Abstract
Introduction
Carrying a load can be broadly divided into two types: symmetrical and asymmetrical. The best way to carry a load is carrying a backpack over both shoulders [1–4]. Additionally, 10–15% body weight (BW) was found to be the optimal weight to carry [4–10]. Nowadays carrying a handbag is more commonly seen in real life. Excessive load and load location was reported to affect posture leading to musculoskeletal problems [4, 11–13]. Females often carry many belongings. In addition, the females’ muscle strength is generally lesser than males [14]. Then, the females are subject to soft tissues strain or injury [15, 16]. This study was conducted an investigation the optimal weight of carrying a handbag to prevent the musculoskeletal problems. The study investigated spinal curvature, center of pressure (COP), muscle activities of upper trapezius and trunk muscles and discomfort when carrying a handbag for seven minutes in females.
Material and methods
Thirteen Thai healthy females with right-hand dominant and aged 18–30 years were recruited. Each participant had a body mass index (BMI) less than 24.9 kg/m2. None of them had a spinal deformity,previous fracture or surgery. The participants signed consent forms approved by the Committee of Center of the Ethical Reinforcement for Human Research, Mahidol University. The participants wore a sleeveless shirt with posterior splits. Electrodes of electromyography (EMG) were attached at the left and right of upper trapezius, erector spinae and rectus abdominis muscles. Markers were placed on the spinous processes of 7th cervical (C7), 1st thoracic (T1), 12th thoracic (T12), 1st lumbar (L1) and 2nd sacrum (S2) vertebrae and the right tragus (Fig. 1). The participants were randomly assigned to the four carrying weight patterns: no load, bag weight of 5% body weight (BW), 7% BW and 10% BW. A bag size in this study was 31 centimeters wide and 35 centimeters high. The participants walked barefoot on the platform (FDM, Zebris Medical GmbH) with preferable speed for seven minutes and then rested for seven minutes. A video camera recorded the marker on the right side during 0-1st, 3rd-4th, and 6th-7th minute walk. The experimental setting was shown in Fig. 2. The sampling rate of video camera was 25 Hertz. The EMG and FDM data were recorded for a minute at the same period of time. The sampling data were 1,000 Hertz and amplified by an 8-channel EMG (TeleMyo 2400T G2). The participants reported the discomfort scale in their bodies by choosing number ‘0’ (no discomfort) to ‘10’ (the worst discomfort imaginable) as shown in the Numerical Rating Scale (NRS). The WinFDM program was used to digitize and calculate spinal curvature in the selected frame for each period of time. The COP displacement was calculated by this program. The raw surface EMG was processed into average of muscle activities (AEMG) by Noraxon MyoResearch XP 1.06. All data were compared among these four carrying weight patterns within the participant. SPSS 19.0 was used to analyze the data. The statistical significance level was set at p < 0.05. Komolgorov-Smirnov Goodness of fit test was applied to investigate data distribution. The variables including the spinal curvature and COP were tested for the differences with two-way analysis of variance with repeated measure and Bonferroni was used as a post-hoc test. If the data were non-normal distribution, the Friedman test was used. Then, comparative analyses were performed, applying t-test for comparing data between the left and right sides of all AEMG or the Wilcoxon signed ranks test for variable that did not meet normal distribution. The difference in discomfort scale was tested by theFriedman Test.
Results
The participants’ characteristics data were shown in Table 1.
Spinal curvature
A craniovertebral (CV) angle is the angle formed by a line connecting the spinous process of C7 to the tragus of the ear and the horizontal line drawn through that spinous process [4]. A small CV angle indicates forward head posture. Positive values of thoracic angle and lumbar angle indicate thoracic kyphosis and lumbar lordosis, respectively.
There were no significant differences of any angles among carrying four weight patterns and three times period (p > 0.05) and no interaction effect on them (Table 2).
Center of pressure (COP)
For anteroposterior (AP) direction, the greater value indicated forward shift and the lesser value indicated opposite direction. For mediolateral (ML) direction, the positive value indicated a shift to the right and the negative value indicated the shift tothe left.
There were no statistically significant differences of the COP in the AP and ML directions and no interaction effects on load and time (p > 0.05) (Table 3).
Symmetry of upper trapezius and trunk muscles
When carrying no load, the upper trapeziusmuscle was found to display significant differences of asymmetrical muscle activities during 0-1st and 3rd-4th minute walk (p = 0.023 and 0.034). Loads 5%, 7% and 10% BW caused significant differences of asymmetrical muscle activities between the left and right sides of upper trapezius and erector spinae muscles during all durations (p < 0.05) (Table 4).
Discomfort area
The right upper trapezius muscle was the muscle that the participant complaint most discomfort when carrying loads 5%, 7% and 10% BW. The right finger flexor muscle was the second most complaint during carrying loads 7% and 10% BW for seven-minute walk.
Discussion
This study aimed to investigate the effects of four weight patterns of handbag carrying (no load, 5% BW, 7% BW, and 10% BW) while walking continuously on the spinal curve, COP, muscle activities of upper trapezius and trunk, and discomfort. Although the results of this study showed no significant difference in the spinal curve while carrying the handbag, the trend of spinal angle adaptation was found.
The cervical spine tended to be flat when carrying a greater load (such as 7% and 10%) at seven minutes. Such a load caused greater lumbar angle whenever carrying. The lumbar lordosis decreased afterward. The thoracic angle did not show any changes while carrying a bag. However, the cervical angle adaptation from this study was different from that reported in the previous study. There was a forward head posture of carrier when carrying one-strapped, two-strapped backpack or bag after walking [17]. The possible reason for the controversial results may be due to the participants’ head positions. If the participants turned their head to one side, the CV angle would have been distorted. Nevertheless, a significant difference of the CV angle was not found.
COP
Normally, one has an oscillated sway of the COP from forward and backward in static and dynamic positions. As seen in the results of this study, the COP in the AP direction displaced from the beginning to the end in similar value when carrying various load patterns. These finding indicated that carrying a bag did not affect the COP in the sagittal plane. According to the previous study [3] that compared carrying a load symmetrically and asymmetrically, no significant difference in the COP displacement in the AP direction was found between the ways of carrying backpack (single strap bag, purse, briefcase). Moreover, no statistically significant differences of the COP displacement in the AP direction were found when carrying the briefcase bag between 10% and 20% BW for 45 seconds.
In addition, the COP shifted in the ML direction during walking to adjust dynamic balance. When carrying no load, the result showed that the COP shifted to the left and right sides with the same value. However, the initial COP shifted to the left side and was still on the left side for the whole period of bag carrying. The COP had lesser displacement at the end of carrying period, compared to the beginning of that period. The greater load had lesser displacement than the lighter one. Moreover, carrying heavy load had a tendency of the COP shifting towards the loaded side that would cause falling. A previous study [18] conducted to investigate the center of mass (COM) while a load was attached to the right pelvis. That study reported that the COM shifted to the right side (the loaded side). Zultowski and Aruin [3] found significant differences of the COP displacement in the ML direction when carrying a single strapped bag 10% and 20% BW and when carrying a briefcase 10% and 20% BW.
EMG
There was asymmetry of both the upper trapezius and erector spinae muscle activities.
As mentioned earlier, there were no significant differences of any spinal angle adaptation while carrying four load patterns. That is, carrying a bag in hand did not disturb spinal angles in the sagittal plane. However, the previous studies [1, 4] showed that carrying load unilaterally caused spinal changes in the frontal plane. The postural control was activated when carrying a handbag. The COM moved lower and laterally from the original position after carrying a load. Then, the neck and trunk bent to the loaded side with right shoulder depression. Spontaneously, the upper trapezius and trunk muscles were recruited to prevent falling. To improve neck posture, the left upper trapezius muscle was recruited. While working against shoulder depression with gravity and load, the right upper trapezius muscle was recruited. The body also tended to collapse to the loaded side. The left erector spinae muscle contracted concentrically while the right erector spinae contracted eccentrically to stabilize the trunk. Normally, the COM was settled in front of the sacrum bone. The rectus abdominis seemed to be less responsible for stabilizing trunk. However, the right rectus abdominis muscle may be recruited to counterbalance the left erector spinae muscle. The left rectus abdominis muscle seemed to be less affected when carrying loads for a long time.
Overall, asymmetrically significant differences in muscle activities between the left and right upper trapezius were found when carrying all load patterns (no load, 5%, 7% and 10% BW). Significant differences in muscle activities of the erector spinae muscle between the left and right sides during carrying 5%, 7% and 10% BW were found. However, the left and right upper trapezius contracted asymmetrically, as shown in this study. The activity of the right upper trapezius was lower than the left side during no load. The muscle fibers in the dominant side changed their muscle properties to slow fiber for improving endurance when doing activities [19]. The left upper trapezius played a role to maintain balance while the standing balance was disturbed on the right hand.
Discomfort area
The discomfort of the right upper trapezius and right finger muscles were reported for all participants. These muscles had an important role in this study – the former elevated the shoulder against the load while the latter hold the bag. The prolonged muscle contraction led to poor intramuscular circulation and collected waste product [20]. The longer the carrying period, the more discomfort was experienced. Therefore, carrying a handbag with unload, 5%, 7% and 10% BW for seven minutes did not cause the body to collapse and fall. However, this study showed asymmetry of the upper trapezius and erector spinae muscles when carrying load on one side. Prolonged carrying a too heavy handbag will lead to musculoskeletal problems. More research needs to be done to focus on the optimal weight for carrying a briefcase in men.
Conclusion
This study was carried out to investigate the effects of carrying a handbag on spinal curvature, COP, upper trapezius and trunk muscle activities, and discomfort. Prolonged and asymmetrical carrying of a heavy load did not disturb the spine in the sagittal plane. However, asymmetry between the left and right sides of the upper trapezius and erector spinae muscles was observed. The right upper trapezius and finger flexor muscles were the most discomfort area duringcarrying the handbag. This study showed that the optimal weight of asymmetrical bag carrying should not be greater than 10% BW. Continuously carrying a too heavy handbag for a long period of time will cause weight-related musculoskeletal problems.
Limitations
This study recruited only female participants as females tend to be subject to have more musculoskeletal problems than males. The video camera was set up to observe the changes of spinal curvature in the sagittal plane. Therefore, combined movements in other planes, for example, spinal rotation would be missed. The video recorder was on the right side which might be suitable for the right-handed participant.
Conflict of interest
The authors have no conflict of interest to report.