Laura Peeters

General discussion |119 6 (MVIC) task to determine the joint torque, stiffness does not limit performance. Third, since the trunk is a central segment, function of the head, UE and lower extremities can also influence the trunk movement (for more detail see heading ‘trunk as central segment’). For example, head stability is challenged more when moving the trunk (chapter 2). If neck muscle strength is lower in SMA patients compared to DMD patients, SMA patients might reduce the trunk movement, or be more conservative in moving the trunk to prevent instability of the head. The same applies to the lower extremities as they contribute to sitting stability [10]. If lower extremity function is worse in SMA patients compared to DMD patients, more muscle effort might be required from the trunk to stabilize the body when performing seated tasks. Fourth, there could be a difference in the function of the deep trunk muscles responsible for trunk stability. Demands on trunk stability are less when leaning and pushing against a force sensor (MVIC tasks) than during voluntary UE movement. Last but not least, in the healthy control group we found a decrease in trunk movement with age when performing tasks (chapter 3). SMA patients were generally adults and might therefore initially use less trunk movement to perform tasks compared to the younger DMD patients. However, these results are difficult to generalize across the patient populations. SMA patients seemed to have a weaker trunk in the population tested, but also had worse UE function compared to the DMD patients. Nevertheless, even if UE function was comparable between both groups, differences in trunk function could still be anticipated between these patient groups. For instance, imaging studies for lower extremity, pelvis and UE muscles show differences in muscle atrophy and fatty infiltration patterns between patients with DMD and SMA [11-14]. However, studies evaluating back or abdominal muscles are minimally available. Two research groups found that the rectus abdominis and external oblique muscles are affected in late disease stages in both DMD and SMA type 3 patients [11, 15], and Sambrook, et al. [14] showed that the posterior spinal muscles are involved in the disease at mid-disease stage in SMA patients. Therefore, new imaging studies are necessary to gain more insight in the trunk muscle weakness patterns in both groups. This would improve the understanding of our results and provide insight in disease progression in relation to the trunk. More in-depth research into trunk stability itself can also provide more insight in the differences in trunk function that we observed. For instance, muscle reflex response times might be relevant to study, since reflexes can be absent or reduced in SMA patients [16] (impacting the maintenance of stability) and can therefore differ from DMD patients.