Cite this asKuu S, Pedak K, Port K (2019) The relationship between postural components and muscle strength balance among 9 to 14-year old children. Arch Sports Med Physiother 4(1): 010-015. DOI: 10.17352/asmp.000011
Study was to examine the differences between the strength of linked agonist and antagonist muscles according to their location and postural role. Study included 102 schoolchildren: age 11,28±1,55 (x̅±SD); BMI 18,87±3,71. Body posture was assessed visually in the sagittal plane (neck, chest, shoulders, upper back, trunk, abdomen, lower back position) and in the frontal plane (head, shoulders, spine, hips, feet, arches position) using the New York State Posture Rating Chart. Each component was rated on a scale 5 (normal), 3 (slightly deviated), 1 (markedly deviated). Maximal isometric muscle strength was measured with manual dynamometer on the muscles: neck flexors and extensors, upper and lower part of pectoralis major, middle and lower part of trapezius, trunk flexors and extensors, hip flexors and extensors, hip adductors and abductors. Children were divided into two groups: normal posture and deviated posture. We found that the detrimental effect on the posture of the divergence in strength between opposing muscles was dependent on their location. There were cases of pronounced strength imbalance among children with normal posture, for example in relationship of hip extensors to hip flexors, while deviated posture group showed negligent imbalance. On the other hand, children with postural deviation appear to have imbalances in other locations. For example, in the relationship of the pectoralis group to the trapezius. Likely explanation is that there are normal differences between anterior and posterior muscle groups, but posture related problems seem to be dependent on the strength imbalance of antagonistic muscle groups at a specific location.
Poor posture is a common problem among schoolchildren. The most frequently occurred defects in frontal plane are scoliosis  and protruding scapulae [2,3], in sagittal plane there is increase in lumbar lordosis [1,2] and thoracic kyphosis [2,4].
Already in 2004, Kopecky brought out: “The main reason for the defective posture is muscular dysbalance, the so-called upper-cross syndrome and lower cross syndrome manifested in keeping the head stuck forward, a round back, outstanding shoulder blades, high pelvis inclination accompanied with lumbar lordosis, and in many cases with loose abdominal muscles.“ It is known that upper crossed syndrome is related to the weakness in the neck flexors in front of the neck and also weakness in the middle and lower trapezius together with the rhomboid muscles on the back side of the chest, but on the other hand with shortened and tight upper trapezius together with levator scapulae on the back side of the neck and pectoralis minor and major in front of the chest . Likewise, weak and tight muscles are crossed on lumbar region, identified as lower crossed syndrome. There are thoracolumbar extensors and iliopsoas together with rectus femoris in the shortened and tight muscle group, and abdominals together with gluteus medius and maximus on the weak group . Poor posture is also related to a lower value in isokinetic strength of the trunk muscle  and also to a lower endurance capacity of trunk muscles, especially for hyper-lordotic spine . But we cannot find studies describing the ratios of muscle strength and grouping that assess the size of differences between the agonists and antagonists according to the impact on deviations in the posture. However, the agonist/antagonist ratio of muscular strength may play an important role in clinical functional analysis and rehabilitation .
With the aim of exploring the issue, purpose of the study was to examine differences between strength of linked agonist and antagonist muscles according to their location and role in the posture.
This study was carried out in spring 2014, in an Estonian private school, at a school doctor’s office. Study informed and inquired approval from all parents of participating pupils from 3rd to 7th grade. As a result, study involved only volunteers whose parents had signed the consent sheet and who did not have any musculoskeletal injury. The assessment of posture was a part of annual school health check. The posture and muscle force of all subjects were evaluated by experienced physiotherapist.
The cross-sectional study included 102 schoolchildren (44 boys and 58 girls), ages 9-14 years (x±SD:11,28±1,55). Each subject filed a results registration sheet with questions about age and health. Study session started with introduction of measurements and methods, followed by registration of body weight and height, and subsequently the assessment of posture with measurements of strength. Table 1 depicts anthropometric characteristics of the study subjects.
Body posture was assessed visually by the New York State Posture Rating Chart , using the plumb line and grid. Table 2 shows assessed components by 5 points criteria.
Maximal isometric muscle force was measured with manual dynamometer (BASELAINE, Hydraulic LCD Push-Pull Dynamometer). The maximum isometric force tests were performed on the therapy board where the subject´s body was stabilized to ensure that the muscle or muscle group being tested is isolated. The subject initiated and exerted a force against the dynamometer (that was firmly hold by the tester) until it began to move. Tested muscles or muscles groups and positions of testing are given in Table 3. Each muscle group was tested three times, and the best result was recorded.
All data were collected and analyzed using SPSS Statistic (IBM Corporation, USA). Independent samples T-test was used to assess the group differences (normal posture versus postural deviation). p<0.05 indicated statistical significance. The results were expressed as means and standard deviations (±SD).
Most frequent postural deviations on sagittal plane (Table 4) were as follows: neck forward, chin out (52% of all children) and shoulders forward (51.9% of all children), and on frontal plane: one shoulder higher (70.5% of all children), one hip higher (52% of all children), feet pronation (58.8% of all children) and arches lower, feet flat (70.5% of all children).
Children were divided into two groups according to assessment of the posture: normal posture (NP) and deviated posture (DP). Comparing the force ratios of postural groups (Table 5), there were statistically significant differences (p<0.001) between NP and DP in ratios of neck flexors and extensors in relationship to the head position.
There were statistically significant differences between NP and DP groups in force ratio of pectoralis major upper part and trapezius middle part on the right side in relationship to the shoulder position as in sagittal as in frontal plane; however, no significant differences were found in same ratios on left side. There were no statistically significant differences in pectoralis major lower part and trapezius lower part ratio related to the shoulder position in sagittal plane. But there was statistically significant difference between NP and DP group in the same ratio in frontal plane, but only on left side.
Typically, subjects did not show significant differences between NP and DP groups in the strength ratios of trunk flexors and extensors, in hip flexors and extensors, in hip adductors and abductors in relationship to the spine, trunk and pelvic positions accordingly. Just only one statistically significant difference was found in the ratio of hip adductors to abductors on left side at hip position if one hip was higher.
Comparison of position of feet (pronation) found statistically significant differences (p<0.001) between NP and DP in the ratios of strength of hip extensors to flexors on both sides, but no significant differences in strength were registered between the groups regarding ratio of hip adductors to abductors.
In everyday life, most noticeable defects of posture are forward head position and rounded shoulders together with protruding scapulae. Current study confirms the above observations. Neck forward was found among 52% of all children and shoulders forward was exhibited among 51.9% of all children. According to Rosa et al., , their study found the forward head position with 66.7% of the schoolboys and 58.3% of the schoolgirls. On the other hand Asl and Savucu , found that only 20.6% of male students at the age between 11-16 had forward head. Kratenová et al., , found that 50% of all children have protruding scapulae, and Penha, João, Casaratto, Amino and Penteado , brought out that shoulder protraction is very widespread among 10 years old students, 82%.
Forward head position and rounded shoulders together with protruding scapulae are related to upper crossed syndrome. Based on Janda’s view , in the cases of upper crossed syndrome the cause seems to be related to weak neck flexors in front side of the neck and tightened and shortened neck extensors in back side of the neck. Our study indicated that neck extensors are approximately 30% stronger than neck flexors and there is no difference among schoolchildren between normal posture group and deviated posture group in the sagittal plane. But there is significant difference (p<0,001) between posture groups in frontal plane at head position. Force ratio of neck flexors and extensors is 0.51 on deviated posture group what means that neck extensors are two time stronger than neck flexors. We can hypothesize that if the force ratio between neck agonist and antagonist is ca 2/3 then it does not cause problems in the posture and does not bring along forward head position, but if the force ratio is ½ then it can cause deviations in the posture. Quite often the physiotherapists associate the forward head position with headaches but Weber Hellstenius , found that among 10-13-year old students the headache is not related with forward head posture.
In cases of upper-crossed syndrome the common observations are weak middle and lower trapezius together with rhomboid muscles on the back side of the chest and tight pectoralis minor and major in front of the chest . We compared muscle strength between pectoralis major upper part and trapezius middle part and also between pectoralis major lower part and trapezius lower part. And we found that the differences of strength are bigger for the lower part. The force ratio of trapezius against pectoralis was 0.31 to 0.42. It means that pectoralis major lower part is approximately three time stronger than trapezius lower part, but it may not yet cause deviations in the posture. We got only one significant difference between normal and deviated posture groups in the strength ratio of trapezius lower part to pectoralis major lower part, and that in frontal plane at shoulder position on the left side. We found interesting that on both sides of the body the deviated posture group had higher ratio in strength rising the possible explanation that persons with deviated posture have enhanced balance between muscle strength between trapezius and pectoralis major. The same tendency can be seen in ratios of strength between trapezius middle parts to pectoralis major upper part in frontal plane on both sides of body, but differences in upper part muscle strength are not so much pronounced like these were in lower part of the chest. The ratio of trapezius middle part to pectoralis major upper part varies from 0.53 to 0.71 taken all measures together, in both sagittal and frontal plane. One discrepancy on the ratios of chest muscles between lower and upper part is this that on upper part there were two significant differences between ratios of normal and deviated posture group, both on right side. But in sagittal plane force ratios of deviated posture group, at shoulder position were smaller. This seems to indicate to a higher imbalance between trapezius and pectoralis muscles.
We also examined posture to antagonistic muscle group balance from the perspective of lower crossed syndrome. It is known that lower crossed syndrome is related with weak abdominal and tight as well to shorten back muscles on upper part and also with weak gluteal and tight and shortened hip flexors muscles on lower part .
We assessed the ratio of strength of trunk flexors to trunk extensors. We compared the groups of normal posture and deviated posture at four different positions: the spine position in frontal plane; the trunk position, the abdomen position and the lower back position in sagittal plane. We observed that the trunk extensors were stronger (approximately 5-16%) than the trunk flexors, however there was no significant differences between posture groups. Fortunately, trunk posture deviations were registered less frequently, among fewer than half of test subjects. We registered the laterally curved spine on 39,2%, the trunk inclination on 37,3%, the protruding abdomen on 35,3% and the hollow lower back on 46,1% of the cases. These findings agree with other investigators, for example: the increased lumbar lordosis occurs on 32% of schoolchildren by Kratenova et al., , on 51,3% of male students by Asl and Savucu  and on 31%, on average, by Dejanovic et al., . We could not confirm the results of Kim et al. (2006) who found among adults that bigger imbalance between trunk extensors and flexors is significantly related to bigger lordotic curve which in turn can predict potential low back pain in the future. On the other hand, Lee et al., , brought out that if the extensor strength is more balanced to the flexors and even weaker than the flexors then it might be a risk factor for the low back pain later in life.
Similarly, to the ratio of strength of trunk flexors/extensors, we did not register significant differences between normal posture and deviated posture groups comparing the ratios of hip extensors to flexors. We studied these ratios at two position: lower back position in sagittal plane and hips position in frontal plane. Looking these results at the position of lower back one could see that on normal posture group the ratio of hip extensors to flexors was more balanced, difference between the muscle strength was around 5-9%. Observing the same ratio on deviated posture group revealed bigger difference between extensors and flexors (14-18%). This may indicate that children who have hollow lower back seem to demonstrate higher imbalance of strength between hip flexors and extensors, thereby the flexors are stronger then the extensors. But on the other hand, at hips position in frontal plane we saw that children in deviated posture group demonstrated strength-wise more balanced muscles, showing difference between hip flexors and extensors around 5-9% while among the normal posture group the same indicator was 15-18%. The problematic hips level – one hip was higher than the other – was observed on 52% of all assessed subjects. But Rosa et al., , did not find any change by pelvic alignment in frontal plane for schoolchildren.
Hips level link to lower back position, both are related also with hip abductors and adductors [5,14]. We evaluated the ratio of strength of hip adductors to abductors and registered bigger difference between agonist and antagonist muscles - hip abductors were 31-40% stronger compared to adductors. At the lower back position this ratio was fairly equal, but at hips level position there was a larger disparity between normal and deviated posture group. The deviated posture group had more balanced muscles compared to the normal posture group. We also registered a statistically significant difference between the hip adductors/abductors strength ratios of normal and deviated posture group on the left leg.
Assessment of muscular strength with manual dynamometer revealed that hip flexors are stronger than hip extensors and hip abductors are stronger than hip adductors. Previously have been used isokinetic dynamometer to evaluate muscle strength and strength ratios of agonist and antagonist muscles. Therefore, the relationship between agonist and antagonist muscles may show distinctive outcomes to previous studies with different methodological approach. For example, Kushner, Reid, Saboe and Penrose , tested ballet dancers and found out that the hip extensors are approximately 28% stronger than hip flexors and hip adductors were 24% stronger than hip abductors. On the other example, Tis, Perrin, Snead and Weltman , tested female runners and got quite equal results between hip flexors and extensors. By concentric test – the hip extensors were 2% stronger and by eccentric test the hip flexors were 3% stronger .
In addition to the position of hips we assessed the same ratios (hip extensors/flexors, adductors/abductors) at feet position, too. Feet pronation is fairly widespread deviation of posture, in our study we registered the pointed out feet on 58,8% of cases. Ilić and Đurić . registered among 40% of girls and 53,3% of boys change in Achilles tendon position – Disortion in Pes planus.
It is known that straight ankles and correct feet arches are dependent on the status of anterior and posterior tibialis muscles on the calf . Present approach to relate the problems within ankle joint to hip muscles was based on the concept of whole-body fascial and myofascial linkage, also known as “anatomical trains”. The calf and hip muscles are linked to each other in the Superficial Back Line and the Superficial Front Line . With our study we found that depending on the position of the feet there were significant differences (p<0.001) between the hip extensors/flexors ratios of strength in normal and deviated posture group at both sides of the body. It was more remarkable that on deviated posture group (at feet position) hip extensors and flexors muscle strengths were equal (the ratios were 0.97 on right side and 1.0 on left side). At the same time, the ratios on normal posture group were 0.71 and 0.78, accordingly. Unfortunately, we were unable to find similar study outcomes .
In addition, we compared the strength ratios of hip adductors/abductors at feet position, but did not register significant differences between the posture groups. We did observe that hip abductors were 33-36% stronger compared to hip adductors despite differences in feet positions.
Two groups of 9-14 years old children with normal and deviated posture demonstrated significant differences in the strength ratios between antagonistic muscles of the postural muscle groups. The impact of the above ratio on the posture was dependent of location of the muscle group. There were cases of pronounced strength imbalance among children with normal posture, for example in comparison of hip flexors to hip extensors, while deviated posture group showed negligent imbalance. On the other hand, postural deviation appears to be related to imbalances in other locations, for example in comparison of the pectoralis group to the trapezius etc. Likely explanation is that there are normal differences between anterior and posterior muscle groups, but problems arise based on the location.
This topic definitely needs further follow-up studies to find out how big the agonist and antagonist muscle strength disparity could be in different body regions without causing the postural deviations.
This study has been coordinated by the Ethics Committee of Tallinn University.
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