Effects of Walking Speed and Backward-Directed Resistive Force Applied to the Center of Mass in Young Adults

Background/Objectives: This study evaluated whether walking speed and backward-directed resistive force interact to systematically alter knee joint kinematics and activity of knee flexor and extensor muscles in healthy adults. Methods: Fourteen active young adults (18–35 years) without lower-extremity injuries walked in a randomized order at 0.5 and 1.0 m/s while experiencing backward-directed resistive forces ranging from 0% to 25% of body weight. Knee joint kinematics, stride characteristics, and electromyographic (EMG) activity of the vastus lateralis, vastus medialis, and biceps femoris were collected and quantified. Results: No significant speed × force interactions were observed for step kinematic outcomes (p > 0.05). Walking speed significantly affected stride length and stride time (p < 0.05), whereas force did not (p > 0.05). For knee stance phase kinematics, force significantly affected outcomes (p < 0.05), whereas speed and the speed × force interaction did not (p > 0.05). For EMG activity, speed and the speed × force interaction was not significant for any muscle (p > 0.05). Resistive force significantly affected activation of all three muscles (p < 0.05). Conclusions: Backward-directed resistive forces alter knee kinematics and EMG activity, with effects becoming evident at approximately 15% of body weight regardless of walk speed. These findings suggest that posteriorly directed resistance to the center of mass at slower walk speeds may provide a novel approach for modifying knee joint kinematics and lower-extremity muscular activity while walking on a stable, level surface.