The Effect of Pes Planus on Balance Ability in Individuals with Chronic Ankle Instability—A Pilot Study

Background/Objectives: Pes planus is characterized by loss of medial longitudinal foot arch, resulting potentially in dysfunction in balance. Chronic ankle instability (CAI) is related to sensorimotor control deficits. Both of these two musculoskeletal disorders have a diminishing effect on joint proprioception. The present study examined the impact of flatfoot on balance in individuals with CAI. Methods: A total of 28 students (15 men, 13 women; 18–23 years, M = 20.46, SD = 1.07) were assigned to CAI with pes planus (n = 15) or CAI only (n = 13). Balance was assessed using the Y-balance test (YBT) and modified star excursion balance test (mSEBT) in three directions (anterior, posteromedial, and posterolateral), alongside the Cumberland ankle instability tool (CAIT). Group differences were analyzed with independent t tests or Mann–Whitney U tests (α = 0.05). Results: The findings of the study did not show statistically significant differences between the two groups in the balance variable [mSEBT/anterior left foot (t = 0.239, p = 0.865); mSEBT/posteromedial left foot (t = −0.048, p = 0.562); mSEBT/posterolateral left foot (t = 0.164, p = 0.258); mSEBT/anterior right foot (t = −0.433, p = 0.748); mSEBT/posteromedial right foot (t = 0.745, p = 0.606); mSEBT/posterolateral right foot (t = 0.263, p = 0.680); YBT/anterior left foot (U = 96.00, p = 0.93); YBT/posteromedial left foot (U = 94.50, p = 0.87); YBT/posterolateral left foot (U = 96.00, p = 0.93); YBT/anterior right foot (U = 95.50, p = 0.92); YBT/posterolateral right foot (U = 82.50, p = 0.45)]. However, a trend towards significance was found as patients with flatfeet had a weaker performance in balance tests [posteromedial direction of the YBT for the right foot (U = 70.00, p = 0.12)]. Conclusions: Although pes planus did not seem to affect the balance ability of individuals with CAI, future studies should confirm the relationship of pes planus and CAI with a larger group, including variables such as ankle range of motion, muscle strength, and functional activity level. A better understanding of the above relationship may lead to more precise diagnostic processes and more efficient therapies in CAI.