Purpose: We used methods from dynamical system analysis to investigate the effect of carrying external loads on the stability of the locomotive system and sagittal plane kinematics. We hypothesized that carrying an additional load at the waist would 1) decrease the dynamic stability of the locomotive system and 2) cause changes in the location of the Poincaré map's equilibrium point for the hip, the knee, and the ankle joint kinematics.
Methods: Lower extremity kinematics were recorded for 23 subjects as they walked on a treadmill at their preferred speed while carrying external loads of 10%, 20%, and 30% of their body weight around their waist. Gait stability was evaluated by computing the eigenvalues of the locomotive system at the instance of heel contact and midswing. Changes in the hip, the knee, and the ankle's equilibrium point of the Poincaré sections were used to determine whether there were changes in the joint kinematics while carrying external loads.
Results: No significant differences in sagittal plane stability were found between the respective load carrying conditions (P > 0.05). Significant changes (P < 0.05) in the equilibrium points of the hip and the knee were found at heel contact and midswing.
Conclusions: The data suggest that humans are capable of maintaining sagittal plane stability while carrying loads up to 30% of their body weight.