Introduction The knee joint of a conventional knee-ankle-foot orthosis (KAFO) is locked during walking. This study aims to evaluate the effects of stance phase knee flexion on the kinematics, spatiotemporal parameters, and energy consumption of KAFO users. Materials and Methods A retrofitting drop lock that allows 12° of stance flexion in a single-axis knee joint was developed (called the semiflexion knee joint). Four subjects with quadriceps weakness volunteered to participate in the study. Gait analysis was conducted for the locked knee and the semiflexion knee orthosis using a calibrated eight-camera three-dimensional motion capture system. Preceding each trial, participants were given 5 days of gait training with the corresponding KAFO. Subjective feedback about the performance of the new KAFO was collected. Results Walking with the semiflexion KAFO did not significantly improve the speed (p = 0.161), cadence (p = 0.232), and stride length (p = 0.95) compared with walking with a locked KAFO. Although not significant (p = 0.132), trends of Physiological Cost Index (PCI) seemed to reduce in all subjects while walking with the semiflexion knee. Conclusions Using the KAFO with stance flexion did not significantly improve any of the locked knee gait parameters. However, there was a trend of reduction in the PCI score while walking with the semiflexion KAFO, and it also improved the ease of doing some activities of daily living. Copyright © 2018 American Academy of Orthotists and Prosthetists.