Midlife mobility assessment from the pocket: agreement and consistency between smartphone placement configurations

Subtle declines in mobility may emerge during midlife and can be detected using smartphone-based motion measures. Instrumented assessment of balance and functional tasks commonly uses lower-back placement, which requires device fixation and may limit convenient real-world monitoring. This study compared mobility outcomes obtained from a practical front-pocket/thigh placement with those from a standardized lower-back placement during static balance, sit-to-stand (STS), and Timed Up and Go (TUG) tests. Forty-six adults aged 40-64 years completed static balance, 30-second STS, and TUG assessments during simultaneous smartphone recording at the lower back and pocket/thigh. Outcomes included two- and three-dimensional sway, STS repetition count, average repetition duration, lower limb power, and maximal vertical velocity, as well as TUG total duration and peak yaw angular velocity during both turns. Associations, intraclass correlation coefficients, and Bland-Altman analyses were used to characterize the relationship and agreement between the two placements. Balance outcomes showed strong correlations between placements (rs=0.83-0.93), with moderate-to-excellent consistency for most conditions, although pocket/thigh sway values were systematically higher. TUG total duration showed strong correlation and good absolute agreement (rs=0.895; ICC=0.881), and turning velocity showed good consistency during the first and the second turns. STS repetition count and average repetition duration demonstrated strong correlations between placements (r=0.996 and 0.985, respectively), with excellent absolute agreement for repetition count and excellent consistency for average repetition duration (ICC=0.993 and 0.982, respectively). In contrast, STS power and maximal vertical velocity showed weaker correlations and consistency (r=0.709 and rs=0.382; ICC=0.600 and 0.417, respectively). Pocket/thigh smartphone placement supports practical mobility assessment across balance, STS and TUG. Although some movement-derived measures showed placement-dependent differences in absolute values, relative performance patterns were generally preserved across placements. These findings support the development of pocket-based smartphone assessment for simplified and potentially unsupervised mobility monitoring in midlife, while placement-sensitive outcomes require further validation against external reference measures.