Introduction:​ Ankle joint complex (AJC) injuries are common during sports and activities. Following an AJC injury, many patients are classified with functional ankle instability (FAI) and increased predisposition to re-injury. Many studies have suggested that impaired proprioception or motor control might be important factors in FAI. The purpose of this study was to assess the effects of FAI on AJC dynamics and motor control during a lateral hop movement (LHM).

Methodology:​ Two groups were studied:​ 1) Normal — no prior AJC injuries (n=12) and 2) FAI — previously injured subjects with FAI (n=12). Kinematics were recorded with eight cameras, ground reaction forces were collected with two force platforms, and an electromyographic (EMG) system was used to collect myoelectric signals. The LHM consisted of multiple lateral-medial hops over an obstacle (width 72.5 cm, depth 25.5 cm, height 14.3 cm) onto adjacent force platforms. Each subject was instructed to perform as many lateral hops as possible during the 6-s trial. AJC kinematics, forces, AJC moments, centre of pressure displacements, AJC dynamic stiffnesses, and muscle activities were measured throughout the LHM. Means, standard deviations, and 95% confidence intervals of the differences were calculated for each of the outcome measures.

Results:​ Means (SD) of the outcome measures that showed significant differences between the FAI and Normal groups were dorsiflexion AJC positions at contact FAI 82.4º (6.4) and Normal 75.2º (10.1);​ tibialis anterior normalized muscle activity at contact FAI 0.27 (0.21) and Normal 0.16 (0.13);​ dorsiflexion AJC moment at contact FAI –0.26 Nm/kg (0.21) and Normal –0.13 Nm/kg (0.12);​ maximal lateral force during pre-impact phase FAI 1.2 N/kg (2.4) and Normal 2.6 N/kg (1.4);​ maximal lateral centre of pressure displacement FAI 14.4 mm (15.6) and Normal 34.9 mm (25.5).

Conclusions:​ The FAI group revealed greater tibialis anterior muscle activity and dorsiflexion AJC position at contact and less maximal lateral force and lateral centre of pressure displacements during contact. The differences between the groups may have been a genetic predisposition to AJC injuries, a pre-existing difference in task performance, a consequence of injuries, or a compensatory adaptation to previous injuries.