Scientists have established that school children who use backpacks should avoid loads of more than 10 per cent of their body weight and those who use trolleys, 20 per cent of their body weight.
To date, weight recommendations have been established for ordinary school backpacks, as they are the most widely used type in the school context worldwide.
For the study, published in the Applied Ergonomics journal, 49 primary school children were assessed.
A kinematic analysis of the children (posture of the trunk and lower limbs) was conducted while (i) they walked freely, carrying no weight, (ii) carrying a traditional backpack, and finally (iii) pulling a backpack trolley with different loads (10 per cent, 15 per cent, and 20 per cent of their respective body weights).
For the analysis, a three-dimensional optical motion capture system was used, similar to those used in animation films and video games.
In collaboration with researchers from Liverpool John Moore University, scientists from the University of Granada in Spain used statistical techniques to analyse the full kinematics curves, based on tracing point trajectories.
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The findings of the study indicate that the greatest alterations when using trolleys or backpacks are produced in the proximal extremities (hip and trunk), while there is little difference in the kinematics of the distal extremities (knee and ankle).
However, pulling the backpack trolley produces fewer changes in the child's kinematics and, therefore, resembles more closely their movement when walking free of any load, compared to carrying the backpack, even when it weighs very little.
As an overall conclusion, the study corroborates that schoolchildren who use backpacks should avoid carrying loads greater than 10 per cent of their body weight.
When pulling a school backpack trolley, the child should avoid carrying any load greater than 20 per cent of their body weight, according to the study.