Biomechanical analysis of sit-to-stand motion in children with backpack load

Abstract

Sit-to-stand (STS) motion is a highly coordinated and energy demanding task ofdaily activities. The primary objective of this study was to investigate the effects of backload on the sagittal plane kinematics and kinetics of STS motion in healthy children. The secondary objectives were to determine the limbs which may be more prone to damageand to suggest a critical value of back load relative to the body weight. Fifteen healthychildren (8 males, 7 females, mean age 9.6 ± 1.2) participated in the study to perform STSmotion in three conditions: (1) with no back load (2) with a back load of 10% of the body weight (BW) and (3) with a back load of 20% of the BW. The motion was performedusing a fixed bench height at a self-selected speed. Kinematic and kinetic data werecollected via a 6-camera motion analysis system and 2 force plates.The present results led us to four major conclusions reflecting the effects of back load on the STS motion: (1) The neuromuscular system is concluded to adjust thedurations of the individual phases rather than that of total STS in order to adapt themotion to the new mechanical conditions. (2) Subjects followed a "trunk flexion strategy"in the loaded cases by shifting the new center-of-mass both forward and downward presumably to ease the control of the motion and to reduce the risk of falling. (3)Different back load levels affect different joints. Increasing the load to 20% BW causedsignificantly higher ankle moment. On the other hand, even 10% BW load producedsignificantly higher knee moment. However, no major effect of back load was shown on hip moment and power. (4). Back loading causes higher forces and increases the range ofeccentric activity of gastrocnemius and soleus muscles by leading to much higher anglesof ankle dorsiflexion. Therefore, the calf muscles and the achilles tendon, were concludedto be the most prone elements of the muscle-tendon complexes of the lower extremity to damage while performing STS motion with back load.|Keywords: Sit-to-Stand Motion, Backpack Load, Children, Motion Analysis.