Introduction

Chronic neck pain caused by degenerative spinal disorders is an increasing global problem and the association with reduced neck muscle strength (NMS) and forward head/neck posture are generally accepted as being important factors [1–11]. They have become more frequent among younger ages, especially due to computer use and as a consequence of minor whiplash trauma [12–15]. Neck muscle strengthening in the rehabilitation of chronic neck pain has already been investigated in adults [16–22]. There is a paucity of data on neck posture, mobility and NMS in school aged children and the correction of neck posture by restoring neck muscle strength in children has not been published. Objectives of this survey were to develop methods for head, neck and shoulder posture, neck mobility, neck muscle strength measurement among schoolchildren and furthermore to establish correlations between data in different age groups. The aim of measuring neck muscle strength in nodding is to evaluate the strength of deep neck flexors which are predominantly responsible for neck posture. Exercises for strengthening neck muscles, performed at schools was also investigated for their effect.

Subjects and methods

Subjects

In this study 428 boys and girls aged 9, 12 and 16 years old in the same proportion were surveyed. Informed consent was obtained from all children and their parents and approval was also obtained from the local ethics committee.

Methods

  1. 1.

    Anthropometric data included measuring weight and height that were calculated to form the BMI values. The neck length was measured by a tape measure as the distance between spinal processes of C 2–7 first strained (A) and then laid on to the neck curvature (B). The A/B values were calculated as the neck lordosis index.

  2. 2.

    To measure neck posture digital photos were taken while the subjects were sitting in a neutral head position [22]. The digital photos were evaluated using a computer software program for three angles [2, 4] as follows (see Fig. 1):

    1. a.

      Craniovertebral angle (CVA) between the line connecting the tragus and C 7 spinal process and the x‑axis,

    2. b.

      Head tilt angle (HTA) between the line connecting the tragus and the base of the nose and the y‑axis,

    3. c.

      Shoulder angle (SHA) between the line connecting the acromion and C 7 spinal process and the x‑axis.

  3. 3.

    The isometric muscle strength in nodding, flexion, extension, left and right lateral flexion was measured using a strain gauge dynamometer. The device was fixed on one side to the wall and on the other side to the subject’s head by a helmet. The subjects were stabilized using a belt on their thighs and held at their shoulders by a physiotherapist who controlled the procedure in order to make sure only the neck muscles were used. The subjects pulled the dynamometer with their head and always turned in the appropriate direction. The maximal isometric force “breaking force” was measured in flexion, extension, lateral flexion to the right and to the left and also in nodding (that meant chin tuck with neck extended). Each motion was performed for 10 s, with resting intervals of 20 s and was repeated three times. The measurements were expressed in Newton, the device’s measuring range was 1–250 N, with a minimum of 1 N. An average of the efforts in each direction was calculated and was summed up to a value of total neck muscle strength.

  4. 4.

    Mobility was measured in degrees by a goniometer constructed as a combination of an inclinometer and a compass [23]. This instrument is able to perform measurements in flexion, extension, lateral flexion and rotation to both sides and in nodding. Nodding is defined as the flexion motion in the atlanto-occipital joint. Each test was repeated three times and the average value was calculated.

  5. 5.

    The resulting descriptive data were analyzed for correlations.

  6. 6.

    The data of strength values were evaluated by cluster analysis for strength categories according to the age groups and the 12-year-old subjects, who were found to be “weaks” took part in a 25 min isometric neck muscle strengthening program. The exercises were performed at schools, 2 times per week for 2 months and after that their neck muscle strength and neck posture were measured.

  7. 7.

    Reliability study: The reproducibility of strength and mobility measurements was between 8–15 %, based on calculating the coefficients of variation. The significance was calculated by the t‑test and Mann-Witney test whereas the correlations were tested by Spearman’s rho test.

Fig. 1
figure 1

Diagrammatic representation of a neck posture at the age of 9 years, b at the age of 12 years and c at the age of 16 years

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Results

1. Anthropometric data

  • The subject’s weight, height and the calculated BMI values according to the age and sex showed a proportional increase with age (Table 1).

  • The neck length and the new neck lordosis index (Table 2).

  • The neck lordosis index correlated to body height and neck length (Table 3).

  • The BMI was proportional to the muscle strength, whereas it was inversely proportional to mobility and to the CVA values (Table 4).

Table 1 Anthropometric data
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Table 2 Neck lordosis data (in 12-year-olds, n = 138)
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Table 3 Correlations of neck lordosis index (12-year-olds n = 138)
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Table 4 Correlations of BMI values (12-year-olds n = 138)
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2. Neck mobility

  • Between 9 and 12-year-olds there were no significant differences; however with the 16-year-olds a significantly reduced mobility was found compared to both the 9 and the 12-year-olds. The mobility in nodding was proportional to the mobility measured in the other directions (Fig. 2).

  • The mobility significantly correlated to the CVA values, while to a lesser extent to the muscle strength and was inversely proportional with the BMI, SHA and HTA values (Table 5).

Fig. 2
figure 2

Neck mobility data (9, 12 and 16-year-olds)

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Table 5 Correlations of neck mobility (12-year-olds n = 138)
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3. Neck posture

3.1. Angles characterizing neck posture

(Values from Table 6)

  • The CVA values were found to be reduced on average of 8° between ages 9–16 years that represented more forwarded neck posture.

  • The SHA values were found on average 13° higher with age, that represented more protracted shoulder.

  • The HTA values were found on average 1.6° higher with age, that represented more extended head posture.

Table 6 Neck posture data
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3.2. Correlations within angles characterizing posture

  • The CVA was proportional to muscle strength, while to a lesser degree to mobility and inversely proportional to BMI and SHA (Table 7).

  • The SHA was inversely proportional to CVA, neck length, muscle strength and proportional to HTA (Table 8).

  • The HTA was proportional to SHA and inversely proportional to CVA (Table 9).

Table 7 Correlations of CVA values (12 yr olds n = 138)
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Table 8 Correlations of SHA values (12-year-olds n = 138)
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Table 9 Correlations of HTA values (12-year-olds n = 138)
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4. Neck muscle strength

4.1. Comparison of neck muscle strength values

  • The muscle strength values were increased in different age groups with age. The total neck muscle strength average value with the 9‑year-old age group was 96.65 N, with the 12-year-old age group 133.4 N and with the 16-year-old group 141.25 N. (Table 10)

  • A slight difference (approximately 10 N) was found between the 9 and 12-year-old age groups while practically no differences were found between the 12 and 16-year-olds. Impact of gender on neck muscle strength (Table 11)

  • The average total muscle strength values were calculated by cluster analysis and categories have been distinguished, as “weak”, “medium” and “strong”. This showed that there was a higher percentage of “weaks” with the 16 and 12-year-olds than with the 9‑year-olds, while the “strong” distribution was equal between the age groups (Table 12).

Table 10 Neck muscle strength data
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Table 11 Neck muscle strength values as to gender in different age groups
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Table 12 Proportion of summed neck muscle strength values in different age groups (by cluster analysis)
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Table 13 Neck muscle strength values of 12-year-old “weaks” before and after exercises
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The values for 9‑year-old girls were slightly (only 1–2 N plus) higher than those of the boys but those of the 16-year-old boys were much stronger (average of 30 N plus) than the girls.

4.3. Correlations concerning neck muscle strength values

  • The neck muscle strength was found to be in direct proportion to CVA and inversely to SHA and HTA (Table 7, 8, 9).

  • The neck muscle strength highly correlated to BMI whereas its correlation was on a lower significance level to neck length (Table 4).

5. Effects of neck muscle strengthening exercises

The “weak” category of children based on cluster analysis took part in the strengthening exercise program at their schools and the control measurements showed significant improvement in all directions (Table 13). After the completion of the strengthening exercise program, significantly higher CVA and HTA values, while lesser SHA values represent the correction of neck posture (Table 14).

Table 14 Neck posture values of 12-year-old “weaks” before and after exercises
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Conclusion

Anthropometric data

Cervical lordosis was correlated to body height and neck length. Subjects with higher BMI values were stronger but they were less mobile and they had more forward neck posture (in the 12 year age group).

Neck posture

The results proved that neck posture deteriorated with age, i.e. the neck became more forward and the shoulders more protracted between ages 9 and 16 years (see Fig. 1a–c).

The “good” (i. e. the “neutral”) neck posture was in correlation with mobility, neck muscle strength and “good” shoulder posture (i. e. lesser SHA angle).

The “bad” (protracted) shoulder posture was in correlation with neck muscle weakness.

Head posture

The higher HTA value represents what can be seen in everyday life that the forward neck posture used to be associated to extended head. That faulty posture could be the explanation as a pathomechanism of upper cervical syndrome.

Neck mobility

Between the 9 and 12-year-old age groups there were no significant differences in mobility but the mobility of the 16-year-olds is significantly reduced compared to the 9 and 12-year-olds.

Neck muscle strength

The neck muscle strength of the 16-year-olds was relatively reduced compared to 9‑year-olds. The muscle strength was found to be weak especially in nodding.

Strengthening exercises performed at schools

Strengthening exercises performed at schools over 8 weeks, 2 times a week resulted in a 2-3-fold increase in muscle strength, especially in nodding. Thus muscle strength can be restored in a relatively short period of time thus making correction of neck posture possible.