linguistic:paraphrasing
Torso

MH1 preserves 4 neck, 6 thoracic, and 2 lumbar vertebrae; and MH2 preserves 2 neck, 7 thoracic, 2 lumbar, and 1 sacral vertebrae. The lordosis (humanlike curvature) and joints of the neck vertebrae, indicating similar head posture to humans. However, the overall anatomy of the neck vertebrae is apelike, and point to a much stiffer neck. A. sediba lacks a humanlike brachial plexus (which is identified in some A. afarensis), and the human brachial plexus is responsible for nerves and muscle innervation in the arms and hands enhancing motor control. Like humans, A. sediba appears to have had a flexible lumbar series comprising 5 vertebrae—as opposed to 6 static vertebrae in non-human apes—and exhibiting lumbar lordosis (human curvature of the spine) consistent with habitual upright posture. However, A. sediba seems to have had a highly mobile lower back and exaggerated lumbar lordosis, which may have been involved in counteracting torques directed inwards while walking in the hyperpronating gait proposed for A. sediba. MH1 preserves 2 upper thoracic, 1 mid-thoracic, and 3 lower thoracic ribs; and MH2 4 consecutive upper-to-mid-thoracic, and 3 lower thoracic ribs joined with the vertebrae. This indicates that A. sediba had an apelike constricted upper chest, but the humanlike anatomy of the pelvis may suggest A. sediba had a broad and humanlike lower chest. The narrow upper chest would have hindered arm swinging while walking, and would have restricted the rib cage and prevented heavy breathing and thereby fast walking or long-distance running. In contrast, A. sediba seems to have had a humanlike narrow waist, repositioned abdominal external oblique muscles, and wider iliocostalis muscles on the back, which all would improve walking efficiency by counteracting sideward flexion of the torso.