The impact of different bag-carrying modes on postural stability and spatiotemporal parameters of adolescents: A cross-sectional study

Abstract

Background/purpose

Previous research studied the effects of load carriage in adolescents did not focus on balance as an essential indicator in walking activity. Therefore, this study examined how different bag-carrying techniques affected adolescent balance and gait spatiotemporal parameters.

Methods

By using the HUMAC balance system and a GAITRite system, the postural balance and gait spatiotemporal parameters were assessed in 32 adolescents of both genders, while carrying a traditional backpack, messenger bag, and one-shoulder strap bag, weighing 15% of body weight.

Results

The sensory integration of balance using firm and foam surfaces with open and closed eyes during messenger carrying was significantly less than their values during backpack and one-shoulder strap conditions (P < 0.05), and no significant difference between backpack and one-shoulder strap conditions (P > 0.05). Regarding limits of stability and weight shifting, the value of messenger was significantly greater than their values during backpack and one-shoulder strap conditions (P < 0.05), and no significant difference between backpack and one-shoulder strap conditions (P > 0.05). The step and stride lengths during carrying the backpack was significantly longer than the messenger and one-shoulder strap bags (P < 0.05). The cadence during carrying the one-shoulder strap bag was significantly higher than backpack and messenger bag (P < 0.05).

Conclusion

In adolescents, carrying the backpack improved the walking spatial parameters in comparison to the messenger and one-shoulder strap bags. The messenger bag has a better impact on sensory integration of balance than the backpack and the one-shoulder strap. Hence, the messenger bag style preserves speed while maintaining balance.

Keywords

Backpack balance load carrying messenger bag one-shoulder bag walking

Introduction

Inappropriate use of backpacks is not healthy for anyone, especially for children who are more vulnerable to damage because of how their bodies are still developing and growing.¹ The prevalence of musculoskeletal pain among school children (12–15 years) was found to be 67%, and the school bag type was one of the contributing factors.² The school students have a high incidence of back pain due to the weight of the bag and way of carrying.³

There are various styles of school bag designs, including traditional backpacks, shoulder bags, and hand-carried bags. Despite the harmful effects of carrying loads announced by professional organizations concerning unequally distributed heavy loads, students still carry shoulder bags on one shoulder.⁴ The use of backpacks represents a daily routine of carrying load style.⁵ Significant kinematic changes in the child's ankle, knee, and hip joints were seen as the backpack load increased, especially when the backpack burden exceeded 15% of their body weight.⁶

The double-sided bag restores body balance and head posture to a state that is similar to no load even when carrying a bag that weighs 15% of body weight.⁷ However, changing head posture and increasing the normalized value of the anterior–posterior (AP) shear force may result in the development of postural abnormalities.^8,9 Not only at the level of gait, upper and lower musculoskeletal injuries, but also backpack load may have a negative effect on balance control.¹⁰

Numerous reviews^11–13 were concerned with the effect of backpack style more than other load carriage methods. Several variations in backpack carriage protocols could explain variations in results between studies, including walking speed, which is one of the outcomes that still has a conflict in the evidence.¹² In contrast to double pack loads, the backpack carriage considerably decreased postural stability after a 10-min walking exercise. For the best balancing control, it is preferable to distribute heavy loads between the front and the back.¹⁴ The one-shoulder bag produces an uneven displacement away from the load, which results in a significant postural deviation.¹⁵

The balance as an important outcome measure in the walking activity, especially with external loads, was not targeted in the reviews, although it was discussed in some studies on 8–12 years students.¹⁶ While the balance was measured in another study with different load carriage styles in college students,^14,17 its impact on adolescent is still needs further studies to make evidence of the best way of carrying. So, this study investigated the influence of different styles of carrying bags on balance and gait spatiotemporal parameters in adolescents.

Materials and methods

Study design

This is a cross-sectional study with repeated measurements on one group that was carried out between December 2022 and March 2023.

Participants

Thirty-two adolescents from both genders (14–18 years) were recruited from three secondary schools in Giza region through posters, online social media, and verbal invitations, which were posted for duration of two months. The volunteers were screened for eligibility before the commencement of the study. A pilot study was done on eight participants before starting the actual study to estimate the effect size. The sample was determined as 30 participants according to effect size = 0.37, alpha = 0.05, and power = 0.85 by using (G*Power 3.1 software). Forty participants were screened for eligibility, only 32 participants were found meeting the inclusion criteria. The participants and their parents received a full explanation of the procedure and a consent form signed by the parents approving the child's participation. It was approved by the local institutional review board of the Faculty of Physical Therapy (No: P.T.REC/012/ 003135), Cairo University, and conducted in accordance with the Declaration of Helsinki.

The participants were physically active, they should carry their school bags daily, able to comply with the researcher's instructions, and not overweight or obese. According to the standard definition for adolescents overweight and obesity worldwide.¹⁸ The cutoff point of the mean body mass index (BMI) for overweight and obese adolescents with age ranged from 14 to 18 years is 22.62–25 kg/m² for boys and 23.34–25 kg/m² for girls. The BMI of the participants ranged from 17.21 to 21.55 kg/m². Adolescents with visual or auditory defects, musculoskeletal disorders in upper or lower extremities, postural deviation such as scoliosis and kyphosis, surgical interference, or neurological problems were excluded from the study. The demographic data of the subjects are shown in Table 1.

Table 1.

Demographic characteristics of subjects.

Variables	Male, n = 13(Mean ± SD)	Female, n = 19(Mean ± SD)
Age, years	16.31 ± 1.44	16.79 ± 1.51
Weight, kg	60.15 ± 2.48	48.58 ± 1.26
Height, cm	171.85 ± 1.77	160.32 ± 2.38
body mass index, kg/m²	20.36 ± 0.63	18.91 ± 0.78

SD: standard deviation.

Each adolescent participated in three loading conditions: traditional backpack, messenger bag and one-shoulder strap bag. The bag weight was 15% of body weight.¹⁹ The backpack model was traditional. It is a soft backpack with no internal framing or back support, two padded adjustable shoulder straps, no internal compartments, one size only and with no waist, chest, or load (side) compression straps. It had dimensions of (39.37 cm in height × 31.75 cm in length × 13.97 cm in width). The backpack positioning in the back is considered the thoracic vertebrae (T8 and T9) as the medium point. The backpack weight, body mass, and height were measured by electronic weight and height scale.²⁰ A backpack worn over one shoulder, particularly the dominant shoulder, is known as a one-shoulder strap bag. It includes characteristics and dimensions of the aforementioned backpacks. When only one strap was used, the backpack was positioned at midthorax (T2) level. As a result, the participants were able to walk with a gait that was quite similar to their usual gait.²¹ The bony landmarks of the thoracic vertebrae were determined according to the procedure of Bernhard and Stelzenmueller.²²

The messenger bag is a cross-bag style that is carried on one shoulder across the body. It was carried in a square bag with dimensions (width: 30 cm, height: 34 cm, and a 12 cm-wide) shoulder strap. The dominant shoulder was used when wearing both the cross-body bag and the shoulder bag.²³ Figure 1 shows the three loading conditions. The order in which an individual entered or finished the three loading conditions was randomized by individuals picking lots. An assistant set up the bags, paying attention to maintaining an even vertical and horizontal load distribution to prevent load concentration, and gave the participants five minutes to walk around the lab to be more accustomed to the carrying conditions.

Figure 1.

Carrying conditions: (a) backpack, (b) one-shoulder strap, and (c) messenger bags.

Procedure

Balance assessment

The participants were evaluated for their balance using the HUMAC balance system and spatiotemporal parameters of gait using the GAITRite system during walking with different conditions of load carriage. This balance system was represented as a reliable method for assessment of static and dynamic balance (EN 60601-1-2, class type [BF] equipment, CE Conformity to MDD 93/42/EEC).²⁴ The study's assessment procedure of body balance has utilized three tests; limits of stability (LOS), weight shift test and modified clinical test of sensory integration of balance (mCTSIB).

Limits of stability test: Used to assess postural control. Before being given the chance to perform the test to make sure the participants were comfortable with it before the measurement, they should stand on the platform and record their anatomical zero. The participant had to learn that the movable purple point was a representation of his/her body, and it was moving in response to this movement. Following that, the participant followed the lighted target with his/her body and still there until another target was lighted, and so on. The target depicts how the body can travel in eight various directions: forward, front/right, forward/left, right, left, and back, as well as back/right and back/left. The test result's typical percentile is 65%.²⁵

Weight shift test: Using this test, the participants’ ability to control changing their center of pressure from right to left, anterior to posterior, or anywhere in between without leaving the target zone was evaluated. The participant was initially told to stand on the platform, record their anatomical zero, and move the purple point so that it would strike the green vertical line and turn it red. Then, go to the second vertical line after ten hits and keep the spot in the target area (two transverse purple lines) for the remaining 30 s.²⁵

Modified clinical test of sensory integration of balance: Used to evaluate the subject's ability to maintain balance when their eyes were open or closed. After instructing the subject to stand on the platform, the therapist established the positions of their two feet, and the anatomical zero. The participant was instructed to focus on a target on the wall placed at the level of his/her eyes for 30 s (it has been established that 30–60 s is an adequate amount of time for data analysis).²⁶ Then, he/she was instructed to close their eyes for another 30 s on a firm surface before repeating the process on a foam surface for a final 30 s. The normal percentile of these subtests is eye open firm surface (92%), eye closed firm surface (90%), eye open foam surface (88%), and eye closed foam surface (79%).

Gait assessment

The gait spatiotemporal parameters were measured by the GAITRite System (CIR Industries, Clifton, NJ, USA). The walkway is a carpet of 5 × 0.9 m with pressure sensors embedded into the carpet. The sensors are activated by the mechanical force of the subjects (80 Hz sampling frequency). Subjects wore flat shoes and were directed to walk at a self-paced speed.²⁷ This system is a valid and reliable tool for determining several gait parameters in healthy subjects, including spatial parameters (step and stride lengths), temporal parameters (step time and cadence), and derived measures of velocity.^28,29

Before the actual measurement, each participant conducted 5-min walks to get acclimated to the system and to retain constant gait patterns. Then, he/she completed three walking trials at a self-preferred pace with their own footwear for data collection.²⁷ The measured spatiotemporal gait parameters were step length (cm), stride length (cm), walking velocity (cm/s), and cadence (steps/min). The data from the three experimental trials were averaged together to be evaluated for the dominant side (except for velocity and cadence). Measurements were taken in three carrying conditions while the adolescent was wearing their own shoes.^30,31

Statistical analysis

Data were analyzed using a Statistical Package for Social Sciences (Armonk, NY: IBM Corp.) version 20.0. Prior to statistical analysis, the Shapiro–Wilk test demonstrated that the data were normally distributed (P > 0.05), which was essential before conducting the parametric analysis of the data. Repeated measures analysis of variance was used to examine the effect of carrying conditions on sensory integration, postural control, controlling center of pressure, and spatiotemporal parameters. For all statistical tests, (α) level of 0.05 was considered statistically significant.

Results

The value of sensory integration of balance using firm surface with open and closed eyes during messenger carrying condition was significantly less than their values during backpack and one-shoulder strap carrying conditions (P = 0.001, 0.033, 0.001, 0.042, respectively). No significant difference between backpack and one-shoulder strap carrying conditions (P = 0.156, 0.794, respectively).

The value of sensory integration using a foam surface with open and closed eyes during messenger carrying condition was significantly less than their values during backpack and one-shoulder strap carrying conditions (P = 0.003, 0.018, 0.001, 0.024, respectively). No significant differences between backpack and one-shoulder strap carrying conditions (P = 0.752, 0.138, respectively).

Regarding LOS and weight shifting, the value of messenger carrying condition was significantly greater than their values during backpack and one-shoulder strap carrying conditions (P = 0.028, 0.001, 0.034, 0.001, respectively). No significant difference between backpack and one-shoulder strap carrying conditions (P = 0.482, 0.841, respectively) as shown in Table 2.

Table 2.

Values of sensory integration, limits of stability and weight shifting during carrying conditions.

Parameters	Backpack (mean ± SD)	Messenger (mean ± SD)	One-shoulder strap (mean ± SD)	F ratio	P value
Eye open firm surface (%)	86.78 ± 4.15	79.56 ± 7.37	84.16 ± 5.57	11.72	0.001*
Eye closed firm surface (%)	84.13 ± 5.22	77.56 ± 8.81	82.44 ± 6.30	8.92	0.001*
Eye open foam surface (%)	78.00 ± 6.36	71.69 ± 7.38	76.66 ± 4.95	9.27	0.001*
Eye closed foam surface (%)	77.06 ± 7.01	68.28 ± 7.62	73.53 ± 5.88	12.49	0.001*
Limits of stability, (%)	31.94 ± 9.56	37.66 ± 8.60	29.25 ± 8.02	10.76	0.001*
Weight shifting, (%)	86.5 ± 9.55	92.5 ± 7.02	83.88 ± 9.47	8.19	0.001*

*Significantly (P < 0.05), SD: standard deviation.

The step and stride lengths during carrying the backpack were significantly longer than the messenger and one-shoulder strap bags (P = 0.018, 0.001, 0.012, 0.001, respectively) and the step and stride lengths during carrying the messenger bag was significantly longer than the one-shoulder strap bag condition (P = 0.025, 0.017 respectively). However, the cadence during carrying the one-shoulder strap bag was significantly higher than backpack and messenger bag (P = 0.001, 0.049, respectively), and cadence during carrying the messenger bag was significantly higher than the backpack condition (P = 0.030). Regarding walking velocity, there was no significant difference between the three carrying conditions (P = 0.955) as shown in Table 3.

Table 3.

Values of gait spatiotemporal parameters during carrying conditions.

Parameters	Backpack(mean ± SD)	Messenger(mean ± SD)	One-shoulder strap(mean ± SD)	F ratio	P value
Step length, cm	53.10 ± 6.01	51.97 ± 5.45	50.65 ± 5.62	21.90	0.001*
Stride length, cm	106.94 ± 10.77	105.03 ± 11.10	102.19 ± 11.39	34.52	0.001*
Cadence, step/min	105.96 ± 8.77	108.61 ± 7.87	111.64 ± 8.26	16.14	0.001*
Walking velocity, cm/s	83.39 ± 10.92	83.17 ± 10.76	83.27 ± 11.90	0.003	0.955

*Significantly (P < 0.05), SD: standard deviation.

Discussion

The results showed that the balance is affected in the same way whatever carrying backpack or one-shoulder strap. While the one-shoulder strap bag affects balance more than the messenger bag carrying this may be attributed to the effect of one-shoulder strap bag in developing lateral deviation of the trunk toward the loaded side. This is consistent with the findings reported that while carrying a messenger bag over one-shoulder, some subjects experienced decreased base of support (BOS) due to shifting to one side, which causing a narrowing of the BOS during walking.²¹

The study revealed that backpack carrying had less effect on balance than messenger bag. This may be due to AP instability caused by carrying a backpack, which leads to forward trunk leaning as compensatory postural adjustment for a loaded bag, which directed the center of pressure posterior to maintain balance. This finding agrees with Singh and Koh³² who stated that wearing a backpack causes a larger trunk lean. They concluded that when carrying a load, the forward trunk inclination balances the posterior shift of the body and backpack system's combined center of mass to prevent balance loss caused by a decrease in it, particularly in the AP direction.

Healthy children (12 years) needed a stronger propulsive power to restore balance after swinging while carrying a 10-kg backpack on one shoulder. Asymmetry in gait for braking and propulsive forces was also brought on by carrying a 10-kg, one-strap backpack. A two-strap backpack concealed this asymmetrical gait. It was found that using a one-strap backpack hurts children's gait kinetics, and they should carry their backpacks on both shoulders.³³

The results of the mCTSIB revealed a significant difference in the four variables (Eye open firm surface, eye closed firm surface, eye open foam surface, eye closed foam surface). There was decreased value of messenger carrying condition than backpack condition. These findings revealed that the decreased values of the sensory integration test and increased value of weight shift and LOS of normal reference in the three carrying conditions of school bag influenced body stability and sensory integration of balance but with different degree, in which messenger backpack was considered the best way of carrying with less affection on balance than a backpack and one-shoulder strap bag. The theory that the trunk bends toward the unloaded side to account for the disruption from the external load has been used to explain the biomechanical adjustments of humans sustaining unilateral loads. Through level walking, the response of the postural system to an asymmetric load is not constant. It most certainly has to do with the subject's handedness, task experience, and load characteristics.³⁴

Furthermore, the current findings revealed that the step and stride lengths during carrying the backpack were significantly longer than the messenger and one-shoulder strap bags. While carrying the one-shoulder strap bag, the cadence was significantly higher than a backpack, and messenger bag. While the cadence during the messenger bag was significantly higher than backpack condition. Conversely, there was little to no variation in the walking speed between the three carrying conditions. A previous study reported no significant difference in BOS, stride length, or velocity when carrying on one or two shoulders compared with the unloaded baseline walk. In contrast, stride length was reduced when a backpack was carried over one-shoulder when compared to the baseline walk,²¹ which was inconsistent with the current findings. This contradiction may be attributed to their participants being children (12–13 years) not adolescents. However, it was reported that the children walked with longer stride, stance, and double stance when carrying a backpack (on one- or two-shoulders) than when walking without one.³³ However, they calculated the weight of the backpack using a percentage of his body weight. Their participants walked on a treadmill, not at their normal pace as presented in the present study.

It is reported that the load carriage decreased the stride length in comparison to unload condition. But the stride length of the double strap bag style was still more than the other styles and the messenger was the lesser length.³⁵ Their findings concurred with the current study's results that neither carriage condition's velocity showed a discernible difference. When compared to the velocity while no backpack was worn, there was no appreciable variation in velocity when subjects wore the backpack over one-shoulder or two-shoulders. The velocity somewhat decreased from the baseline values when individuals carried the backpack over one shoulder. However, the velocity increased from the normal baseline values when subjects carried the backpack over two shoulders.²¹

The center of gravity will move from the front to the back of the BOS when carrying a backpack. According to recent findings, adolescents who are carrying backpacks must spend more time on both feet during gait to manage the weight.³⁶ Another study discovered no significant difference in the three walking conditions for stride length, stride time, step length, step time, gait speed, and cadence (without a backpack, with one strap; and with two straps),³⁷ which disagreed with the current results. This difference may be referred to that the target population of this study was an adult (22 years) that they may have different biomechanical strategies for load compensation that differ from teenage of the current study.

When carrying a bag by two methods (one-sided or cross-body), the activation of the upper trapezius and erector spinae muscles differs, the cross-body carrying appears to have a far less effect on spinal posture than same-sided carrying since the right upper trapezius and left erector spinae muscles showed much less activity during cross-body carrying.³⁸ The increased trunk forward inclination in space seemed to be mostly caused by trunk flexion in relation to the pelvis, which explained the increase in trunk flexion observed at 15% bodyweight backpack.¹⁹

It appears that alterations in the gait parameters, such as stride length and time, as well as in the cadence and velocity, suggest a significant change in the gait pattern of scholars. These behaviors indicate that the gait was performed with a higher velocity, length, and support of the lower extremities on the ground, favoring the maintenance of body balance.²⁰ This strategy was obvious in a study that showed the most significant alterations during walking with a backpack, weighing 15%, and 20% of the body weight, were observed in the hip joint.⁶ This strategy suggests significant adaptations in gait spatiotemporal parameters to maintain the center of gravity within BOS limits, favoring body balance despite an increase in load.¹¹

Limitations

A lack of control over the choices of participants’ footwear. The subject's gait can be measured while wearing shoes of their own choosing despite the lack of standardization in shoe types.²¹ The study's results are not entirely representative of reality in this carrying condition because the adolescents typically carry backpacks for periods of time far longer than the testing sessions. The present study only used 15% of body weight, making it impossible to extrapolate the results to other weights heavier or less than 15% of body weight. Finally, during the one-shoulder carry, the outcomes of the current investigation were not analyzed based on which shoulder the participant wore the bag. Because subjects may carry the backpack on either shoulder, there was insufficient sample size to analyze how the shoulder they chose for the one-shoulder carrying condition affected their gait.

Conclusion

Carrying the backpack with a load 15% of the adolescent's body weight improved the walking spatial parameters in comparison to the messenger and one-shoulder strap bags of the same weight. However, the messenger bag has a better impact on sensory integration of balance than the traditional backpack and one-shoulder strap bag, with no appreciable difference in speed between the three carrying conditions. Hence, the messenger bag style preserves speed while maintaining balance.

Footnotes

Acknowledgments

The researchers would like to thank everyone who took part in the study.

ORCID iDs

Amr A Abdel-aziem

Dalia M Mosaad

Rania G Hegazy

Ethical approval

The ethics committee of Faculty of Physical Therapy, Cairo University approved this study, which was given the number P.T.REC/012/ 003135.

Author contributions

Conception and design: Amr A Abdel-aziem and Dalia M Mosaad; Methodology: Dalia M Mosaad and Rania G Hegazy; Writing – original draft preparation: Amr A Abdel-aziem, Dalia M Mosaad, and Rania G Hegazy; Writing – review and editing: Amr A Abdel-aziem, Dalia M Mosaad, and Rania G Hegazy. All authors have critically reviewed and approved the final draft and are responsible for the content and similarity index of the manuscript.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data availability statement

On reasonable request, the corresponding author will provide the information supporting the study's findings.

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