Background The in-vivo status of lateral ankle joint complex laxity and stiffness in chronic ankle instability (CAI) is unclear and may contribute to repeated injury.

Objective To determine if differences exist in beginning- and end-range stiffness during anterior loading between various ankle injury groups.

Design Cross-sectional.

Setting Biomechanics Laboratory.

Participants From 52 recreational athletes, 40 meeting the inclusion criteria were divided into control (n = 17), coper (n = 7), mechanical ankle instability (MAI; n = 6) and functional ankle instability (FAI; n = 10) groups (17 males; 23 females; 22.2 ± 3.2 years, height 168.4 ± 10.3 cm, body mass 68.3 ± 15.8 kg). Controls (no ankle injury) and copers (1 sprain from which they recovered) reported Cumberland Ankle Instability Tool (CAIT) scores ≥29. MAI and FAI groups had repeated episodes of giving way and CAIT scores ≤24. The MAI group had anterior displacement >18 mm with arthrometer testing with the FAI group having <18 mm.

Interventions Three trials of an instrumented arthrometer anterior load to 150N were applied.

Main outcome measurements Anterior displacement (mm) at 150N and stiffness (N/mm) values at beginning- (40–60N) and end-range (125–150N) were extracted and averaged. One-way ANOVAs with Tukey post-hoc comparisons (α = 0.05) were applied.

 Results Groups were not different in age, height, or body mass. The MAI group had significantly lower stiffness values at beginning-range (3.1 ± 0.6 N/mm, p < 0.007) than controls (4.4 ± 1.0 N/mm) and FAI (4.8 ± 0.6 N/mm) and at end-range (4.3 ± 0.5 N/mm, 5.5 ± 0.9 N/mm, 6.0 ± 0.6 N/mm, respectively, p < 0.01). There were no other group differences.

Conclusions The MAI group demonstrated decreased stiffness at beginning- and end-range compared to control and FAI groups. There were no differences in stiffness among control, coper, and FAI groups. For the MAI sub-group with laxity, decreased stiffness may indicate poor joint complex response to loading. Changes in ankle joint complex laxity and stiffness may play a role in repeated injury in some sub-groups of CAI.