An ´alisis y discusi ´on

A primary requirement of an ENG recording system is to reduce the residual EMG, removing it completely, if possible. The TT offers this possibility by adjusting the gains of the first stage amplifiers, possibly using an automatic feedback control system. This approach has the potential to eliminate the high-order filters conventionally used to reduce interference such as EMG and improve the signal to noise ratio [4]. The new arrangement (the screened tripole), while still allowing adjustment, as in the TT, also features a higher inherent signal-to-interference ratio. The ST arrangement therefore allows a feedback control system that rejects EMG artefacts to converge more rapidly than if employed with a TT. This is convenient since the rejection of the EM G must occur within the time window of adjacent stimulation pulses, typically 50ms [1] in FES applications.

The amplitude o f the ENG signal recorded from a ST will be less if the two recording (active) end electrodes are positioned away from the ends of the cuff. However, this is only critical when the cuff length is less than the wavelength of the action potentials determined by the largest fibres within the nerve bundle. Simulation results show that for a cuff length equal to the wavelength of the action potential, the single fibre action potential amplitude maximises when the outer electrodes in a tripolar configuration are near the ends of the cuff [10].

Connecting the end electrodes by a wire is useful even if the end electrodes are placed at the ends of the cuff. Although currently this connection has been implemented outside the cuff in the QT, future cuff developments will allow the wire connection to be included in the cuff walls so full advantage can be taken from the reduction in the field gradient. Also multiple electrodes, i.e., any tripolar electrode configuration can be positioned inside the active region, which can be defined as the region between the connected end electrodes.

It has been demonstrated that an increase in cuff length effectively decreases the gradient of the interfering field within the cuff, increasing the S/I ratio [11]. The use of a screen, as demonstrated in the ST, has the same effect as increasing the overall cuff length as shown in Fig. 4.4. The main reason for the significant improvements between short and long cuffs is that there is high resistance to current flow in long cuffs

compared to short cuffs. As a result a larger fraction of the interfering current flows outside the cuff in long cuffs compared to short cuffs. Also the ends of the cuff are further away for the interfering source in long cuffs resulting in smaller potential differences at the two ends.

In this paper a simple one-dimensional lumped impedance model was used as in [5]. However because of end effects and non-linearity in the field [11], in particular for small cuffs, this is only an approximation of the field inside the cuff. However the analysis and general conclusions are still valid as demonstrated by the inhomogeneous model.

4.7

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