Contrasting effects of finger and shoulder interpersonal light touch on standing balance.

Sway is reduced by light nonsupporting touch between parts of the body and a fixed surface. This effect is assumed to reflect augmentation of sensory cues for sway by point-of-contact reaction forces. It has been shown that movement of the contact surface can increase sway relative to an earth-fixed contact. Light touch contact with another person, for example, holding hands, affords a moving contact due to partner sway. We asked whether interpersonal light touch (IPLT) would increase sway relative to standing alone. We expected effects on sway to vary as a function of the site of contact and the postural stability of each partner. Eight pairs of participants, standing in either normal bipedal or tandem Romberg stance with eyes closed and using IPLT (finger to finger or shoulder to shoulder) or no contact, provided 4 trials of 30-s duration in each of 12 posture-touch combinations. Sway (SD of the rate of change of upper trunk position at C7) was reliably less with IPLT compared with no contact, with two exceptions: in normal stance, shoulder contact with a partner in tandem stance, and in tandem Romberg stance, finger contact with a partner in the same stance, increased sway. Otherwise, the reduction in sway was greater with shoulder than with finger contact. Measures of interpersonal synchronization based on cross-correlations and coherence analysis between the partners' C7 movements suggest different control factors operate to reduce sway in IPLT with the hand or shoulder contact.