Synergic control of the minimum toe clearance in young and older adults during foot swing on treadmill walking in different speeds.

BACKGROUND: The vertical toe position at minimum toe clearance (MTC) in the swing phase is critical for walking safety. Consequently, the joints involved should be strictly controlled and coordinated to stabilize the foot at MTC. The uncontrolled manifold (UCM) hypothesis framework has been used to determine the existence of synergies that stabilize relevant performance variables during walking. However, no study investigated the presence of a multi-joint synergy stabilizing the foot position at MTC and the effects of age and walking speed on this synergy.

RESEARCH QUESTIONS: Is there a multi-joint synergy stabilizing MTC during treadmill walking? Does it depend on the persons' age and walking speed?

METHODS: Kinematic data from 23 young and 15 older adults were analyzed using the UCM approach. The participants walked on a treadmill at three speeds: slow, self-selected, and fast. The sagittal and frontal joint angles from the swing and stance legs and pelvis obliquity were used as motor elements and the vertical toe position at MTC was the performance variable. The variances in the joint space that affected (VORT , 'bad' variance) and did not affect (VUCM , 'good' variance) the toe position at MTC and the synergy index (ΔV) were computed.

RESULTS: The ΔV>0 was revealed for all subjects. Walking speed did not affect ΔV in older adults, whereas ΔV reduced with speed in young adults. ΔV was higher for older than for young adults at self-selected and fast speeds, owing to a lower VORT in the older group.

SIGNIFICANCE: The vertical toe position at MTC was stabilized by a strong multi-joint synergy. In older adults, this synergy was stronger, as they were better at limiting VORT than young adults. Reduced VORT in older adults could be caused by more constrained walking, which may be associated with anxiety due to walking on a treadmill.