|Year : 2022 | Volume
| Issue : 5 | Page : 563-568
Neck Muscles' Responses to Cradle, Cross-cradle and Football Breastfeeding Hold Positions in Nursing Mothers: A Preliminary Study
CP Ojukwu1, CG Okpoko1, IT Ikele2, IJ Ilo3, SS Ede1, NE Anekwu4, AJ Okemuo1, EM Anekwu5, SU Ugwu6, CN Ikele1
1 Department of Medical Rehabilitation, University of Nigeria, Enugu Campus, Nigeria
2 Department of Anatomy, University of Nigeria, Enugu Campus, Nigeria
3 Department of Nursing Sciences, University of Nigeria, Enugu Campus, Nigeria
4 Department of Obstetrics and Gynaecology, Alex Ekwueme Federal University Teaching Hospital, Abakaliki, Ebonyi State, Nigeria
5 Department of Physiotherapy, Federal Teaching Hospital, Abakaliki, Ebonyi State, Nigeria
6 Department of Medical Physiology in College of Medicine, University of Nigeria, Enugu Campus, Nigeria
|Date of Submission||16-Oct-2021|
|Date of Acceptance||11-Mar-2022|
|Date of Web Publication||19-May-2022|
Dr. I T Ikele
Department of Anatomy, University of Nigeria, Enugu Campus
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Background: Breastfeeding-related Neck Pain (BFNP) is prevalent in nursing mothers and cuts across the utilization of different breastfeeding (BF) hold positions. Biomechanical considerations to highlight ergonomically safe BF positions for the prevention of BFNP have not been previously studied. Aim: This study was designed to compare the electrical activities of selected neck muscles across three breastfeeding hold positions [cradle (C1), cross-cradle (C2), and football (FB)]. Materials and Method: Surface electromyographic analyses of four neck muscles (right and left components of each trapezius and sternocleidomastoid) were performed respectively during three BF trials with different BF hold positions (C1, C2, and FB) in 22 nursing mothers. For all the trials, the mothers nursed from the left breast. Results: Across the three BF trials, the electrical activities of each of the neck muscles did not vary significantly (P > 0.05). Furthermore, the left muscular components showed predominantly higher activities, as compared to their right components. Conclusion: The breastfeeding hold position adopted during BF tasks may not be a determinant of BFNP in nursing mothers. Further studies to ascertain the biomechanical implications of the utilized BF holds are recommended.
Keywords: Breastfeeding hold, electromyography, neck muscles, nursing mothers, positions
|How to cite this article:|
Ojukwu C P, Okpoko C G, Ikele I T, Ilo I J, Ede S S, Anekwu N E, Okemuo A J, Anekwu E M, Ugwu S U, Ikele C N. Neck Muscles' Responses to Cradle, Cross-cradle and Football Breastfeeding Hold Positions in Nursing Mothers: A Preliminary Study. Niger J Clin Pract 2022;25:563-8
|How to cite this URL:|
Ojukwu C P, Okpoko C G, Ikele I T, Ilo I J, Ede S S, Anekwu N E, Okemuo A J, Anekwu E M, Ugwu S U, Ikele C N. Neck Muscles' Responses to Cradle, Cross-cradle and Football Breastfeeding Hold Positions in Nursing Mothers: A Preliminary Study. Niger J Clin Pract [serial online] 2022 [cited 2022 Jul 7];25:563-8. Available from: https://www.njcponline.com/text.asp?2022/25/5/563/345583
| Introduction|| |
Breastfeeding (BF) is the act of transferring milk from the mother to the child. It is one of the most effective ways to ensure a child's adequate health and survival. According to the World Health Organization (WHO), BF is a normal and suitable method of providing nutrients for the healthy growth and development of the child. BF is recommended to be initiated within the first hour of life and that the infant should be exclusively breastfed for the first 6 months of life. It is believed to be the first step that ensures that the child gets a healthy and continuous source of nutrients.
BF provides short and long term benefits to both mother and child. Some of these benefits include protection against acute and chronic illnesses, savings in health care costs, creation of an intimate relationship between mother and child, reduction of neonatal complication risks, respiratory illness and other childhood diseases. It reduces infant mortality and morbidity rates, and aids the development of intellectual and motor skills, among others. In addition, maternal benefits include reduced risk of postpartum hemorrhage, ovarian cancer, and postmenopausal cancer.
Despite the infant and maternal benefits of BF, there are some related adverse health effects including breast soreness, engorgement, and musculoskeletal pain, other maternal discomforts, blocked ducts, and mastitis. BF also reduces bone mineral density in women. BF-related musculoskeletal pain may be attributed to hormonal influence on the musculoskeletal tissues in combination with the biomechanical and ergonomic stress of childcare-related activities. In addition, one of the major predisposing factors of BF-related musculoskeletal pain is the adoption of poor body posture during BF tasks.,,
Posture-related musculoskeletal pain is usually of sustained awkward posture and excessive repetition of tasks, which put stress on the musculoskeletal tissues. As in the case of BF, prolonged periods of inappropriate postures can lead to end range loading of muscles and deformation of normal body soft tissues. For improved efficiency and effectiveness of BF, there are some recommended BF positions which include the cradle (C1) hold, cross-cradle (C2), clutch or football (FB) hold, reclined and side-lying positions.
The three commonly utilized BF positions (C1, C2, and FB) are widely practiced cross-culturally., They are typically performed with the mother in a sitting position. C1 and C2 are similar in that the baby is positioned on the side lying on the mother's thigh while facing her breast. However, there are slight differences in the mother's upper limb positions, relative to the nursing breast and baby. For the C1 hold, the baby is supported with the upper limb (ipsilateral limb) closer to the nursing breast while the hand of the other upper limb (contralateral limb) is used to support the breast. Conversely, the C2 hold requires that the baby is supported by the contralateral limb while the ipsilateral limb supports the breast from the underside in a U-shaped hold, guiding the baby's mouth to the breast. The FB hold requires a slightly different approach. With the mother in a sitting position, the baby is positioned supine in the space between the mother and the armrest of the chair. In this case, the nursing breast points inferiorly toward the baby's face and is being supported by the contralateral limb. The baby's head is then supported within the palm of the ipsilateral limb. These positions are used interchangeably but recommendations to utilize one position over the other may be made in specific scenarios. For instance, C2 is recommended for infants who have problems with latching while FB is preferable for mothers with recent cesarean sections or large breasts.
To ensure bonding and close monitoring during nursing sessions, mothers are often observed to be gazing at their infants with a sustained flexed neck posture. Considering the duration and frequency of BF, we envisaged that this neck-down posture might be a risk factor for BF position-related neck pain (BFRNP) reported among nursing mothers. In addition to other BF position-related musculoskeletal disorders reported among nursing mothers,,,,, BFRNP has been identified as the commonest in more recent studies., Understanding the biomechanical implications of the commonly utilized BF positions, relative to the incidence of BFRNP and other musculoskeletal disorders is very important. Such knowledge will guide BF practices among nursing mothers as well as recommendations for BF position modifications in clinical settings. In a previous study, electromyographic activities of four trunk muscles (right and left components of the erector spinae and external oblique muscles) in response to these three commonly utilized BF positions were studied. Findings showed that the FB hold position might be more biomechanically efficient, as compared to the C1 and C2 holds. Unfortunately, no other study has evaluated neck muscle responses relative to these BF positions, considering the prevalence of BFRNP reported in previous studies., We also hypothesize that neck muscle activity will vary across C1, C2, and FB trials, considering their maternal body orientation characteristics. Therefore, this study was designed to compare the electrical activities of some neck muscles (sternocleidomastoid and trapezius), in response to the C1, C2, and FB hold BF positions. The trapezius was selected to represent the posterior neck muscles while the sternocleidomastoid was a representative of the anterior neck muscles. Both muscles are superficial neck muscles and are considered convenient for a surface EMG study as opposed to deep muscles which may require more invasive assessment procedures.
| Materials and Methods|| |
Twenty-two (22) apparently healthy, nursing mothers (18-40 years) who are currently breastfeeding (BF) their infants were involved in this repeated measures observational study, according to procedures approved by the University of Nigeria Health Research and Ethics Committee. A total of 20 participants were estimated for the analysis of variance at the degree of freedom (dfb) = 1, to achieve 96% (0.96) power with a moderate to a large effect size of 0.60 at an alpha level of 0.05 in a preliminary power analysis. All participants met the inclusion criteria for neck electromyography (EMG) measurement and gave written informed consent prior to participation in the study. All the participants were right-handed. Others with known orthopedic and neurological conditions of the neck and upper body were excluded from this study.
Each woman underwent three BF trials in three different positions, including C1, C2, and FB holds. Pictorial and video representations of each BF hold were availed to them to enable a thorough understanding of the BF trials, prior to the interventions. For electrode placements, the left and right components of the sternocleidomastoid (SCM) and upper trapezius (UT) muscles were located and marked off. Pairs of 12 mm diameter Ag-ACl reusable surface electrodes (Verity Medicals, UK) were placed on the marked surfaces after wiping them with alcohol swabs with an inter-electrode distance of 20 mm., For the UT muscles, electrodes were also placed in line with muscle fibers approximately midway between the anterior border of the upper trapezius and posterior midline (two-thirds the distance from the spinous process of the C7 vertebrae and the lateral edge of the acromion)., Each EMG channel had a separate ground electrode.
Raw EMG signals were recorded via two dual-channel EMG apparatuses (Myoplus2, Neurotrac System, Verity Medicals, Hamsphire, UK), which amplified and sampled the EMG inputs at 1000 Hz. For precision purposes, a wide band-pass filter (3db Bandwidth) which guarantees that 50 Hz and 60 Hz frequencies would not interfere with muscle activity recordings was utilized. This filtering permits an accuracy of 0.1 microvolts for surface EMG measurements. Signals were digitally recorded and displayed on a synchronized PC using Neurotrac software (version 5.0.117).
Each participant underwent the three BF trials in a random sequence generated on a Latin square to control for fatigue and carry-over effects. Descriptions of the set-up for each BF hold during the three trials were as reported in. The infants were placed on the left side and the mothers nursed from the left side in all the trials for uniformity purposes. Muscle activities were measured continuously for 5 minutes from the beginning to the end of each trial. To ensure adequate rest in between trials, a rest period of one hour was ensured between subsequent trials. Once the position had been learned and adopted by the participant, the EMG activities of their neck muscles were measured and recorded with a two minutes rest in between trials.
Raw EMG data for each muscle were normalized to the peak activity obtained during the three trials for each individual. The ratios of right SCM to left SCM (RT SCM: LT SCM ratio) muscle activation, as well as right UT to left UT (RT UT: LT UT ratio) muscle activation, was used to make inferences about postural symmetry/asymmetry. A ratio of >1, indicated that there was more muscle activity in the RT SCM or RT UT, respectively; a ratio of <1, indicates more muscle activity in the LT SCM or LT UT, respectively, while a ratio equal to 1 indicates symmetry/equal muscle activity on both sides of the body.
Data were summarized with descriptive statistics of mean and standard deviation while inferential statistics of repeated measures within group Analysis of Variance was used to compare the normalized EMG values of each muscle across the three trials. P value was set at 0.05. Data were analyzed with Statistical Package for Social Sciences (SPSS) software (version 20 SPSS, Inc. Chicago, Illinois).
| Results|| |
Mean age, height, weight, body mass index, and bust circumference of the participants were 29.32 ± 4.49 years, 1.65 ± 0.06, 76.00 ± 12.53 kg, 27.95 ± 4.30 kgm2, and 100.32 ± 7.17 cm, respectively.
From [Table 1], there were no significant differences in the EMG activities of each of the right trapezius (P = 0.610), left trapezius (P = 0.193), right sternocleidomastoid (P = 0.137), and left sternocleidomastoid (P = 0.667) muscles across the three BF positions. The symmetry ratios of each of the SCM (P = 0.493) and UT (P = 0.291) muscles did not vary significantly, [Table 2]. Respectively, FB and C2 hold marginally elicited the least asymmetry ratios in the UT and SCM muscles.
|Table 1: Comparison of the values of the EMG activities of the selected neck muscles in the three selected breastfeeding positions (n=22)|
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|Table 2: Comparison of the values for the asymmetry ratio of neck muscles in the three selected breastfeeding positions (n=22)|
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| Discussion|| |
This study compared the electrical activities of the sternocleidomastoid and upper trapezius muscles across the cradle (C1), cross-cradle (C2), and football (FB) breastfeeding hold. The scarcity of related studies was a major challenge to the discussion of this study's findings. Contrary to our hypothesis, neck muscle activities did not vary significantly across the studied breastfeeding trials. The varying maternal body orientation across the three BF positions formed the basis for our hypothesis. In a previous related study, the activities of the back muscles differed significantly across these three BF positions. Thus, the present study findings contradicted theirs such that despite the neck motions involved during each BF task, related muscular activities were not significant.
A few factors may be responsible for the contradictory findings between the present study and the previous one. First, the population of this study is nursing mothers who have been engaging in the act of BF prior to this study while the previous related study involved young nulliparous females. There is a possibility that the survivor effect, may have contributed to the physiological responses observed in the nursing mothers as physiological conditioning may occur with consistent performance of physical tasks. Considering the prolonged nature of BF tasks, nursing mothers may have adapted to some of the musculoskeletal responses of BF since repeated stimulus is a factor of physiological adaptation. Thus, the observed response in nulliparous females in the previous study may not have been similar to such conditioning. Second, the previous related study simulated BF with an infant dummy of uniform size and weight throughout the study whereas, the present study involved life infants of different anthropometric characteristics. Considering that for all the trials in the present study, only one breastfeeding pillow was used to support the infants, the varying anthropometric characteristics of these infants will undoubtedly affect the orientation of the mother and infant. For instance, larger infants will definitely be positioned closer to the nursing breast while smaller ones will appear farther from the breast. A close approximation of the infant to the nursing breast will likely result in a decreased neck flexion to maintain a gaze with the infant while the neck range of motion may appear lesser with a smaller infant. As a result, neck flexion in the present study was invariably non-uniform, as compared to the previous study that controlled for extraneous variables by the utilization of a uniform infant dummy size. Nevertheless, the choice of nursing mothers in the present study was to gain a clearer picture of the BF-related responses of the neck musculature.
Despite the presence of non-significant differences in muscular activities across the three BF trials in the present study, we observed some marginal differences revealing higher activities in the right and left components of both muscles during the FB and C2 holds respectively. The FB hold involved supporting the left with the right hand, while the C2 hold involved supporting the left breast with the left hand. As a result, the ipsilateral neck muscles of the supporting upper limbs seemed most active during the FB and C2 holds. This aroused our consideration of possible relationships between upper limb position and the activities of the neck muscles as previous studies had suggested synergistic activities between upper limb and neck muscles., However, more studies to better elucidate the relationships between arm positions and BF-related neck pain are recommended.
The present study also showed that predominantly, left components of the studied muscles showed higher activities than their right counterparts for each muscle. We attributed these findings to the fact that for uniformity purposes, all the participants were nursed from the left breast. Thus, while the infants were positioned on their mothers' thighs, their head positions were situated to the left side of their mothers while the feet were towards her right side. To maintain a gaze with the infant during BF, the mothers adopted a forward neck flexion posture with slight left lateral rotation and side flexion. Forward neck flexion is a function of the bilateral contraction of the sternocleidomastoid muscles while left lateral rotation is initiated by the unilateral contraction of the right SCM. The left upper trapezius muscle functions as a left side flexor of the neck, alongside other superficial back muscles. Considering that forward flexion dominated the neck orientation in this BF scenario, bilateral contraction of the SCM should be inherent in that task. Since the forward neck flexion was with more emphasis on the left side of the maternal body where the infant's head was positioned, there is a possibility of expecting more muscular action from the left SCM. The slight left side flexion also implicated the higher activities recorded for the left UT muscle, as compared to its right component. Sustained neck postures in flexed and rotated positions during BF will undoubtedly result in stress and excessive tension in the muscles around the neck and shoulders with a possibility of muscular imbalance, postural abnormalities, and myofascial pain syndrome. This suggests the need for educating mothers on alternating nursing breasts within and between nursing sessions.
The predominance of activities in the left components of both studied muscles suggests possible asymmetry between the actions of both muscles. Co-contraction of muscles is a prerequisite for muscular and postural balance, biomechanical stability, and prevention of abnormal postural compensations during a task. Failure of muscles to co-contract results in abnormal muscle lengths, while performing a task which can lead to muscle fatigue, decreased range of motion as well as biomechanical alterations with resultant musculoskeletal pain. The three BF trials studied were characterized by asymmetry in muscular activities with the neck muscles ipsilateral to the nursing breast being more active. Comparing the asymmetry ratios of each of the SCM and UT across trials showed no significant differences. However, the FB hold elicited the least asymmetry ratio in the UT muscles, while the highest SCM muscles asymmetry was observed during the C2 hold.
Our study failed to consider the baseline activities of the studied muscles prior to the trials. This information would have enabled a better understanding of the study outcomes. Second, failure to establish a uniform distance between the nursed infant and the head was a major study limitation. Considering that the anthropometric characteristics of the infant affected how close or far away they were positioned from the breast, some level of uniformity should have been established by altering the sizes of the BF pillow so as to achieve a closer range of neck forward flexion among the participants. In essence, further studies should ensure that the neck ranges of motion per trial are standardized for all the participants to eliminate the confounding effects of joint range of motion (ROM) on the studied muscular activities. Cohort studies exploring other biomechanical changes associated with BF with subsequent applications in real-life scenarios are further recommended.
| Conclusion|| |
Neck muscle activities did not differ across the cradle, cross-cradle, and football breastfeeding hold positions. Neck muscle ipsilateral to the nursing breast showed higher activities than their contralateral components. Maternal education on postural correction during breastfeeding, alternating nursing breasts, and inclusion of neck stretches into their breastfeeding routines is highly recommended.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
| References|| |
Mbada CE, Oyinlola FC, Olatunbosun TO, Awotidebe TO, Arije OO, Johson OE, et al
. Is baby-friendly breastfeeding mother-friendly? J Womens Health Phys Ther 2013;37:19-20.
Victoria CG, Barros AJ. Effect of breastfeeding on infant and child mortality due to infectious diseases in less developed countries: A pooled analysis. Lancet 2000;355:451.
World Health Organization. Global Strategy on Infant and Young Child Feeding. 2003.
Chantry CJ, Eglash A, Labbok M. Position on breastfeeding. Breastfeed Med 2008;3:267-71.
Leon-Cava N, Chessa L, Jay R, Luann M. Quantifying the Benefits of Breastfeeding: A Summary of the Evidence. Washington DC: Pan American Health Organization; 2002.
Kramer MS, Guo T, Platt RW, Sevkovskaya Z, Dzikovich I, Collet JP, et al
. Infant growth and health outcomes associated with 3 compared with 6 months of exclusive breastfeeding. Am J Clin Nutr 2003;78:291-5.
Ogbonna C, Okolo SN, Ezeogu A. Factors influencing exclusive breast-feeding in Jos, Plateau State, Nigeria. West Afri J Med 2000;19:107-10.
Akobeng A, Heller R. Assessing the population impact of low rates of breastfeeding on asthma, celiac disease and obesity: The use of a new statistical method. Arch Dis Child 2007;92:483-5.
Robenblatt K, Thomas D. Lactation and the risk of epithelial ovarian cancer. Int J Epidemiol 1993;22:192-7.
Kinpikul N, Srichandr P, Poolthong N, Thavarungkul N. Factors affecting low back pain during breastfeeding of Thai women. World Acad Sci Eng Technol 2010;72:289-92.
World Health Organization. Infant and Young Child Feeding. Model Chapter for Text Books for Medical Students and Allied Health Professions. 2009. p. 65-77. NS125.
Grimes P, Legg S. Musculoskeletal disorders (MSD) in school students as a risk factor for adult MSD: A review of the multiple factors affecting posture, comfort and health in classroom environments. J Hum Environ Syst 2004;7:1-9. doi: 10.1618/jhes7.1.
Borg-stein J, Dugan SA. Musculoskeletal disorders of pregnancy, delivery and postpartum. Phys Med Rehabil Clin N Am 2007;18:459-76.
Nuńez DE, Keller C, Ananian CD. A review of the efficacy of the self-management model on health outcomes in community-residing older adults with arthritis. Worldviews Evid Based Nurs 2009;6:130-48.
Ojukwu CP, Anyanwu GE, Nwabueze AC, Anekwuh F, Chukwu SC. Prevalence and associated factors of work related musculoskeletal disorder among commercial milling machine operators in Southeastern Nigeria markets. Work 2017;58:473-80.
Mbada C, Olowokere A, Faronbi J, Faremi F, Oginni M, Oyinlola AF, et al
. Breastfeeding profile and practice of Nigerian mothers: A cross-sectional survey. Int J Reprod Contracept Obstet Gynecol 2014;3:969. doi: 10.5455/2320-1770.ijrcog20141218.
Palikhe S, Yirong M, Choi BY, Lee DE. Analysis of musculoskeletal disorders and muscle stresses on construction workers' awkward postures using simulation. Sustainability 2020;12:5693. doi: 10.3390/su12145693.
Mbada C, Olowookere A, Faronbi J, Oyinlola AF, Faremi F, Ogundele A, et al
. Knowledge, attitude and techniques of breastfeeding among Nigerian mothers from a semi-urban community. BMC Res Notes 2013;6.552. doi: 10.1186/1756-0500-6-552.
United Nations International Children Emergency Fund. Global Strategy for Infant and Young Child Feeding. Geneva: World Health Organization; 2008.
Foxman B, Schwartz K, Looman SJ. Breastfeeding practices and lactation mastitis. Soc Sci Med 1994;38:755-61.
Colson SD, Meek JH, Hawdon JM. Optional positions for the release of primitive neonatal reflexes stimulating breastfeeding. Early Hum Dev 2008;84:441-9.
Goyal RC, Banginwar AS, Ziyo F, Toweir AA. Breastfeeding practices: Positioning, attachment (latch-on) and effective suckling – A hospital-based study in Libya. J Family Community Med 2011;18:74-9.
Hobbs AJ, Mannion CA, McDonald SW, Brockway M, Tough SC. The impact of caesarean section on breastfeeding initiation, duration and difficulties in the first four months postpartum, BMC Pregnancy Childbirth 2016;16,90. doi: 10.1186/s12884-016-0876-1.
Forster DA, McLachian HL, Lumley J. Factors associated with breastfeeding at six months postpartum in a group of Australian women. Int Breastfeed J 2006;1:18. doi: 10.1186/1746-4358-1-18.
Ip S, Chung M, Raman G, Chew P, Magula N, DeVine D, et al
. Breastfeeding and maternal and infant health outcomes in developed countries. Evid Rep Technol Assess (Full Rep) 2007;:1-186.
Rani S, Habiba U, Qazi W, Tassadaq N. Association of breastfeeding positioning with musculoskeletal pain in postpartum mothers of Rawalpindi and Islamabad. J Pak Med Assoc 2019;69:564-6.
Ezeukwu OA, Ojukwu CP, Okemuo AJ, Anih CF, Ikele IT, Chukwu SC. Biomechanical analysis of the three recommended breastfeeding positions. Work 2020;66:183-91.
Cohen J. Statistical Power Analysis for the Behavioural Sciences. 2nd
ed. Lawrence and Erlbaurn Associates; 1988. p. 101.
Sommerich CM, Joines SM, Hermans V, Moon SD. Use of surface electromyography to estimate neck muscle activity. J Electromyogr Kinesiol 2000;10:377-98.
Netto KJ, Burnett AF. Reliability of normalization methods for EMG analysis of neck muscles. Work 2006;26:123-30.
Shin SJ, Yoo WG, Kim TY. Effects of different overhead work conditions on the neck and shoulder muscle. J Phys Ther Sci 2012;24:197-9.
Richardson D, Wing S, Steenland K, Mckelvev W. Time-related aspects of the healthy worker survivor effect. Ann Epidemiol 2004;14:633-9.
Chevrier J, Picciotto S, Esien EA. A comparison of standard methods with g-estimation of accelerated failure-time models to address the healthy-worker survivor effect: Application in a cohort of autoworkers exposed to metalworking fluids. Epidemiol 2012;23:212-9.
Pittino F, Kliegl KM, Huckauf A. Subjective, physiological and behavioural responses towards evaluatively conditioned stimuli. Cogn Emot 2018;32:1082-96.
Gizzi L, Muceli S, Petzke F, Falla D. Experimental muscle pain impairs the synergistic modular control of neck muscles. PLoS One 2015;10:e0137844. doi: 10.10.1371/journal.pone. 0137844.
Lee S, Choi YH, Kim J. Effects of the cervical flexion angle during smartphone use on muscle fatigue and pain in the cervical erector spinae and upper trapezius in normal adults in their 20s. J Phys Ther SCi 2017;29:921-3.
Bordoni B, Varacallo M. Anatomy, head and neck, sternocleidomastoid muscle. In StatPearls. StatPearls Publishing, Treasure Island (FL) 2018.
Moore KL, Agur AM, Dalley AF. Essential Clinical Anatomy. 5th
ed. Baltimore, MD: Lippincott Williams & Wilkins; 2011.
Kim JH, Lee HS, Park SW. Effect of the active release technique on pain and range of motion of patients with chronic neck pain. J PhysTherSci 2015;27:2461-4.
Granata K, Lee P, Franklin T. Co-contraction recruitment and spinal load during isometric trunk flexion and extension. Clin Biomech (Bristol, Avon) 2006;20:1029-37.
Lin JZ, Lin YA, Lee HJ. Are landing biomechanics altered in elite athletes with chronic ankle instability. J Sports Sci Med 2019;18:653-62.
[Table 1], [Table 2]