Spinal muscular atrophy patients present with weakness, greater in the proximal muscles, leading to scoliosis and limited upper extremity function. The purpose of this study is to identify unique aspects of these patients and to understand how spinal fusion affects their function. Forty patients underwent Harrington or Luque rod instrumentation with functional evaluations preoperatively and 2 and 5 years postoperatively. Biomechanical assessment of function is important. Flexibility of the spine is functionally advantageous because distal strength is used to align weaker proximal segments. Postoperatively, lack of spinal flexibility resulted in a decline in gross motor function and increased use of UE aids due to a change in the trunk position in the weaker patients. The stronger patients' activities were maintained. Earlier mobilization in patients with Luque procedures did not improve postoperative function.