Physical Therapy Korea

Forward head posture (FHP) is defined as an excessive anterior positioning of the head in the sagittal plane, beyond the normal alignment [1-3]. This postural disorder is acknowledged to be attributable to conditions involving prolonged exposure to fixed postures, such as extended periods of sitting in front of computers, using phones, and driving [4,5]. Despite the presence of a potential link between FHP and several pathologies, such as thoracic hyper kyphosis, improper muscle activation, and local spinal malalignment resulting from degenerative changes or post-surgical kyphosis, the precise nature of this connection remains elusive [6]. In a cross-sectional study involving university students, the prevalence of FHP was reported as 63.96% [7], while another study documented FHP occurrence in adolescents ranging from 52% to 68% and in young adults spanning from 11.4% to 67% [8]. Due to the inherent ambiguity in its biomechanics and its notably high prevalence, an array of biomechanical studies have been undertaken to explore the determinants influencing the progression of FHP [9-11].

When examining the biomechanical alterations commonly linked to FHP, researchers often segment the region spanning from the occiput to first thoracic vertebra (T₁) into two distinct segments for analytical purposes: the upper cervical spine and the lower cervical spine. For the analysis of kinetic changes associated with the formation of FHP, several parameters in the cervical region are considered, including muscle length, muscle strength, and muscle thickness. The evaluation of muscle length is typically conducted through in vitro studies utilizing cadaver specimens. In contrast, the changes in muscle thickness during resting and contraction phases are assessed using ultrasound imaging techniques [9-11]. Additionally, a laboratory model incorporating cadavers is used to investigate the kinematic changes in the craniocervical vertebrae and subaxial spinal region associated with FHP [12]. In biomechanical studies of this nature, the procedure begins with computed tomography scans conducted for each vertebra spanning from occiput to T₁. Following this, static three-dimensional (3D) models are constructed using the acquired data. For the development of dynamic 3D models, a testing apparatus is utilized. Using the data obtained through optoelectronic motion measurement, a dynamic 3D model is constructed. The experimental setup used for the kinematic analysis of FHP using this method is shown in Figure 1 [13].

Figure 1. The experimental setup used for the kinematic analysis of forward head posture. (A) Computed tomography scan, (B) static three-dimensional (3D) model, (C) Testing Apparatus, (D) Optotrak motion data, (E) dynamic 3D model, (F) anatomical literature from Gray H. Anatomy of the Human Body. Philadelphia, PA: Lea & Febiger; 1918, and (G) 3D muscle model. Adapted from the article of Khayatzadeh et al. (Phys Ther 2017;97(7):756-66) [13].

In cadaver studies, the comparison between neutral posture and cervical sagittal alignment abnormalities is typically based on evaluating the values of sagittal vertical axis (SVA) and T₁ tilt angle [14,15]. Khayatzadeh et al. [13] reported in their study that they based their measurements for neutral posture on the values of C₂–C₇ SVA = 17 mm and T₁ angle = 23°. In cadaver studies, the biomechanics of an increase in SVA is investigated while maintaining a constant T₁ tilt angle. However, it should be noted that the T₁ angle also undergoes changes in conjunction with FHP [16]. This situation poses a limitation in cadaver studies. In the study by Patwardhan et al. [12], which investigated FHP using a lab model, a mechanical constraint was used to ensure that occiput orientation remained consistent with horizontal gaze. In vivo studies, however, do not include such a constraint, which could alter the biomechanics [12].

On the other hand, the utilization of 9-axis inertial measurement units (IMUs) in kinematic research has exhibited a steady increase in recent years. In the utilization of IMUs for kinematic analysis of FHP, there are no constraints applied to any anatomical structure. Additionally, the reliability of IMU employed for spinal flexion-extension kinematic analysis has been substantiated [17]. In Fercho et al. [18]’s study, they employed four IMUs to investigate the biomechanics of FHP. According to the results of this study, the biomechanics of FHP contradicts what has been widely accepted in the literature until now [16,18]. During the sitting session, a total of 33.33 ± 13.56 degrees of flexion was observed in the C₀–C₁ joint, while 3.30 ± 10.10 degrees of extension were reported in the C₂–C₇ vertebral levels, indicating the development of a FHP [18]. However, this study contradicts the current literature, as it reports results opposite to those found in studies using cadaveric specimens, where FHP is indicated to cause hyperextension in the craniocervical region and flexion between the lower cervical spinal vertebrae. In cadaver studies, there is an experimental setup that helps analyze the biomechanics of FHP formation while adhering to this experimental arrangement, but the biomechanics may vary depending on the factors contributing to FHP formation. In the context where compensatory hyperextension might develop in the craniocervical vertebrae to counteract flexion in the subaxial region caused by thoracic kyphosis, acknowledging the potential for flexion to occur in the craniocervical vertebrae when tasks such as knitting are performed and the eyes are directed downward, it becomes evident that in vitro studies have inherent limitations. In contrast to cadaver studies, research conducted using IMUs allows for the investigation of the biomechanics of FHP development in individuals without any constraints, enabling examination for each individual separately. Therefore, utilizing IMUs in biomechanical studies enables a comprehensive observation, facilitating a clearer understanding of deviations in the sagittal plane.

However, there was only one study conducting the kinematic analysis of FHP using the IMU system, and there is no study available for comparison. Therefore, the objective of this study was to analyze all the studies conducted until 2024 on the kinetic and kinematic analysis of FHP, with the aim of reviewing whether there are biomechanical inconsistencies. By synthesizing findings from various studies, this review seeks to assess whether the observed kinetic and kinematic aspects of FHP align coherently with each other. The analysis will contribute to a deeper understanding of the biomechanical characteristics associated with FHP and may provide insights for future research and intervention strategies.

In this study, we hypothesized that there would be discrepancies between kinetic and kinematic findings, suggesting that the biomechanics of FHP formation may not be uniform but rather may involve different biomechanical alterations.

1. Search Strategy

This study was conducted following the guidelines of Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). The flow diagram illustrating the selection process of studies searched in databases for inclusion in this study is presented in Figure 2. The relevant studies were investigated on PubMed and Google Scholar databases and were limited to publications in the English language from 2012 to 2024. Interest in the biomechanics of FHP has shown an upward trend since 2012, accompanied by advancements in analysis methods. Both to incorporate more studies and to standardize the analysis method, the year 2012 has been accepted as the starting point among authors. The keywords and keyword combinations entered into the databases for this study, aligned with its purpose, were “forward head posture OR cervical sagittal alignment OR cervical spine AND biomechanics OR kinetic analysis OR kinematic analysis.”

Figure 2. PRISMA flow diagram of study selection.

2. Study Selection

All authors participated in the selection process. At the initial stage of the research, the authors conducted separate searches in databases. After removing duplicate works from the studies found by the authors, the screening process was conducted. We performed the initial screening by examining the titles and abstracts of the remaining studies. Excluding studies whose titles and abstracts indicated that they did not meet the inclusion criteria of our study, the authors proceeded with the screening process. In this study, the inclusion criteria for investigating biomechanical changes in individuals with FHP were studies comparing the thickness of muscles in the cervical region between individuals with FHP and those without FHP, studies comparing muscle strength in the cervical region, studies investigating muscle length in cadaver studies, and studies exploring kinematic analysis of cervical vertebrae in the presence of FHP. The exclusion criteria for this study were studies published before 2012 and written in a language other than English, studies in which it is not clear whether participants included in the research process have spinal pain, studies concentrating solely on the biomechanical analysis of either the upper or lower cervical region, and studies for which full-text access is not available, and additionally, review studies.

3. Data Extraction

In order to articulate the findings of the acquired studies in a tabular format, the research has been systematically categorized into five distinct subheadings: criteria for recognizing FHP, general characteristics of participants, parameters, evaluation tools, and mean outcomes. In the process of tabulating the studies, all authors have reached consensus, and disagreements among authors have been resolved using more appropriate expressions.

Following the selection process presented in Figure 2, after eliminating duplicate research in the databases, a total of 2,562 articles were narrowed down to 1,809 for the next stage, which involves reviewing titles and abstracts. Among these, only 34 studies were considered suitable for in-depth examination through full-text reading, aligning with the specific objectives of our research. Thirteen of these studies were excluded from our analysis due to the lack of clear disclosure regarding participants’ spinal pain status or cervical surgery history in their exclusion criteria. Additionally, in six studies, participants were excluded because the analysis focused solely on either the upper or lower cervical region while analyzing FHP. Six studies were excluded because full-text access could not be obtained. As a result, a total of 24 studies could not be included in our investigation. Consequently, a total of nine studies were included in our investigation aimed at exploring the biomechanics of FHP [9-13,18-21].

1. Description of the General Characteristics of the Selected Studies

The general characteristics of the selected studies are presented in Table 1. Three out of the nine studies were conducted by analyzing specimens of cadavers [10,12,13]. In these studies, a total of 29 cadaver specimens were included, and the focus was on testing the segmental angular motion or muscle lengths associated with the FHP. Only in the study by Lin et al. [10], the severity of FHP was considered, and using craniovertebral angle (CVA) measurements, the study investigated the changes in muscle lengths based on the severity of the FHP. The parameters of the other six studies that included living participants were as follows: segmental angular motion, cervical muscle strength, cervical muscle thickness, difference in thickness change of extensor muscles during contraction and relaxation, and active range of motion (ARoM) [9,11,18-21]. Two of the studies conducting kinematic analysis were cadaver studies, employing a specialized test apparatus to assess segmental angular motion [12,13]. In the other two kinematic analysis studies, one utilized an IMU, while the other employed the cervical range of motion (CROM) device to test the ARoM [18,21].

Table 1 . General characteristics of the selected studies.

Study	Criteria for recognizing FHP	General characteristics of participants	Outcomes	Main outcomes
Khayatzadeh et al. [13], 2017	SVA	13 Fresh-frozen cadaveric cervical spine specimens (9 males, 4 females) Age = 54 ± 15 years	Muscle length, segmental angular motion	Kinematic changes: C0–C2: Hyperextension occurs C2–C7: Flexion occurs Muscle length changes: Shortened muscles: Occipital extensor muscles Cervical flexor muscles Elongated muscles: Occipital flexor muscles Cervical extensor muscles
Patwardhan et al. [16], 2015	SVA	10 Cadaveric cervical spines (occiput–T1) Age = 54 (21–59) years	Segmental angular motion	Forward head displacement: Displacement: 4 cm forward Resulting extension: Approximately 12 degrees of extension between the occiput and C1 Approximately 12 degrees of extension between C1 and C2 Resulting flexion: Approximately 10 degrees of flexion below C5–C6 Study findings on SVA (sagittal vertical axis): Subaxial spinal vertebrae (below C2): Increased SVA results in flexion Axial vertebrae (C0–C2): Increased SVA results in hyperextension
Lin et al. [10], 2022	1. CVA for NHP = 55 2. CVA for slight FHP = 55° > and < 45° 3. CVA for severe FHP 45° > and 35° <	6 Cadavers(4 males and 2 females) Age = 86.2 ± 8.7 years	Deep neck muscle length	Comparison of neutral posture to slight (FHP): Shortening observed: Upper SSC muscle RCP muscles Lengthening observed: Longus capitis muscle Splenius cervicis muscle Comparison of neutral posture to severe FHP: Shortening observed: All occipital extensors (excluding OCS) Lengthening observed: All cervical extensor muscles Superior oblique part of the LCo muscle Comparison of slight FHP to severe FHP: Elongation observed: Superior oblique part of the LCo muscle
Fercho et al. [18], 2023		N = 25(11 females and 15 males) Age = 23.36 ± 2.79 years	Segmental angular motion	Sitting posture while using a phone: C0–C1 Joint: 33.33 degrees of flexion Subaxial spinal segments: 1.05 degrees of extension Standing while using a phone: C0–C1 region: 27.50 degrees of flexion Subaxial spinal segments: 2.50 degrees of flexion Walking while using a phone: C0–C1 region: 32.03 degrees of flexion Subaxial vertebrae: 3.30 degrees of extension
Eun et al. [19], 2020	CVA < 48°	FHP = 24 (15 males) CVA = 44.8° ± 2.0° NHP = 27 (10 males) CVA = 52.5° ± 3.0° Age = 32.6 ± 4.8 years (for all participants)	Muscle strength (UCE, LCE, UCF, LCF)	Statistical observations: Significant decreases: Strength of LCE Strength of UCF Non-significant changes: Strength of LCF Strength of UCE Significant increase in ratio: LCF strength to LCE strength in the FHP group No significant change in ratio: UCF strength to UCE strength
Bokaee et al. [11], 2017	CVA < 48°	FHP = 35 females Age = 24.94 years (5.13) CVA = 43.76° NHP = 35 females Age = 25.18 years (5.52) CVA = 54.26°	Cervical muscle thickness (RCP, OCS, SSC, SCM, and LCo)	Statistical observations: Significant increase: Thickness of the SCM muscle in the FHP group Non-significant changes: Increases in the thickness of other muscles (RCP, OCS, SSC, LCo) in the FHP group
Goodarzi et al. [9], 2015	CVA	FHP (n = 20) CVA = 43.43° ± 2.58° Age = 21.30 ± 2.36 years NHP (n = 20) CVA = 55.90° ± 2.25° Age = 21.85 ± 2.87 years	Extensor muscles thickness at rest (multifidus, SSCe, SSC, Sca and UT)	Statistical observations on muscle groups: No statistically significant changes: Occipital extensor muscles Cervical extensor muscles Thickness observations in the occipital extensor muscle group: Muscles found to be thinner: Sca muscle SSC muscle p-values: The p-values for the Sca and SSC muscles indicated that these muscles were thinner compared to other muscles in the occipital extensor group.
Goodarzi et al. [20], 2018	CVA < 49°	FHP (n = 20) CVA = 43.43° ± 2.58° Age = 21.30 ± 2.36 years NHP (n = 20) CVA = 55.90° ± 2.25° Age = 21.85 ± 2.87 years	Difference in thickness change of extensor muscles during contraction and relaxation (multifidus, SSCe, SSCa, Sca and UT)	Change in muscle thickness between rest and isometric muscle contraction: Statistically significant decrease: SSC muscle within the occipital extensor muscle group
Quek et al. [21], 2013	CVA	N = 51 (29 females, 22 males) Age = 66 ± 4.9 years (60–78) CVA = 45.6° ± 6.7° (31–59)	ARoM for upper and general cervical rotation and cervical flexion	CVA was found to be significantly correlated with increased cervical flexion (Spearman r = 0.30) and general rotation RoM (r = 0.33), but no significant association was observed with upper cervical rotation RoM (r = 0.15).

FHP, froward head posture; NHP, neutral head posture; ARoM, active range of motion; SVA, sagittal vertical axis; CVA, craniovertebral angle; UCE, upper cervical extensor; LCE, lower cervical extensor; UCF, upper cervical flexor; LCF, lower cervical flexor; RCP, rectus capitis posterior; OCS, oblique capitis superior; SCM, sternocleidomastoid; LCo, longus coli; UT, upper trapezius; Sca, splenius capitis; SSC, semispinalis capitis; SSCe, semispinalis cervicis..

2. Contradictions Obtained During the Comparison of the Studies

The comparison of results among the nine studies revealed inconsistencies in their findings. The contradictions were as follows:

1) In the study conducted by Khayatzadeh et al. [13], it was reported that while the SVA value was 26 mm, there was a 2% shortening of the cervical flexor muscle, longus colli, compared to neutral posture. However, contrary to this finding, Lin et al. [10] reported a 4% lengthening of the longus colli muscle in their study.

2) Patwardhan et al. [12]’s study, in addition to Khayatzadeh et al. [13]’s two cadaver studies, reported that FHP leads to hyperextension in the craniocervical spinal region and flexion in the subaxial spine. However, in the study conducted by Fercho et al. [18], it was reported that the analysis of participants’ head posture using IMU revealed that FHP resulted in flexion in the craniocervical region kinematically.

3) In the study by Lin et al. [10], it was reported that when the CVA was adjusted to range from 55° to 45°, significant changes in the lengths of both the rectus capitis posterior and semispinalis capitis muscles were reported. However, in the study by Bokaee et al. [11], despite the average CVA being 43.76°, no significant changes were reported in these muscles.

4) Contrary to the purported statistical significance observed in kinetic analyses concerning alterations in the longus capitis and suboccipital muscles associated with FHP, Quek et al. [21] have indicated, in their study, a lack of alteration in upper cervical rotation, a function attributed to these muscles [13]. Therefore, there appears to be a discrepancy between kinetic and kinematic analyses in the literature regarding the development of FHP and the changes in these muscles. The significant changes observed in kinetic analyses in Khayatzadeh et al. [13]’s study were not supported by the kinematic analysis in Quek et al [21]’s study.

5) In Goodarzi et al. [20]’s study, which investigated thickness changes in extensor muscles during isometric contraction, only the semispinalis capitis muscle exhibited a significant decrease. This finding aligns with Fercho et al. [18]’s study, which also indicated a larger flexion angle at the craniocervical junction than extension in the lower cervical region. However, in contrast, Bokaee et al. [11]’s investigation reported changes only in the lower cervical region, while Quek et al. [21]’s study, consistent with Bokaee et al. [11]’s findings, indicated no alteration in upper cervical rotation range of motion. Quek et al. [21], reported an association between FHP and changes in lower cervical flexor and rotation range of motion. Thus, a divergence in findings exists among these studies regarding the specific regions affected by kinetic changes in relation to FHP.

The aim of this study was to critically evaluate research conducted between 2012 and 2024 investigating the kinetic and kinematic changes in individuals with FHP, and to assess the consistency of these studies with one another. It has been observed that out of the nine studies included in this review, a total of five discrepancies were noted.

The results presented in the studies conducted by Khayatzadeh et al. [13] and Lin et al. [10] using cadaver samples regarding the length of cervical flexor muscles appear to be conflicting with each other. The results presented for the occipital flexor, extensor, and cervical extensor in these two studies support each other. Khayatzadeh et al. [13]’s computerized model study contradicts the findings of two qualitative studies, which are in line with the study by Lin et al. [10], supporting elongation in cervical flexor muscles [22,23]. Additionally, Lin and colleagues reported in their study that there was a significant elongation of cervical flexor muscles after the CVA reached 45°, with no significant elongation observed between 55° and 45°. The superior part of the longus colli muscle inserts onto the anterior tubercle of the atlas [24]. The elongation of the superior part of the longus colli muscle may be attributed to the increasing severity of FHP, which could be a result of increased craniocervical hyperextension. In Lin et al. [10]’s study on muscle length, it can be inferred that FHP may be associated with hyperextension in the upper cervical region. However, a definitive inference cannot be made regarding its association with flexion in the lower subaxial region. Both muscle shortening and lengthening are influential factors on muscle force generation, and it is widely accepted in the literature that there exists an optimal muscle length for maximal force production [25]. Nonetheless, the elongation observed in both cervical extensors and flexors can lead to muscle weakness in the cervical region. In the literature, the inclusion of stretching exercises for cervical flexor muscles is recommended in clinical practice for the presence of FHP [26]. When such an inference is made, the time allocated for cervical flexor stretching in the treatment process can be reallocated for cervical flexor muscle strengthening exercises. Nam and Kwon [27], in their studies, reported a statistically significant increase in the activation of the trapezius and serratus anterior muscles in individuals with FH and rounded shoulder posture. Based on these studies, it can be inferred that incorporating cervical flexor strengthening exercises and exercises aimed at reducing the activation of cervical extensor muscles into the treatment protocol for individuals with FHP may be beneficial. This treatment method exhibits inconsistency with the recommendation for strengthening cervical extensor muscles proposed by Eun et al. [19].

In the study conducted by Fercho using IMUs, the report of flexion occurring in the upper cervical region due to smartphone usage contradicts previous studies that have established a relationship between FHP and craniocervical hyperextension [12,18]. Fercho et al. [18] additionally observed that the vertebral levels from C₂ to C₇ remained almost stationary. Given the findings of this study, it is imperative to consider prioritizing the therapeutic intervention within the cervical region extending from the occiput to the axis. A heightened focus on this area during the treatment regimen, as opposed to the lower cervical region, may offer a potentially more efficacious approach.

In this study, conflicts 3, 4 and 5 support each other. Conflict 3 is supported by the study of Lin et al. [10], which reported significant biomechanical changes in the semispinalis capitis and Rectus capitis posterior muscles even in the presence of slight FHP. However, Bokaee’s study [11] contradicts this, stating that there were no biomechanical changes for these two muscles at the CVA of 43.76°, which is considered severe FHP intensity according to Lin et al. [10]’s study. In Fercho et al. [18]’s study, it was reported that FHP primarily originates from the upper cervical region, with no great change observed in the lower cervical spinal vertebrae. This finding is supportive of Lin et al. [10]’s work. Similarly, this finding aligns with Khayatzadeh et al. [13]’s study, which reported the largest change in muscle length occurring in the craniocervical region. Additionally, Goodarzi et al. [9] reported, in his study on muscle thickness, that despite being statistically insignificant, the p-value for occipital muscles in individuals with FHP was smaller than that for muscles in the cervical region. Considering these four studies, it could be inferred that FHP predominantly initiates from the upper cervical region. However, Quek et al. [21]’s investigation corroborated the findings of Bokaee et al. [11], suggesting a positive correlation between CVA and cervical flexion. Nevertheless, the study did not find any association between upper cervical rotation and other variables. The muscles considered responsible for upper cervical rotation are the longus capitis and suboccipital muscles [28]. Upon reviewing these six studies, it is evident that the literature lacks sufficient evidence to make inferences regarding the initial region affected in the formation process of FHP and to comment on the most affected region. Upon scrutinizing nine studies conducted in this systematic review, inconsistencies are apparent in the kinetic and kinematic analyses of FHP. These inconsistencies lead to the inability to reach conclusions on three important issues: (1) A clear interpretation cannot be made regarding from which cervical region FHP primarily begins to exhibit biomechanical changes; (2) It is ambiguous whether the most affected region is the upper cervical spinal region or the lower cervical spinal region; and (3) There is a discrepancy regarding whether FHP in individuals involves hyperextension or flexion in the craniocervical region.

If it is accepted that the inconsistency between kinetic and kinematic studies is due to the variability in the biomechanical mechanism of FHP formation from person to person, then it can be acknowledged that the treatment methods used in the therapeutic process for individuals with FHP may vary from person to person. Therefore, making decisions about the most appropriate treatment for each individual’s cervical region after detailed kinetic and kinematic analysis could result in a more effective treatment. However, such an inference would require further biomechanical studies, particularly utilizing IMU-based research, to better understand the biomechanics of various factors involved in FHP formation. Additionally, during the treatment of the cervical region, it is essential to maintain good posture throughout the entire body, not just in the cervical area. A study investigating the relationship between the lumbar curve and head posture in the sitting position reported that preserving the physiology of the lumbar spine, characterized by the position of the L3 vertebra, is necessary to achieve correct head posture [29]. Analyzing the overall posture of individuals prior to treatment can have positive effects on the treatment process.

This study was subject to several limitations. Firstly, the inclusion of only nine studies restricts the breadth of analysis. Secondly, the differing parameters used for analysis across these studies present a challenge for comparative interpretation. Thirdly, the limitation arises from the scarcity of studies utilizing IMU, with only one such study available, thereby constraining the interpretive scope.

Conflicts have been identified among kinetic and kinematic studies conducted on FHP. These conflicts may stem from the possibility that the biomechanics of FHP formation could vary from person to person. With such an assumption, the treatment process may exhibit variability from individual to individual. However, for a more comprehensive analysis, further biomechanical studies on this topic are necessary.

YD would like to express her appreciation for the financial support of the Ministry of Education, which made it possible to obtain a master’s degree through the Korean Government Scholarship Program.

None to declare.

No potential conflicts of interest relevant to this article are reported.

Conceptualization: YD, EP, EC. Data curation: YD, EC. Methodology: YD, EP, EC. Project administration: YD, EP, EC. Supervision: EP. Visualization: YD, EC. Writing - original draft: YD. Writing - review & editing: EP.