Laura Peeters

The trunk in Duchenne Muscular Dystrophy | 87 4 This indicates that back muscle function plays a more important role than thought, so the arm might not be the only limiting factor accomplishing tasks. It is likely that compensatory trunk movements are limited by increasing back muscle activity with disease progression, due to which patients lose the ability to accomplish the task. The percentage of trunk muscle capacity used when sitting upright was already two times higher in patients with Brooke scale 1 compared to HC and this normalized activity level is even higher when performing tasks. This indicates early trunk muscle weakness in relation to motor function, which contrasts with previous studies indicating that trunk function is good in the ambulatory phase [5, 6]. When a higher percentage of the maximum muscle capacity is used, this leads to faster development of fatigue and possibly to overloading of the muscles [20]. Clinicians should take this increased muscle activity into account for function assessment and development of interventions. Proper seating, back rests or the use of other trunk supportive devices can reduce trunk muscle fatigue during the day [21]. However, it is important that patients are still able to move their trunk, despite increased activity, to accomplish tasks independently. Also physical muscle strength training might reduce fatigability [22]. There are several limitations to this study. The sample size was small when subcategorizing the DMD patients based on Brooke scale. Therefore, the power to detect differences in trunk ROM between these categories may have been too low. Furthermore, only patients with relatively good arm function could perform the more difficult tasks, which reduced statistical power. The control group was not completely matched with the DMD patients in terms of gender. However, there were no significant differences between boys and girls in the HC group. The normalized trunk muscle activity was based on standardized seated MVIC tasks, which probably does not correspond to the actual maximal values for trunk muscle activity. As a consequence, 100% muscle activity does not necessarily correspond to the maximum capacity, but is likely an overestimation. However, since the MVIC tasks were standardized across all participants, it showed that DMD patients used significantly more muscle activity compared to healthy subjects. Reaching distances were based on the distances that could be reached without moving the trunk. Consequently, the reaching distances varied between subjects and groups. In general, patients with weaker arm muscles reached towards shorter distances, however even though the distance was shorter they showed increased trunk movement compared to HC. Lastly, as described before [10], reaching distance and height were set based on subjects’ sitting posture. Small changes in posture could already influence the distance and height and cause variability between tasks within and between subjects. Since we were interested in self-selected movements of the trunk, we did not choose to standardize sitting posture.