BACKGROUND: Reduced gait speed is common following traumatic brain injury (TBI). Several studies have found that people with TBI display increased lateral movement in their center of mass while walking. It has been hypothesized that reduced gait speed following TBI is a consequence of increased caution and postural instability, but reduced ankle power generation at push-off may also play a contributing role. OBJECTIVE: To determine whether postural instability or reduced muscle power were associated with reduced gait speed following TBI. METHODS: A convenience sample of 55 people with TBI receiving physiotherapy for gait disorders were assessed using 3D gait analysis at self-selected and fast walking speeds. A comparison group of 10 healthy controls performed walking trials at a speed matched to the mean TBI self-selected speed and at a fast walking speed. RESULTS: When matched for speed, people with TBI walked with similar cadence and step length but with reduced ankle power generation at push-off and increased hip power generation both in early stance and in preswing compared with healthy controls. Width of base of support and postural instability were also significantly increased for people with TBI. The differences between the 2 groups at the matched speed remained for the fast speed condition. Postural stability did not deteriorate with increasing gait speed in either group. CONCLUSION: Reduced gait speed following TBI appears to be attributable to biomechanical deficiencies such as reduced ankle power generation rather than reduced postural stability and increased caution.