Gait impairment remain one of the main problem for most of chronic hemiplegic stroke patients. After the 6 first months, stroke patients are classically described with moderate or no improvement for motor skills. However, physiothertapy is very often delivered to try to increase motor performance. Here, we want to compare the effectiveness of a flexor and extensor paretic knee muscular strengthening program with conventional physiotherapy delivered by the local physiotherapist of the patient, in a very ecological way. Patients will be randomly affected to one group with the same amount of physiotherapy (at least 3 times weekly for 6 weeks). Assessment will be done blindly by another physiotherapist, before, after the 6-week treatment and 6 weeks later to evaluate immediate and remaining benefits of each treatment.