Chronic stroke survivors tend to be inactive, often with sarcopenia, and have decreased physical function and activities of daily living. Muscle atrophy and weakness differ between sarcopenia patients and stroke patients. Therefore, it is difficult to evaluate physiotherapy and intervention for sarcopenic patients with stroke. The purpose of this study was to identify muscles that cause muscle weakness and muscle atrophy in stroke sarcopenia patients. The subjects were 117 chronic stroke survivors who were 65 years or older. Subjects were determined using the criteria of the Asian Working Group on Sarcopenia in 2019 to determine the presence of sarcopenia and were classified into sarcopenia group (SG, n = 60) and non sarcopenia group (nSG, n = 57). Atrophy assessments obtained unaffected lower limb muscle thickness (iliopsoas, gluteus maximus, gluteus medius, hamstrings, quadriceps femoris, tibialis anterior, triceps surae) using B-mode of transverse ultrasound imaging. Strength assessments obtained unaffected lower limb muscle strength (flexion, extension, abduction, adduction, external rotation and internal rotation of hip joint, flexion and extension of knee joint, planter flexion and dorsiflexion of ankle joint) using handheld dynamometer. We conducted a Student's t-test to compare the two groups. A P-value of P < 0.05 and power >0.8. We conducted with the approval of the ethics committee of Aichi Saiseikai Rehabilitation Hospital (201,908). SG had muscle atrophy in all muscles compared to nSG (P < 0.05, power >0.8). SG had muscle weakness in all joint direction compared to nSG (P < 0.05, power >0.8). In particular, extension of knee joint and planter flexion of ankle joint muscle weakness, quadriceps femoris and triceps surae muscle atrophy occurred (P < 0.01, power >0.8). Assessment and intervention of skeletal muscle in stroke sarcopenia patients should focus on the knee joint and ankle joint. The authors declare no conflicts of interest associated with this manuscript.