PURPOSE: To examine the reliability and validity of a mobile device application for measuring balance

RATIONALE: Balance plays a crucial role in a person’s ability to engage in meaningful occupations across the lifespan. Many conditions alter the ability to maintain balance during daily life activities, which limits function and engagement. Occupational therapists assess balance through activity participation, although changes can be difficult to evaluate objectively. Force platforms are the gold standard for objective measurement of postural sway but are costly with limited portability. Handheld mobile devices may provide a convenient and accurate means to objectively assess balance.

DESIGN: Cross-sectional design evaluating test–retest reliability and concurrent validity of a mobile device application (app)

PARTICIPANTS: Twenty-eight adults (mean age = 30.3, standard deviation = 12.1; range = 18–59 yr), without known orthopedic, musculoskeletal, or neurologic conditions participated in the study.

METHOD: Participants completed a series of stances on a force platform (AMTI 1000, Advanced Mechanical Technology Inc.), while concurrently holding a mobile device (Apple iPod Touch 5th generation, Apple Inc.) against the chest. Sway (Sway Medical, LLC), a mobile device app approved by the Food and Drug Administration to assess postural sway, recorded motion of the trunk based on acceleration changes of the iPod and calculates a representative score after each trial (0–100 scale).

Center-of-pressure measures beneath the feet were calculated from force platform data.

Participants completed a series of 10-s stances (standing with feet together, standing tandem with the dominant foot in front, and single-leg stance on the dominant foot) and repeated the stances four times: twice with eyes open, twice with eyes closed.

ANALYSIS: Test–retest reliability was assessed using an intraclass correlation coefficient (ICC). Pearson product–moment correlations were used to assess concurrent validity between Sway and force platform measures.

RESULTS: The test–retest reliability of Sway scores was comparable to force platform measures. The Sway scores calculated during tandem stance with eyes open produced the lowest ICC value (.211), and force platform measures were high (ICC range = .553–.640) for the same test. Sway scores for tandem stance with eyes closed (.569) and single-leg stance with eyes open (.359) produced higher ICC values than force platform sway measures (.507 and .243, respectively). Pearson correlations evaluating concurrent validity produced significant correlations (p < .05) between Sway scores and force platform measures during tandem (rs = –.425 to –.615) and single-leg stances (rs = –.598 to –.661). All tests conditions, except feet-together stances, produced moderate to moderately strong correlations between Sway scores and force platform measures.

DISCUSSION: Lower Sway scores were associated with larger force platform values in all trials, indicating good agreement between measures. Differences in measurement location (i.e., postural corrections detected at the sternum with Sway, while under the feet with the force platform) may explain lack of strong agreement for some stances.

Clinicians should consider appropriate balance stances (tandem or single-leg stances for minor instability, feet together for severe instability) and visual conditions (eyes open or closed) for specific populations. Although results are promising, further research must be conducted across health conditions and contexts.

IMPACT STATEMENT: Mobile apps, including Sway, may provide clinicians with a portable, cost-effective, and accurate means to evaluate functional balance, ultimately allowing for improved engagement in meaningful activities for the people we serve.