Data from: Complementary mechanisms for upright balance during walking

Lateral balance is a critical factor in keeping the human body upright
during walking. Two important mechanisms for balance control are the
stepping strategy, in which the foot placement is changed in the direction
of a sensed fall to modulate how the gravitational force acts on the body,
and the lateral ankle strategy, in which the body mass is actively
accelerated by an ankle torque. Currently, there is minimal evidence about
how these two strategies complement one another to achieve upright balance
during locomotion. We use Galvanic vestibular stimulation (GVS) to induce
the sensation of a fall at heel-off during gait initiation. We found that
young healthy adults respond to the illusory fall using both the lateral
ankle strategy and the stepping strategy. The stance foot center of
pressure (CoP) is shifted in the direction of the perceived fall by ≈2.5
mm, starting ≈247 ms after stimulus onset. The foot placement of the
following step is shifted by ≈15 mm in the same direction. The temporal
delay between these two mechanisms suggests that they independently
contribute to upright balance during locomotion, potentially in a serially
coordinated manner. Modeling results indicate that without the lateral
ankle strategy, a much larger step width is required to maintain upright
balance, suggesting that the small but early CoP shift induced by the
lateral ankle strategy is critical for upright stability during
locomotion. The relative importance of each mechanism and how neurological
disorders may affect their implementation remain an open question.