Laura Peeters

118 | Chapter 6 GENERAL DISCUSSION Trunk function in DMD and SMA DMD and SMA are often seen as comparable conditions, since both diseases are characterized by progressive muscle weakness, proximally more than distally, and disease onset is (generally) during childhood [7, 8]. This thesis shows to what extent these disorders are comparable in terms of trunk function. Specifically, trunk function in DMD and SMA patients compared to healthy controls (HC) is described in chapters 4 & 5, respectively. It is remarkable that the DMD patients showed increased trunk movement when performing seated tasks and the SMA patients did not. We stated that DMD patients used this increased trunk movement to compensate for their reduced arm function. However, UE function was reduced in both groups compared to HC and the included SMA patients generally had worse UE function compared to the DMD patients (based on the Brooke scale). The percentage of trunk muscle capacity used when performing tasks was comparable between both groups, despite the smaller trunk motions in the SMA group. This suggests that the included SMA patients had on average worse trunk function and likely needed more muscle effort to stabilize the trunk compared to DMD patients. No literature was found to confirm or contradict this hypothesis. Nevertheless, the decreased active trunk flexion and extension in SMA patients compared to DMD patients that we observed substantiates this hypothesis. Yet, this hypothesis was not supported by the median trunk joint torques, as these were comparable between both groups, indicating that the difference in trunk function cannot solely be explained by differences in trunk muscle strength, thus, that other factors must play a role too. Several other factors could have had an influence on trunk function and could help to explain why the trunk seemed weaker in the included SMA patients compared to the DMD patients when performing tasks, while maximum trunk torque was comparable. First, there was a large difference in body weight and trunk length between the SMA and DMD patients. The median body weight was 74.5 kg (IQR 56.6-88.1 kg) for SMA patients compared to 48 kg (IQR 40-52 kg) for DMD patients, and the median trunk length was 61 cm (IQR 52-67 cm) for SMA patients compared to 50 cm (48- 56 cm) for DMD patients. Both higher trunk mass and length would result in larger torques needed to balance the trunk in the same inclination angle against gravity. Therefore, SMA patients were likely functionally weaker than DMD patients, despite similar maximum trunk torques. Secondly, there could be a difference in spinal stiffness; more SMA patients had a scoliosis which could be related to increased stiffness. Therefore, more force is needed to move the parts of the spine involved in the deformity.[9] When performing a maximum voluntary isometric contraction