Proprioceptive Acuity of the Ankle is Higher in Plantarflexion than in Dorsiflexion

Intact ankle proprioception is essential for the control of balance and gait. This study determined ankle position sense acuity for plantarflexion and dorsiflexion. In two separate assessments, the right ankle of 30 healthy young adults was passively rotated from neutral joint position to a 15° reference position and a smaller comparison position in either plantar- or dorsiflexion. Subsequently, participants verbally indicated which position felt more flexed. After 25 trials, a psychometric function was fitted to the respective response-stimulus size difference data for each participant. Two outcome measures were derived: a Just-Noticeable-Difference (JND) threshold as a measure of systematic error and an Uncertainty Area (UA) indicating random error. Analysis showed that 70% (21/30) of participants exhibited lower JND thresholds for plantarflexion. Mean JND threshold and median UA were both significantly lower in plantarflexion when compared to dorsiflexion (p = 0.008, d = 0.52; p = 0.001, $r_b$ = 0.58) indicating that ankle proprioceptive acuity was not uniform for sagittal plane ankle motion. We discuss differences in plantar and dorsiflexor muscle mechanoreceptor density and central proprioceptive signal processing as possible reasons for the observed differences in acuity and highlight the importance of understanding movement-specific proprioceptive acuity for designing effective rehabilitation protocols.