Ergonomics Vol. 51, No. 6, June 2008, 890–901

Changes in neck muscle electromyography and forward head posture of children when carrying schoolbags M.H. Kima, C.H. Yib*, O.Y. Kwonb, S.H. Chob and W.G. Yooc a

Department of Rehabilitation Therapy, The Graduate School, Yonsei University, Republic of Korea; Department of Physical Therapy, College of Health Science, Yonsei University, Republic of Korea; c Department of Physical Therapy, College of Biomedical Science and Engineering, Inje University, Republic of Korea

b

This study tested the effects of three alternative types of backpack on head posture and neck muscle electromyography (EMG) in children. Four loading conditions were tested: no pack; a backpack; a double pack; a modified double pack (designed with a backpack and a front pack weighing 10% and 5% of body weight, respectively). Dependent variables were neck muscle activity, forward head angle and forward head distance (the perpendicular distance from C7 to a vertical line through the tragus of the ear). Fifteen children were asked to walk at a speed of 0.8 m/s on a treadmill. The EMG activity of upper trapezius, sternocleidomastoid and midcervical paraspinals muscles and the forward head angle and forward head distance were all significantly higher when carrying a backpack than for the other conditions. When carrying a double pack, there was a backward head posture characterised by an increased negative forward head angle, decreased forward head distance, increased sternocleidomastoid EMG signal and decreased midcervical paraspinals EMG signal, compared to carrying no pack. When carrying a modified double pack, the forward head angle and forward head distance decreased when compared to carrying a backpack. These findings indicate that the modified double pack minimises postural deviation. Keywords: electromyography; forward head posture; schoolbag; children

1.

Introduction

Proper posture is considered to be a state of musculoskeletal balance that involves a minimal amount of stress or strain to the body (Griegel-Morris et al. 1992). Kendall et al. (2005) described a standard for normal sagittal alignment involving the theoretical straight line formed by the points of reference consisting of the lobe of the ear, the seventh cervical vertebra, the acromion, the greater trochanter, just anterior to the midline of the knee and slightly anterior to the lateral malleolus. Deviation from normal alignment (i.e. postural abnormality) suggests the presence of imbalance and abnormal strain on the musculoskeletal system (Braun 1991). Alignment has been considered ‘poor’ when the head is held forward in relation to the trunk and characteristics referred to as poor include ‘forward head’, ‘poke chin’ and ‘rounded shoulders’ (Raine and Twomey 1997). A forward head posture is considered a cervical musculoskeletal variation that has been associated with shortening of the posterior neck extensor muscles and tightening of the anterior neck muscles (Fernandez-de-las-Penas et al.

*Corresponding author. Email: [email protected] ISSN 0014-0139 print/ISSN 1366-5847 online Ó 2008 Taylor & Francis DOI: 10.1080/00140130701852747 http://www.informaworld.com

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2005). The forward head posture also implies a relatively extended upper cervical spine and a relatively flexed lower cervical spine (McKenzie 2006) and has been associated with neck and shoulder pain (Haughie et al. 1995). Cureton (1941) found that the mean sagittal plane head posture angle (also called the craniovertebral angle, measured between a line through the seventh cervical spinous process and the tragus of the ear and a horizontal line through C7) in more than 600 men was 53.68 anteriorly and Raine and Twomey (1997) reported the value to be 48.98 anteriorly in 160 adults. Any decrease in this angle (i.e. increase of the forward head posture) has been suggested to increase both neck and shoulder pain (Moore 2004). The epidemiological and clinical literature identifies strong associations between spinal posture and the use of a backpack (Jull et al. 2002, Di Palma et al. 2005). Backpacks of various types are widely used by hikers, soldiers and schoolchildren. Recreational hikers commonly carry subsistence and shelter items in backpacks (Fletcher 1974) and foot soldiers often walk for long distances carrying extremely heavy backpack loads (McCaig and Gooderson 1986). A number of backpack-carrying studies have involved the military and have focused on the physiological, biomechanical and medical aspects (Knapik et al. 2004, Christie and Scott 2005). Although researchers have studied various backpack types and designs with the aim of preventing injuries associated with prolonged load carriage in hikers and soldiers, few have focused on backpack use by schoolchildren (Chansirinukor et al. 2001). Repetitive static and dynamic loading of the spine constitutes a risk factor for lowback, shoulder and neck pain not only in adults but also in children (Balague et al. 1999, Chansirinukor et al. 2001, van Gent et al. 2003). Pascoe et al. (1997) found that the prolonged carrying of heavy backpacks could lead to symptoms of body soreness, aches, pains and tiredness in children. The child’s spine differs from the adult spine in two important respects: (i) a child’s skeleton has large amounts of cartilage that is susceptible to repetitive micro trauma, weakness of which decreases soft-tissue flexibility, induces muscle imbalances and can also lead to injury (Micheli and Fehlandt 1992); (ii) the highest rate of growth occurs in schoolchildren when they are 10–15 years of age (Rowland 1996) and they are thought to be less able to withstand the stresses that the adult spine can cope with (Grimmer and Williams 2000). Recognition of the importance of studies involving children has led to reports on the effects of backpacks on postural changes and musculoskeletal impairments (van Gent et al. 2003). However, many of these studies have focused on segments of the back such as trunk forward lean (Cottalorda et al. 2004), back muscle activity (Motmans et al. 2006) and back pain (Skaggs et al. 2006) and there have been few reports on neck posture and neck muscle activity whilst carrying a backpack. Both back and neck segments are influenced by backpack loads (Grimmer et al. 2006). Abnormal postural changes due to the carrying of a backpack may induce round shoulder, forward head posture, changes in neck muscle activity and muscle fatigue. Moreover, Brattberg (2004) reported that risk factors such as load carriage, abnormal neck posture and neck muscle fatigue may induce headache in schoolchildren. Based on this background, this study assessed the electromyography (EMG) activities of the neck muscles and the head posture in children aged 9–11 years while they were carrying three alternative types of schoolbag (backpack, double pack and modified double pack) and compared these with carrying no pack. 2. Methods 2.1. Subjects A total of 15 children (10 boys and five girls) with a mean age of 10.3 years (9–11 years), a mean weight of 33.64 kg (29.6–40.0 kg), a mean height of 142.67 cm (134.6–151.5 cm) and

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a mean BMI of 16.52 kg/m2 (15.0–18.2 kg/m2) participated in this study. They were selected from 21 children who attended a local elementary school, of whom six children (one boy and five girls) resigned during the treadmill gait testing. The subjects were healthy and reported no musculoskeletal pain that would interfere with their performance in the experiments. All subjects were right-handed. Before the schoolbag testing, the subjects and their parents were informed about the purpose, procedures and applications of the study and parental agreement was obtained. Ethical approval was obtained from the Yonsei University Faculty of Health Sciences Human Ethics Committee and the subjects provided written informed consent prior to their participation. 2.2. Study design To determine the effects of different schoolbag designs, the neck muscle activity and neck posture during the carrying of schoolbags were studied in the four trials involving the subjects walking on a treadmill: with no pack; a backpack; a double pack; a modified double pack (as shown in Figure 1). The subjects wore lightweight long-sleeve shirts, socks and sports shoes. In the starting position on the treadmill each subject stood comfortably erect with the knees extended and the feet separated by approximate shoulder width. All subjects were asked to walk at a speed of 0.8 m/s, which is a comfortable walking speed for children, and to keep their head looking straight ahead for 5 min. During this time, the following gait parameters were recorded: (i) the amplitudes of the EMG signals of the upper trapezius (UT), sternocleidomastoid (SCM) and midcervical paraspinals (MPS); (ii) the positions of markers attached to the C7 spinous process and tragus of the ear (for motion analysis). The EMG and kinematic data were obtained from the last 30 s of each 5min data collection period. This interval was chosen as the interval for analysis due to the significant difference found between the first 30 s and the last 30 s of the 5-min treadmill walk in a pilot study. The test order was assigned randomly to prevent any test-order effect, and muscle fatigue was avoided by providing a 10-min rest interval between trials for each subject,

Figure 1. pack.

Carrying the various schoolbags. (A) Backpack; (B) double pack; (C) modified double

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during which the subject sat on a comfortable chair. Before the commencement of the experiment, each subject was given ample time to familiarise themselves with the use of the treadmill for the walking trials. 2.3. Schoolbag parameters The schoolbag load weight used in this study was 15% of body weight, since the loadweight carried by school-age children reportedly varies from 10% to 20% (Grimmer and Williams 2000, Whittfield et al. 2001, Sheir-Neiss et al. 2003, Limon et al. 2004). Preparative books and weights (1 kg) were used to achieve this load weight. 2.3.1. Backpack condition The centre of the backpack used in this study (Figure 1a) weighing 15% of body weight was placed between the T11 and T12 levels on each subject (Korovessis et al. 2005). The subjects adjusted the shoulder straps of the schoolbag according to their personal comfort preferences. 2.3.2. Double pack condition The double pack consisted of a front pack and a backpack, both of the same type as that used in the backpack condition and each weighing 7.5% of body weight. The centre of the front pack was positioned at the umbilicus of the abdomen (Figure 1b) and the subjects adjusted the shoulder straps according to their personal comfort preferences. 2.3.3.

Modified double pack condition

The modified double pack was especially made for this study and consisted of two types of packs; these were loaded so that the backpack and front pack weighed 10% and 5% of the body weight, respectively. The front pack was half the size of the backpack and its shoulder straps were adjusted by the primary researcher. The front pack was positioned on the sternum (not the abdomen) and its centre was located at the xiphoid process of the sternum (Figure 1c).

2.4.

Experimental equipment

2.4.1. Electromyography system The EMG data were collected using a Biopac MP100WSW (Biopac System, Santa Barbara, CA, USA) data acquisition system and a Bagnoli EMG system (Delsys, Boston, MA, USA). Ag-AgCl surface EMG electrodes (3 6 2 cm) were positioned at an interelectrode distance of 20 mm. Electrode sites were prepared using skin abrasion and by cleansing the area with alcohol. The active surface electrodes were aligned approximately parallel to the direction of the muscle fibres. The electrode sites were located on each subject’s dominant right side as follows (Cram et al. 1998): (i) UT, 2 cm lateral to the midline drawn between the C7 spinous process and the posterolateral acromion; (ii) SCM, 2 cm distal to the muscle insertion at the mastoid process; (iii) MPS, 2 cm lateral to the midline of the spine approximately at the C4 level. All EMG signals were amplified, bandpass (20 Hz to 450 Hz) and bandstop (60 Hz) filtered, and digitised at 1000

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Hz using AcqKnowledge software (Biopac System). The root mean square values of the raw data were calculated, with the amplitude normalised to reference isometric voluntary contractions (RVC) rather than to the maximal voluntary isometric contractions. This was done to reduce the risk of injury or residual muscle soreness, especially in the neck and shoulder (Harms-Ringdahl and Ekholm 1986). The reference isometric contractions of the SCM and MPS were determined with the subjects looking straight ahead while keeping the neck in line with the spine in the supine position (Babski-Reeves et al. 2005). The UT reference isometric contractions were determined by holding the arms abducted 908 in the frontal plane and parallel to the floor (Babski-Reeves et al. 2005). Subjects completed three voluntary isometric 5-s exertions with a 1-min rest period between contractions. The EMG data collected during walking on the treadmill for 5 min are expressed as the %RVC. 2.4.2.

3-D Motion analysis system

A 3-D ultrasonic motion analysis system (CMS-HS; Zebris, Medizintechnik, Isny, Germany) was used to measure the forward head angle and the forward head distance (Figure 2). The positions of the two markers were sampled at 20 Hz during walking on the treadmill for 5 min. The markers were visible to the measuring sensor, which consisted of three microphones used to record the ultrasonic signals. The forward head angle was defined as the angle between a horizontal line at C7 and a line from the tragus of the ear to the spinous process of C7 (Fernandez-de-las-Penas et al. 2005). The measured angles were normalised to 08 relative to the starting position, namely, neutral position, where a positive forward head angle larger than 08 indicated an increase in the forward head posture, and a larger negative head angle indicated an increase in the backward head posture. The forward head distance was defined as the perpendicular distance from C7 to a vertical line through the tragus, with an increased value of this distance compared with the starting position, indicating the presence of forward head posture. The collected kinematics data were analysed by Win-data software (ver. 2.19; Zebris).

Figure 2.

Measurements of the forward head angle (A) and forward head distance (B).

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Statistical analysis

To test for differences in muscle activities, forward head angles and forward head distances when carrying the various schoolbags, repeated-measures ANOVA was used to determine if there was a significant effect. For a significant main effect, Bonferroni’s correction was performed to identify the specific mean differences. Differences were defined as significant at p 5 0.05.

3.

Results

3.1. Upper trapezius electromyography The normalised EMG data are presented in Table 1. When compared to walking without a pack, the EMG activity of the UT was significantly higher while carrying the backpack, double pack and modified double pack (p 5 0.05) (Figure 3). The EMG activity did not differ significantly between these three packs.

Table 1. Electromyographic muscle activities (% reference isometric voluntary contractions, mean + SD) (n ¼ 15).

Upper trapezius Sternocleidomastoid Midcervical paraspinals

No pack

Backpack

Double pack

Modified double pack

p

23.5 + 2.35 14.0 + 7.11 26.4 + 9.36

33.3 + 4.25 22.8 + 12.53 38.0 + 14.96

39.5 + 12.20 24.5 + 11.21 22.2 + 7.49

35.2 + 6.46 17.7 + 10.50 28.4 + 11.45

50.001 0.001 50.001

Figure 3. Mean neck muscle activation levels during carrying (with Bonferroni’s correction for multiple comparisons applied). RVC ¼ reference isometric voluntary contractions; UT ¼ upper trapezius; SCM ¼ sternocleidomastoid; MPS ¼ midcervical paraspinals. Note: *Significantly different from no pack condition (padj 5 0.05/6), **Significantly different from modified double pack condition (padj 5 0.05/6), Error bars represent SD.

896 3.2.

M.H. Kim et al. Sternocleidomastoid electromyography

As can be seen in Table 1, the SCM EMG activity increased when the load was carried by the backpack or double pack above that without a pack (p 5 0.05, Figure 3) and was significantly lower for the modified double pack than for the backpack or double pack (p 50.05). In addition, the SCM EMG activity did not differ significantly between the no pack and the modified double pack conditions. 3.3.

Midcervical paraspinals electromyography

The normalised EMG activity of the MPS, shown in Table 1, increased in order of double pack5no pack and modified double pack5backpack, with the increase being most significant for the backpack (p 5 0.05) and with the decrease being most significant for the double pack (p 5 0.05). There was no significant difference between the modified double pack and no pack (Figure 2). 3.4.

Neck posture

Table 2 shows the forward head angle and forward head distance. The one-way ANOVA revealed significant overall differences between conditions. The mean forward head angle was most significantly increased when carrying the backpack (p 5 0.05) (Figure 4). The mean forward head angle was lowest for the double pack. Carrying the modified double pack had the least effect when compared with the no pack condition (p 4 0.05) (Figure 4). The mean forward head distance was greater for the backpack than for the no pack condition, or for the double pack and modified double pack conditions (Table 2). The mean forward head distance for the double pack was smaller than for the no pack condition (p 5 0.05) (Figure 4). There was no significant difference between the double pack conditions. 4.

Discussion

The purpose of this study was to characterise the comparative neck muscle activity, forward head angle and forward head distance for the control condition when not carrying a backpack and when carrying the backpack, double pack and modified double pack. The measured EMG activities of three muscles (UT, SCM and MPS) were significantly higher when carrying the backpack than when not carrying a backpack. Moreover, the forward head angle and forward head distance increased significantly for the backpack. The centre of gravity of the body is normally located slightly forward of the lumbosacral joint (Mihelj et al. 2000). Bobet and Norman (1984) reported that the addition of a pack load on the back resulted in the combined centre of gravity of the body plus pack shifting backward

Table 2.

Forward head angle and forward head distance (mean + SD) (n ¼ 15).

Angle (8) Distance (mm)

No pack

Backpack

Double pack

Modified double pack

p

4.1 + 3.49 27.1 + 7.24

7.5 + 5.34 43.3 + 17.89

70.7 + 5.06 9.0 + 13.35

5.0 + 3.43 31.4 + 14.41

0.004 0.001

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Figure 4. Mean forward head angles and forward head distances during carrying (with Bonferroni’s correction for multiple comparisons applied). NP ¼ no pack; BP ¼ backpack; DP ¼ double pack; M-DP ¼ modified DP. Note: *Significantly different from NP condition (padj 5 0.05/6), **Significantly different from M-DP condition (padj 5 0.05/6), Error bars represent SD.

and creating extension moments, which were counterbalanced by both a forward trunk lean and a forward head shift (Goh et al. 1998). These explanations support the backpack increasing the forward head posture in the present study. Theoretically, an anterior shift in the centre of gravity of the head elicits the head and neck postural reflexes involving the vestibulocollic (Wilson et al. 1995), cervicocollic (Peterson et al. 1985) and cervical-facet mechanoreceptors. These respond to the forward head position of the postural stimulation by actively orienting the trunk’s centre of gravity under the head’s centre of gravity (Morningstar et al. 2004). Although this postural change maintains efficient body locomotion by minimising the energy expenditure (Adkin et al. 2000), this abnormal posture can induce musculoskeletal pain when it is sustained (Pascoe et al. 1997, Babski-Reeves et al. 2005). For the double pack, the UT and SCM EMG activity was significantly higher than without a backpack or with the modified double pack. The forward head angle was significantly increased for the double pack, but this was in the opposite direction. This indicates that the head was shifted backward relative to the starting position. As the head

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moves backward, beyond the neutral position, it is considered that the SCM activity increases greatly to counteract the head backward movement. Motmans et al. (2006) also reported that the trunk posture was shifted backward while carrying a front pack and Fiolkowski et al. (2006) reported that carrying a front pack resulted in the head being moved backward relative to the control condition without a backpack. Similarly, the results of the present study for the double pack are consistent with the results of these front pack studies. It was thought that the distance between the front pack (at the abdomen) and spine would be greater than that between the backpack and the spine because the abdominal region comprises the abdominal cavity and many fat and muscular sheets (Meuwly and Gudinchet 2004), and so the flexion moment would be larger than the extension moment for an equivalent weight loading (given that moment ¼ force 6 distance). The net effect would result in a backward head posture to compensate the flexion moment when carrying the double pack, similar to the effect of carrying a front pack. The modified double pack was designed to improve the restrictions of the double pack. First, distributing the pack weight between the back and front in the ratio of 2:1 was designed to improve the load distribution relative to using the double pack (for which the load distribution was 1:1). Second, the front pack of the modified double pack was designed to be smaller than the front pack of the double pack, since Knapik et al. (1997) reported that the double pack could result in inhibition of movement, reduction of respiratory ventilation and discomfort due to restriction of the abdomen. In the present study, the modified double pack improved the restriction of the abdomen associated with using the double pack. For the modified double pack, it was found that UT EMG activity significantly increased and that SCM and MPS EMG activity were slightly, although not significantly, increased when compared with the no pack condition. The UT EMG activity was thus exposed to a constant load in all conditions, resulting in increased muscle activity. The mean forward head angle and forward head distance were significantly lower for the modified double pack than for the backpack and appeared to be slightly but not significantly increased relative to the no pack condition. As a result, it was shown that carrying the modified double pack had little effect on forward head posture, when compared to the forward head effect with the backpack and the backward head effect with the use of the double pack. This represents evidence that the modified double pack minimised forward head posture. There are data indicating that sustaining the head in protraction leads to pain over the dorsal aspect of the cervical and upper thoracic spine (Harms-Ringdahl and Ekholm 1986). This may be explained by the increase in the joint shear stress of the spine due to the neck shifting forward of its normal posture and with the compressive forces on the neck increasing due to the additional weight of the head. Fabris et al. (2004) suggested that applying a significant load to the spine for a sustained period results in deformation of and abnormal changes to spinal tissues, which can become permanent. These changes may often result from unilateral myofascial forces acting on the musculoskeletal system, jamming spinal facet joints and irritating sensitive joint receptors. Whilst the generation of musculoskeletal pain by load carrying remains controversial (Cardon and Balague 2004, Burton et al. 2006), abnormal postures induced by carrying a pack have been considered as possible risk factors for musculoskeletal pain in schoolchildren (Negrini and Negrini 2007). The present study indicates that using a modified double pack designed for an improved load distribution may encourage the adoption of erect postures that are present in the no pack condition.

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Yoo et al. (2006) reported that the posture of the trunk and pelvis, which are more proximal, affect the posture of the head and shoulder, which are more distal. However, the present study only found the changes that occurred in the head and neck and hence future investigations should focus on not only the motion of the head and neck of children carrying a schoolbag, but also the posture of the trunk and pelvis. The use of the treadmill in this study allowed a consistent walking speed and simulated the environment of walking in the street. In general, kinematic data and EMG activity obtained at both slow and fast speeds do not differ between treadmill walking and floor walking (Myrray et al. 1985). However, the data on the similarity of effects between treadmill walking and ground walking are limited. Also, the data were obtained during a short recording time from a small sample size, so that longer duration changes in the neck muscle EMG and the forward head posture were not addressed. Such a further study will need to recruit many schoolchildren and to record the EMG and postures for longer durations. 5.

Conclusions

This study showed that carrying a backpack increased forward head posture and that the double pack induced a backward head posture beyond the neutral position. However, the use of a modified double pack decreased the sagittal deviation of the head and neck posture for the children. It is considered that the modified double pack minimises the forward head posture by redistributing the loads carried. Acknowledgements This research was supported by the Regional Research Center Program, which was conducted by the Ministry of Commerce, Industry and Energy of the Korean Government.

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