Influence of school bag carrying on gait kinetics.

The purpose of this study was to determine the effect of different methods of backpack carrying on gait kinetics in children, using a new treadmill that allowed three-dimensional measurement of right and left leg ground reaction forces (GRFs). Forty-one healthy children, with a mean age of 12 years, participated in this study. The mean height was 152 cm and the mean weight 40 kg. The three trials consisted of walking on the treadmill at the speed of 3.5 km/h, first without a backpack and then carrying a 10 kg school bag on the right shoulder or on both shoulders. For each carrying condition GRFs were recorded, averaged, and analyzed for 30 steps. Stride, stance, double stance, thirteen specific GRF parameters and the symmetry index were measured. The right leg produced higher propulsive fore-aft forces than the left one, whatever the walking conditions. For the two maximum peaks and the average vertical force during stance, a statistical difference was found between walking without a backpack and carrying a backpack on one or two shoulders (one or two shoulder carrying > no backpack) but never between one-shoulder and two-shoulder carrying. The children increased their stance and double stance when walking with a backpack compared with walking without a pack. The symmetry index increased with one-strap carrying (compared with no backpack and two-strap carrying) for the maximum force during the breaking phase (Fy1) when it decreased for the maximum propulsive horizontal force before taking-off (Fy2). Children should be advised to carry their backpack on two shoulders rather than use a one-strap backpack.