Laura Peeters

108 | Chapter 5 future. Treatment with for example Spinraza is currently evaluated with the use of the Hammersmith Functional Motor Scale, but this does not discriminate between different body segments and does not give insight in the benefits for performing activities of daily living [6, 25]. This study has several limitations. First, while we covered a broad range of the clinical spectrum of SMA, it was statistically not possible to compare for example SMA type 2 or type 3 patients, or patients with or without spinal fusion surgery due to the small sample size. It would be interesting to investigate in more detail how differences between subtypes affect task performance. Secondly, the control group was not age matched with the patients with SMA. This might have had an effect on the maximum joint torque and maximum active trunk ROM, as muscle strength and joint flexibility decrease with ageing (starting around 50 years) [26, 27]. However, differences found between the HC and patients with SMA were very high and cannot be solely attributed to age. Furthermore, the reported ROM values during the maximum ROM tasks are active ranges based on unsupported seating and it should be noted that several participants reported that they were afraid of falling when moving further. Lastly, the percentages presented for normalized muscle activity are likely an overestimation, since standardized MVIC tasks were performed from a seated position which likely resulted in lower absolute maximum muscle activity signals. However, this position was chosen so patients could perform the MVIC tasks and because it corresponded with the position in which the movement tasks were performed. In conclusion, due to degeneration of motor neurons, patients with SMA need a greater percentage of their maximum muscle capacity to generate the same amount of force as HC. This study was the first to quantify the effects of this in performance of seated tasks. Maximum trunk joint torque and active trunk ROM were significantly reduced in patients with SMA. Further, increased normalized trunk muscle activity, without increased trunk ROM, was seen when performing daily tasks. Co-contraction of the trunk muscles is very likely present. This indicates that patients with SMA use more of their muscle capacity to maintain trunk stability compared to healthy controls. Clinicians should take trunk function into account when assessing function and interventions, as using a high percentage of the maximum muscle capacity may result in fatigue and muscle overloading. On the other hand, 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.