Significant impairment in hand function is found prevalent among stroke survivors, but its effective restoration is challenging due to the complexity of the functional tasks of the hand. In this study, we proposed a novel approach to improve functionality of the hand post-stroke by providing 'subject-specific' patterns of targeted assistance to impaired musculotendon of the hand during functional task performance. Eleven chronic stroke survivors participated in the experiment in which a biomimetic device provided targeted assistance to the impaired hand musculotendons. The proposed subject-specific approach was found to restore proper mechanics of two functional tasks, hand open and palmar pinch, as quantified by the joint kinematics and fingertip force direction. The range of motion and joint coordination was improved for the hand open task, and the fingertip force was directed towards normal direction by the targeted assistance of impaired muscles. The assistance pattern was significantly different across subjects, indicating significant between-subject variability in the impairment characteristics that affect task performance. The proposed method could enable an effective 'subject-specific' training method that restores proper mechanics of functional hand tasks for stroke survivors.