Interpersonal interactions for haptic guidance during balance exercises

Highlights

• Passive support mode demonstrated its advantages in increased strength of the IPC.

• Active support mode decreased the postural sway to a greater extent.

• More partnership based methods should be considered for balance rehabilitation.

• Postural control can be responsive to social factors.

Abstract

Background

Caregiver–patient interaction relies on interpersonal coordination during support provided by a therapist to a patient with impaired control of body balance.

Research question

The purpose of this study was to investigate in a therapeutic context active and passive participant involvement during interpersonal support in balancing tasks of increasing sensorimotor difficulty.

Methods

Ten older adults stood in semi-tandem stance and received support from a physical therapist (PT) in two support conditions: 1) physical support provided by the PT to the participant’s back via an instrumented handle affixed to a harness worn by the participant (“passive” interpersonal touch; IPT) or 2) support by PT and participant jointly holding a handle instrumented with a force-torque transducer while facing each other (“active” IPT). The postural stability of both support conditions was measured using the root-mean-square (RMS) of the Centre-of-Pressure velocity (RMS dCOP) in the antero-posterior (AP) and medio-lateral (ML) directions. Interpersonal postural coordination (IPC) was characterized in terms of cross-correlations between both individuals’ sway fluctuations as well as the measured interaction forces.

Results

Active involvement of the participant decreased the participant’s postural variability to a greater extent, especially under challenging stance conditions, than receiving support passively. In the passive support condition, however, stronger in-phase IPC between both partners was observed in the antero-posterior direction, possibly caused by a more critical (visual or tactile) observation of participants’ body sway dynamics by the therapist. In-phase cross-correlation time lags indicated that the therapist tended to respond to participants’ body sway fluctuations in a reactive follower mode, which could indicate visual dominance affecting the therapist during the provision of haptic support.

Significance

Our paradigm implies that in balance rehabilitation more partnership-based methods promote greater postural steadiness. The implications of this finding with regard to motor learning and rehabilitation need to be investigated.

Introduction

Falls and fall related injuries in older adults are a public health issue [1,2]. Balance exercises, however may reduce falls risk [3]. In balance rehabilitation, a physical therapist (PT) manipulates the provision of sensory cues during sensorimotor training to facilitate motor learning, and control of body balance [[4], [5], [6]].

The factors governing sensorimotor interactions between therapist and client, however are poorly understood [7]. Interpersonal sensorimotor interaction can be classified into cooperation and collaboration [8]. In contrast to collaborative interactions that do not integrate a priori role assignments, roles are assigned a priori to each participant in cooperative interactions. For example during balance exercises, this can lead to an allocation of sub-tasks, such as provision of haptic balance support by a therapist and reception by the client involved in the balancing task [9].

Additional tactile feedback is a reliable approach to augment control of body balance [10]. In the traditional paradigm (“active” light touch), a participant is controlling the upper limb directly, which is contacting the external haptic reference [11]. Hereby, the movement degrees of freedom of the contacting limb are used for precision control of the contact force with the control of body sway as a separate process [12]. In addition to the haptic feedback signal, the output of fingertip control could serve as a signal to control sway [13]. In non-manual, “passive” light touch, the contact is delivered to a participant’s body segment. A participant is less able, to control the precision by which the contacting force is applied [13]. Here, the movement degrees of freedom available to a participant for controlling the contact force are limited by the current postural degrees of freedom, thereby creating a direct equivalence between control of body sway and precision of the contact.

Passive light touch with an earth-fixed reference results in proportional sway reductions in the range of 20%–30% [13]. This is similar to what has been reported in studies involving fingertip light touch [i.e. 14]. Interpersonal fingertip touch (IPT) leads to lesser sway reductions of around 9–15% [9,[14], [15], [16], [17]]. The reason for this diminished effect could lie in the fact that the contact reference is not earth-fixed but shows own motion dynamics, which might make disambiguation of the haptic signal in terms of own sway-related feedback more challenging. Johannsen et al. [9] assessed “passive” IPT in neurological patients as well as chronic stroke and reported sway reductions between 15%–26%. In stroke patients, passive, trunk-based IPT [9], nevertheless, seemed more beneficial than fingertip IPT [16].

In our study, we directly contrasted the effects of active and passive support modes on body sway in a therapeutic setting. We measured the interaction forces between a physiotherapist and participants and characterized the interpersonal postural coordination (IPC) between both partners. We predicted that the participant would demonstrate the greatest sway reductions when passive IPT was provided to the trunk with no involvement in contact precision control. We increased the sensory challenges imposed by the balance task (foam surface, eyes closed, pitch head movement) and assumed that with increasing difficulty, the benefit of IPT would increase as well potentially in interaction with the specific IPT mode.