RESULTS: Linear Kinematics

Joint Position. We compared linear displacement of the hip joint during the two climbing tasks to better understand the difference in translation of the body. The stair climbing motion had a much larger vertical displacement compared to the Stairmaster motion. The maximum vertical displacement of the hip marker in stair climbing was 0.33 m and 0.1 m in Stairmaster and the maximum horizontal displacements were 0.58m and 0 m, respectively. During the Stairmaster movement, the horizontal displacement of the hip stayed constant throughout the movement, because the subject did not move in a horizontal direction. During stair climbing, however, the subject moved forward 0.58 m during one cycle.

Figure 8. Position of the hip joint in the Stairmaster (left) and the stair climbing (right). The solid line represented the vertical displacement and the dashed line represented the horizontal displacement. The dotted line represented the resultant displacement. Positive values in the vertical directions corresponded to an upward motion. Negative values in the horizontal direction corresponded to a forward motion.

Joint velocity. We compared linear velocity of the hip joint during the Stairmaster and stair climbing motions to better understand the difference in velocities of the body. The vertical velocities of the Stairmaster ranged from -0.5 m/s to 0.5 m/s while the vertical velocities of the stair climbing ranged from 0 m/s to 0.85 m/s. The horizontal velocities of the Stairmaster was approximately 0 m/s while the horizontal velocities of the stair climbing ranged from 0 m/s to -0.5 m/s. The maximum vertical velocity of the hip marker in stair climbing was 0.85 m/s and 1 m/s in Stairmaster and the maximum horizontal velocity was 0.5 m/s and 0.3 m/s, respectively. The average horizontal velocity during stair climbing was -0.33 m/s and the average vertical velocity was 0.75 m/s.

Figure 9. Velocity of the hip joint in the Stairmaster (left) and the stair climbing (right). For both the Stairmaster and stair climbing movements, the solid line represented the vertical velocity and the dashed line represented the horizontal velocity. The dotted line represented the resultant velocity. Positive values in the vertical direction corresponded to an upward motion. Negative values in the horizontal direction corresponded to a forward motion.

Center of mass. The center of mass was calculated for the trunk, thigh, and shank in the starting positions of the Stairmaster climbing and the stair climbing motions. The center of mass was located on the anterior side of the body just above the hip for both the Stairmaster climbing and stair climbing motions during the first frame. The center of mass for the Stairmaster climbing was located in the same horizontal position as the ankle joint indicating that the subject was in an upright position. However, the center of mass for the stair climbing motion was located forward of the ankle joint, due to the horizontal component of the movement.

Figure 10. Location of the center of mass (CM) in the Stairmaster climbing (left) and the stair climbing (right). The body segments displayed are the trunk, thigh, and shank.