Medical Engineering & Physics 35 (2013) 1141–1149 Contents lists available at SciVerse ScienceDirect Medical Engineering & Physics jou rn al h om epage: www.elsevier.com/locate/medengphy Reconfiguration of the upper extremity relative to the pushrim affects load distribution during wheelchair propulsion Joseph M. Munaretto a,∗ , Jill L. McNitt-Gray a,b , Henryk Flashner c , Philip S. Requejo d a Department of Biomedical Engineering, University of Southern California, Los Angeles, CA, USA b Department of Biological Sciences, University of Southern California, USA c Department of Aerospace & Mechanical Engineering, University of Southern California, USA d Rehabilitation Engineering Program, Rancho Los Amigos National Rehabilitation Center, Downey, Los Angeles, CA, USA a r t i c l e i n f o Article history: Received 18 November 2011 Received in revised form 18 November 2012 Accepted 6 December 2012 Keywords: Biomechanics Model simulation Multijoint control Upper extremity Joint kinetics Net joint moment Net joint force Manual wheelchair propulsion a b s t r a c t Objective: Repetitive loading during manual wheelchair propulsion (WCP) is associated with overuse injury to the upper extremity (UE). The aim of this study was to determine how RF redirection and load distribution are affected by changes upper extremity kinematic modifications associated with modifi- cations in seat positions during a WCP task. The aim of this study was to determine how RF redirection and load distribution are affected by upper extremity kinematic changes associated with seat position adjustment during a WCP task. Design: Dynamic simulations using an experiment-based multi-link inverse dynamics model were used to generate solutions for redistributing UE mechanical load in different seating positions without decre- ments in WCP task performance. Methods: Experimental RF and kinematic data were collected for one subject propelling at a self-selected speed and used as input into the model. Shoulder/axle distance, wrist angular position, and RF direction were systematically modified to simulate how the mechanical demand imposed on the upper extremity (elbow and shoulder net joint moments (NJMs) and net joint forces) may vary. Results: Load distribution depended on UE orientation relative to the wheel. At peak force, lower shoul- der/axle distances and more anterior wrist positions on the pushrim allowed for more extended elbow positions and reduced total NJM load. Interpretation: Simulation results incorporating subject-specific data may provide mechanically based information to guide clinical interventions that aim to maintain WCP performance and redistribute load by modifying RF direction, seat configuration and hand/rim interaction. © 2012 IPEM. Published by Elsevier Ltd. All rights reserved. 1. Introduction Individuals with lower extremity or spinal cord injury rely on manual wheelchair propulsion (WCP) to maintain independence and community participation. Unlike more expensive powered wheelchairs, manual wheelchair use promotes upper extremity strength and cardiovascular conditioning as part of activities of daily life. Manual WCP entails the generation of a tangential force at the hand/rim interface during hand contact to accelerate the mass and regulate the momentum of the entire system (wheelchair user and chair) [1]. Chronic use of manual WCP exposes the upper extremity (UE) to repetitive loading from the reaction forces (RFs) at the hand/pushrim interface. Mechanical loading experienced by the UE during WCP has been associated with the high incidence ∗ Corresponding author at: University of Southern California, 3560 Watt Way, PED 107, Los Angeles, CA 90089-0652, USA. Tel.: +1 440 376 0761. E-mail address: jmunaretto@gmail.com (J.M. Munaretto). of acute and overuse shoulder injuries in manual wheelchair users [2–4]. Disability – secondary to the primary injury – contributes to a loss of autonomy and a decrement in quality of life. Experimental results indicate that there is more than one solution to satisfy the mechanical objective of this propulsion task [5–9]. The causal rela- tionship between manual WCP techniques and UE injury patterns remains unknown. Knowledge of the advantages and disadvantages of differ- ent WCP techniques can aid clinicians when making decisions regarding pre- and re-habilitation interventions that aim to main- tain the client’s functional performance while reducing injury risk. Simulation studies indicate that maintaining the tangen- tial component of the reaction force (RF) and altering the radial component provide multiple solutions for redistributing the mechanical loading across the upper extremity without a decrement in propulsion speed [9]. RF redirection combined with alteration in upper extremity segment positions and configu- rations is expected to provide additional solutions for UE load redistribution. 1350-4533/$ – see front matter © 2012 IPEM. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.medengphy.2012.12.002