Electromyographic activity of the vastus intermedius muscle in patients with stiff-knee gait after stroke. A retrospective observational study

•VI activity was measured by fine wires during stiff-knee gait of stroke patients.•VI abnormal activity is frequent in stroke patients with SKG.•VI may affect the stance phase thus preventing knee flexion.•VI may affect the swing phase thus braking knee flexion.•VI activity should be assessed to assist in the clinical decision-making processes. Stiff-knee gait (SKG) in hemiplegic patients is often due to an inappropriate activity of the quadriceps femoris. However, there are no studies in literature addressing the vastus intermedius (VI) involvement in SKG. In this study, VI activity was analyzed in a sample of 46 chronic stroke patients with SKG, during spontaneous gait. VI activity was recorded by fine-wire electrodes inserted under ultrasound guidance then confirmed by electrical stimulation. The measured VI activity was compared to the normal reference pattern reported in literature and classified (e.g. premature, prolonged). The occurrences of abnormal activations during each sub-phase of the gait cycle were assessed. VI activity presented an abnormal timing in 96% of the sample. The most common pathological pattern (in 46% of the sample) was the combination of premature and prolonged VI activation. Nearly 20% of patients presented a continuous activity. A pathological activation in patients was found for 91% in mid stance, for more than 50% in terminal stance and pre-swing and for 37% and 70% in initial- and mid-swing. Results indicate that abnormal VI activity is frequent in patients with SKG. Hence, VI activity should be included in the assessment of SKG to assist in the clinical decision-making processes.