Laura Peeters

1 General Introduction | 11 stability, trunk control greatly determines the precision of UE movement [5]. Trunk movement and trunk stability also influence movement of the head and vice versa. Head orientation is generally kept constant in space for fixation of gaze on the target and for visual feedback of task performance [7]. This means that the head often shows a countermovement relative to the trunk movement. TRUNK IMPAIRMENT Trunk function is impaired in patients with a flaccid trunk. A flaccid trunk is typically associated with (severe) muscle weakness due to primary muscle disease (e.g. Duchenne muscular dystrophy (DMD)) or motor neuron disease (e.g. spinal muscular atrophy (SMA)) [8, 9], but can also be present in patients with central neurological disorders with bilateral paresis, like cerebral palsy or spinal cord injury [10]. Besides impairment of trunk function due to muscle weakness, (structural) deformities of the spine and/or rib cage can influence trunk function. When these disorders become symptomatic during (early) childhood and trunk function becomes impaired, two additional factors may negatively affect motor capacity. First, in typically developing Superior nuchal line Nuchal ligament Semispinalis capitis Semispinalis cervicis Semispinalis thoracis Multifidus Intertransversarii Levator costarum Rotatores Splenius cervicis Splenius capitis Semispinalis capitis External occipital protuberance Semispinalis capitis Longissimus capitis Longissimus cervicis Iliocostalis cervicis Iliocostalis thoracis Longissimus thoracis Spinalis thoracis Iliocostalis thoracis Longissimus lumborum Iliocostalis lumborum Erector spinae Cervical Thoracic Lumbar Sacrum Coccyx A. B. Figure 1 Illustrations of the trunk skeleton with the five different segments of the spine (A) and origin and insertion of several back muscles (B) (reprinted with permission of Wolters Kluwer Health, Inc. [6]).