Spastic paretic stiff-legged gait is a frequently encountered gait problem in patients with traumatic brain injury, as well as in many other patients with upper motor neuron disease. Formerly, spasticity of the quadriceps was considered to be the sole cause of stiff-legged gait. Quantitative gait analysis, however, may implicate hip flexor weakness or poor ankle mechanics as the cause of stiff-legged gait. We discuss the use of an algorithm to evaluate stiff-legged gait in traumatic brain injury using a quantitative gait analysis system such that the specific etiology of stiff-legged gait can be identified and can serve as the basis of a treatment plan.