Laura Peeters

The trunk in spinal musclar atrophy | 107 5 trunk movement did not increase. We expected to find increased trunk movement to compensate for reduced arm function, as for example was visible in patients with Duchenne muscular dystrophy [13]. But, although deltoid muscle activity level was close to 100% of MVIC, trunk ROM did not increase. As a consequence, patients will be restricted in their workspace and therefore in performing daily activities. The fact that patients with SMA did not increase their trunk ROM, although normalized shoulder muscle activity was very high, suggests that patients with SMA need more of their trunk muscle capacity to maintain stability in order to perform the upper extremity movements [18]. To gain more insight in mechanisms underlying the increased normalized muscle activity when performing the reaching and daily tasks, we analyzed the absolute muscle activity. This showed similar absolute muscle activity levels of the back muscles, indicating comparable back muscle activation during task performance in SMA and HC (in combination with comparable trunk ROM). Noteworthy, this still resulted in increased percentages of normalized muscle activity in patients with SMA since the absolute maximum muscle activity was decreased. On the other hand, the absolute abdominal muscle activity was significantly increased in patients with SMA, which could indicate co-contraction of the abdominal muscles during task performance and would support the hypothesis above. The co-contraction can be caused by recruitment of more motor units needed to generate enough muscle force to maintain trunk stability and/or recruitment of larger motor units due to re- innervation in SMA [19]. Using increased percentages of the maximum muscle capacity and co-contraction causes earlier development of fatigue and increased risk of muscle overloading [18, 20]. Since scoliosis is related to muscle weakness and fatigue, clinicians should pay high attention to trunk function in children with SMA [21]. But also in general for functional assessment and development of interventions, there should be more awareness for the great loads on trunk muscles required to perform simple manual tasks. Interventions to reduce muscle fatigue during the day can be applied, like proper seating, use of trunk supportive devices, or physical muscle strength training to reduce fatigability [22, 23]. Rigid trunk orthoses are not recommended, because these restrict important trunk movements that are necessary to perform daily tasks. Additionally, being able to move could also prevent the muscles from degenerating faster due to disuse [3, 24]. New supportive devices that allow movement and reduce load on the trunk are needed. For the first time in patients with SMA, a quantitative insight in trunk function was obtained. The results were consistent with clinical experience on trunk function and can therefore support clinical decision making. Furthermore, the method used in this study gives opportunities to evaluate interventions in a quantitative manner in the