Laura Peeters

Trunk involvement in performing upper extremity activities | 37 2 Stabilizing the head in space (i.e., not with respect to the trunk) was the most common strategy used by healthy children and adults [4, 13]. This strategy facilitates the fixation of gaze on the target while reaching, and thus optimizes visual feedback of task performance. For patients with a flaccid trunk it appears difficult to stabilize their head in space because of trunk instability, as was shown by Saavedra, et al. [42] in children with CP (GMFCS levels I-III). Head stability improved when external trunk support was given to these patients; nevertheless, some of them still showed head instability during trunk support probably due to reduced control of the neck muscles. Unfortunately, no research was available on head-trunk interaction in patients with other conditions, such as SCI or NMD. Thus, further research should take into account head stability problems due to both trunk instability and neck-head stability. Based on the literature, it is beyond doubt that normal interaction between trunk, head and UE movements typically develops during childhood [12, 13]. Hence, the developmental stages with age of trunk-head-arm interactions must be kept in mind when looking at neurological patients with a flaccid trunk during childhood. It is, however, difficult to give precise age boundaries for the various stages, as movement strategies vary between different planes and age groups [13]. Variability seems to be highest in the sagittal plane when reaching to nearby targets in healthy children below the age of 10. This implies that when a physician examines a 7-year-old child with CP reaching forward to a nearby target and observes trunk extension, this might be interpreted as ‘abnormal’ compared to a healthy adult, but (s)he should realize that approximately 30% of the healthy children of the same age show the same movement strategy. Postural stability and influence of reaching direction The definition of (postural) stability varied among the included studies or was sometimes even lacking. The most common definition was keeping or returning the center of mass over the base of support while performing self-initiated actions [18, 37]. Thus, ‘stability’ refers to a ‘dynamic’ situation, whereas ‘balance’ most often refers to a ‘static’ situation. Using the same definitions in research is important to be able to validly compare data and increase our understanding of postural control in various disorders. Two types of strategies can be distinguished to maintain trunk stability during reaching in patients with a flaccid trunk. To minimize the demands on trunk control, patients can either reduce the proximal degrees of freedom of the reaching synergy or they can reduce the movement speed of the arm. Both strategies are seen in patients with CP [26, 28, 30, 43]. While reduction of the proximal degrees of freedom reduces motor complexity, it also narrows the workspace because movement of the trunk