Which two strategies reduce downward displacement of the COM?

Which two strategies reduce downward displacement of the COM?

• A. Frontal plane hip rotation and stance knee flexion
• B. Frontal plane pelvic rotation and stance knee flexion
• C. Hip abduction and knee extension
• D. Horizontal plane pelvic rotation and sagittal plane ankle rotation

Answer: (D)

When we look at reducing how much the body's center of mass moves up and down, the pelvis and ankle play two key roles. Horizontal plane pelvic rotation helps by allowing the pelvis to twist as the leg swings, which keeps the trunk aligned and distributes momentum in a way that prevents a large vertical dip with each step. This rotation effectively minimizes vertical oscillation because the upper body doesn’t have to rise and fall as much to reorient itself for the next step. The ankle’s movement in the sagittal plane acts like a gentle rocker. Controlled plantarflexion and dorsiflexion during stance adjust the leg’s effective length and the timing of weight transfer, smoothing the path of the COM downward and then upward in a continuous, forward-directed arc. This sagittal plane ankle rotation dampens vertical fluctuations while maintaining forward progression. Movements in other planes or simple joint actions like knee extension or hip abduction don’t directly provide that combination of pelvic mass distribution in the horizontal plane and the ankle rocker in the sagittal plane, so they’re less effective at reducing downward COM displacement.

When we look at reducing how much the body's center of mass moves up and down, the pelvis and ankle play two key roles. Horizontal plane pelvic rotation helps by allowing the pelvis to twist as the leg swings, which keeps the trunk aligned and distributes momentum in a way that prevents a large vertical dip with each step. This rotation effectively minimizes vertical oscillation because the upper body doesn’t have to rise and fall as much to reorient itself for the next step.

The ankle’s movement in the sagittal plane acts like a gentle rocker. Controlled plantarflexion and dorsiflexion during stance adjust the leg’s effective length and the timing of weight transfer, smoothing the path of the COM downward and then upward in a continuous, forward-directed arc. This sagittal plane ankle rotation dampens vertical fluctuations while maintaining forward progression.

Movements in other planes or simple joint actions like knee extension or hip abduction don’t directly provide that combination of pelvic mass distribution in the horizontal plane and the ankle rocker in the sagittal plane, so they’re less effective at reducing downward COM displacement.