Laura Peeters

68 | Chapter 3 laterally. In the sagittal plane, the strategy to move the head in opposite direction of the trunk was most frequently present. However, also a quite substantial percentage of participants did move the head in the same direction as the trunk in several tasks. Variations in strategy might be explained by the relatively small trunk movements, which do not strongly influence the gaze on the object when the head would not move relative to the trunk at all. Movement onset of the head and the trunk generally seemed to be earlier than the movement onset of the arm when reaching. Only in a few forward reaching tasks, there was also a significant difference between head and trunk onset, resulting in the onset sequence “head-trunk-arm”. These findings correspond to previous literature [19, 20], however, the variability of movement onset was very large in our study. This could be caused by the chosen method in this study; we did not instruct participants to sit as quietly as possible before the start. Especially for the younger, more energetic subjects it was difficult to sit quietly. We did ask the participants to look ahead at the beginning of each trial, but especially younger children did not always comply. We tried to eliminate these movements unrelated to the task performed, by excluding the trials in which subjects already moved their head or trunk at the start of the recording before performing the task. The following considerations should be taken into account when developing new trunk or head supportive devices. Allowing movement between the pelvis and lower lumbar segment is of importance for all movement directions. Based on the relative motions of the lower thoracic segment, allowing movement between the lower thoracic and upper lumbar segments is important for both lateral bending and axial rotation. In additions, since movement of the upper thoracic relative to the lower thoracic segment is quite substantial when bending laterally and when flexing forward, some movement should also be allowed between upper and lower thoracic segments. Although the four trunk segments taken into account in this study still represent a simplification of reality, this analysis provides insight in the minimal degrees of freedom that should be allowed for performance of daily tasks. For a head supportive device, it is important to realize that the head is often moving in the opposite direction of the trunk. Therefore, supportive devices should allow for head rotations independent of the trunk movement. When developing actuated trunk and head supportive devices, they cannot be controlled based on the arm movement when timing of movement is seen as important factor, since the trunk and head generally started to move prior to the arm movement. Several other limitations of this study warrant some discussion. First, reaching distance and height were set based on the sitting posture of the subject at the given moment, while small changes in posture may influence reaching distance and height. This may have resulted in variance between trials within and between subjects. However,