Purpose A biomechanical study, in which imaging modalities are used to strictly include patients with concurrent lateral ankle instability (LAI) and osteochondral lesions of the talus (OLT), is needed to demonstrate the static and dynamic ankle range of motion (ROM) restriction in these patients, and determine whether ankle ROM restriction can be corrected postoperatively. Methods Eight patients with concurrent LAI and OLT treated with the arthroscopic modified Brostrom procedure and microfracture were recruited from June 2019 to January 2020. Patients were assessed using outcome scales, static ankle ROM, and a stair descent gait analysis for dynamic ankle ROM, a day prior to surgery and one year postoperatively. Eight healthy subjects were assessed using the same modalities upon recruitment. Operative outcomes and variables during stair descent were documented and compared among the preoperative, postoperative, and healthy groups. A curve analysis, one-dimensional statistical parametric mapping, was performed to compare the dynamic ankle kinematics and muscle activation curves over the entire normalised time series. Results The functional outcomes of patients with concurrent LAI and OLT were significantly worse than those of healthy subjects preoperatively, but were partially improved postoperatively. Patients had decreased static and dynamic ROM preoperatively, and static ROM did not significantly increase postoperatively (preoperative, 39.6 +/- 11.3; postoperative, 44.9 +/- 7.1; healthy, 52.0 +/- 4.6; p = 0.021). Patients showed increased dynamic ankle flexion ROM (preoperative, 41.2 +/- 11.6; postoperative, 53.6 +/- 9.0; healthy, 53.9 +/- 3.4; p = 0.012) postoperatively, as well as increased peroneus longus activation (preoperative, 35.8 +/- 12.0; postoperative, 55.4 +/- 25.1; healthy, 71.9 +/- 13.4; p = 0.002) and muscle co-contraction of the tibialis anterior and peroneus longus (preoperative, 69.4 +/- 23.4; postoperative, 88.4 +/- 9.3; healthy, 66.2 +/- 18.1; p = 0.045). Conclusions Patients with concurrent LAI and OLT had decreased static and dynamic sagittal ankle ROM and altered neuromuscular activation patterns. The arthroscopic modified Brostrom procedure and microfracture did not significantly increase the static sagittal ankle ROM. However, the dynamic sagittal ankle ROM, peroneus longus activation and muscle co-contraction of the tibialis anterior and peroneus longus increased postoperatively.