Laura Peeters

10 | Chapter 1 Unconsciously we move our trunk very often during the day when performing seated activities, for instance while reaching forward, writing a note, eating, looking over our shoulder when driving a car, et cetera. Can you imagine how this will change if we would not able to move our trunk and what the consequences would be for moving our arms? This thesis focuses on trunk function and the interaction between trunk, upper extremity and head movements when performing seated daily tasks in patients with neuromuscular disorders. Before further introducing this topic, trunk anatomy and function in healthy people are discussed for a basic understanding. THE TRUNK AS CENTRAL SEGMENT The trunk (or torso) is the central segment of the body which connects the legs (“lower extremities (LE)”), arms (“upper extremities (UE)”) and the head. Although it is often captured as one segment, it is a complex, multisegmented part of the body. The skeleton of the trunk consists of the ribcage and the spine, which is caudally connected to the iliae through the sacroiliac joints. The spine consists typically of 33 vertebrae that are divided into different regions: the cervical spine (7 vertebrae), thoracic spine (12 vertebrae), lumbar spine (5 vertebrae), sacrum (5 fused vertebrae) and the coccyx (4 fused vertebrae) (Figure 1A). The large number of joints provide the spine with substantial flexibility, which implies that the trunk cannot be seen as one, rigid segment. The cervical spine is also called the neck and rotations of its vertebrae considerably contribute to overall head movement [1]. Deep back muscles (e.g. rotators, multifidus, interspinales, and intertransversarii) are assumed to be involved in stabilizing the spine, whereas the superficial muscles (e.g. erector spinae, oblique abdominal muscles, and rectus abdominis) are mainly responsible for generating the large moments of force needed to hold and move the trunk against gravitational forces (Figure 1B). The origin of several back muscles can be found at the sacrum and pelvis, which emphasizes the close connection between the trunk and the pelvis. Since the trunk is the central part of the body, it is indispensable when performing daily tasks. In a seated position, the trunk interacts with the UE as part of a kinematic chain and by providing a stable base. Involvement in the kinematic chain is most pronounced when reaching beyond arm length distance, as moving the trunk enlarges the workspace [2]. But trunk displacement is also observed when reaching within arm’s length, or when performing other daily tasks than reaching [3]. In addition, the trunk adapts its posture to maintain stability during voluntary UE movements. UE movement results in displacing the body center of mass, thereby initiating a perturbation to posture and body balance [4]. As a result, in terms of