Laura Peeters

Summary |117 6 These high levels could lead to fatigue and overloading. Normalized muscle activity also increased until a task could not be performed anymore. This might indicate that back muscle function plays a more important role than previously thought, and the UE might not be the only limiting factor for accomplishing tasks. Additionally, trunk and shoulder joint torques were significantly decreased (by 52% and 63%, respectively) in DMD patients compared to HC, and so was the active trunk ROM in all movement planes. Joint torques were already decreased in early disease stages. To conclude, due to increased compensatory trunk movement, demands on trunk muscles are increased in DMD patients, and this is compounded by trunk muscle weakness. Therefore, clinicians should take the increased load on trunk muscles into account when assessing function and when developing interventions such as seating adjustments or physical exercise training. Additionally, if supporting the trunk restricts (compensatory) trunk movements, this will likely cause limitations in accomplishing tasks independently, and could accelerate muscle decline due to disuse. The aim of chapter 5 was to investigate trunk function during seated upper extremity tasks in patients with SMA type 2 and 3. Seventeen patients with SMA participated and they were compared with a HC group above the age of 12 years old (n=15, a subgroup of the study described in chapter 3), because the majority of the SMA patients were adults. We expected to find similar results to DMD patients, since patterns of muscle weakness are often described as comparable. However, trunk ROM did not differ between SMA patients and HC when performing the tasks. So, SMA patients did not use compensatory trunk movements when performing seated tasks, although normalized deltoid activity was close to 100% of MVIC in all tasks. The normalized trunk muscle activity was significantly increased in all muscles of SMA patients when performing these tasks. The average muscle activity was almost twice as high for back muscles and 4 times higher for abdominal muscles. This indicates that SMA patients need high levels of trunk muscle capacity to maintain stability when performing UE movements. Consistent with these findings, we found decreased active trunk ROM in SMA patients compared to HC in all movement planes during the active ROM tasks, but comparable percentages of maximum muscle capacity for the muscles counteracting gravitational moments. So, comparable muscle effort coincided with less movement in SMA patients, which is not surprising since the force generating capacity is reduced due to loss of motor neurons. Decrease in maximum muscle capacity was also reflected in significantly decreased maximum trunk and shoulder joint torques in SMA patients compared to HC. Therefore, similar to DMD, clinicians should take trunk function into account when assessing overall function in SMA and when designing interventions, as increased muscular effort to perform tasks could result in fatigue and muscle overloading. Again, one must bear in mind that restrictions in trunk movement will likely cause limitations in accomplishing tasks independently and might accelerate muscle decline due to disuse.