Effect of the use of backpacks with different weights in trunk acceleration in children: A behavioral analysis

Abstract

The shoulder bag is the most widely used implement for transporting school supplies. About 25% of school children complain of musculoskeletal pain, often associated with excessive weight of school backpacks (Ries et al., 2012). Poor backpacking habits persist throughout the school years and, consequently, lead to chronic musculoskeletal problems, which may worsen into adulthood (Jardim, 2013). Although these symptoms are associated with other factors, it is assumed that these may be related to the weight overload, in which these children are systematically exposed. For this reason, a petition was submitted to the Assembly of the Republic in Portugal, where it was suggested that definitive legislation should be adopted to ensure that children's backpacks should not exceed 10% of their body weight and that schools should regulate weekly the minimum school activities.

The theory of dynamic systems assumes that motor behavior is a consequence of a network of codependent systems (e.g., musculoskeletal, nervous, respiratory, etc.), from which motor patterns emerge through processes of self-organization (Glazier et al., 2003; Kelso, 1995; Williams et al., 1999).

The objective of this study is to verify if the children show postural readjustment (parameter of order) when constrained to the backpack with different weights (control parameter).

The sample consisted of sixteen children aged between 8 and 9 years of the first cycle of basic education. Each child carried out a 5-meter course with a backpack, under the following conditions: i) without added weight; ii) with 3kg added; and (iii) with 5 kg added. The three-dimensional acceleration values ​​were recorded in g units, collected through the Physics Toolbox Suite (Vyera Software) application installed on a ZTE smartphone. The data were filtered and smoothed in each of the components of the three-dimensional axis in MatLab software (The MathWorks, Inc), through a High-Pass filter. The Friedman test (χr2) was used to compare the conditions, followed by the Wilcoxon (T) test, with Bonferroni correction.

No significant changes were observed in the vertical (χr2 = 1.625, ns) and anteroposterior (χr2 = 0.375, ns) components. However, for the mediolateral component, there was a significant difference between conditions (χr2 = 15.500, p0.001); attributable to a significant reduction of the acceleration of the condition without additional load to that with addition of 3kg (T = -2.741, p≤0.01), and the condition with no additional load for the with addition of 5kg (T = -3.206, p≤0.001). Since the backpack is attached to the trunk and has a series of degrees of freedom allowed by the spine, we deduce that children seek a more stable motor solution when subjected to additional weight, probably by reducing degrees of freedom. In this case, the process of dimensional compression must be considered as a constraint (Santos et al., 2015).