Phase Identification of Sitting Pivot Wheelchair Transfers

INTRODUCTION The ability to perform sitting pivot transfers (SPTs’) amongst people with Spinal Cord Injury (SCI) is a crucial factor in the maintenance of an independent and high functioning lifestyle [1]. This high reliance on the upper arms to lift and support the weight of the body throughout the transfer is partially responsible for the high prevalence of shoulder pain [2]. In order to obtain a better clinical understanding of the biomechanics of the upper limbs through the duration of a transfer it is commonly delineated into three distinct phases. The phases being: pre-lift phase (also known as the preparatory phase), lift phase (where the arms are predominantly weight bearing), and the descent/post lift phase. Gagnon et al[3] summarized the various kinematic and kinetic parameters used in delineation of transfer phase and found that no consensus existed as to an optimal technique for phase identification. This is in part due to the different experimental set ups that are currently being used to study SPTs. The purpose of this paper is twofold 1) Describe an identification technique which incorporates force information in combination with marker position data to delineate the different phases of a transfer and 2) Compare peak resultant shoulder forces between phases. METHODS Subjects: Eight male participants with paraplegia provided informed consent prior to participation in the study. The inclusion criteria were: SCI at C4 level or below that occurred over one year prior to the start of the study, able to independently transfer to/from a manual wheelchair without human assistance or assistive devices, over 18 years of age, and free from upper extremity pain that influenced their ability to transfer. Experimental Protocol: Participants used their personal wheelchairs (WCs) to transfer to and from a bench. For all transfers the wheelchair was positioned and secured at a comfortable angle to an adjustable height tub bench (Figure 1). The bench was adjusted to be level with the subject’s wheelchair seat. The platform shown in Figure 1 contains two force plates (Bertec Corporation, Columbus, OH), one beneath the wheelchair and one beneath the tub bench. The wheelchair and bench were secured to the platform. A steel beam attached to a 6-component load cell (Model MC5 from AMTI, Watertown, MA) was positioned to simulate a wheelchair armrest. A seven camera 3D motion capture system and Nexus 1.6.1 software (Vicon Peak, Lake Forest, CA) measures the position of reflective markers placed on the back, chest, and arms during the transfer. The cameras are positioned around the platform so as to minimize marker drop out. Kinematic and kinetic data were collected at 60 Hz and 360Hz respectively.