Evaluation of an ergonomically designed schoolbag: Heart rate variability and body discomfort rating

Abstract

BACKGROUND:

Unsuitable schoolbags may stress the spine and promote poor body posture, particularly for school students. Global recommendations have suggested that schoolbag weight must not exceed 10% of a healthy student’s body mass, which would need continuous monitoring and enforcement.

OBJECTIVES:

The present study presents a comparison between an ergonomically designed schoolbag, which helps reduce the potential effects of carrying a load, and a commercial one.

METHODS:

A total of 30 healthy male students were recruited for this experiment. Independent variables determined were schoolbag type (ergonomically designed and commercial schoolbags) and three load levels based on body mass percentage (i.e., 10%, 15%, and 20% of body mass). Heart rate variability (HRV) and body discomfort rating were then measured.

RESULTS:

Our results showed that the developed schoolbag promoted enhanced subjective measures and HRV response at 15% and 20% of body mass. Participants who wore the developed schoolbags experienced significantly lesser neck, shoulder, upper and lower trunk discomfort than those who wore the traditional ones. Changing the load percentage from 10% to 15% caused an increase in heart rate among participants carrying a commercial schoolbag but a decrease in heart rate among those carrying the developed schoolbag.

CONCLUSIONS:

The findings presented herein suggest introducing strategies for reducing the potential impact of load carrying through the combined effect of new educational inventions and policy changes.

Keywords

Backpack carried load children ergonomics

1 Introduction

Carrying a schoolbag is a regular activity for students across various educational levels from the first year of school until the last year of university. However, studies have shown that this practice has a detrimental impact on student’s health, such as back pain [1 –3]. Nonetheless, the relationship between schoolbag use and back pain has remained uncertain and has been an area of great interest among medical and safety societies given that it increases physical stress and risk of injury [4]. Back injury rates have been among the most costly and prevailing health problems considering the high probability of future back injuries [5]. Studies have shown that carrying a weight more than 10% of their body mass could cause musculoskeletal disorders and problems and physical fatigue, which negatively affect students’ concentrate during learning [6, 7].

Research has shown that between 30% and 70% of young students exhibit lower back pain, which may vary based on population age, definition of pain, and type of research design [8, 9]. Guardians, health and safety professionals, and school principals have emphasized wearing schoolbags by tightening both straps to reduce the probability of back risk [10]. Several studies [11 –15] have revealed that schoolchildren with back pain wore heavier schoolbags relative to their body weight than those without back pain. Furthermore, data has shown that the association between back pain and carried load was higher among male than female students, while Negrini and Carabalona [16] found that 79.1% of the students who participated had heavy school bags, which caused back pain and fatigue in 46.1% and 65.7% of the patients, respectively. Moreover, Goodgold et al. [17] noted that 55% of all students carried a load of more than 15% of their body mass, among whom 67% reported back pain. Mackie et al. [18] concluded that these high loads impose an increased risk of injury on children. A study by Dockrell et al. [19] introduced the relationship between individual physical and psychosocial risk factors and schoolbag-related musculoskeletal stress among primary school children in Ireland.

Similarly, Khan et al. [20] evaluated the frequency of neck, shoulder, and back pain in school children carrying heavy schoolbags and found that those who used double straps and carried bags on both shoulders experienced fewer pain symptoms. Azabagic et al. [21] who studied musculoskeletal disorders among primary school children in Bosnia and Herzegovina, showed that the incidence of musculoskeletal pain was approximately 48%. Furthermore, a statistically significant correlation had been observed between total schoolbag weight and acute right shoulder pain using the length of time at which the schoolbag was used. Aprile et al. [22] found a relationship between back pain and schoolbag use in terms of intensity, differences between students, prevalence, and predisposing variables, showing that over 60% of the participants experienced pain. Moreover, a logistic model showed that adolescent girls were at higher risk for intense pain when compared to male students. The length of time at which the schoolbag was carried had been identified as a strong predictor of back pain, whereas schoolbag load had been determined to have a weak effect on back pain [3]. After describing the anthropometric variables, degree of disability, schoolbag weight, and quality of life in girls and boys aged 11 to 17 years, Macedo et al. [23] found that boys had more moderate disabilities and better quality of life than girls with regard to psychosocial health, physical health, and physical and emotional functioning. Notably, low back pain had also been identified as the primary factor impacting an individual’s quality of life.

Apart from its weight, a schoolbag’s center of gravity height on the student’s back has received increasing attention [24]. To select a suitable method for load carriage, the position of the schoolbag is critical [25]. Generally, Orloff and Rapp [26] reported that posture changes were reduced when wearing a backpack in a low position instead of a high position on the back, which is typically recommended. Several other studies have identified variables during schoolbag carriage, including schoolbag weight, carriage process, time spent on carrying schoolbags, and strap length, among other factors [27 –35]. Such considerations were found to affect the risk of musculoskeletal symptoms, such as pain and discomfort, in primary school children [34]. Several types of research have shown that carrying heavy schoolbags reduces walking time [28 –30], increases trunk forward lean [30 –32], cardiorespiratory responses [33], force exertion on spinal intervertebral discs [34], and foot–ground pressures [35]. The reduction in walking speed and time can be attributed to the combined effect of taking shorter steps (lower step length), fewer walking steps per minute, and more time on both feet instead of just one foot. For instance, Proffitt et al. [36] found that college students carried a schoolbag weighing 16% –20% of their body masses while walking distances farther than those walked by similar students not wearing a schoolbag.

Moreover, previous research on children has shown that contact pressures under shoulder belts increases dramatically when back loads fall between 10% and 30% of their body mass [37]. This shoulder belt pressure is higher than the pressure thresholds over which cutaneous blood flow is interrupted [38]. Schoolbag belts often compress the anterior portion of the shoulder wherein the brachial plexus, axillary artery, and vein are located [39]. Thus, pressure on such tissues may influence hand/arm sensation and circulation. In a study evaluating the difference in pressure transmission effect and decompression between soft and hard bags with increased load weight, Zhou et al. [40] concluded that shoulder to backpressure value increased significantly along with increasing schoolbag load; however, although the increase in schoolbag load was linear, that for backpressure was exponential.

Some schoolbag designs that harness better results and reduce the back pain among students have been available [41]. Southard and Mirka’s study [42] reported that a considerable portion of the carried load could be transferred directly to the pelvis, thereby reducing muscle forces supporting the carrying load. Accordingly, 30% of the schoolbag’s vertical weight could be transferred to the hips utilizing a hip belt with a framed schoolbag [43]. Another drawback of schoolbags without a strong hip strap was that the straps of the schoolbag itself placed stress on the shoulders. In addition, Holewijn [44] reported that a conveyed load could be moved using a flexible waist frame to reduce pressure on the shoulder skin considering that the skin on the waist was much less sensitive to pressure than that on the shoulder area. A study by Reid et al. [45] demonstrated that adding lateral hardening rods to the side edges of the attached schoolbag system could move a portion of the load from the shoulders and back to the hips, resulting in reduced weight supported to the torso.

Mosaad and Abdel-Aziem [46] compared the effects of two types of schoolbag designs, namely a traditional schoolbag and a double-sided bag, on body balance and neck angle in children. Accordingly, they found that carrying the bag on both sides allowed the children to maintain body balance and normal neck position similar to those in children not carrying a load. Atier et al. [47] introduced a new concept for a schoolbag wherein two compartments were created for school supply placement, one in front and another in the posterior portion of the trunk, with handles added to join these compartments. Two elastic rods were posted on the back of the schoolbag compartment, with Velcro serving as a belt to be fixed at the front compartment. One limitation of this schoolbag design was its compartment size and handle, which were limited to children of a specific anthropometric range. Mallakzadeh et al. [48] developed a new schoolbag design with non-flexible straps to determine differences between normal gate and that while carrying a load in two schoolbags. Accordingly, they found low anterior shoulder muscle discomfort in approximately 60% of participants carrying both schoolbags. Moreover, more than 80% of the participants stated neck discomfort in both schoolbags.

Previous studies have concluded that excess loads produce an effect on the arm and can be transferred to prevent back injuries. In an earlier work using the same approach, Amiri et al. [49] designed a comfortable schoolbag with a user-focused design method; however, they did not consider ideal weight in their schoolbag design. Thus, the present study considered ideal weight when designing a schoolbag.

Mohammadi et al. [50] considered anthropometric dimensions when designing a schoolbag and compared them across various school levels. Accordingly, variations in chest breadth, shoulder width, waist breadth, Laparoscopic and Robotic-assisted Laparoscopic Sacral Colpopexy (LC-RSC) curve length, the seven cervical vertebrae and the fifth lumbar vertebrae (C7–L5) distance and curvature, right and left sacrum distance, the tenth thoracic vertebrae, and the third lumbar vertebrae (T10–L3) distance, and right and left apex of scapula distance had been observed among levels. No significant variations had been found for other measurements. Sen [51] reported changes in perceived exertion after treadmill walking with a side pack and schoolbag. Moreover, they found a considerable increment in body discomfort rating with a schoolbag and left, and right sides pack after treadmill walking. Heart rate variability (HRV), one of the most common physiological measures [52], can also assess physical stress. According to the National Heart, Lung, and Blood Institute, stress monitoring offers information regarding how the heart functions during physical stress. Tanaka et al. [53] reported that HRV analysis showed promise indicators in detecting physical fatigue and identifying different physical activities. The current study’s principal motivation was to compare a developed schoolbag, which helps reduce the potential effects of the carried load, and a well-known commercial one available in the local market considering HRV and subjective measure.

2 Materials and methods

The present study primarily aimed to evaluate the developed schoolbag design [54, 55] by measuring the effects of carrying the developed schoolbag on HRV and perceived exertion rating (PER). Moreover, this study also compared the developed schoolbag with a commercial one available in the local market. We hypothesized that the developed schoolbag would help reduce the negative effects on the cardiac system while also lessening discomfort.

2.1 Participants

The minimum number of participants required was estimated based on Equation 1 proposed by The International Standards Organization (ISO 15535:2012) “General requirements for establishing anthropometric databases,” with a 95% confidence interval for the 5th and 95th percentiles: $N ⩾ {(3.006 \times \frac{cv}{α})}^{2}$ (1) where:

N is the number of samples needed,

CV is variation coefficient,

and α is the desired relative accuracy in percentage.

Therefore, a relative accuracy of 5% is desired for the 5th and 95th percentiles, while coefficient variation values were calculated as provided by Ramadan and Al-Shayea [4]. Consequently, the minimum required sample size for participation was 30 students considering the weight parameter.

A total of 30 healthy male students with an average (standard deviation) age of 25.9 (2.634) years, stature height of 1.68 (0.046) m, and body weight of 70.94 (9.12) kg were recruited for this study. University students were selected due to their availability and relevance to the study area. All participants were healthy without back pain problems or other musculoskeletal injuries. Those with a pre-existing orthopedic condition; history of musculoskeletal disease; spine injury; leg, head, lung, and/or heart disorders; or allergic reactions to gels and adhesive materials were excluded from the experiment.

Given that human subjects were involved, informed consent was obtained before conducting the experiments. The Institutional Review Board (IRB) of the University was responsible for assessing possible physical or psychological risks associated with research involving human participants for their protection by ensuring the implementation of ethical practices. The aforementioned body reviewed research in accordance with the following approval criteria:

Assessing the risk/benefit ratio

Ensuring that informed consent was appropriately obtained

Verifying that recruitment methods were not misleading

Ensure selection of subjects was equitable and justified

The visiting physician was involved with the experiment to ensure the integrity of the subjects as required by IRB. Prior to study participation, each participant completed an integral health history form and written consent form, which was approved by the IRB of the University (IRB Research Project Approval No. E-18-3451).

2.2 Experimental design

To assess the efficiency of the developed schoolbag, we compared it to a typical schoolbag. After a considerable evaluation, the present study opted to include two types of schoolbags as an independent variable to determine which type would promote the best outcomes while carrying different loads. The percentage of load carried, that is, 10%, 15%, and 20% of participants’ body masses, was considered the second variable. As such, a within-subjects design was used in which each participant was asked to walk at a comfortable self-speed along a specific path around the lab (e.g., length of 54 m). They were asked to walk wearing both types of schoolbags containing three different loads (one at a time at a specific weight level). Six experimental treatments were conducted to evaluate HRV and PER for each participant.

2.3 Experimental variables

2.3.1 Independent variables

This experiment utilized two types of schoolbags to determine which was more effective in reducing heart effort and back pain. The developed schoolbag consisted of two-sided pockets attached with the back pocket to form a new schoolbag type. The new backpack was similar to the Back-T-pack model with compression straps [56] with adding a back pocket, as shown in Fig. 1a. The other type of schoolbag used was that most commonly used in the community, as shown in Fig. 1b. Details regarding the two schoolbags are presented in Ramadan and Al-Tayyar study [54].

Fig. 1

a. Two-side pockets attached with back pocket to form a new schoolbag. b. Commercial backpack.

The other independent variable was the percentage of load carried (i.e., 10%, 15%, and 20% of the student’s body mass) [4, 54]. Academic books with various weights (i.e., 0.1, 0.2, 0.5, 1, 2, and 4 kg) were used as load, utilizing different sizes and book groupings tied together to provide more weight. The load applied during the experiment was evenly distributed among all three pockets of the schoolbag.

2.3.2 Dependent variables

The dependent variables included HRV and PER.

2.3.2.1. Electrocardiography signal response An electrocardiograph (ECG) is a graph that displays electrical potential changes occurring between electrodes mounted on the skin of the patient to illustrate cardiac activity. Spreading electrical currents produces various potentials at different body locations that can be measured by electrodes mounted on the skin [57]. ECG signals were recorded using a one-channel MT-ECG-1 preamplifier connected to two eight-channel Bio-monitor ME6000 to evaluate the changes in HRV parameters associated with carrying a schoolbag (developed and commercial) at three different loads. The MT-ECG-1 preamplifier has three lead wires with Ag/AgCl solid adhesive pre-gelled electrodes, which are placed on the participants’ chests to record ECG signals without muscle contraction artifacts occurred during schoolbag carrying. ECG signals were acquired and amplified with the help of Mega Win software at a sampling rate of 1000 Hz.

ECG HRV analysis is a sensitive diagnostic instrument for evaluating the function of the autonomic nervous system under stress states. Moreover, HRV was measured while paying particular attention to time- and frequency domain measurements, which can be a responsive and sensitive indicator of efficient and stress-mediated changes in sympathetic and parasympathetic conditions. Among healthy subjects, the ratio between low frequency “LF” (0.04–0.15 Hz) and high-frequency “HF” (0.15–0.4 Hz) components (LF/HF ratio) of HRV spectra represents a measure of sympathovagal balance. The standard deviation of all normal to normal intervals has been used as a representative of overall autonomous activity for both sympathetic and parasympathetic influences. Moreover, LF was mainly used as a representative of sympathetic modulation activities, while HF, root mean square of successive difference (RMSSD), and the percentage of successive intervals that differ by more than 50 ms (pNN50) were utilized as representatives of vagal modulation activity. Finally, the LF/HF ratio was used to indicate sympathovagal balance.

The Kubios HRV Analysis Program 2.2 was used to process ECG signals [58]. Recorded ECG signals were pre-processed to remove artifact beats by eliminating RR intervals that ranged more than 25% between two consecutive RR intervals. Exclusive RR intervals were replaced using linear interpolation to ensure consistent data length (i.e., resulting in the same total number of beats). The smoothness prior to the process with a Lambda value of 1000 was used to eliminate disturbing low frequency basic pattern components [59, 60].

Signals were also subjected to frequency domain analysis using fast Fourier transformation. Quadratic 4 Hz interpolation converted the non-equidistant beat-by-beat time series into a new time series sampled at equidistant intervals. The signals were then detrended to eliminate possible patterns, after which the average was removed to derive variance from signals. With a 256 s window and 50% overlap, Welch’s periodogram function was used to produce rhythmic oscillation spectra of power over a frequency range of 0.0–0.4 Hz with a resolution of 0.015 to 0.02 Hz. Welch’s periodogram was placed for whole standard divisions (no predefined frequency bands).

Time domain analyses: For time domain analysis, mean RR intervals (mRR), mean heart rate (mHR), standard deviation of all RR intervals (SDRR), standard deviation of the mean RR intervals in all parts of the data (SDHR), RMSSD, total number of pairs of consecutive standardized RR intervals differing by more than 50 ms in the data (NN50), and pNN50 were calculated in this study.

Frequency domain analyses: For HRV frequency domain analysis, the following indicators of HRV were calculated: the absolute power of the band of frequencies less than 0.04 Hz (VLF), frequencies between 0.04 and 0.15 Hz (LF power), frequencies between 0.15 and 0.4 Hz (HF power), LF/HF ratio, normalized LF (LFn.u.; LF/total power), normalized HF (HFn.u.; HF/total power), and total power (included in the range of 0.0–0.4 Hz).

2.3.2.2Body discomfort rating Body discomfort rating was evaluated while considering locally perceived discomforts in the shoulder, upper back, lower back, and neck. This determined the sensation of comfort while carrying the schoolbag during the activity through different levels, which would help compare both types of schoolbags [61]. Four 12 cm line drawings associated with each reported body part were visually presented to the participant through which they expressed their underlying discomfort in terms of pain sensation. A 6-point scale ranging from 0 (no discomfort) to 6 (extreme discomfort, almost highest) was used to assess discomfort per region. Each participant was asked to rate any discomfort by marking an asterisk in the specified line at the start of each treatment and immediately after completing each run. Importantly, we highlight that using aforementioned subjective measure was designed to somehow mirror the objective measure (ECG).

2.4 Instruments

Developed and commercial schoolbags have been utilized to carry different loads. The eight-channel Bio-monitor ME6000 and MT-ECG-1 preamplifier were selected in this experiment to record ECG signals provided by the Mega Win 3.0.1 software (Mega Electronics Ltd., Kuopio, Finland). HRV was also determined using Kubios HRV program v2.2 (University of Western Finland, Finland).

Given our experimental design for the loads (i.e., 10%, 15%, and 20% of participants’ body mass), various sets of reference weights had to be used to control load accuracy considering that the participants’ weights varied. Calibration sets ranged from 100 g to 2.5 kg, with a 100 g unit being the smallest used. Anthropometric dimensions of the participant were measured using the following instruments:

fixed anthropometry (0–2100 mm with curved measuring branches and straight probes);

sliding caliper (length of 0–200 mm with a depth of 0–50 mm, Martin type);

spreading caliper with rounded ends (0–600mm); and

fiber glass tape (Dean, 0–1500 mm).

A balance scale (Seca 708, ±0.1 kg) was utilized to measure participant’s body weight, while a calculator was used to calculate 10%, 15%, and 20% of the participant’s body mass. Other materials and instruments included 70% isopropyl alcohol swabs, tissues, cotton swabs, adhesive bandages, and Ag/AgCl solid adhesive pre-gelled electrodes for acquiring ECG signals (Ambu A/S, Denmark).

2.5 Experimental procedures

This section comprehensively describes the steps for the experimental comparison between the designed and developed schoolbag. The experimental procedure was initiated by an invitation to tender issued and distributed at King Saud University to attract adult participants (Fig. 2). In the announcement, participants were asked to participate in pre- and six sessions lasting for 45–120 min. Participants’ weight was measured to calculate load levels used for the experiment (10%, 15%, and 20% of the participant’s body mass). After receiving the responses, participants were scheduled. In the opening session, participants were welcomed to the experimental station at the Human Factor Lab of the Industrial Engineering Department of King Saud University. Each participant received a full explanation regarding the aim of the investigation, steps involved, and time taken for each session. Thereafter, participants were given the opportunity to inquire about the research without hesitation and were made aware of their right to terminate their involvement at any stage of the experiment. Participants were then tested for health condition and allergies to any materials, which lasted for around 10 min.

Fig. 2

Experimental procedures.

After welcoming participants and describing their intent and procedure, participants received a consent form to read and sign within 5 min. Subsequently, demographic and anthropometric data, such as stature height, shoulder breadth, elbow breadth, hip breadth, elbow–shoulder height, elbow sitting height, shoulder sitting height, chest circumference, abdominal circumference, and weight, were obtained and recorded. Thereafter, the skin on the chest of the participant was cleaned using 70% isopropyl alcohol swabs, after which ECG electrodes were attached.

To minimize learning effects, each participant performed every experimental session in a randomized order, which was subsequently established. Prior to a treatment session, the participant was required to undergo PER assessment for several body parts. After filling the schoolbag with the weight measured according to the experiment’s randomization table, the investigator assisted the participant in wearing the school bag and asked him to walk for 5 min around the lab perimeter. The walking starting point was marked 3 m from the bottom corner of the laboratory. A minimum 5 min rest interval among sessions was allowed to avoid fatigue. Upon completing the assigned session, the participant was asked to reassess his exercise ratings using descriptive scales for the assigned treatment. After completing all sessions, the electrodes were removed, and the participant received a gift and was thanked for their participation. Also, the investigator informed participants to withdraw at any time during the experiment.

2.6 Experimental statistics

Statistical analysis was conducted with the significance level threshold set to 0.05 (type I error). A within-subjects design had been used to determine the main interaction effects of schoolbag type and percentage of load carried on HRV. A simple effect technique was used where interaction occurs [62]. The design assumptions (normality using the Kolmogorov–Smirnov test, variance homogeneity using Mauchly’s sphericity test) were tested to ensure the reliability of the results obtained during statistical analysis. The significance level for Mauchly’s test should be greater than 0.05. A violation of Mauchly’s test suggested that the variance calculations may be distorted, resulting in an inflated F-ratio. In such cases, the degrees of freedom (df) were corrected using the Greenhouse–Geisser sphericity (ɛ) estimates [63]. The Friedman test was used to determine group differences when the dependent variable being evaluated was ordinal, such as PER tests. In case of a significant effect, Wilcoxon signed-rank tests were used to illustrate differences between variable levels. In addition, the effect size was estimated based on a partial eta-squared value (η²) to reflect the variance percentage of independent variables due to a particular independent variable. IBM Statistics Package for Social Sciences software (version 23, IBM Corp., Armonk, NY, USA) was utilized for all statistical analyses.

3 Results

3.1 Body discomfort rating

Participants who wore the developed schoolbags experienced significantly lesser neck, shoulder, upper and lower trunk discomfort than those who wore the traditional ones (p < 0.002, p < 0.0001, p < 0.0001, and p < 0.0001, respectively; Table 1), using Wilcoxon signed-rank tests regardless of weight carried. Substantial individual variations were observed as indicated by high standard deviations. Furthermore, load carried had considerable effects on the participants’ neck, shoulder, upper trunk, and lower trunk discomfort using Friedman tests [χ² (2) = 24.649, p < 0.0001; χ² (2) = 43.074, p < 0.0001; χ² (2) = 23.016, p < 0.0001; and χ² (2) = 17.391, p < 0.0001, respectively (Table 1)]. Wilcoxon signed-rank tests to differentiate among load levels showed that a 20% load promoted greater neck, shoulder, and lower trunk discomfort compared to a 15% load (p < 0.008, p < 0.0001, and p < 0.035, respectively) and greater neck, shoulder, upper trunk, and lower trunk discomfort compared to a load of 10% (p < 0.0001, p < 0.0001, p < 0.0001, and p < 0.001, respectively). Similarly, a 15% load promoted greater neck, shoulder, and lower trunk discomfort compared to a 10% load (p < 0.008, p < 0.001, p < 0.0001, and p < 0.015, respectively).

Table 1
Results of subjective body discomfort ratings

Mean (Standard deviation) Statistics p (Z) values

Variable Backpack type Carried loads Backpack type Load percentage

Levels body parts Developed Commercial 10% 15% 20% Developed Commercial 10% vs. 15% 10% vs. 20% 15% vs. 20%

Neck discomfort 0.1889 (0.616) 0.4833 (0.949) 0.0583 (0.227) 0.2833 (0.739) 0.6667 (1.099) 0.002 –3.143 0.008 (–2.65) 0.0001 (–3.88) 0.008 (–2.67)

Shoulder discomfort 1.0289 (1.113) 1.572 (1.369) 0.6917 (0.824) 1.208 (1.180) 2.002 (1.402) 0.0001 –3.745 0.001 (–3.40) 0.0001 (–5.31) 0.0001 (–4.17)

Upper back discomfort 0.3611 (0.779) 1.0722 (1.5389) 0.2417 (0.593) 0.9167 (1.557) 0.9917 (1.323) 0.0001 –4.722 0.0001 (–3.93) 0.0001 (–4.40) 0.399 (–0.844)

Lower back discomfort 0.05 (0.2127) 0.4444 (0.8361) 0.0667 (0.312) 0.2500 (0.628) 0.4250 (0.828) 0.0001 –4.154 0.015 (–2.43) 0.001 (–3.22) 0.035 (–2.11)

Feet discomfort 0.0778 (0.403) 0.0667 (0.391) 0.0333 (0.181) 0.05 (0.287) 0.1333 (0.596) 0.655 –0.447

	Mean (Standard deviation)	Statistics p (Z) values
Neck discomfort	0.1889 (0.616)	0.4833 (0.949)	0.0583 (0.227)	0.2833 (0.739)	0.6667 (1.099)	0.002	–3.143	0.008 (–2.65)	0.0001 (–3.88)	0.008 (–2.67)
Shoulder discomfort	1.0289 (1.113)	1.572 (1.369)	0.6917 (0.824)	1.208 (1.180)	2.002 (1.402)	0.0001	–3.745	0.001 (–3.40)	0.0001 (–5.31)	0.0001 (–4.17)
Upper back discomfort	0.3611 (0.779)	1.0722 (1.5389)	0.2417 (0.593)	0.9167 (1.557)	0.9917 (1.323)	0.0001	–4.722	0.0001 (–3.93)	0.0001 (–4.40)	0.399 (–0.844)
Lower back discomfort	0.05 (0.2127)	0.4444 (0.8361)	0.0667 (0.312)	0.2500 (0.628)	0.4250 (0.828)	0.0001	–4.154	0.015 (–2.43)	0.001 (–3.22)	0.035 (–2.11)
Feet discomfort	0.0778 (0.403)	0.0667 (0.391)	0.0333 (0.181)	0.05 (0.287)	0.1333 (0.596)	0.655	–0.447

3.2 Electrocardiography responses

Electrocardiography includes several responses related to time and frequency domain indices, some of which have a momentous effect either by independent variables or by their interactions. Our analysis showed that mRR, mHR, RMSSD, LF (ms2), and HF (ms2) were significantly was affected (Table 2).

Table 2
Mean (SD) and statistics values of the HRV

Parameters Mean (SD) Statistics p (η2)

Backpack type Developed schoolbag Commercial schoolbag Backpack type (η2) Percent lifted load (η2) Interaction (η2)

% Carried load 10% 15% 20% 10% 15% 20%

mRR (ms) 607.36 (62.56) 620.32 (62.59) 607.47 (59.76) 620.79 (62.25) 604.28 (60.89) 594.12 (66.85) 0.012 (0.20)^a 0.0001 (0.33)^c 0.0001 (0.48)^c

SDRR (ms) 16.38 (5.02) 16.71 (4.69) 15.91 (5.27) 16.96 (5.39) 15.86 (5.59) 15.33 (5.20) 0.457 (0.019) 0.016 (0.133)^a 0.170 (0.053)

mHR (bpm) 99.97 (10.46) 97.85 (10.28) 99.86 (10.28) 97.78 (10.29) 100.44 (10.49) 102.41 (11.90) 0.007 (0.228)^b 0.0001 (0.317)^c 0.0001 (0.510)^c

SDHR (ms) 2.92 (0.61) 2.86 (0.58) 2.87 (0.73) 2.98 (0.57) 2.88 (0.71) 2.84 (0.59) 0.697 (0.005) 0.170 (0.059) 0.843 (0.006)

RMSSD (ms) 11.83 (5.11) 11.75 (5.10) 10.51 (4.64) 10.66 (4.98) 10.500 (4.92) 9.89 (4.70) 0.0001 (0.517)^c 0.0001 (0.296)^c 0.414 (0.030)

NN50 1.13 (3.39) 1.30 (4.11) 0.80 (1.67) 1.20 (3.21) 1.00 (2.96) 0.63 (2.28) 0.196 (0.057) 0.157 (0.124) 0.880 (0.009)

pNN50 (%) 0.30 (0.79) 0.30 (0.95) 0.18 (0.38) 0.28 (0.74) 0.23 (0.69) 0.15 (0.53) 0.214 (0.053) 0.121 (0.077) 0.869 (0.004)

TINN 86.17 (28.43) 83.67 (24.49) 81.17 (30.65) 86.83 (28.12) 82.00 (31.17) 75.67 (25.45) 0.388 (0.026) 0.004 (0.173)b 0.583 (0.018)

HVR 5.14 (1.78) 5.00 (1.55) 4.85 (1.52) 4.86 (1.64) 4.84 (1.74) 4.85 (1.79) 0.069 (0.109) 0.524 (0.022) 0.546 (0.021)

VLF (ms2) 16.95 (9.54) 19.34 (12.68) 17.17 (10.38) 19.09 (10.47) 14.49 (10.58) 15.10 (9.31) 0.147 (0.71) 0.442 (0.028) 0.057 (0.099)

LF (ms2) 181.44 (98.92) 179.5 (110.91) 171.300 (106.76) 192.65 (145.42) 160.5 (120.19) 35.97 (28.36) 0.0001 (0.485)^c 0.0001 (0.392)^c 0.0001 (0.343)^c

HF (ms2) 40.47 (40.48) 42.09 (42.95) 32.17 (35.74) 39.52 (43.29) 35.43 (43.18) 7.25 (6.11) 0.001 (0.339)^b 0.0001 (0.282)^c 0.014 (0.137)^a

LF/HF 7.21 (4.03) 6.29 (2.77) 7.25 (6.12) 7.55 (5.17) 7.62 (4.64) 8.13 (3.72) 0.062 (0.115) 0.485 (0.025) 0.232 (0.794)

Parameters	Mean (SD)	Statistics p (η2)
% Carried load	10%	15%	20%	10%	15%	20%
mRR (ms)	607.36 (62.56)	620.32 (62.59)	607.47 (59.76)	620.79 (62.25)	604.28 (60.89)	594.12 (66.85)	0.012 (0.20)^a	0.0001 (0.33)^c	0.0001 (0.48)^c
SDRR (ms)	16.38 (5.02)	16.71 (4.69)	15.91 (5.27)	16.96 (5.39)	15.86 (5.59)	15.33 (5.20)	0.457 (0.019)	0.016 (0.133)^a	0.170 (0.053)
mHR (bpm)	99.97 (10.46)	97.85 (10.28)	99.86 (10.28)	97.78 (10.29)	100.44 (10.49)	102.41 (11.90)	0.007 (0.228)^b	0.0001 (0.317)^c	0.0001 (0.510)^c
SDHR (ms)	2.92 (0.61)	2.86 (0.58)	2.87 (0.73)	2.98 (0.57)	2.88 (0.71)	2.84 (0.59)	0.697 (0.005)	0.170 (0.059)	0.843 (0.006)
RMSSD (ms)	11.83 (5.11)	11.75 (5.10)	10.51 (4.64)	10.66 (4.98)	10.500 (4.92)	9.89 (4.70)	0.0001 (0.517)^c	0.0001 (0.296)^c	0.414 (0.030)
NN50	1.13 (3.39)	1.30 (4.11)	0.80 (1.67)	1.20 (3.21)	1.00 (2.96)	0.63 (2.28)	0.196 (0.057)	0.157 (0.124)	0.880 (0.009)
pNN50 (%)	0.30 (0.79)	0.30 (0.95)	0.18 (0.38)	0.28 (0.74)	0.23 (0.69)	0.15 (0.53)	0.214 (0.053)	0.121 (0.077)	0.869 (0.004)
TINN	86.17 (28.43)	83.67 (24.49)	81.17 (30.65)	86.83 (28.12)	82.00 (31.17)	75.67 (25.45)	0.388 (0.026)	0.004 (0.173)b	0.583 (0.018)
HVR	5.14 (1.78)	5.00 (1.55)	4.85 (1.52)	4.86 (1.64)	4.84 (1.74)	4.85 (1.79)	0.069 (0.109)	0.524 (0.022)	0.546 (0.021)
VLF (ms2)	16.95 (9.54)	19.34 (12.68)	17.17 (10.38)	19.09 (10.47)	14.49 (10.58)	15.10 (9.31)	0.147 (0.71)	0.442 (0.028)	0.057 (0.099)
LF (ms2)	181.44 (98.92)	179.5 (110.91)	171.300 (106.76)	192.65 (145.42)	160.5 (120.19)	35.97 (28.36)	0.0001 (0.485)^c	0.0001 (0.392)^c	0.0001 (0.343)^c
HF (ms2)	40.47 (40.48)	42.09 (42.95)	32.17 (35.74)	39.52 (43.29)	35.43 (43.18)	7.25 (6.11)	0.001 (0.339)^b	0.0001 (0.282)^c	0.014 (0.137)^a
LF/HF	7.21 (4.03)	6.29 (2.77)	7.25 (6.12)	7.55 (5.17)	7.62 (4.64)	8.13 (3.72)	0.062 (0.115)	0.485 (0.025)	0.232 (0.794)

^aSignificant effect at p < 0.05; ^bsignificant effect at p < 0.005; ^csignificant effect at p < 0.0005.

3.2.1 Time domain analysis

ANOVA showed that schoolbag type markedly affected mRR [F (1, 29) = 7.243, p < 0.012, η² = 0.2], suggesting that the commercial schoolbag caused a smaller mean R-R interval than the developed schoolbag. Moreover, load percentage was also found to significantly affect m-RR [F (1.79, 51.981) = 14.284, p < 0.0001, η² = 0.33], implying that the mRR decreased as carrying load increased.

Schoolbag type and load percentage interaction was found to have a significant effect on mRR [F (1.96, 56.948) = 26.652, p < 0.0001, η² = 0.479], revealing that mRR was greater when a participant used the commercial schoolbag at a load of 15% and 20%. At a load of 10%, the effect of the commercial schoolbag on heart rate decreased (Fig. 3). Pairwise comparison showed that at a load of 15% and 20%, the developed school bag served various purposes.

Our data showed that load percentage significantly affected STDRR [F (2, 54.290) = 4.44, p < 0.018, η² = 0.133]. As shown in Fig. 4, STDRR decreased as carried load increased, irrespective of schoolbag type.

Schoolbag type was found to have a significant effect on mHR [F (1, 29) = 8.58, p < 0.007, η² =0.228], demonstrating that the commercial schoolbag bag promoted a higher mean heart rate (beat per minute) than the developed schoolbag. Our result also showed that load percentage significantly affected mHR [F (1.722, 49.938) = 13.442, p < 0.0001, η² = 0.317], suggesting that heart rate increased as the carried load increased from 10% to 20%.

Fig. 3

Effect of schoolbag type and carried load interaction on mean RR intervals.

Fig. 4

Effect of carried load percentage on the standard deviation of all normal R wave to R wave intervals.

In addition, mHR was significantly affected by the interaction between schoolbag type and load percentage [F (1.958, 56.779) = 30.238, p < 0.0001, η² = 0.51]. Pairwise comparison showed that the developed schoolbag promoted a lower mean heart rate at load levels of 15% and 20% compared to the commercial one. However, the commercial schoolbag promoted a lower mean heart rate at a load level of 10% compared to the developed one, as demonstrated in Fig. 5.

Fig. 5

Interaction effect on mean heart rate.

In the case of RMSSD, the interval distance of HRV was computed. Accordingly, our results showed that HRV was significantly affected by schoolbag type and load percentage [for schoolbag type, F(1, 29) = 31.08, p < 0.0001, η² = 0.517; for load percentage, F(1.95, 56.438) = 12.166, p < 0.0001, η² = 0.296, respectively]. A comparison showed that the developed schoolbag had a higher mean (11.364) than the commercial one (10.35), implying that developed schoolbag required lesser cardiac effort. On the other hand, mean RMSSD decreased as load percentage increased [mean for 10%, 15%, 20% = 11.246, 11.125, and 10.2, respectively], as shown in Fig. 6.

Fig. 6

Effect of backpack type and percentage of carried loads on root mean square of successive difference.

3.2.2 Frequency domain analysis

Frequency domain analysis of the HRV indices indicated that schoolbag type had a significant effect on LF power [F (1, 29) = 27.308, p < 0.0001, η² = 0.485] and HF power [F (1, 29) = 14.858, p < 0.001, η² = 0.339]. This suggests that carrying the developed schoolbag promoted lower LF and HF power compared to carrying the commercial schoolbag. In addition, load percentage was found to have a significant influence on LF power [F (1.99, 57.71) = 18.73, p < 0.0001, η² = 0.392] and HF power [F (1.616, 46.856) = 11.415, p < 0.0001, η² = 0.282], suggesting that an increase in carried load caused a decrease in LF and HF power regardless of schoolbag type.

Moreover, the interaction between schoolbag type and load percentage had a significant effect on LF power [F (1.58, 45.80) = 15.11, p < 0.0001, η² = 0.343] and HF power [F (1.759, 51.01) = 4.59, p < 0.018, η² = 0.137] wherein the developed schoolbag promoted a higher LF at a load of 15% and 20% and a lower LF at a load of 10% (Fig. 7), as well as a higher HF at a load of 10%, 15%, and 20% compared to the commercial one (Fig. 8).

Fig. 7

Effect of schoolbag type and load percentage interaction on LF power.

Fig. 8

Effect of schoolbag type and load percentage interaction on HF power.

4 Discussion

The current study’s primary objective was to compare an ergonomically developed schoolbag with a commercial one experimentally. The reported result was similar to the results obtained in other studies [54, 55] in which the developed schoolbag provided significantly greater comfort to participants. Furthermore, the study’s result showed an inverse relationship between load percentage and schoolbag comfort. Indeed, this may be considered an expected result and could be used as default verification. Moreover, the interaction between schoolbag type and load percentage significantly affected shoulder and lower back discomfort. This finding is consistent with the results of Al-Shahry et al. [64], who reported that 70% and 18% of the sample students suffered from shoulder and back, respectively.

The current study revealed that the developed schoolbag promoted decreased pressure on the shoulders. Notably, the difference between both types of schoolbags was not significant at a load of 10%. The significant point suggested herein would be that the developed schoolbag’s considerable effects (less pressure) increased together with increasing load percentage. This point might indicate that the developed schoolbag design was more appropriate for heavy loads [55]. Specifically, our results found that using the developed schoolbag reduced participant’s shoulder discomfort by 52.80% regardless of the weight carried. Nonetheless, as the load percentage increased from 10% to 15%, participants discomfort increased to 74.70%, subsequently jumping to 94.55% when the load level increased to 20%.

The current study results showed a significant difference in cardiovascular responses, particularly heart rates at levels of 15% and 20% while carrying a commercial schoolbag. Accordingly, our findings suggested that heart rate significantly increased when participants carried a commercial schoolbag. Heart rate also increased as the load increased from 10% to 20%. The results mentioned above agree with the findings of other studies [4, 65] who reported that the higher the increase in load, the higher the heart rate.

The present study also found that carrying a high load using the commercial schoolbag promoted an increase in heart rate, whereas carrying the same loads using the developed schoolbag promoted a slight heart rate decrease. Furthermore, low loads (10%) and schoolbag type (developed/commercial) had no statistically significant heart rate effect. Moreover, changing the load percentage from 10% to 15% caused an increase in heart rate among participants carrying a commercial schoolbag but decreased heart rate among those carrying the developed schoolbag. Therefore, these results suggested that carrying the commercial schoolbag with high loads produced high stress and discomfort, whereas carrying the developed schoolbag with high loads produced low stress and discomfort. Moreover, our findings suggest that the schoolbag load percentage interaction contributed the most to stress compared to other variables (51.7%), followed by load percentage (31.7%), schoolbag type (22.8%). Those results agreed with those results reported by Ramadan and Al-Shayea [4].

The rise in heart rate among those carrying the developed schoolbag at 10% body mass might be attributed to the friction between the body and the three pockets. This friction is caused by high movement due to less weight, which promoted more discomfort. Among those carrying the developed schoolbag, the R–R interval decreased at 10% of body mass, and increased at 15% and 20% of body mass, suggesting that the developed schoolbag demands more effort at 10% of body mass. This might be a consequence of friction augmentation, resulting in high movement due to less weight, which would cause more discomfort between the body and three pockets.

Moreover, our results showed that the HF and LF feature decreased significantly among participants carrying a commercial schoolbag when the percentage load increased from 10% to 20%, demonstrating that additional stress was generated when the participants carried a high percentage load in the commercial schoolbag. On the other hand, the HF and LF features remained constant among participants carrying the developed schoolbag. The effect of respiration on heart rate reflects the HF power used to represent parasympathetic activity [66]. Respiratory sinus arrhythmia has been used to distinguish HF power [67]. Meanwhile, LF has been linked to blood pressure control, which is used to represent sympathetic activity. Heart rate has been shown to increase with excessive parasympathetic modulation [66], which is likely to occur during a commercial schoolbag task. The key finding of this study was that HRV indices, namely HF and LF, were a more sensitive measure of overall stresses, which was consistent with the findings presented in several other studies using either prolonged exposure, such as in Perini & Veicsteinas and Collins et al. [68, 69], or short-term psychophysical stressors [70].

5 Study limitations

However, our results may be limited in terms of study population and region. The present study directs future research to use the same study objectives for investigating the impact of schoolbag load within a different community, and possibly over a prolonged period of time. Likewise, educational institutions, policymakers, and the Ministry of Health must work collaboratively for better results.

Apart from the conclusions drawn from an earlier hypothesis, the current study did not evaluate overweight, younger, or female students wearing schoolbags. Therefore, such populations have more significant uncertainty and postural changes that could lead to the study of high-risk communities and should therefore be examined for hazards in future research.

6 Conclusions

According to the percentage of body mass, the developed schoolbag was evaluated at three weight levels (i.e., 10%, 15%, and 20%). The study revealed that the developed schoolbag performed better in terms of subjective measurement and ECG responses at 15% and 20% of body mass but not 10% of body mass. Numerous studies [2 , 13] have reported that many students carry more than 10% of their body mass. Nevertheless, the developed design had been verified to be practical, taking another step toward the optimum design.

On the other hand, the current study revealed that a more than 10% increase in the carried weight would lower back pressure by distributing the abdominal muscles’ load. The developed schoolbag design accomplished this by having both side pockets to the backpack, which made it function substantially better. Moreover, the contact area of the straps is raised to reduce the student’s contact pressure. However, several points for improvement have been noticed and should be considered in future research. First, the back pocket could be pushed to the top side of the body, which would reduce pressure on the back. Moreover, developing straps from different materials is highly recommended to reduce pressure on the shoulders. Creating more user-friendly methods for reducing possible size problems is also suggested, creating more tension due to the friction problems.

The schoolbag weights using in this experiment differed from one participant to another, considering that the weight was calculated based on their body mass. Therefore, using three fixed weights in future research could produce very different results. Finally, the performance of the developed design had been verified. Accordingly, the developed schoolbag distributed weight through the two-side pockets and applied weight to the body via the new design’s two upper and lower straps. However, the evolved design enabled the body to spread the weight and prevent concentration on particular areas, which is essential for reducing the risk associated with schoolbag carrying.

Footnotes

Acknowledgments

The authors gratefully acknowledge all the participants who helped them conduct the study. They would also like to extend their appreciation to the research center of the College of Engineering at King Saud University for its support.

Conflict of interest

The authors declare no conflict of interest.

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	Mean (Standard deviation)					Statistics p (Z) values
Variable	Backpack type		Carried loads			Backpack type		Load percentage
Levels body parts	Developed	Commercial	10%	15%	20%	Developed	Commercial	10% vs. 15%	10% vs. 20%	15% vs. 20%
Neck discomfort	0.1889 (0.616)	0.4833 (0.949)	0.0583 (0.227)	0.2833 (0.739)	0.6667 (1.099)	0.002	–3.143	0.008 (–2.65)	0.0001 (–3.88)	0.008 (–2.67)
Shoulder discomfort	1.0289 (1.113)	1.572 (1.369)	0.6917 (0.824)	1.208 (1.180)	2.002 (1.402)	0.0001	–3.745	0.001 (–3.40)	0.0001 (–5.31)	0.0001 (–4.17)
Upper back discomfort	0.3611 (0.779)	1.0722 (1.5389)	0.2417 (0.593)	0.9167 (1.557)	0.9917 (1.323)	0.0001	–4.722	0.0001 (–3.93)	0.0001 (–4.40)	0.399 (–0.844)
Lower back discomfort	0.05 (0.2127)	0.4444 (0.8361)	0.0667 (0.312)	0.2500 (0.628)	0.4250 (0.828)	0.0001	–4.154	0.015 (–2.43)	0.001 (–3.22)	0.035 (–2.11)
Feet discomfort	0.0778 (0.403)	0.0667 (0.391)	0.0333 (0.181)	0.05 (0.287)	0.1333 (0.596)	0.655	–0.447