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Weight varying effects of carrying schoolbags on electromyographic changes of trunk muscles in twelve-year old male students.


Surface electromyography (EMG), a non-invasive method for the neuromuscular system investigation, is commonly used in ergonomic research to study the appearance of local muscle fatigue. (1,2) A significant linear relationship was found between external load increments and EMG signal increments. (3) Some studies have shown that more than 75 percent of junior students in the elementary and secondary schools of Italy and France are carrying their schoolbags in excess of 10% of their body mass (BM). (4-6) A general guideline of a limit of 10% BM in carrying loads was initially proposed by Voll and Klimt in 1977, in order to avoid muscular injuries. (7)

Inapplicable equipments as well as over weight in maturation age may lead to spinal disorders. If school students experience a backache in childhood, they will be exposed to worse consequences in adulthood. (8-10)

Imbalance of the shoulder that has been affected by carrying an over-loaded bag due to the weakness of the upper trapezium (UT) muscle is a common cause of these disorders. (8) Sharifah et al (2009) showed that carrying a heavy load of 15% and 20% of BM would cause a significant increase in the trunk inclination angle. (11)

Many studies have shown that chest flexion, decrease in action of erector spine muscle, increase in activation of RA muscle and tachycardia may be caused as result of carrying over-loaded backpacks. (12) Furthermore, prolonged reputational motions, with high force in unsuitable postures, result in incorrect habitual behaviors which in turn may lead to skeletal disorders. (4) During sustained arm tasks, it has been largely indicated that UT, a muscle with main function the stabilizing of shoulder girdle, presents EMG signs of fatigue. (13,14)

Some studies have shown that the increase in the height of a load location on the back results in an acute increase of muscle activation. (15) In response to this change, subjects naturally shift their trunk segments either backward or forward in order to counterbalance the load of the backpack. (11) Many researchers found out that when the load of the backpack is increased, the angle of the trunk will move forward. (16-18) Some researchers suggest that the normal load of a bag is 10 percent or 20 percent of BM (9,12) , while more studies have shown the normal load of a bag is between 10-15 percent of BM. (8,16) So, by providing the appropriate guidelines for safe postures and by introducing the suitable tools, lead to decrease physical disorders in future. (19) The risk factors for musculoskeletal discomfort associated with schoolbag carrying include the combined effects of heavy loads as well as load shape and size; important is, the duration of carrying the load and the position of the load on the body. (20)

Due to the fact that scholars hold divergent ideas about this issue, further research seems to be necessary. It is, thereby, our intention to achieve more information with detailed EMG information.

The purpose of this study was to determine the effect of backpack carrying with difference loads, on trunk muscles' EMG findings.



Twelve sedentary males with no history of neuromuscular or orthopedic disorder volunteered in the study (height: 146.83[+ or -] 11.3 cm; body mass: 40.91[+ or -] 7.7 kg; age: 12.58 [+ or -] 1.4 years, right-handed). None of them had practiced any physical activity for more than 5 h per week. They were used to carrying a two-strap backpack, going to school daily. The procedures of the whole experiment were explained to the subjects and their parents before the test begins. The study was approved by the local Ethics Committee and conducted in accordance with the declaration of Helsinki. The students were recruited from the local area to participate in the study.


MMT8 Biomedical Telemetry System

This system made by MIE Company. It includes a device for sending EMG signals. A fascia is bonding with MMT and girdles around the subject's waist. MMT is designed for remote control. The valid distance that this device supports, is 100 meters. The device has an 8- Socket input that is located on the left and right sides of the device. Some believe that when we are putting the electrodes the biggest signal can be registered as the motor point. There are two ways to find this motor point: first method is to determine the electrical stimulation which has greatest muscle activity with the least stimulation signal. Second method is the recording of EMG when the biggest signal will be deteccted. While putting the electrodes on the muscles, there was kept a distance of 1 centimeter among the electrodes. Then the leads were connected to a 4K amplifier.

When a muscular contraction and movement would be succeeded, the position of the surface electrodes would be expected to change -in regard to the contracted muscle, so the electrodes were positioned in the way put in the real test time. For the same reason, an anti-allergic glue was applied on the skin so that noise and movements would be minimized. The electrodes were put in a parallel direction with the muscular fibers for succeeding the appropriate conduction, and were stabilized with the use of an electrode gel. The studied students were asked to clean their bodies and shave their skin well before entering the lab.

While recording the EMG signals, the air conditioning devices and electrical appliances were turned off to prevent disturbance. After preparing students and cleaning the specified places on their bodies to put the electrodes, electrodes were connected at four muscle motor points. The way that electrodes were put on the target muscles, was based on the Deluca and Bassamajian method. So each electrode was put on muscular nerve area, with its end close to the end tendon, and in a parallel direction to the muscular fibers. The muscle position was identified through capturing isometric muscular contraction and on the basis of MMT method and simultaneous touch method and by observing its end. (21)

The area where the electrodes were put for the mentioned muscles, consisted of:

Upper trapezius: was put in the middle of space between acanthoid apophysis And seventh neck vertebra (C7) and posterior border of achromiom apophysis and the direction of trapezius line.

Central part of rectus abdominis: opposite of navel. Central fibers of rectus abdominis.

Upper pectoralis major: on the external part and on the 1/3 distance between macro toberosyty of gzyphoid apophysis in the position of 90 degree of arm abduction.

After the electrode positioning, the students were prepared for the maximum voluntary isometric contraction test. It must be mentioned that this test was taken as the reference date for comparison with muscular contraction in carrying backpack position.

Then the students were prepared for standing position for taking the backpack carrying test. In this test, three types of weights for carrying bye student were specified. The weights were 10, 15 and 20 percent of a studied student's weight respectively, it was done in sequential way, and signals for each weight were recorded for 15 seconds in static position. The weight that each student was carrying was chosen randomly, so that the result would not be affected by fatigue arising from previous load. The contents of backpack were completely simulated with usual loads carried in backpack position. In this part, each student after carrying his weight, had 1 minute of rest before carrying the following weight considered as recovery time and the date for each student was registered by the computer.

Statistical Analysis

Group results were presented as mean[+ or -]SD. Statistical analysis was performed using the dependent t-test for comparison of electromyography of trunk muscles. Values of p<0.05 were considered significant. Statistical analyses were performed using the 17th version of SPSS for Windows.


Table 1 summarizes the characteristics of the study group. Table 2 shows that there is not significant difference between carrying bag with 10% and 20% of BM in RA. However, there is a significant difference between carrying bags with 10% and 20% of BM for RA. Likewise, there is a significant difference among carrying bags with different loads of BM in UT, ES and UP muscles (Table 2). Based on the comparison of the EMG findings of the studied muscles, UT had the maximum activation and RA had the minimal activation while carrying a bag (Table 3, Figure 1).



Carrying an over-loaded backpack may cause deviations in natural postures and can increase the stress at the lower back. (11) When a person flexes his body with a trunk moving forward, the pelvis and the lumbar spine contribute to the motion in a way that arches the spine, shortening the distance between the front of the ribs and the pelvis. (22)

The results of the current study showed that EMG findings are changing with increasing the loads; however, the activation of the pectoral major decreased while increasing the loads. The possible mechanism for this EMG change is an increase in spine curve with an increase in load that causes a decline in pectoral major stress. (23) The comparison between a load of 10 and 20 percent of BM showed a significant difference in EMG of RA. The contraction of the abdominis muscle against and backward of the upper body can be attributed to cession for this different. (24) Likewise, through increasing a load up to 15 percent of BM, the UT was increased; on the other hand, by increasing a load to 20 percent of BM causes decrease in UT EMG activity.

As we know, the UT plays a major role in scapula elevation and handles backpack stress on scapulas; therefore through increasing bag load the level of activation of UT decreases. (21) Also, with increasing backpack load, the strain exerted by backpack is transferred to lower parts of the trapezius and other body muscles, and this change of force exertion results in decreasing of UT activation. (21) To explain the EMG changes in ES and RA found in the current study, it should be mentioned that these muscle play a great role in the extension of trunk and the resistance against extension with an increase of load. Other main findings of the current study show that the EMG findings respond to the different levels of load, with a maximum level of activation to be succeeded in the max load in all studied muscles, UT, ES, UP and RA respectively. The maximum EMG of these muscles happens in exerted loud of 20 percent of BM, in accordance to other studies. (9,12,21,23)

In the current study and in agreement to the international literature there was not any difference between muscles EMG in backpack carrying in comparison to dangling a bag in the front of body, but it was found a significant EMG difference in back pack carrying rather than carrying bag with left or right shoulder (asymmetry carrying). (25,26) In this position, the activation level of muscles decreases significantly and in the current study RA showed more activation than ES; it has been found that the maximum normalization of EMG findings of ES decreases by 3% in comparison to symmetry carrying. (27,28)


The results of the present study indicate that the best position for carrying schoolbags is backpack carrying, because carrying backpack leads to decrease in flexor muscles activity and increase in extensor muscles activity; so that in situations that students have to carry a bag with overload, specific training for the improvement of power of extensor muscles, might eliminate the possibility of trunk disorders.


This study was conducted in the Department of Sport Physiology Laboratory, faculty of Physical Education and Sport Sciences, Shahid Chamran University of Ahvaz, Iran. The authors would like to sincerely thank all participants in this study and the staff of Physical Education and Sport Sciences faculty.

Conflict of interest: None declared.


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Corresponding author: Abdolhamid Habibi,

Department of Sport Physiology,

Shahid Chamran University,

Golestan Boulevard,

Ahvaz, Iran

Table 1. Subject characteristics.

    age            high             weight         No. of
 SD     mean     SD     mean      SD      mean
 1.4    12.58   11.3    146.8    7.7     40.91       12

Table 2. The effect of carriage with different loads on the
electromyography changes in muscles.

     ES                      UP     RA              UT         Muscle
SD      mean    SD      mean    SD      mean    SD      mean

4.88    15.36   3.41    11.54   1.59    3.78    2.58    30.17   10%
4.78    17.84   3.09    10.59   4.44    1.91    2.02    31.54   15%
4.04    11.24   3.08    9.78    5.52    2.19    1.96    26.38   20%

Table 3. Comparison of the electromyography changes in
muscles with different loads.

    20%             15%             10%          Muscle
SD      mean    SD      mean    SD      mean

1.96    26.38   2.02    31.54   2.58    30.17     UT
5.52    2.19    4.44    1.91    1.59    3.78      RA
3.08    9.78    3.09    10.59   3.41    11.54     UP
4.04    11.24   4.78    17.84   4.88    15.36     ES
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Author:Habibi, Abdolhamid
Publication:Archives: The International Journal of Medicine
Article Type:Report
Geographic Code:7IRAN
Date:Oct 1, 2009
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