Study design: Pretest-posttest design. Objectives: To investigate mechanisms by which short-term resistance training (6 weeks) increases strength of partially paralysed muscles in people with spinal cord injury (SCI). Setting: Community-based setting, Sydney, Australia. Participants: Ten community-dwelling people with partial paralysis of elbow flexor, elbow extensor, knee flexor or knee extensor muscles following SCI (range 5 months to 14 years since injury). Methods: Muscle architecture and strength were assessed before and after participants underwent a six week strength-training program targeting one partially paralysed muscle group. The outcome of primary interest was physiological cross sectional area (PCSA) of the trained muscle group measured using magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI). Other outcomes were changes in mean muscle fascicle length, muscle volume, pennation angle, isometric strength and muscle strength graded on a 13-point scale. Results: The mean increase in maximal isometric muscle strength was 14% (95% CI, −3 to 30%) and 1.5 points (95% CI, 0.5 to 2.5) on the 13-point manual muscle test. There was no evidence of a change in muscle architecture. Conclusion: This study is the first to examine the mechanisms by which voluntary strength training increases strength of partially paralysed muscles in people with SCI. The data suggest that strength gains produced by six weeks of strength training are not caused by changes in muscle architecture. This suggests short-term strength gains are due to increased neural drive or an increase in specific muscle tension.