TITLE: Effect of backpack position on foot weight distribution of school-aged
children

  ABSTRACT:
[Purpose] In the present study, we aimed to determine the effects of backpack position on
foot weight distribution of standing school-aged children. [Subjects] Thirty school-aged
children volunteered to participate in this study. [Methods] The subjects randomly
performed four types of carrying a backpack: no backpack (condition-1), carrying a
backpack at C7 (condition-2), carrying a backpack at 10 cm below C7 (condition-3), and
carrying a backpack at 20 cm below C7 (condition-4). [Results] Statistically significant
differences were noted in the anterior and posterior pressure values, and in the
anterior-to-posterior ratio, among the four conditions (p < 0.05). Post-hoc analysis
indicated that the pressure value of condition-4 was significantly lower in the anterior
foot region and higher in the posterior foot region than in condition-2 and condition-3.
In addition, the anterior-to-posterior ratio was lower in condition-4 than in condition-2
and condition-3. [Conclusion] These findings suggest that carrying a backpack in a higher
position, with fastening of the shoulder strap, may be more favorable for normalizing the
foot weight distribution.

BODY.INTRODUCTION:
Backpacks are indispensable, as people regularly use them for a long time to carry objects
that are used daily1). Therefore, avoidance of postural aberration, by practicing
appropriate backpack usage, is vital. Recently, it has been suggested that the prolonged use
of backpacks by school-aged children aggravates musculoskeletal conditions2, 3). In
particular, the increased prevalence of spinal deformities, such as scoliosis and kyphosis,
and/or pain and discomfort including low back pain and shoulder pain, may be associated with
backpack-carrying habits and the backpack weight4,5,6). Occasionally, poor distribution of backpack weight and ineffective
absorption of this load leads to postural changes, leading to musculoskeletal
impairments3). Hence, school-aged
children should be educated and made aware of these concerns to protect their spinal
health7).  To our knowledge, although the use of backpacks has a major influence on the spinal
condition of school-aged children, the majority of studies have focused on the effects of
backpack carrying on posture and movement during standing and walking of adults8, 9).
Due to this lack of research on juvenile subjects, it is challenging to establish a resource
for educating children on the manner in which their backpacks should be correctly used.
Hence, we sought to determine the effects of backpack use on children rather than review the
outcomes of existing adult studies. Thus, we aimed to assess the effects of backpack
position on the foot weight distribution of standing school-aged children.

BODY.SUBJECTS AND METHODS:
Thirty healthy children (14 males, 16 females) volunteered to participate in this study.
The mean age of the children was 8.43 ± 0.50 years, their mean height was 128.57 ± 6.06 cm,
and their mean weight was 30.53 ± 4.17 kg. Only the subjects who fulfilled the following
criteria were included in this study: no orthopedic, neurological, or cognitive impairments
that could have influenced the study procedure or results; no discrepancy in leg length; and
no regular daily exercise. All subjects signed a written consent form before study
participation, and this study was approved by the Institutional Review Board of Cheongju
University.  Foot weight distribution during standing was recorded using a force plate (FDM-s system,
Zebris, Germany). This consists of a square plate (1.5 m × 1.5 m) containing 17,000
micro-sensors which detect foot pressure. A cable connects the force plate to the main unit,
and a computer system with custom computer software for analyzing the measured data. The
equipment evaluates the weight pressure distribution of the foot, and analyzes the load
distribution of the anterior and posterior foot regions. We also calculated the
anterior-to-posterior load ratio. Subjects were asked to stand still on the plate, with
their feet shoulder length apart and their arms at sides, while looking at a round target
attached to wall located 2 m in front of them. After zero-point calibration, measurement was
performed for 1 min and data were averaged over three trials separated by 1 min rest
intervals.  Based on the study of Oh and Choi10),
four types of backpack (height: 34 cm, width: 25 cm, and depth: 13 cm) (BP-X031, PROSPECS,
Republic of Korea) carrying conditions were studied. The backpack position was adjusted
using the shoulder strap of the backpack: condition-1, no backpack; condition-2, carrying a
backpack at C7; condition-3, carrying a backpack at 10 cm below C7; and condition-4,
carrying a backpack at 20 cm below C7. The sequence of the four conditions was randomly
determined to avoid an interaction effect of each condition. The randomization process
involved blindly drawing a card from an envelope, containing 4 cards marked 1, 2, 3, or 4.
We adjusted the weight of each backpack to 15% of the subject's bodyweight (average backpack
weight of the subjects: 4.58±0.63), based on a suggestion made by Al-Khabbaz et al.2), who investigated the effects of backpack
weight load.  SPSS 12.0 was used to analyze all data. Data are presented as mean ± standard deviation
(SD). One-way repeated analysis of variance was used for data analysis. Statistical
significance was accepted for values of p<0.05. When statistical significance was
identified, the differences in the pairwise comparison were examined using the post-hoc
Bonferroni adjustment.

BODY.RESULTS:
Table 1Table 1.Comparison of anterior and posterior pressure values and anterior-to-posterior
ratios of the four backpack positionsCondition-1Condition-2Condition-3Condition-4Anterior pressure values39.45±10.9642.04±12.1640.64±13.5137.50±11.18†‡Posterior pressure values60.56±10.9657.96±12.1659.36±13.5162.51±11.18†‡Anterior-to-Posterior ratio0.72±0.390.82±0.480.80±0.540.65±0.31†‡Condition-1: no backpack; Condition-2: carrying the backpack at C7; Condition-3:
carrying the backpack at 10 cm below C7; and Condition-4: carrying the backpack at
20 cm below C7. †Significant difference from condition-2.
‡Significant difference from condition-3. displays the foot pressure results of each condition. Statistically
significant differences were noted in the anterior and posterior pressure values, as well as
the anterior-to-posterior ratio among the four conditions (p < 0.05). Post-hoc analysis
indicated that the pressure values of condition-4 were significantly lower in the anterior
foot region and higher in the posterior foot region than in condition-2 and condition-3. In
addition, the anterior-to-posterior ratio was lower in condition-4 than in condition-2 and
condition-3.

BODY.DISCUSSION:
The excess weight load of a backpack causes asymmetry in the anterior and posterior load
distribution, eliciting forward leaning of the upper trunk to adapt postural agitation and
maintain postural balance8). Although the
anterior-to-posterior pressure ratios did not significantly differ among the conditions, the
ratios of condition-2 and condition-3 were greater than those of condition-1, implying that
the weight load was greater in the anterior foot region and less in the posterior region. In
condition-3, weight load appeared to be concentrated to a greater extent on the posterior,
rather than the anterior foot region. A lower backpack position results in the increase of
the moment arm generated by the upper trunk movement axis. Therefore, it may be difficult to
maintain the appropriate pressure ratio on the anterior and posterior foot regions. Similar
effects have been observed with increased backpack weight, indicating that carrying a
backpack at a higher position, with fastening of the shoulder strap, may be better for
normalizing foot weight distribution than other conditions. Our results are supported by the
study performed by Gong et al8).  Previous studies have reported that forward leaning of the upper trunk increases to a
greater extent, when subjects carry a backpack in a lower position, with a longer shoulder
strap10, 11). These studies indicated that the weight load is concentrated on
the anterior foot region, which is inconsistent with our present study's findings. However,
those studies examined walking of subjects, not standing of subjects, as in the present
study. Given that additional load from a backpack can impede forward movement of the body
during walking, forward trunk leaning may be an effective strategy for reducing energy
consumption10, 11). Therefore, caution should be exercised when comparing our results
with those of these previous studies, as their results require the understanding of the
dynamic walking mechanism rather than that of static control that is behind the present
study's findings.  We believe that carrying a backpack in the upper position may lead to safer use of the
backpack, and may help prevent musculoskeletal discomfort in those who regularly use a
backpack. Our findings suggest that adjusting the backpack position, by fastening the
shoulder strap, is a useful strategy for the prevention of musculoskeletal problems in
school-aged children.  This study had certain limitations, which should be addressed in further research. First,
as the subjects of the present study included only school-aged children, the findings cannot
be generalized to other age groups. Second, we did not record long-term follow-up data and
the findings may not reflect long-term backpack use. Finally, the lack of other measures
such as electromyography or kinematic analysis may be factors limiting the establishment of
a more definite conclusion on the effects of backpack position.

Question: Compared to Carrying a backpack at C7 what was the result of Carrying a backpack at 20 cm below C7 on Anterior-to-posterior ratio?
Evidence: Post-hoc analysis
indicated that the pressure value of condition-4 was significantly lower in the anterior
foot region and higher in the posterior foot region than in condition-2 and condition-3.
Answer: significantly decreased