Laura Peeters

86 | Chapter 4 increases with age, and joint torque does not increase with body weight, function decreases [15]. Indeed, when we corrected trunk joint torque for body weight in DMD patients, we found a significant decrease with Brooke scale, implying that functional trunk strength does decrease with disease stage. Increased trunk lateral bending and/or flexion-extension was found in DMD patients in all tasks. It is remarkable that this was even found when reaching within arm length distance, since the reaching distance could be shorter for patients. DMD patients likely reduce shoulder and upper arm muscle activity using increased trunk lateral bending towards the non-dominant side to reduce shoulder flexion and abduction. By leaning towards the non-dominant side, the dominant shoulder and arm are automatically positioned higher so less shoulder muscle effort is needed to lift the arm for reaching. Opposite to what was initially expected, the increased ROM in trunk flexion-extension was mainly in extension direction. This could mean that the DMD participants lean backwards in order to keep balance, as is also seen in patients with spinal cord injuries [16], or that patients extend their spine from an initially more slumped posture. This also positions the shoulder higher to reduce shoulder muscle effort and allows for a greater ROM of the shoulder [17]. These compensatory trunk movements are likely crucial to accomplish a task when arm function is insufficient. Compensatory trunk movements are also seen in children with cerebral palsy when performing daily tasks and were related to decreased upper extremity function [18, 19]. Unexpectedly, we did not find a significantly larger increase in compensatory trunk movements with task difficulty (e.g. object weight) in DMD patients compared to HC. It could be that patients already use the most optimal strategy in the easiest tasks (e.g. reaching nearby without weight) and therefore further increasing trunk movements is not beneficial. Alternatively, trunk function could limit increasing the compensatory movements as muscle activity levels did approach the maximum values. However, the median change in trunk ROM was often twice as high in DMD patients compared to HC. It is therefore also possible that we did not find a significant increase due to lack of statistical power, also due to the fact that DMD patients with less good arm function could not perform the more difficult task. No significant differences were found in trunk ROM between patients with different scores on the Brooke scale, although it was expected that compensatory trunk ROM would increase with Brooke scale. This is likely caused by small numbers of subjects in all categories. Normalized muscle activity was significantly higher in patients with DMD compared to HC for all muscles and all tasks. Normalized muscle activity also increased until the task could not be performed. Despite possible overestimation due to non- maximum MVIC, we found that normalized back muscle activity was around 100% when the maximum arm muscle (e.g. deltoid and trapezius) activity was reached.