Synchronization of passive interpersonal light touch and body control responses during walking

Passive interpersonal light touch (PILT) provides postural coordination cues to the body of a contact receiver through the fingertips of a contact provider. However, the manner in which the fingertip information of the PILT contact provider affects the gait performance of the contact receiver remains unclear. We aimed to investigate body control responses such as trunk sway and smoothness to PILT during walking to clarify the synchronization of fingertip–body movements in the context of the experience the PILT provider in body manipulation. In total, 17 healthy volunteers (21.4 ± 1.1 years old, 9 females) performed 10-m walk tests under three conditions: walking without touch (NT), walking with PILT administered by contact provider with extensive rehabilitation experience in manual body manipulation (PILTE), and walking with PILT administered by contact provider with no experience (PILTN). A force sensor on the middle finger of the contact provider and inertial sensors on the third lumbar and seventh cervical spinous processes of the contact receiver were used to gather data pertaining to contact force, direction of motion, walking speed, triaxial root mean square, harmonic ratio (HR), autocorrelation, and phase locking value (PLV) to estimate the synchronization between the inertial and fingertip sensors for each condition. The PILTE group showed significantly higher walking speed and HR values in the anteroposterior (AP) direction than those of the NT and PILTN groups. Additionally, the PLVs were significantly higher in the mediolateral (ML) direction in the PILTE group than in the PILTN group. In contrast, applying PILTE significantly reduced the PLVs in the AP direction compared with that observed for PILTN. These findings suggest that the fingertips of skilled contact provider and the shift in the center of gravity of the contact receiver synchronize in the ML direction, which affects walking speed and smooth movement of walking patterns because of increased HR in the AP direction. Additionally, physical responses during walking to PILT administered by contact providers may change depending on the experience of the contact providers. Thus, the quantification of touch skills may potentially aid in administering effective walking assistance.