External loading alters lower extremity kinetics, kinematics, and muscle activity in a distribution-specific manner during the transition from stair descent to level walking.

Excess body mass is thought to be a major cause of altered biomechanics in obesity, but the effects of body mass distribution in biomechanics during daily living tasks are unknown. The purpose of this study was to determine how increasing body mass centrally and peripherally affects lower extremity kinematics, kinetics, and muscle activation when transitioning from stair descent to level gait. Fifteen normal weight volunteers descended a staircase at a self-selected pace under unloaded, centrally loaded, and peripherally loaded conditions. Spatial-temporal gait characteristics and lower extremity joint kinematics, kinetics, and mean electromyography amplitude were calculated using 3D motion analysis. Both central and peripheral loading reduced gait velocity. Peripheral loading increased time spent in stance phase, increased step width, and reduced step length. At the hip joint, peripheral loading reduced peak hip extension and adduction angle. Conversely, central loading reduced peak hip flexor moment. Both central and peripheral loading increased peak knee flexion angle, but only peripheral loading increased peak knee extensor moment. Central and peripheral loading increased mean electromyography amplitude of the medial gastrocnemius, but only peripheral loading increased mean electromyography amplitude of the semitendinosus and the vastus medialis. Increasing mass centrally and peripherally differently affects spatial-temporal gait characteristics and lower extremity joint kinematics, kinetics, and electromyography when transitioning from stair descent to level gait. Body mass distribution may be an important factor for obesity-induced biomechanical alterations and should be considered when developing biomechanical models of obesity. • Increasing body mass, regardless of distribution, reduces stair descent velocity • Adding mass to thighs, but not the trunk, negatively impacts spatial-temporal parameters during stair transition • Hip and knee kinematics and kinetics are differently affected by adding mass to the trunk and thighs • Adding mass to the thighs, but not the trunk, increases EMG activity of the knee flexors and extensors • Distribution of mass is an important determinant of kinematics, kinetics, and muscle recruitment during stair transitions [ABSTRACT FROM AUTHOR]