Abstract
Introduction
Many adults, schoolchildren, and teenagers use backpacks for carrying a variety of objects. Students make use of it to carry their personal belongings, laptops, and books or even sport equipment to school. An ideal backpack creates the possibility of high load transmission by distributing the load on the back area. Current backpack designs are either frameless, or they have an external or internal frame. The favorite form of a backpack is made out of a cloth carried on body’s back and fixed with two straps that go over the shoulders to hold the load. Two kinds of usual school backpacks are an Australian design, which includes two main compartments, a comprehensive back padding system and compression straps; and a British design with expandable side pockets and an internal waterproof bag for carrying sports clothes [1].
Most backpack users are students aged 7 to 25 who it during school time. Permanent inclination of the spine may occur due to this considerable time of backpack usage. The permanent inclination and abnormal deviation in spinal column causes discomfort and injuries of the inter-vertebral discs. One of the most frequent spinal disorders is a deviation in the sagittal plane because of external forces exerted on the spine [2]. Investigations show that carrying back load in short time using an improper backpack can be harmful like long time usage. Chow et al. [3] studied the alteration of spinal curvature in adults immediately after backpack wearing and they found bad effects in short term backpack usage.
The magnitude, distribution pattern, and position of load are important factors which should be considered by researchers because of their role in spinal disorders. Some of the biomechanical researchers concentrated on determination of the proportionate and safe weight for different ages and they recommended limitations for load [4, 5]. Bauer and Freivalds [6] used EMG system to find acceptable backpack loads based on physiological and psychophysical measurements for middle school students. They found that 10% of body weight for backpacks was safe for students and increasing it to 20% caused significant change in trunk flexion. In another study, effects of the load on gait and discomfort in muscles were investigated by Simpson et al. [7]. Furthermore, the effects of backpack’s load as 0, 10, 15 and 20% of body weight, on stride length and temporal parameters, trunk lean angles, and trunk motion range were analyzed by Hong and Cheung [8]. The results for 9 and 10-year old primary school children indicated that weights of more than 15% of body weight changes trunk inclination significantly and it does not seem to be safe.
The weight of a backpack produces a load, which is directly applied to the spine via the shoulder straps. Several backpack carriage instructions represent the correct manner of backpack lifting and adjusting to the body. The style of backpack wearing may affect the spine and cause pain, discomfort, or in some cases injury. So, several studies were performed to depict the best position of backpack against the spine and the effects of changing it. The effects of backpack load position on physiological responses in infantry soldiers were investigated by Liu [9]. They found that the load should be close to the trunk to prevent spinal inclination but it can affect lung function, so there is the optimum place for load on the back. Brackley et al. [10] evaluated the forward lean of trunk in 15-year old children by displacing load position on the back. Their results illustrated that the low position of load made fewer changes in trunk forward lean. Instead of finding the best position for load on the back, several designers were ambitious to find optimum distribution of the load. Rugelj and Sevsek [11] compared two different load distribution patterns, backpack and waist jacket, for both male and female. Effects of backpacks on posture sway were significant in comparison to waist jacket and there was no difference between male and female. Combination of weight and load position and the requirement for backpack using by school students were investigated [12–14]. A systemic review of the literature has been done [15] that introduces some backpack articles, which can answer the main question about thesafe loads.
Discomfort and fatigue in muscles are the other problems which are influenced by backpack. Discomfort in different regions of body such as: Lumbar region [16] and, upper and lower body muscles [7, 17] are obtained by EMG technique. Physiological parameters, which adversely affect by back load, are gait [18–20], trunk muscle activity [21], energy consumption [22], stance stability [23] and cardiopulmonary function [24]. Wood and Orloff obtained the changes in heart rate through backpack carrying [25]. They found that the human heart rates were different for each backpack design.
Many manufacturers are trying to design new backpacks specifically for student usage. Some of their techniques were backpacks quality progression by separating pack compartments and using compression straps to close the load near center of the body. Although there are many studies on comparison of different types of backpacks [26, 27], they only persist on primitive designs and none of them have compared the current backpacks with modified structure which can be suitable for students in all generations. The effects of different kinds of backpacks on kinematics and dynamics of students walking has already been investigated [1, 28]. Besides a biomechanical view, ergonomic aspects and appearance of backpacks were considered by designers. Amiri et al. [29] demonstrated that proper designs for 7–9 year old scholars reduce the effective load on shoulders, neck and waist. Backpacks that are designed in this way, in addition to diminishing the effective load, would be acceptable for scholars.
In this study, both biomechanical and architecture views were considered to modify the structure of conventional backpacks. The new suggested design was a non-flexible structure, which clung to the canvas bag. The effects of remodeling in backpack structure on biomechanical parameters of human gait have been studied. Beside study inclination in the spinal column which was the major factor in this perusal, the change in velocity and lateral displacement during walking were obtained. Likewise, discomfort in muscles, which are influenced by backpack usage, were investigated based on standardized questionnaire form. The new design, which is proposed by this study, contains a unique strap structure that shows the possibility of reducing the side effects of backpack carrying. The main concentration was made on the trunk angle and its flexion or extension because of the back load. The results of this investigation could help to develop the opinion that restriction in spinal column inclination may prevent serious injuries during back load carrying.
Method
These experiments were designed to examine two backpacks with different strap structures. Three main parts of this test were backpacks, subjects, and gait analysis.
Backpack
In this research, two backpacks with different strap structures were compared. The first one, backpack type (A) was a bag with non-flexible straps and the other one, backpack type (B) was a backpack with two flexible straps and a comprehensive back pad. These two types of backpacks are shown in Figs. 1a and 1b. For the backpack type (A), load directly transfers to the shoulders through its rigid straps. This non-flexible structure diminishes the momentum at lumbar region of the back. As it is indicated in Fig. 1a, strap curvature was shaped close to spine curvature and there were two flexible drawstrings to prevent backpack movement in transverse plane. The straps of backpack type (A) were wide enough to distribute the pressure on shoulders and also a white glass wool layer had been added to the internal part of them to absorb dynamic forces, which could be produced through walking. The backpack type (A) was an experimental sample that need further options to be prepared for school or college usage. Backpack size was appropriate for participants of this experiment. The backpack type (B) was a frequent frameless type with flexible straps, which transfer load to shoulders with high momentum in lumbar region due to free movements.
Participants
Nine healthy male college scholars participated in this investigation. Means and standard deviations (SD) of age, height and weight were 24.6 (1.1) years, 179.6 (4.94) cm and 76.41 (13.05) kg, separately. Subjects were asked about the times that they wear backpacks per week. Responses depicted that 33.3% of participants did not use backpack and the others use it at least two times a week. To select the participants among healthy students, both types of prior or current back injury and pain were our exclusion criterion.
Gait analysis
Motion analysis system
Participants’ gait patterns were recorded by a video camera system. Three cameras (Basler Industrial Cameras, Pilot Series, piA640-210gc, 210 fps, Japan) were fixed on two sides of a walkway and captured data from markers. The 3D coordinates of each marker were obtained in all frames through gait in a global coordinate system by cameras.
Markers
Markers were attached to 5 body points for recording their kinematics data by cameras during gait. Kinematics analysis of these points shows the backpack effects on the gait pattern. Hence head, neck and trunk movement are significantly influenced by backpack usage, markers were attached to the forehead, ear, shoulder, hip and the 7th vertebra of the cervical region of spinal column (C7) to investigate the changes and calculate the magnitude of flexion in these regions.
To obtain the flexion of head, neck and trunk, three imaginary members, which link body joints, were defined in SIMI motion analysis software (Fig. 2). Member (a) is an imaginary line that links the forehead to C7 joint, member (b) links the ear to the shoulder joint and member (c) links the shoulder joint to the hip joint. In Fig. 2, α, β and γ are the angles of members a, b and c with respect to the transverse plane, respectively. Although more markers need to be attached for evaluating exact values of flexion or extension, these numbers of markers were used to reduce the enormous amount of calculations without great effect on this study results. In fact, rotations of these imaginary members were used to compare backpacks effect on head, neck and spine. The marker attached to shoulder joint is appropriate for estimating lateral displacement. Furthermore, the shoulder and hip joint markers give information about the velocity. The average of these two velocities seemed to be a good approximation of trunk velocity. However, information extracted from markers were used to compare different effects of backpacks on gait pattern.
Backpack load
Backpacks were filled with the usual items carried by students such as: Laptop, books, and clothes. Backpack weight was 10% of the subject body weight, approximately, which is a safe load for college students [6]. The effects of different weights were not investigated in this study. Therefore, the backpack weight was constant during different experiment steps for each subject.
Gait
Five different test parts were considered for participants as follows:
(Normal gait)
Individuals change their manner of walking when they carry a backpack and their gait pattern changes by using different designs of backpacks. To determine the normal gait of the participants, in this part, their gait without carrying a backpack was recorded by cameras. The dynamic data acquired from this part were compared with the same from the other parts to identify the effects of each backpack on biomechanical parameters of the gait.
After recording the normal gait, backpacks were tested. At first, backpack type (A) was tested in two steps that each one took 15 minutes. Considering the results of pre-tests, they showed that the selected times periods for the tests were enough to appear the effects of the backpack carrying on the body. Then, participants were asked to rest and do some exercise to eliminate the effects of carrying backpack type (A) on their bodies, before carrying backpack type (B). Finally, the participants accomplished the test in two steps carrying backpack type (B), 15 minutes for each.
Step 1 (first 15 minutes)
In this step, participants were asked to walk on the treadmill with backpack. Initial velocity of treadmill was 1.5 m/s and it increased 0.5 m/s in 5 minutes increments. It means that the participants walked the first, second and third 5 minutes of this step with constant velocities of 1.5, 2 and 2.5 m/s, respectively. These various velocities were used to give time to the participants to adapt themselves with backpack, gradually and also, prevent disturbance in their gait for high velocity. In addition, it seems more reliable that the participants walk with different velocities like their natural walking. At the end of the step, the participants walked on the walkway when they were carrying a backpack and their gait was recorded by cameras.
To obtain information about discomfort in the muscles, the participants responded to a modified version of questionnaire [1, 7]. Seven body regions were selected and the participants were asked about the discomfort they experienced in the muscles (Fig. 3). They were neck, shoulders (posterior and anterior), lumbar and upper and lower leg muscles that are involved when individuals carry backpack. At this step of the experiment, the participants were asked to explain if they feel any discomfort in these muscles by backpack usage. Three grades of low, medium and severe were considered for the participants to depict their feelings about the level of discomfort.
Step 2 (Second 15 minutes)
Again, the participants were asked to wear backpack and walked for 15 minutes on treadmill. In this step, velocity was fixed at 3 m/s during the test. The dynamic data of the markers were recorded by cameras and again participants were asked to fill out the questionnaire at the end of the step.
Range of motion
In backpack designing, free movement of hands and trunk flexion or extension are the major factors which should be considered. In addition to the questions about discomfort in muscles, after second step of testing each backpack, the participants were asked to move their hands, trunk, head, legs and try to pick up a book from ground. The non-flexible structure of backpack type (A) may cause several limitations in body movement. So, participants were asked to see how these limitations influencing their body movements. Their feelings regarding free movements of these body regions and the way they bended to pick up the book were reported to compare the backpacks.
Statistical analysis
At first, raw data of each step were collected and filtered by SIMI motion analysis software. This software automatically detects and omits data, which abnormally is placed out of the average range. This kind of data was produced in several frames because of high frequency in getting information by cameras or jump in markers which was not avoidable. In this research, only the filtered data were analyzed, which was between two stance phases of left leg. In other words, just one stride of each step was considered to decrease the time of data collecting and computational processes. Means and standard deviations of biomechanical parameters (head, neck and trunk flexion, velocity and lateral displacement) for all participants in each step were calculated for statistical analysis.
The one-way repeated ANOVA was used to show if effect of a backpack on biomechanical parameters of gait was significant. P-value less than 0.05, presents significant difference between using backpacks (A) and (B). In this study, all analysis was implemented using SPSS software.
Results
Trunk head and neck angle
The means (SD) of angles α, β and γ, which are indicated in Fig. 2, were calculated for normal gait and two steps of each backpack and illustrated in Figs. 4–6, respectively. The results of both backpacks after first and second 15 minutes could be compared with the same values in normal gait to notice how each backpack changes the biomechanical parameters of the normal gait. The difference between γ value in the normal gait and each step, which is shown in Fig. 4, indicates flexion or extension in the trunk because of backpack usage. In the steps that the participants carried backpacks, angle values which are lower than those in normal gait, indicate a flexion in the related member and conversely, values higher than normal gait shows extension. According to (γ) values in Fig. 4, backpack type (A) caused an extension in first step and a flexion in second step, while backpack type (B) caused flexion in both steps. Angles (α) and (β) were used to obtain flexion or extension produced by backpacks in head and cervical regions. Qualification of these angles is similar to the angle (β). Figure 5 shows extension in head for two types of backpacks at both steps. Figure 6 indicates that for both steps backpack types (A) and (B) caused extension and flexion in neck,respectively.
Lateral displacement and velocity
The mean (SD) of coordinates of the shoulder joint in each frame was calculated for a complete stride and it is illustrated in Fig. 7. Values higher than the normal gait show external increases while lower ones show internal increase in lateral displacement. Figure 7 shows deviation of both backpack types (A) and (B) in external side and internal side,respectively.
The mean (SD) of all absolute velocities of markers, which were attached to shoulder and hip joints were calculated for all the subjects during the test of the normal gait and both steps. In Fig. 8, velocity had a minimum value of 1.45 m/s in the first step with backpack type (A) and a maximum of 1.5 m/s in the second step with backpack type (B).
Discomfort in muscles
There was a sheet for each participant, where their feeling about discomfort grade in muscles was reported. The percentage of the reports about low, medium, and severe discomfort in muscles for both backpacks was obtained (Fig. 9). Each bar in Fig. 9 shows the percent of low, medium and severe discomfort reported in a special muscle for a backpack. As is shown in Fig. 9, the only report about severe discomfort was in shoulders (both ant. and post.), which 6% of participants reported after they carried backpack type (B).
Range of motion
All participants demonstrated no difficulty moving their hands when they were carrying the newly designed backpack. For both backpacks, the same range of trunk extension was seen, but a significant difference was indicated in range of trunk flexion where non-flexible straps prevent highly flexion. When participants were asked to pick up a book from the ground separately, 77.8% and 44.4% of backpack type (A) and (B) users did the work properly. In this task, “properly” means that they flexed their knees to pick up a book and reduced pressure on spine column.
Statistical analysis
Means ( ± SD), F-ratios and p-values derived for each source of variance for the kinematic data generated during the gait are presented in Table 1. P-values less than 0.05 (P < 0.05) shows significant difference. The main concentrate was on the difference between each step and the normal gait. As it is marked by star in Table 1, the significant change was just in the trunk where the angle γ significantly changes during the first and second steps of carrying backpack (B).
Discussion
Adverse effects of backpacks on gait are clear but the solution to decrease the effects is not [5]. In this study, instead of recommending a safe load [4–8] or an optimum position for the backpack against spine [9–14], a change in the structure of backpack straps is proposed to decrease the side effects of backpack carrying. As it was shown by some researchers such as Mackiea et al. [1] or Schmidt and Docherty [28], backpacks, which are different in their structures, have different effects on human gait. So, both kinematic and physiological parameters, which significantly change during backpack carrying, were compared for two backpacks with different strap structure to see benefits of the new design.
The spinal inclination or forward lean value was considered as a reference to show how backpacks affect the body [3, 10]. For the first and second 15 minutes with backpack type (A), γ was 1.11 degrees higher and 0.63 degrees lower than normal gait, separately. On the other side, for the first and second 15 minutes with backpack type (B), γ was 3.74 and 2.93 degrees higher than normal gait, respectively. These values of trunk angle are comparable with study by Simpson et al. [7].
Interesting point about backpack type (A) was an extension of 1.11 degrees in the first step. Although previous studies mostly found flexion in trunk, Al-Khabbaz et al. [30] reported trunk extension in an analysis of backpack usage. This extension may be useful for individuals with permanent inclination in their spine region. In the second step with backpack type (A), the trunk approximately turned back to its normal state where it caused just 0.63 degrees flexion that was less than the flexion produced by backpack type (B). Backpack type (B) caused a flexion in both steps which can cause disorders in the spine in long usage [2]. Lower change in (γ) value of the normal gait by backpack type (A) in compare with significantly change in (γ) value by backpack type (B) (Table 1) can be an advantage for the non-flexible structure. According to Figs. 5, 6 and Table 1, there was not any significant change in head and neck. The small differences between them, especially the flexion in neck by backpack type (B) and extension by backpack type (A), came from backpack design. It seemed that non-flexible structure prevented the flexion in neck.
Chow et al. [3] found the tendency in the trunk to turn back into its normal position after removal of the carrying load. According to the results indicated in Figs. 4–6 for both backpacks, the participants explicitly try to turn their trunk back to the normal situation through each step. It means that the body tends to adapt itself with the external load and it is not dependent on how the load is applied. In contrast, in the test by Hong and Cheung [8] for 3mintes walking, they indicated that the forward inclination of trunk would increase as the walking distance increased.
Although participants had symmetric movements in the transverse plane in the normal gait, there were lateral displacements from side to side in their gait when they were carrying the backpack [20]. Figure 7 indicates a shift in shoulder marker coordinates value toward the outside for backpack type (A), and inside for backpack type (B), respectively. The effects of backpacks on lateral displacement were Low (Table 1). Lateral displacement completely depends on the distance between two straps, which in backpack (A) was wider and is a good reason for the outside displacement.
External load decreased the walking velocity as indicated in Fig. 8. Although differences in velocity are not significant (Table 1) [7, 18], Fig. 8 indicates that the changes in the velocity during walking with backpack type (A) were more than backpack type (B). Calculated velocities are in the range of values obtained by Rugelj and Sevsek [11].
Discomfort reports in the involved muscles [7] during backpack carrying (Fig. 9) indicate that backpack type (A) was more popular for participants. Participants believed that both backpacks caused low discomfort in the anterior region of the neck muscles. The number of participants who believed low discomfort in neck (post.) and lumbar regions, was higher for the users of backpack type (A) in comparison with type (B). The neck flexion, which is shown in Fig. 6 proves that neck flexion in backpack type (B) is more than backpack type (A). This is the reason for more medium discomfort reports in backpack type (B). Similar to Simpson et al. [7], reports about feeling discomfort in shoulders were more than other regions. In some cases, participants reported a severe discomfort in the shoulder regions, when they were carrying backpack type (B). It seems that because of the non-flexible straps pressure, reports about medium discomfort in shoulders (ant.) for backpack type (A) were more than backpack type (B). This pressure could be reduced by using wider straps in that region. Lumbar and posterior shoulder reports show the advantages of new straps design where these straps attempted to hold the spine in natural position without rotation and prevent muscle involvement in backpack carrying. Lower legs muscles data indicated equal results for using both backpacks, and most of the participants believed that discomfort in this region was low. For upper leg regions, the number of participants who believed in low discomfort was higher for users of backpack type (B) in compare with users of backpack type (A). Although the participants were selected from healthy students, physical condition of their bodies was an important factor and influenced their reports.
Finally, the participants did not have any problem with freely moving neck, head, arms and legs. But there was a problem for the participants who were carrying backpack type (A) in highly trunk flexion. When trunk is in flexion condition and subjected to back load, spine is highly in pressure. The proposed test for the participants to pick up a book from the ground, indicated how this limitation may be useful by decreasing the pressure on the spine in trunk flexion.
Conclusion
Based on this study, it is feasible to compare the effects of using non-flexible straps instead of usual straps in backpacks on biomechanical parameters of the gait. An important topic in backpack design categories is the generated deviation in the spinal column, when individuals try to return their COM to the original place during backpack carriage. Prevention of this inclination is not possible, but the results of this study indicated that it can be reduced by changing the structure of the conventional backpacks.
One of the limitations in this study was the backpack size. The prepared backpack for this test was in one size and it could not adjust to everyone’s body. So, it was a problem to choose appropriate participants. In addition to backpack size, its load was constant and the effects of different loads by this backpack were not investigated. Instead of asking about discomfort in muscles by questionnaire form, by using EMG system one can determine the pressure in muscles. Although results from EMG are more reliable, because of the complexity in extracting exact data from EMG systems, we preferred to use questionnaire form. For the future research, the effects of weight should be considered to propose a more reliable design of the backpack. Furthermore, pressure measurements beneath the straps, an analysis of energetics (dynamics) and real quantification of the overall effects of the new design could be the part of this study which were missed.