Assessing functional load symmetry: A case for a 15 % threshold in healthy young adults

Symmetry is often defined as a limb difference of less than 10 %, a threshold originally established using sport-specific hop tests. However, its applicability to daily functional tasks in healthy populations remains unclear. This study aimed to evaluate differences among common symmetry indices and assess whether 10 % or 15 % thresholds are appropriate for healthy young adults during everyday activities.

72 young healthy adults were enrolled. A load sensing insole (200 Hz) was used to collect plantar loading data during level walking (LW), stair ascent (SA), stair descent (SD), and sit-to-stand (STS). Bilateral peak impact force (PIF) and average loading rate (ALR) were used to calculate limb symmetry with three indices (Absolute Symmetry Index (ASI), relative Limb Symmetry Index (rLSI), absolute Normalized Symmetry Index (aNSI)). A chi-squared analysis determined the appropriateness of the 10 % or 15 % threshold. Absolute agreement ICC values assessed agreement between symmetry indices.

10 % symmetry threshold was only appropriate for PIF symmetry in SA and SD (χ2 = 3.84, α = 0.05). In contrast, a 15 % threshold captured a substantially higher percentage of trials across all tasks, particularly for PIF in gait-related activities. ICC values demonstrated excellent agreement between the ALR and PIF ASI and rLSI. Poor agreement was observed between ALR ASI and aNSI and ALR rLSI and aNSI during walking and SD task.

15 % threshold may better account for natural variability in symmetry, especially for gait-related tasks. Increasing disagreement between symmetry indices shows the need for careful consideration of the symmetry index choice.