Abstract Stiff knee gait (SKG) is common in hemiplegic patients. The main focus of treatment is rectus femoris (RF) spasticity. The aims of this study were to evaluate the effect of botulinum toxin injection (BTI) in the RF muscle on peak knee flexion during swing phase and its quantitative and functional impact on gait. We also wished to evaluate the correlation between the effects of nerve block and BTI on peak knee flexion. 10 adult hemiplegic subjects (> 6 months post stroke or traumatic brain injury) with SKG and inappropriate RF EMG activity during mid-swing phase were included. 3D gait analysis, clinical and functional assessments (Timed Up and Go test, 10 m walk test, 6 min walk test and the time taken to ascend and descend a flight of stairs) were performed initially, 30 min after anaesthetic block of the RF nerve and one month post BTI. After BTI, there was a significant increase in knee flexion (8° average) and a tendency towards improvement in gait and functional parameters. The effect of the nerve block on peak knee flexion was significantly correlated with the effect of BTI (11° average increase in peak knee flexion after nerve block). We challenge the relevance of RF nerve blocks in this population when EMG and kinematic data are available. Our results indicate that BTI is an effective treatment for SKG in adult hemiplegic subjects, with a significant increase in peak knee flexion, no reduction in hip flexion and a tendency towards functional improvements.