Background: Two populations commonly presenting with equinus gait are Idiopathic Toe-Walkers and children with Cerebral Palsy. Surgical intervention to treat equinus is defined by three zones. Zone three surgery, performed at the Achilles tendon, is most commonly used clinically. There is however, evidence from simulation studies that zone two surgery, performed at the muscle belly, might provide better functional outcomes. The purpose of this study was to investigate the effect of zone two calf-lengthening on post-operative gait in these populations. Methods: A retrospective audit of the Queensland Children's Motion Analysis Service database identified 17 toe-walkers (mean age 10.13 (SD 2.625)) and 11 Cerebral Palsy (mean age 9.72 (SD 4.04)) participants that received calf-lengthening surgery for plantarflexion contracture and had pre- and post-surgery 3D gait analysis. Inverse kinematics, dynamics, and muscle analysis were performed in OpenSim (v3.3) using a modified gait2392 model. Pre to post-surgery comparisons were performed in MATLAB using statistical parametric mapping. Dependent variables included ankle kinematics, powers and muscle-tendon length estimates. Findings: The primary outcome of this study was that ankle dorsiflexion increased in both Idiopathic Toe Walking and Cerebral Palsy groups post-calf lengthening across 90% and 85% of the gait cycle respectively. There was an increase in modelled muscle-tendon lengths, specifically in the medial gastrocnemius, of 78% (toe-walkers), and 100% (Cerebral Palsy) of the gait cycle. Power generation during push-off was not affected. Interpretation: Overall, the results appear to support the efficacy of zone 2 calf-lengthening for children with Cerebral Palsy and Idiopathic Toe Walking.