Is the Sway App a valid and sensitive tool to measure balance in healthy adults?

The intervention was mobilisation of each foot for 6 minutes. The foot mobilisation/preparation technique is based on the Bobath Concept and was performed on both feet by an experienced senior physiotherapist who had previously received formal training in the Bobath Concept. Participants were seated in a chair with their foot supported by the therapist’s thigh. The intervention was tailored to each participant’s presentation with a focus on mobilisation of all the joints of the foot and ankle. This included for example, ankle dorsiflexion, plantarflexion, inversion and eversion, gliding movements between the tarsal and metatarsal bones, and toe flexion and extension movements. Further, sensory and proprioceptive input was provided via stretching, massage and release of soft tissue around the lower leg and foot (including calf and foot muscles). The intervention of foot mobilisation was used to measure validity and sensitivity to change of the Sway App. The Sway App (Sway Medical, Tulsa, OK, USA) is a smartphone application that measures static standing balance using the triaxial accelerometer located in the smart device. As described in the testing protocol below, the Sway App was used by the participant holding the mobile phone uploaded with the Sway App at the front of their chest. The tests used to assess balance were the unilateral stance test (UST) on the left leg (eyes open and eyes closed), and the UST on the right leg (eyes open and eyes closed). These components can be found in the mBESS (modified Balance Error Scoring System) and the CDC 4-stage balanced modules within the App. The Sway App measured balance responses at the same time as those responses were being measured by the gold standard SMART EquitTest System. The protocol for testing, supervised by a member of the research team, follows. 1. Participants were fitted with the SMART EquiTest harness. 2. Participants were asked to stand barefoot on the SMART EquiTest System and were secured with the harness. 3. At the same time, participants were asked to hold a mobile phone with the Sway App at the front of their chest. 4. Participants were given one familiarisation trial. 5. Participants then completed the UST protocol in which they were asked to perform 3 trials of the UST for 10 seconds in each of the four following conditions: a. with eyes-open standing on left leg, b. with eyes-closed standing on left leg, c. with eyes-open standing on right leg, and d. with eyes-closed standing on right leg. 6. During each 10 second period, participants’ balance was measured via both the SMART EquiTest System and the Sway App. 7. Participants then received 6 minutes of foot mobilisation per foot. 8. Immediately post-intervention, participants’ balance was reassessed, and data were collected repeating protocol points 1 to 6.