in height and shaped to fit securely within the pillow shell. Participants selected one or two pads during a pre-test fitting to ensure comfort, allowing for user-specific internal height adjustment while preserving consistent external pillow dimensions.

Dynamically adjustable air-based (DA) condition

The DA condition (Thai patent application No. 2603001301) used an inflatable insert with two longitudinal air chambers supporting the cervical curve and cranial region, each linked to a manual inflation bulb, control valve, and pressure gauge for precise, real-time adjustment. Housed in a non-elastic textile sleeve with a central seam, the chambers maintained alignment and airflow separation. The research team operated the system under technical oversight (K.K. and S.W.). Air pressure was checked before and after each trial, maintaining deviations within 5 mmHg, and all values were logged for reproducibility.

Pillow height for all insert-based conditions was adjusted before testing based on participant comfort after a 15-minute familiarization period, then kept constant during data collection.

Experimental procedure

To enhance clinical relevance, a three-phase protocol (rest, active, and recovery) was employed to capture neuromuscular modulation over time. Unlike prior studies assessing only resting muscle function,^{1, 2} our design simulated low-load cervical demands seen in real life sleep postures and presleep behaviors. The rest phase established baseline SmO₂ and EMG activity. The active phase used low-intensity wrist extension to simulate cervical muscle engagement during activities like reading or smartphone use before sleep. The recovery phase assessed reoxygenation and neuromuscular normalization. This time-based protocol, supported by evidence that such activities induce cervical fatigue within minutes,¹⁷ enabled more nuanced interpretation of adaptive muscular responses compared to static assessments.

Each condition was conducted with participants in a standardized supine position on a physical therapy treatment table, with arms resting alongside the torso. Pillow height for each condition was adjusted based on the participant’s comfort to support the cervical curve and head. Shoulder and head positions were marked to maintain alignment across trials. Posture was standardized using a manual goniometer and positional markers to ensure reproducible alignment across trials. The lower edge of each pillow was aligned with the level of the C7 spinous process, and the pillow’s midline was centered along the participant’s sagittal axis between the shoulders. Adhesive stickers were placed on the bed surface to mark anatomical landmarks at the head, shoulders, elbows, wrists, hips, knees, and ankles, enabling precise repositioning between conditions. Because pillow height was an experimental variable, the cranio-cervical angle was allowed to vary naturally, while overall head-neck and body alignment were maintained consistently throughout testing.

Each session began with five breathing cycles and gentle isometric head pressure to standardize posture. The protocol included a 5-minute initial rest (IR), 90 seconds of low-load wrist extension (active phase, A) with a 0.25 kg dumbbell at 30 bpm to simulate cervical demands during pre-sleep activities (e.g., reading or smartphone use), and a 3-minute recovery phase (post-rest phase, PR). Participants were instructed to maintain focus on a fixed visual point to avoid head and cervical movement and alternated backward counting with the examiner to prevent drowsiness while preserving a relaxed state. To reduce carryover effects, participants transitioned to a seated position for 1 minute, walked on a treadmill for 1 minute, and then sat for another minute. This washout phase helped normalize circulation and neuromuscular state in the SCM and UT muscles. Given the low-load nature of the wrist extension task and its indirect effect on cervical muscles, posture change combined with light ambulation provided sufficient physiological reset to prevent carryover effects between conditions.¹⁸ This 3-minute duration was further supported by previous evidence indicating that cervical and shoulder SmO₂ and EMG activity typically return to baseline within approximately 2-3 minutes following low-load or postural contractions.^{13, 15} All six pillow conditions were tested in a single session using a randomized order, with consistent electrode placement and controlled environment (22-25 °C, dim lighting, minimal noise) to ensure data quality. Data were collected in February 2025 at the Physical Therapy Laboratory, Faculty of Allied Health Sciences, Burapha University, Thailand.

Measures

Muscle oxygenation

SmO₂ was continuously recorded using a wireless NIRS device (Moxy, Fortiori Design LLC, Hutchinson, MN, USA), which quantifies the balance between local oxygen delivery and utilization in skeletal muscle microvasculature.^19-21 Before sensor placement, skin was cleaned with 70% isopropyl alcohol to improve optical coupling. Sensors were positioned on the left side at the mid-belly of the UT and one-third the distance from the sternal notch to the mastoid process for the SCM, aligned parallel to muscle fibers and mirroring EMG electrode sites. Sensors were affixed using medical-grade tape and covered with an opaque black wrap to prevent light interference. SmO₂ was sampled at 2 Hz and collected continuously across all testing phases. Devices were calibrated before each session per manufacturer instructions. Data were exported to Excel and averaged over 10-second windows at 1-minute intervals during the IR1, IR2, IR3, IR4, IR5 and PR1, PR2, PR3 phases, and every 30 seconds during A1, A2, A3 phase. SmO₂ has been validated as a reliable, non-invasive marker of muscle oxidative metabolism during rest and exercise,²¹ with the Moxy system demonstrating strong test-retest reliability (ICC: 0.79-0.92; SEM: 5-9%).^{20, 22}

Electromyographic data

Surface EMG signals were acquired from the SCM and UT muscles on the right side using TELEMyo DTS telemetry (Noraxon Inc., Scottsdale, AZ, USA). The right-sided placement was selected to minimize ECG interference, which can contaminate EMG recordings on the left side due to proximity to cardiac electrical activity.²³ To prevent cross-talk and ensure complementary physiological measurements, EMG and NIRS were recorded contralaterally (EMG right, NIRS left). Participant dominance was determined by self-reported handedness (27 right, 3 left). Baseline analysis confirmed no significant differences in root mean square (RMS) or SmO₂ between dominance groups (P>0.10), indicating no bias in side selection.

The sampling rate was set to 1500 Hz, and data acquisition and signal processing were conducted using MyoResearch XP Master Edition software (v3.8.3). Electrode placement followed SENIAM recommendations,²⁴ using bipolar Ag/AgCl Blue Sensor electrodes (Ambu A/S, Ballerup, Denmark) with a 20-mm interelectrode distance. Electrodes were positioned at one-third the distance from the sternal notch to the mastoid process (SCM) and midway between the acromion and the C7 spinous process (UT). The skin was shaved, lightly abraded, and cleaned with 70% isopropyl alcohol to reduce impedance. Electrodes were affixed using 3M Transpore medical adhesive tape for signal stability. A high-resolution 3D video camera (Intel^® RealSense^™ Front F200, Dell Inc., USA) was positioned to monitor postural consistency during testing.

Neuromuscular modulation was assessed using the Frequency/Fatigue Analysis module in MyoResearch XP (v3.8.3). Unlike static, single-point amplitude analysis, this time-domain approach provides greater sensitivity for detecting dynamic changes in motor unit recruitment and firing frequency over time. It offers a more nuanced understanding of cervical muscle adaptation across different pillow conditions, beyond simple amplitude responses. The RMS slope quantified cumulative motor unit recruitment, while the zero crossing (ZC) slope reflected changes in firing frequency over time.²⁵ Both signals were band-pass filtered (20-500 Hz); RMS data were full-wave rectified and smoothed using a 150 ms moving window. Slopes were calculated at 1-second intervals within each of the three testing phases: IR (5 min), A (90 s), and PR (5 min), with each phase analyzed separately. This approach is particularly suited for low-load, sustained postural conditions such as cervical stabilization during sleep or relaxation prior to sleep onset, where muscular responses evolve over time rather than remaining static.^{26, 27}

Perceive comfort

Subjective comfort was assessed using a 100-mm Visual Analog Scale (VAS), where 0 mm indicated “not comfortable at all” and 100 mm signified “extremely comfortable.” Participants rated comfort immediately after each condition, based on the session experience. This validated method is widely used in ergonomic studies^7-9 and shows strong reliability (ICC>0.80).²⁸

Video recordings were reviewed to confirm posture stability and phase timing. EMG and NIRS signals were inspected for artifacts; trials with excessive noise or dropout were excluded. A blinded researcher extracted data using coded video files. Pillow appearances were indistinguishable across insert-based conditions to ensure masking.

Statistical analysis

A two-way repeated-measures analysis of variance (ANOVA) was conducted to evaluate the effects of pillow condition and time on neuromuscular modulation indices (RMS and ZC slopes) and SmO₂. Subjective comfort ratings (VAS) across pillow conditions were assessed using a one-way repeated-measures ANOVA. Mauchly’s test of sphericity was performed for all repeated-measures ANOVA models. When the assumption of sphericity was violated (P<0.05), the Greenhouse-Geisser correction was applied to adjust the degrees of freedom. Corrected P values and partial eta squared (η²P) were reported accordingly. When significant main effects or interactions were identified, Bonferroni-adjusted post hoc tests were used to identify specific pairwise differences. Mean differences and 95% confidence intervals were reported. Statistical significance was set at α=0.05. Effect sizes were estimated using partial η²P. Trials with missing EMG or SmO₂ data were excluded listwise from their respective analyses; no imputation was performed. All participants were retained in the dataset, with only trials containing complete EMG and SmO₂ data included in the final statistical analysis.

Results

A total of 30 participants (76.7% female) were enrolled, with a mean age of 20.0±1.3 years, BMI of 21.3±2.0 kg/m², body weight of 58.3±7.1 kg, and height of 165.4±7.1 cm. No participants were withdrawn due to lack of relaxation or attention, and the trial was completed as planned without premature termination. Participants were monitored throughout each condition for signs of discomfort or adverse effects. No harms were reported, and all participants completed the study without complications. SmO₂ data for both the SCM and UT were analyzed from all participants. EMG data were analyzed for 28 participants (SCM) and 26 participants (UT), with exclusions due to incomplete or poor-quality signals. The detailed participant flow and exclusion process are presented in Supplementary Digital Material 1 (Supplementary Figure 1).

Muscle oxygenation (SmO₂)

A two-way repeated-measures ANOVA revealed a significant main effect of pillow condition on SmO₂ for both SCM (F=9.01, P<0.001, η²P=0.24) and UT (F=4.67, P=0.001, η²P=0.14). Time also significantly influenced SmO₂ (SCM: F=65.47, P<0.001, η²P=0.69; UT: F=32.91, P<0.001, η²P=0.53). The condition × time interaction was not significant (P>0.05), indicating consistent response trends across phases (Table I).

Table I. —Mean of muscle oxygen saturation (SmO₂).

Phase		Mean±SD						Pillow effect	Time effect	Pillow x time
		No pillow	No adjust	Memory foam	Latex	Polyester	Dynamic air-based
SCM SmO2 (%); (N.=30)
Initial Rest	IR1	69.32±7.45	70.75±7.23	71.77±7.14	71.72±6.85	71.38±6.79	71.49±8.29	F=9.01 P<0.001* η2P=0.24	F=65.47 P<0.001* η2P=0.69	F=1.17 P=0.20 η2P=0.04
	IR2	70.23±7.70	72.79±6.95	73.05±6.72	72.91±6.74	73.34±6.39	73.30±7.78
	IR3	71.96±7.62	74.62±6.62	74.19±6.93	74.33±6.95	75.11±6.36	74.83±7.61
	IR4	72.93±7.51	76.61±5.68	75.53±6.69	75.62±7.01	76.22±6.12	76.33±7.43
	IR5	73.47±6.80	77.23±5.70	76.45±6.60	76.69±6.25	77.09±5.56	76.73±7.19
Active	A1	75.12±6.50	78.57±5.36	78.16±6.69	78.80±5.50	79.64±5.41	79.01±6.93
	A2	75.01±6.69	78.26±5.48	77.79±6.86	78.97±5.68	79.49±5.46	78.63±7.65
	A3	75.12±7.13	77.83±5.51	77.27±6.91	78.53±6.02	78.96±6.01	78.16±7.86
Post Rest	PR1	75.16±6.54	78.08±5.08	77.78±6.57	77.98±5.61	78.66±5.43	78.89±7.41
	PR2	74.11±6.90	77.81±5.05	77.79±5.91	77.95±5.85	78.46±5.61	78.43±7.06
	PR3	74.41±6.95	78.22±5.37	78.05±6.15	78.26±5.50	75.60±10.06	78.95±6.61
UT SmO2 (%); (N.=30)
Initial Rest	IR1	72.41±12.03	75.05±11.15	75.21±10.46	74.68±10.58	75.60±10.06	75.35±10.34	F=4.67 P=0.001* η2P=0.14	F=32.91 P<0.001* η2P=0.53	F=0.88 P=0.70 η2P=0.03
	IR2	73.21±11.40	76.02±10.38	75.99±9.79	74.99±9.94	76.97±8.93	76.21±9.51
	IR3	74.51±10.36	77.03±9.84	77.14±9.56	76.27±9.68	78.23±8.81	77.67±9.00
	IR4	76.36±9.42	79.07±8.71	78.36±9.23	77.27±9.09	78.99±8.50	78.23±8.71
	IR5	77.21±9.81	79.61±8.41	79.15±8.66	77.96±8.97	79.65±8.32	79.50±8.33
Active	A1	78.54±8.49	81.85±7.18	81.41±7.71	80.89±7.65	82.71±6.98	82.05±7.78
	A2	78.23±8.84	80.97±8.12	80.59±7.98	80.34±8.40	82.09±7.77	81.63±8.44
	A3	78.03±9.33	80.23±8.33	79.73±8.23	79.39±9.02	81.25±8.28	80.77±8.78
Post Rest	PR1	78.69±9.04	80.11±8.82	79.65±8.68	78.62±9.00	80.80±8.54	79.95±9.28
	PR2	78.05±8.69	80.24±8.78	79.96±8.08	78.43±9.47	80.21±8.96	79.83±8.55
	PR3	78.49±8.33	80.61±8.36	80.05±8.48	79.00±9.30	81.06±8.46	80.87±8.18

*Two-way repeated-measures ANOVA tested main effects of condition and time. Effect sizes are reported as partial eta squared (η²P). Statistically significant values indicated by P<0.05.

Phase-wise analysis showed that DA and PE achieved the earliest and most sustained rise in SmO₂, peaking by the third minute of the rest phase and maintaining above-baseline levels during recovery. UT responses mirrored SCM patterns, with DA consistently supporting superior oxygenation during both active and post-rest phases (Figure 2, 3).

Figure 2.

—Time-course changes in sternocleidomastoid (SCM) muscle oxygen saturation (SmO₂) for each pillow condition compared to the no-pillow (NP) control. A-E) SmO₂ (%) across all time points (IR1-IR5, A1-A3, PR1-PR3): A) non-adjustable (NA); B) memory foam (MF); C) latex (LT); D) polyester fiber (PE); E) dynamic air-based (DA). Error bars represent ±SD. *Indicates a statistically significant difference (P<0.05) in time comparisons within each condition: 1 = vs. IR1, 2 = vs. IR2, 3 = vs. IR3, 4 = vs. IR4, 5 = vs. IR5. ^#Indicates a statistically significant difference (P<0.05) between the pillow condition and NP at each time point. IR: Initial Rest; A: Active; PR: Post-Rest.

Figure 3.

—Time-course changes in upper trapezius (UT) muscle oxygen saturation (SmO₂) for each pillow condition compared to the no-pillow (NP) control. A-E) SmO₂ (%) across all time points (IR1-IR5, A1-A3, PR1-PR3): A) non-adjustable (NA); B) memory foam (MF); C) latex (LT); D) polyester fiber (PE); E) dynamic air-based (DA). Error bars represent ±SD. *Indicates a statistically significant difference (P<0.05) in time comparisons within each condition: 1 = vs. IR1, 2 = vs. IR2, 3 = vs. IR3, 4 = vs. IR4, 5 = vs. IR5. ^#Indicates a statistically significant difference (P<0.05) between the pillow condition and NP at each time point. IR: Initial Rest; A: Active; PR: Post-Rest.

Electromyographic analysis

Neuromuscular modulation, evaluated through time-based RMS and ZC slopes, exhibited significant phase-dependent changes across all conditions (P<0.05), with moderate-to-large effect sizes (SCM: RMS η²P=0.25, ZC η²P=0.45; UT: RMS η²P=0.22, ZC η²P=0.26) (Table II).

Table II. —Mean±SD values for neuromuscular fatigue indices (RMS and ZC slopes).

Muscle	Pillow condition	Mean±SD			Pillow effect	Time effect	Pillow x time
		IR	A	PR
RMS slope (μV/s)
SCM (N.=28)	No pillow	-0.0003±0.0024	0.0052±0.0150	0.0001±0.0043	F=0.77 P=0.570 η2P=0.03	F=7.13 P=0.025* η2P=0.25	F=0.49 P=0.900 η2P=0.02
	No adjust	-0.0005±0.0032	0.0020±0.0163	-0.0040±0.0125
	Memory foam	-0.0003±0.0039	0.0080±0.0159	-0.0012±0.0054
	Latex	-0.0010±0.0035	0.0069±0.0184	-0.0013±0.0047
	Polyester	0.0001±0.0040	0.0071±0.0201	-0.0027±0.0070
	Dynamic air-based	-0.0004±0.0034	0.0058±0.0144	-0.0022±0.0113
UT (N.=26)	No pillow	-0.0015±0.0035	0.0077±0.0256	0.0016±0.0055	F=0.65 P=0.670 η2P=0.02	F=7.97 P=0.001* η2P=0.22	F=0.75 P=0.670 η2P=0.03
	No adjust	-0.0010±0.0041	0.0086±0.0310	-0.0032±0.0103
	Memory foam	-0.0007±0.0032	0.0104±0.0328	0.0002±0.0040
	Latex	-0.0006±0.0045	0.0039±0.0252	-0.0010±0.0034
	Polyester	0.0002±0.0028	0.0130±0.0263	-0.0018±0.0079
	Dynamic air-based	-0.0008±0.0035	0.0158±0.0451	-0.0019±0.0077
ZC slope (crossings/s2)
SCM (N.=28)	No pillow	-0.0070±0.0509	-0.0983±0.2065	-0.0291±0.0660	F=0.78 P=0.560 η2P=0.03	F=22.37 P<0.001* η2P=0.45	F=1.32 P=0.220 η2P=0.05
	No adjust	0.0150±0.0544	-0.0986±0.2153	-0.0264±0.1824
	Memory foam	-0.0108±0.0343	-0.1286±0.1861	-0.0057±0.0617
	Latex	0.0196±0.0439	-0.0906±0.2107	0.0076±0.0773
	Polyester	-0.0094±0.0472	-0.1894±0.2462	0.0328±0.0771
	Dynamic air-based	-0.0042±0.0406	-0.1294±0.2184	0.0119±0.1156
UT (N.=26)	No pillow	0.0055±0.0237	-0.0230±0.1444	-0.0170±0.0464	F=2.03 P=0.080 η2P=0.10	F=6.43 P<0.001* η2P=0.26	F=1.71 P=0.080 η2P=0.09
	No adjust	0.0020±0.0242	-0.0052±0.1811	-0.0089±0.0355
	Memory Foam	0.0076±0.0331	-0.0984±0.1300	-0.0215±0.0582
	Latex	0.0125±0.0371	-0.0911±0.1798	0.0020±0.0451
	Polyester	0.0024±0.0374	-0.1202±0.2026	-0.0148±0.0721
	Dynamic air-based	-0.0015±0.0238	-0.0614±0.1424	-0.0104±0.0375

Two-way repeated-measures ANOVA assessed main effects of pillow condition, time, and interaction on RMS and ZC slopes. Effect sizes are partial eta squared (η²P). *Statistically significant at P<0.05. IR: initial rest; A: active; PR: post-rest; SCM: sternocleidomastoid; UT: upper trapezius; RMS: root mean square; ZC slope: zero crossing slope.

During the active phase, RMS slopes increased significantly under MF, LT, and DA for SCM, and PE and DA for UT (P<0.05), reflecting elevated motor unit recruitment in response to the low-intensity task. ZC slopes concurrently declined in all support conditions except NP (P<0.05), suggesting an adaptive reduction in firing frequency consistent with efficient neuromuscular modulation (Figure 4).

Figure 4.

—RMS and ZC slope comparisons of muscle activity in the sternocleidomastoid (SCM) and upper trapezius (UT) across pillow conditions and test phases: A) RMS slope value of SCM; B) RMS slope of UT; C) ZC slope value of SCM; D) ZC slope value of UT. Bars represent mean±SD during initial rest (IR), active (A), and post-rest (PR) phases. *Indicates a statistically significant difference (P<0.05) between phases within the same pillow condition. IR: Initial Rest; A: Active; PR: Post-Rest.

In the recovery phase, both RMS and ZC slopes partially normalized, with the DA condition showing the fastest return to baseline, indicating more efficient stabilization. Among fixed-height pillows, PE exhibited the most balanced response, maintaining moderate recruitment with rapid post-activity normalization (Figure 4).

Perceived comfort

One-way repeated-measures ANOVA revealed a significant effect of pillow condition on VAS scores (F=22.72, P<0.001, η²P=0.44. Post-hoc comparisons indicated that all pillow conditions were rated significantly more comfortable than NP (P<0.05. Mean VAS comfort scores were DA: 75.52±16.94 mm, PE: 70.17±19.02 mm, NA: 69.08±19.90 mm, MF: 66.30±17.73 mm, and LT: 61.09±21.64 mm, compared with NP: 36.41±25.01 mm (Figure 5) (Supplementary Digital Material 2: Supplementary Table I, II, III).

Figure 5.

—Average Visual Analog Scale (VAS) comfort ratings across pillow conditions. Bar plots display mean±SD of VAS scores (0-100 mm), with higher values indicating greater comfort. ^#Indicates a statistically significant difference between pillow conditions (P<0.05). For example, ^#NP denotes a significant difference from the no-pillow (NP) condition, ^#DA from the dynamic air-based (DA) condition, and ^#LT from the latex (LT) condition.

Although no established minimal clinically important difference (MCID) exists for pillow comfort, the minimum detectable change (MDC) method, which is recommended as the most reliable approach in cervical spine VAS validation, defines clinically meaningful changes as approximately 21.4 mm for the neck and 29.1 mm for the arm.²⁹ Accordingly, the observed differences (>30 mm for all supported pillows vs. NP) clearly exceed both perceptual and clinical significance thresholds, confirming the practical relevance of these comfort improvements.

Integrated outcome summary

Overall, the DA condition demonstrated the most favorable combination of outcomes, with the highest SmO₂ levels, strongest adaptive EMG slope modulation, and greatest perceived comfort. Among fixed-height inserts, PE showed the most balanced physiological and perceptual profile, providing early oxygenation gains, moderate recruitment, and efficient recovery. MF offered partial benefits, while LT and NA showed limited adaptability, and NP consistently underperformed across all metrics.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Discussion

This study examined short-term physiological and perceptual responses to different pillow height systems under controlled, sleep-relevant conditions. Across all outcomes, DA and PE configurations showed the most favorable trends, with greater SmO₂, adaptive EMG slope modulation, and higher comfort scores compared with NP and NA conditions. Among fixed-height inserts, PE demonstrated the most balanced response, supporting early oxygenation gains, moderate recruitment, and efficient recovery. MF and LT offered partial benefits, whereas NP consistently produced the lowest physiological and comfort outcomes. These findings support the hypothesis that dynamically adjustable support can promote more efficient cervical perfusion and neuromuscular adaptation than static designs, within the limits of an acute laboratory protocol.

This investigation is among the first to combine NIRS-derived SmO₂ with slope-based EMG analysis to concurrently assess metabolic and neuromuscular adaptation during pillow-related postural loading. This integrative approach captures both oxygen delivery and motor-unit behavior, providing a comprehensive perspective on cervical muscle function beyond traditional single-modality assessments. While these results indicate short-term physiological advantages of adjustable and compliant pillow systems, interpretation remains limited to controlled conditions in healthy adults and should not be generalized to habitual sleep without further longitudinal validation.

Muscle oxygenation

Both the SCM and UT demonstrated distinct SmO₂ responses across pillow conditions, reflecting region-specific physiological sensitivity. SCM SmO₂ was more responsive to changes in cervical alignment, consistent with its stabilizing role in maintaining head posture. Improper pillow height, whether excessively high or low, can alter cervical spine curvature, increase muscular tension, and impair microvascular perfusion, leading to transient oxygen deficits under sustained load.^{30, 31} In contrast, UT SmO₂ appeared more sensitive to posterior pressure redistribution, highlighting the vulnerability of posterior tissues to compressive stress in the supine position.^{32, 33}

DA and PE condition produced the most favorable SmO₂ profile, maintaining elevated oxygenation during activity and demonstrating rapid reoxygenation during recovery. These trends suggest more effective perfusion and metabolic recovery, likely facilitated by the DA system’s adaptive air-cell architecture, which redistributes contact pressure and minimizes focal ischemia. This interpretation is consistent with previous findings on hybrid and air-based surfaces enhance local circulation by dynamically adjusting support pressure.^{34, 35} In contrast, LT and MF, though supportive, may lose conformity over time, producing localized stiffness and reduced perfusion, a pattern consistent with earlier observations that stiffer materials yield higher interface pressure and lower comfort.³⁶

Across all conditions, UT SmO₂ values remained higher than NP, emphasizing that even minimal cervical support mitigates posterior tissue ischemia. Perfusion efficiency improved most under materials that distributed pressure broadly rather than concentrating load, confirming that SmO₂ is a sensitive physiological indicator of cervical support adequacy. These findings support prior evidence that viscoelastic or air-based supports help preserve microcirculation during low-load postural tasks.^{10, 37}

Temporally, SmO₂ rose during rest, declined slightly during activity, and rebounded in recovery, reflecting normal oxygen-delivery dynamics of mild postural exertion and subsequent reactive hyperemia. Such consistent phase behavior validates SmO₂ as a reliable proxy of both muscular perfusion and recovery under ergonomic test conditions.

Neuromuscular activation

Slope-based EMG analysis provided an indirect yet informative view of cervical neuromuscular adaptation. RMS slopes reflected cumulative motor-unit recruitment, whereas ZC slopes indicated firing-frequency modulation rather than direct deep-muscle activity. Thus, EMG slope behavior should be interpreted as a surface-level index of motor control, not as a direct measure of deep stabilizer function.

Across conditions, DA, PE, and MF showed steeper RMS and more negative ZC slopes, suggesting greater motor-unit engagement and adaptive load sharing during the active phase. Such activation does not necessarily indicate fatigue but may represent a compensatory strategy to preserve alignment and mechanical stability under dynamic support. This interpretation aligns with previous work showing that moderate increases in muscle activation during postural tasks enhance proprioceptive accuracy and joint control.^{8, 38, 39}

Linking neuromuscular activity to comfort, this pattern may reflect central fatigue modulation: elevated but well-perfused activity tends to maintain sensory coherence and reduce perceived exertion when oxygenation is sufficient.⁴⁰ The concurrent increase in SmO₂ under DA and PE conditions supports this explanation, as greater metabolic recovery likely minimized the subjective discomfort normally associated with sustained activation.

In contrast, NP and NA conditions exhibited minimal EMG slope variation, indicating lower superficial activation but potentially higher reliance on deep or compensatory muscles not captured by surface electrodes. This interpretation remains speculative but emphasizes the indirect nature of EMG slope metrics.

Overall, combining NIRS-derived SmO₂ and slope-based EMG enabled a dual-dimension perspective encompassing metabolic and electrophysiological aspect, revealing subtle adaptive mechanisms that may be observed by amplitude-only EMG analyses.

Perceived comfort and multidimensional implications

Comfort perception in this study reflected an interaction between mechanical conformity, SmO₂, and neuromuscular modulation, rather than any single determinant. DA and PE, which demonstrated both higher SmO₂ and adaptive EMG slope profiles, were rated as most comfortable despite showing greater superficial activation during activity. This apparent paradox supports central fatigue models, which propose that perceived exertion depends more on metabolic sufficiency and sensory feedback integration than on absolute muscle activation.^{40, 41} In other words, when oxygen delivery remains adequate, the central nervous system interprets effort as stable rather than fatiguing, contributing to higher comfort ratings even under mild activation.

SmO₂ recovery patterns further explain this phenomenon: DA and PE facilitated faster post-activity reoxygenation, potentially attenuating afferent fatigue signals from muscle metaboreceptors. This may reduce the sense of tension or heaviness commonly reported under less adaptive support systems. Conversely, NP, despite minimal EMG activity, yielded the lowest comfort ratings, likely due to reduced alignment and increased sensory mismatch between pressure distribution and proprioceptive expectations.⁷

These results highlight comfort as a multidimensional construct influenced by mechanical, circulatory, and neurophysiological factors. Pillows that maintain alignment and promote perfusion may enhance perceived relaxation even when mild muscle engagement occurs. The integrated NIRS and EMG approach offers a novel framework to interpret comfort beyond static postural evaluation, emphasizing the role of sensorimotor and metabolic coupling in ergonomic design.

Nevertheless, interpretation should remain limited to acute, laboratory-based conditions. Longer-term adaptation, habitual use, and sleep-phase influences were not assessed here. Future research should examine how dynamic support systems influence overnight comfort, autonomic regulation, and muscle recovery in populations with cervical musculoskeletal disorders, where altered perfusion and central fatigue processing are clinically relevant.

Conclusions

This controlled, laboratory-based study suggests that pillow insert mechanisms acutely influence cervical SmO₂, neuromuscular modulation, and perceived comfort in healthy young adults. DA and compliant fixed inserts (e.g., PE) were associated with more favorable physiological and perceptual responses compared with other static or absent support. These effects reflect short-term ergonomic and physiological adaptation rather than long-term or established clinical benefit.

By combining NIRS and EMG slope analysis, this study proposes an integrated framework for assessing metabolic and neuromuscular responses to ergonomic interventions. This dual-modality may help clarify how adaptive support systems influence perfusion and neuromuscular control during low-load postural conditions.

Future studies should extend this work by: 1) examining overnight and habitual use to evaluate long-term adaptation; 2) including older adults or individuals with cervical musculoskeletal disorders to assess clinical relevance; and 3) integrating real-time sensing to quantify natural sleep posture variability.

Overall, these findings provide preliminary physiological evidence that DA may acutely enhance cervical support and perceived comfort under controlled conditions, offering a foundation for future translational and longitudinal ergonomic research.

Supplementary Digital Material 1

Supplementary Figure 1

CONSORT flow diagram.

Supplementary Digital Material 2

Supplementary Table I

Within-subject comparisons of Sternocleidomastoid (SCM) muscle oxygen saturation (SmO₂) across pillow conditions.

Supplementary Table II

Within-subject comparisons of upper trapezius (UT) muscle oxygen saturation (SmO₂) across pillow conditions.

Supplementary Table III

Within-subject comparisons of Visual Analog Scale (VAS) comfort ratings across pillow conditions (N.=30).