Laura Peeters

Trunk involvement in performing upper extremity activities | 25 2 INTRODUCTION Control of upper body movement is essential when performing daily activities in a seated position. Trunk control is indispensable during seated activities, because it interacts with control of the upper extremities (UE) and the head by being part of a kinematic chain and by providing a stable base. In the kinematic chain of UE movement, trunk movement enlarges the workspace [1], but trunk displacement is also observed when reaching within arm length [2]. Voluntary UE movement will disturb posture, which is compensated for by postural reactions to maintain stability [3]. The trunk is involved in this postural chain when performing UE movements. Therefore, in terms of stability, trunk control greatly determines the precision of UE movement [4]. With regard to head movement, trunk movement enlarges the range of head motion in space. Lastly, trunk stability is essential for head balance as well as for accurate visual and vestibular control of posture and voluntary movements of (parts of) the body, such as the arm and hand [5, 6]. Trunk control is impaired in patients with a flaccid trunk, affecting their performance of daily activities. In addition, during their development, children with a flaccid trunk have a higher risk in developing scoliosis, which further complicates the interaction between the trunk and UE. A flaccid trunk is typically associated with (severe) muscle weakness due to primary muscle disease (e.g. Duchenne muscular dystrophy (DMD)) or motor neuron disease (e.g. spinal muscular atrophy (SMA)), but it may also be present in patients with central neurological disease with bilateral paresis [7, 8]. For instance, patients with ‘high’ spinal cord injury (SCI) (above thoracic level 6) may have spastic muscles below lesion level, particularly in their extremities, but often their trunk muscles lack normal (reticulospinal and vestibulospinal) control of postural tone mediated by the brainstem via the medially descending spinal tracts [9, 10]. As a result, these patients lack automatic trunk control which, in complete spinal cord lesions, cannot be compensated by the medial corticospinal descending neurons. Likewise, patients with severe (mostly bilateral) cerebral palsy (CP) may suffer from lack of postural tone as well as voluntary control of trunk muscles through lesions of their medially descending corticospinal and bulbospinal tracts [11]. When trunk control becomes impaired early in life, it may severely affect motor development in general and, through delayed and limited motor skills, even affect the cognitive and emotional development in children. Because many of the conditions mentioned above may become symptomatic during (early) childhood and because a substantial proportion of these children will not be able to walk once they have reached adulthood, studying the consequences of trunk impairments for the performance of seated UE activities is of utmost important. Undoubtedly, the interaction of the trunk with the UEs and the head will depend on the type and