The REACTplusNMES Trial: A Double-blinded RCT

REACT-NMES group: The ActiveStep treadmill will be used to deliver slips during all sessions. Each subject will experience three levels of perturbations over 12 sessions (24 slips/session) in progressive ascending way. On the first week, subjects will start with the lowest displacement level (6 cm) and move up to the next level (12 cm) by week 2 if they have \< 5 falls out of 8 slips at the previous level. By week 3, subjects are expected to move to level 3 (24 cm) and train at that for weeks 3 to 6. If subjects don't move up a level, training will continue at the lower level. NMES will be delivered to the vastus lateralis synchronously with the perturbation, which will always occur 50 ms after slip-onset and last for 450 ms including the period between liftoff to touchdown of the first compensatory step.

REACT group: The REACT group will undergo the same reactive balance training (in terms of type, dosage: intensity, frequency) as the REACT-NMES group. The only difference will be that the REACT group will receive ShamNMES for same time after the compensatory step touchdown. The ActiveStep treadmill will be used to deliver slips during all sessions. Each subject will experience three levels of perturbations over 12 sessions (24 slips/session) in progressive ascending way. On the first week, subjects will start with the lowest displacement level (6 cm) and move up to the next level (12 cm) by week 2 if they have \< 5 falls out of 8 slips at the previous level. By week 3, subjects are expected to move to level 3 (24 cm) and train at that for weeks 3 to 6. If subjects don't move up a level, training will continue at the lower level.