Background Objective measurements of postural control are frequently used to examine the causes of, features associated with, and therapeutic interventions for ankle instability. However, researchers have typically used single-session measures to represent postural control at one point in time. Recent studies in a healthy elderly population demonstrated significant variations in day-to-day postural control and suggest that single-session measurement may not truly reflect postural control. We need to investigate patterns of day-to-day variation in postural control in a younger population, the typical age profile included in ankle instability studies.

Objective Investigate the variations between continuous day-to-day clinical measurements of postural control within subjects, and the associations between once-off and continuous daily measurements, in a healthy young population. It was hypothesised that variations exist and a once-off clinical measure may not be representative of an individual’s true postural control.

Design Observational longitudinal cohort study.

Setting University motion capture laboratory.

Participants 24 healthy young adults (9 female, 15 male) aged 18–40 years.

Independent variables Age, time of day (08:00–10:00), duration (40 s) and testing condition (eyes-open versus eyes-closed).

Main outcome measurements Lifestyle questionnaire and 40 s eyes-open/eyes-closed static Wii Balance Board balance tests, on 20 consecutive weekdays

Results Coefficient of variation demonstrated substantial inter-subject differences from 10–131% (eyes-open) and 10–112% (eyes-closed) across variables. Minimal detectable change percentage showed that 22/30 parameters demonstrated acceptable measurement error (<30%). Across mean COP distance, mean sway length, mean sway frequency and sway area, 16/24 (eyes-open) and 11/24 participants (eyes-closed) exhibited statistically significant differences (p < 0.05) between the once-off and the daily measures.

Conclusion Variations in postural control exist in a healthy young population. Depending on testing conditions and specific variables, a once-off measure is not indicative of an individual's true functional state. Therefore, when investigating subtle changes in postural control, long-term monitoring proves to be a superior assessment tool.