Introduction Amyoplasia is the most common type of arthrogryposis multiplex congenita. Some patients may lack active elbow flexion due to aplasia of the forearm flexor muscles. The objective was to identify the optimal age for pectoralis major muscle transfer to improve elbow flexion in children with amyoplasia and estimate outcomes of the procedure depending on the level of spinal cord injury. Material and methods Restoration of active elbow flexion was performed for 34 children with amyoplasia (39 upper limbs) between 2011 and 2020 using partial monopolar pectoralis major muscle transfer. The age of patients ranged between 1.5 and 15.5 years (6.24 ± 4.24 years). The patients were divided into 3 groups depending on the level of spinal cord injury: C6–C7 (n = 4; 11.8 %), C5–C7 (n = 24; 70.6 %), C5–Th1 (n = 6; 17.6 %). The outcomes were estimated at 6 to 99 months (44.53 ± 31.72 months). The patients underwent preoperative and postoperative clinical and neurological examination. The results were statistically analyzed. Results Active elbow flexion improved by 56.8 degrees (p < 0.0001), forearm flexor muscles strengthened by 2.0 points (p < 0.0001) and extension deficit improved by 14.5 degrees (p < 0.0001) were postoperatively statistically significant. Results were rated as good in 15 (38.5 %); as fair, in 8 (20.5 %) and poor in 16 (41 %) cases. Greater differences were found between the group of patients with the level of C6-C7 spinal cord injury in relation to the group of patients with the level of C5-Th1 (p < 0.05). There were no statistically significant differences between patients with C6–C7 and C5–C7 lesion levels (p > 0.05). Children of different age groups showed no differences in the results of treatment (p > 0.05). Conclusion Pectoralis major muscle could be used for active elbow flexion restoration in patients with amyoplasia. The best results were observed in patients with C6–C7, C5–C7 segmental lesions of the spinal cord. There was no correlation between age of patient at the time of surgery and the effectiveness of operation.