DEVELOPMENT OF A PORTABLE SKIN SURFACE STIMULATOR FOR MUSCLE TRAINING – THE NEOPRAXIS ® EXOSTIM S.A. Simcox 1 , G.M. Davis 1 , A.B. Barriskill 2 , M.R. Duncan 2 , J.W.Middleton 3 , R.M. Smith 1 , W. Fisher 3 1 Faculty of Health Sciences, The University of Sydney, 2 Neopraxis Pty Ltd, 3 Northern Sydney Area Health Service. Rehabilitation Research Centre, The University of Sydney, Lidcombe, NSW 2141, Australia. This paper describes the design and development of a prototype multi-channel neuromuscular stimulator for muscle training and fitness. The Neopraxis ExoStim is a skin surface neuromuscular stimulator for training of lower-limb muscles in paraplegic individuals prior to implantation of a FES neuroprosthesis. The purposes of the ExoStim are to maximise muscle development by increasing strength and endurance, to perform basic lower limb functional movements and to familiarise patients with FES training. The initial design specifications include portability (~500g), battery powered output (0-200 mA across 1 kΩ load), 8 channels of biphasic stimulation (charge balanced, constant current), microprocessor control of all stimulation parameters (under “pulse-by-pulse” control), detailed error checking and compatibility with the existing architecture of the implanted neuroprosthesis system. In the preliminary testing and validation, the ExoStim met the design criteria and is now ready for use with human subjects. A modular outline of ExoStim’s components and how the device integrates into the implanted neuroprosthesis hardware and software is described in detail. An important feature of this unit is that basic controlled movements (eg. knee extension/flexion, sit-stand, and cycling) can be programmed into the controller transparently whether the FES stimulator is skin-surface or implanted. Keywords: functional electrical stimulation, paraplegia, muscle strength and endurance training INTRODUCTION Recently, there has been renewed interest in Functional Electrical Stimulation (FES) as a clinically useful tool to promote muscle development and enhance mobility in individuals with traumatic spinal cord injury (SCI). This renewed interest has been in part due to the continued refinement of biomedical technologies over the last 30 years, as well as increased research supporting clinical and functional outcomes for the SCI population. In particular, some recent studies have demonstrated that skin-surface FES training will increase strength and endurance of paralysed muscle (1), improve cardiorespiratory fitness (2) and enhance overall health and well-being (3). These benefits are especially important for individuals who would receive either transcutaneous (4) or surgically implanted (5) FES systems to optimise their functional mobility and augment their strength and fitness. This paper describes the development of a prototype skin-surface multi-channel FES system for enhancing muscle strength and endurance in SCI persons. Although laboratory and commercial FES systems are now quite common, it was our intent to develop a new FES system that would be easily integrated into an existing implanted neuroprosthesis system (the Praxis24 ® system). Thus, our purpose was to develop a neuromuscular stimulator with broad flexibility in terms of FES parameter selection (eg. frequency, pulse duration, duty cycle, etc), yet still relatively inexpensive and easy to use by the clinician.