Introduction Functional electrical stimulation (FES) synchronized with robot‐assisted lower extremity training is used in spinal cord injury (SCI) rehabilitation to promote residual function. Methods Data of SCI inpatients who trained lower limb mobilization on a stationary robotic system were retrospectively analyzed. The primary outcome was the improvement of muscle strength from the first through to the last training session during FES‐induced as well as voluntarily induced flexion and extension. The secondary outcome was the sum score of voluntary muscle function in the lower limbs before and after the training period. Results Data from 72 patients with SCI (AIS A‐D) were analyzed. For extension, FES‐assisted strength increased (p < 0.001) from 25.2 to 44.0 N, voluntary force (p < 0.001) from 24.4 to 39.9 N. For flexion, FES‐assisted flexion (p < 0.006) increased from 14.1 to 19.0 N, voluntary flexion (p < 0.005) from 12.6 to 17.1 N. There was a significant correlation between the increase in FES‐assisted force and voluntary flexion (r = 0.730, p = 0.001) as well as between the increase in FES‐assisted force and voluntary extension (r = 0.881, p < 0.001). The sum score in muscle test increased from 15 to 24 points. Conclusion Robot‐assisted training with FES seems to support the regeneration of residual functions after SCI. This is evidenced by an improvement in motor function and strength in the lower limbs. Results of the increase in FES‐assisted as well as in voluntary force of the lower limbs in patients with subacute spinal cord injury.