We read the article by Deymeer et al.1 with interest. The authors document onset of weakness and describe progression of weakness in 10 patients with spinal muscular atrophy (SMA) type IIIb. We recently published the natural history data of 12 patients with late onset SMA types IIIb and IV. We could not detect deterioration of muscle strength or functional outcome during a mean follow-up of 2.5 years. According to Deymeer et al.,2 this is too short to detect a decline in muscle strength. Manual muscle testing (MMT) sumscores in our patients were lowest in muscle groups innervated by upper lumbar segments (hip flexors and adductors), which confirms their findings. We were intrigued by how the muscle weakness developed in their patients, so we analyzed MMT sumscores of individual muscle groups in our patients. MMT sumscores were lowest in hip flexors (iliopsoas), followed by quadriceps femoris, yet strength in hip abductors, hip extensors, and hamstrings was relatively preserved. …