Patients with stroke exhibit distinct muscle activation features in sit-to-stand motion due to motor deficiency. Muscle activation amplitude is an important feature but has not been clarified due to the lack of a valid normalization method to enable intra-subject comparisons. This study, focusing on the paretic side, examines the change in muscle tension manifested in activation amplitude for a patient with stroke in serial measurements by a novel method based on joint torques. We constructed a musculoskeletal model, calculated joint torques by inverse dynamics, and solved muscle activation by forward dynamics simulation. Results showed that tibialis anterior, gastrocnemius, vastus lateralis, rectus abdominis, and erector spinae muscles on the paretic side showed significant improvement in generating maximum muscle tension after a rehabilitation training for 120 days.