Inclusion Criteria:

• Patient with a stif knee gait (SKG) after a stroke, a post-traumatic cerebral lesion or a multiple sclerosis.
• If the patient had a stroke, it happened at least 4 month ago (before the inclusion).
• The patient has a rectus femoris spasticity ≥ 1 on the modified Ashworth Scale with the hip in extension position.
• A quadricipital muscular force ≥ 2/5 (MRC Scale)
• The patient didn't receive any botulinum toxin injection during the 3 previous months
• No modification of any anti-spastic medication during the inclusion in the actual month (Baclofène, Dantrolène).
• No neurosurgical intervention or ligamentoplasty of the quadricipital ligament during the 6 previous months.
• No recent knee traumatism with knee instability during the 6 previous weeks.

Exclusion Criteria:

• Other comorbidities invalidating the fonctionnal tests necessary for the study like : decompensated cardiac insufficiency, unstable respiratory insufficiency, coagulation troubles, neurocognitive impairments inhibiting the comprehension of isocinetical tests.
• Patient without severe phasic trouble inhibiting the patient's ability to answer the evaluation scales.
• Sensory lesions inhibiting the possibility to evaluate the walk : visual impairment, major proprioception impairment, or cerebellar ataxia.
• Severe muscular deficit : genetic myopathy, psychomotric desadaptation syndrom
• Absolute contraindication to botulinum toxin injections : urinary tract infection ; current treatment by aminoglycosidic antibiotics, severe myasthenia, urine retention in patient who can't receive a urinary catheter.
• Patient with cognitive troubles compromitting the understanding and the realization of the study.