ELECTRODE ASSEMBLY FOR A MYOELECTRIC PROSTHESIS

Abstract

An electrode assembly for a myoelectric prosthesis allowing at least partial decoupling of the movement of the at least one electrode from the electrode housing case and from the prosthesis socket through the mounting of the at least one electrode using at least one biasing element. This allows the at least one electrode to be forced into contact with the patient in the target area using a moveable pressure application structure reducing/eliminating socket motion artifacts because the pressure is transferred toward the target area but the at least one electrode is not otherwise constrained to minimise or avoid the at least one electrode being dragged across the target area through the mounting to the prosthesis socket which causes socket motion artifacts.

Claims

1. An electrode assembly for a myoelectric prosthesis is provided, the electrode assembly comprising: a. An electrode housing case having at least one adjustable mounting portion to mount the electrode housing case relative to a prosthesis socket and thereby mount the electrode assembly relative to a target area on the residual limb; b. At least one electrode mounted relative to the electrode housing case by at least one biasing element to bias the at least one electrode into a home position relative to the electrode housing case and spaced from the target area; c. A pressure transfer unit to transfer pressure to the at least one electrode in a direction toward the target area; and d. A moveable pressure application structure operatively associated with the pressure transfer unit, the moveable pressure application structure mounted relative to the electrode housing case and relative to the pressure transfer unit to apply adjustable pressure to the at least one electrode toward the target area and moveable between at least one lowered position in which the at least one electrode is in contact with the target area and at least one raised condition in which the at least one electrode returns to the home position.

2. An electrode assembly as claimed in claim 1 wherein a shaped internal void is provided within the electrode housing case to contain but not prevent movement of the electrode within the electrode housing case.

3. An electrode assembly as claimed in claim 2 further comprising an electrode holder provided to hold the electrode relative to the electrode housing case and to abut the moveable pressure application structure when in the at least one lowered position.

4. An electrode assembly as claimed in claim 3 wherein the electrode holder is held relative to the electrode housing case using one or more resiliently deformable biasing bodies to allow movement of the electrode holder, and thereby the electrode, relative to the electrode housing case and socket so that the electrode is not dragged over the skin causing socket motion artefacts.

5. An electrode assembly as claimed in preceding claim 1 further comprising a pressure adjustment mechanism including an actuator mounted for user movement relative to the electrode housing case and a resiliently deformable adjustor mounted between the actuator and a portion of the moveable pressure application structure to adjust the pressure applied to the patient in the target area by the electrode.

6. An electrode assembly as claimed in claim 1 wherein the pressure transfer unit is provided at a lower end of the moveable pressure application structure, within the electrode housing case, the pressure transfer unit applying a force toward the patient, but otherwise allowing the electrode freedom of movement relative to the electrode housing case.

7. (canceled)

8. An electrode assembly as claimed in claim 2 wherein the pressure transfer unit is provided at an upper side of the electrode housing.

9. An electrode assembly as claimed in claim 1 wherein the pressure transfer unit is associated with an arcuate abutment portion.

10. An electrode assembly as claimed in claim 1 wherein the pressure transfer unit is associated with deformable unit to abut a portion of the movable pressure application structure.

11. An electrode assembly as claimed in claim 1 wherein the electrode housing case comprises at least one electrode mounting portion, provided adjacent to the opening through the electrode housing case and relative to which the at least one electrode mounted is mounted by the at least one biasing element to bias the at least one electrode into a home position.

12. An electrode assembly as claimed in claim 1 wherein the moveable pressure application structure comprises an abutment plate mounted relative to the electrode housing case using at least two arms.

13. (canceled)

14. (canceled)

15. (canceled)

16. An electrode assembly as claimed in claim 1 wherein the moveable pressure application structure is lockable in the up and/or down position.

17. An electrode assembly as claimed in claim 1 wherein the moveable pressure application structure is associated with at least one biasing assembly to bias the electrode into the electrode down position.

18. An electrode assembly as claimed in claim 17 wherein the at least one biasing assembly comprises at least one biasing element to provide a biasing force.

19. An electrode assembly as claimed in claim 18 wherein the at least one biasing element is selected to adjust the biasing force.

20. An electrode assembly as claimed in claim 17 when dependent from claim 15 wherein the at least one biasing assembly to bias the electrode into the electrode down position is associated with the crank arm.

21. An electrode assembly as claimed in claim 1 further comprising a threaded member provided relative to the electrode housing case to engage with the at least one mounting portion to releasably secure the electrode housing case in position once an optimum orientation is achieved.

22. An electrode assembly as claimed in claim 1 wherein the moveable pressure application structure is mounted relative to the electrode housing case, allowing movement of the pressure application structure toward and away from the patient and for it to be locked in the electrode up configuration.

23. An electrode assembly as claimed in claim 1 wherein the at least one biasing element is selected from the group including a spring, an elongate resilient member or elongate resilient loop.

24. A prosthesis comprising at least one electrode assembly according to claim 1.

25. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0106] In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

[0107] FIG. 1 is an isometric view of an adjustable electrode assembly according to a preferred embodiment, mounted relative to a representation of the prosthesis socket.

[0108] FIG. 2 is a sectional view of the adjustable electrode assembly illustrated in FIG. 1 taken along line A-A.

[0109] FIG. 3 is an isometric view of the adjustable electrode assembly illustrated in FIG. 1 in the electrode down mode (electrode pressed against the skin).

[0110] FIG. 4 is an isometric view of the adjustable electrode assembly illustrated in FIG. 1 in the electrode up configuration (electrode raised for donning and doffing).

[0111] FIG. 5 is an isometric, partially exploded view of the electrode housing case and electrode holder of adjustable electrode assembly illustrated in FIG. 1.

[0112] FIG. 6 is a partially transparent, isometric view of the electrode housing case and electrode holder illustrated in FIG. 5 in an assembled, use configuration.

[0113] FIG. 7 is a view from below the configuration illustrated in FIG. 6.

[0114] FIG. 8 is a graphical illustration of motion artefacts represented as an electromyographic signal in mV acquired via an electrode in an upper limb myoelectric prosthesis in an ‘ideal secured to skin’ condition.

[0115] FIG. 9 is a graphical illustration of motion artefacts represented as an electromyographic signal in mV acquired via an electrode in an upper limb myoelectric prosthesis in a ‘conventional housing’.

[0116] FIG. 10 is a graphical illustration of motion artefacts represented as an electromyographic signal in mV acquired via an electrode in an upper limb myoelectric prosthesis using an adjustable electrode assembly according to a preferred embodiment of the present invention.

[0117] FIG. 11A is a side view of an assembly according to an embodiment in the electrode up configuration.

[0118] FIG. 11B is a side view of the assembly shown in FIG. 11A in the electrode down mode.

[0119] FIG. 11C is an isometric view of the configuration shown in FIG. 11A.

[0120] FIG. 12A is a side view of an assembly according to an embodiment in the electrode up configuration.

[0121] FIG. 12B is a top view of the assembly shown in FIG. 12A.

[0122] FIG. 13A is a top view of an assembly according to an embodiment.

[0123] FIG. 13B is a side view of the assembly shown in FIG. 13A.

[0124] FIG. 13C is a detailed isometric view from one side of an end of the assembly shown in FIG. 13A.

[0125] FIG. 13D is a detailed isometric view from the opposite side to that shown in FIG. 13C.

[0126] FIG. 14A is a side view of an assembly according to an embodiment in the electrode up configuration.

[0127] FIG. 14B is a side view of the assembly shown in FIG. 14A in the electrode down mode.

[0128] FIG. 14C is a top view of the assembly shown in FIG. 14A.

[0129] FIG. 14D is an isometric view of the assembly shown in FIG. 14A.

[0130] FIG. 15A is a side view of an assembly according to an embodiment in the electrode up configuration.

[0131] FIG. 15B is a side view of the assembly shown in FIG. 15A in the electrode down mode.

[0132] FIG. 15C is a top view of the assembly shown in FIG. 15A.

[0133] FIG. 16A is a side view of an assembly according to an embodiment in the electrode up configuration.

[0134] FIG. 16B is a top view of the assembly shown in FIG. 16A.

[0135] FIG. 17A is a side view of an assembly according to an embodiment in the electrode up configuration.

[0136] FIG. 17B is an isometric view of the assembly shown in FIG. 17A.

[0137] FIG. 18A is a side view of an assembly according to an embodiment in the electrode up configuration.

[0138] FIG. 18B is an isometric view of the assembly shown in FIG. 18A.

[0139] FIG. 19A is a side view of an assembly according to an embodiment in the electrode up configuration.

[0140] FIG. 19B is an isometric view of the assembly shown in FIG. 19A.

[0141] FIG. 20A is a side view of an assembly according to an embodiment in the electrode up configuration.

[0142] FIG. 20B is an isometric view of the assembly shown in FIG. 20A.

[0143] FIG. 21A is a side view of an assembly according to an embodiment in the electrode up configuration.

[0144] FIG. 21B is an isometric view of the assembly shown in FIG. 21A.

[0145] FIG. 21C is a top view of the assembly shown in FIG. 21A.

[0146] FIG. 21D is an isometric view from an end of the assembly shown in FIG. 21A.

[0147] FIG. 22A is a side view of an assembly according to an embodiment in the electrode up configuration with the lock engaged.

[0148] FIG. 22B is a side view of the assembly shown in FIG. 22A in the electrode down mode with the lock disengaged.

[0149] FIG. 22C is a side view of the assembly shown in FIG. 22B with the cord handle moved to the electrode down position.

[0150] FIG. 22D is a side view of the assembly shown in FIG. 22A in the electrode down mode.

[0151] FIG. 22E is a top view of the assembly shown in FIG. 22E.

[0152] FIG. 22F is an isometric view of the configuration shown in FIG. 22A.

[0153] FIG. 23A is a top view of an embodiment showing the mounting studs in the centred position relative to the mounting portions.

[0154] FIG. 23B is a top view of the configuration illustrated in FIG. 23A with the mounting studs out of centre relative to the mounting portions.

[0155] FIG. 23C is a top view of the configuration illustrated in FIG. 23B with the electrode housing in a rotated position relative to the mounting portions.

[0156] FIG. 23D is a detailed isometric view of the engagement between the electrode housing and the mounting portions in the embodiment illustrated in FIG. 23A.

DETAILED DESCRIPTION

[0157] The adjustable electrode assembly 10 of the embodiments illustrated in FIGS. 1 to 7 comprises: [0158] a) An electrode housing case 11 having a pair of mounting portions 12 to mount the electrode housing case 11 and thereby the adjustable electrode assembly 10 relative to the prosthesis socket 13 and thereby over a target area on a patient's residual limb; [0159] b) An electrode 14 mounted relative to the electrode housing case 11 and in contact with the patient's body in the target area; and [0160] c) A moveable pressure application wheel 15 operatively associated with a pressure transfer unit 16, the moveable pressure application wheel mounted relative to the electrode housing case 11 and relative to the electrode 14 to apply adjustable pressure to the electrode 14 in a direction of contact with the patient in the target area and moveable between at least one lowered condition (electrode down mode) in which the pressure on the patient's body in the target area is increased and at least one raised condition (electrode up mode) in which the pressure on the patient's body in the target area is lessened or removed.

[0161] In the illustrated form, a pressure adjustment mechanism is provided to act on the lower part of the moveable pressure application wheel 15 including a threaded actuator 17 mounted for user movement relative to the electrode housing case 11 and a resiliently deformable adjustor spring 18 mounted between the actuator 17 and a portion of the moveable pressure application wheel 15 to adjust the pressure applied by the electrode 14 to the patient's body in the target area.

[0162] Typically, the pressure applied will be applied along the Z axis toward the surface of the patient's skin allowing minor movement of the electrode in the five remaining degrees of freedom.

[0163] As shown in FIG. 2 in particular, an electrode holder 19 is provided to hold the electrode 14 relative to the electrode housing case 11. As shown in FIGS. 5 to 7, the electrode holder 19 of the illustrated embodiment is held relative to the electrode housing case 11 using one or more resiliently deformable biasing bands 20 to allow relatively small movements of the electrode holder 19, and thereby the electrode 14, relative to the electrode housing case and socket so that the electrode is not dragged over the skin causing socket motion artefacts.

[0164] In use, a lower portion of the electrode 14 is in contact with the patient, typically the patient's skin in the target area, when the assembly 11 is in a lowered, use condition (illustrated in FIG. 3) to receive myoelectric signals produced by the muscles that remain within the residual limb in order to control the myoelectric prosthesis to which the assembly of the preferred embodiment is mounted.

[0165] In the raised position (illustrated in FIG. 4), the pressure on the electrode 14 is lessened, and in most cases removed entirely by separation of the pressure transfer unit 16, associated with the moveable pressure application wheel 15, from the electrode 14 and/or electrode holder 19.

[0166] As shown, the electrode housing case 11 is attached/mounted to the prosthesis socket 13 over a target area of the patient' body so that the electrode 14 can contact the skin or body of the patient in the target area, through a hole in the prosthetic socket, to receive signals from the target muscle fibres that control movement of the prosthesis.

[0167] The electrode housing case 11 illustrated is mounted relative to the prosthesis socket to allow for adjustment of the orientation of the electrode housing case 11 relative to the socket and thereby relative to the patient's residual limb. The electrode housing case is securable in position once the correct orientation is achieved.

[0168] In the illustrated embodiment, each mounting portion 12 includes an arcuate track opening 23 provided in a flange extending laterally from a lower end of the electrode housing case with a threaded fastener 24 that can clamp the electrode housing case 11 in position as it allows infinite adjustment over the length of the track opening 23. As shown, a mounting portion and mechanism is located on each of two opposed sides of the electrode housing case 11. This configuration allows rotation of the electrode housing case about a substantially perpendicular axis (with reference to the skin surface) without allowing lateral movement.

[0169] The electrode housing case of the illustrated embodiment is formed from two housing portions, a lower housing portion 21 to house the electrode holder 19 and electrode 14 and which includes the mounting portions 12, and an upper housing portion 22 to mount the pressure adjustment mechanism.

[0170] The lower housing portion 21 illustrated is generally rectangular in shape and is also larger than the upper housing portion 22 which is annular and elongate, taking the form of a tubular extension from an upper side of the lower housing portion 21.

[0171] An internal void 25 as illustrated best in FIG. 2 is provided within the lower housing portion 21. The void 25 is generally rectangular in cross-sectional shape because it contains the typically rectangular electrode and electrode holder, although of a slightly larger dimension than the electrode holder allowing some movement of the electrode holder 19 relative to the electrode housing case 11, thereby reducing or eliminating socket movement artefacts, but limiting the amount of the movement within the electrode housing case 11.

[0172] The upper housing portion 22 of the illustrated embodiment is integrally formed with the lower housing portion 21. As mentioned, the upper housing portion 22 of the illustrated embodiment is configured as a tubular portion with a bore 26 therethrough which extends from the top of the upper housing portion 22 to communicate with the internal void 25 in the lower housing portion 21.

[0173] The bore 26 is provided with an internally threaded portion as part of the pressure adjustment to engage the actuator which has a correspondingly externally threaded portion.

[0174] As shown in FIG. 2, a lower portion of the bore 26 also functions as a guide for the resiliently deformable adjustor spring 18 by containing the resiliently deformable adjustor spring laterally, allowing only extension and compression.

[0175] A laterally extending shoulder wall 27 is provided between the lower housing portion 21 and the smaller dimension, upper housing portion 22. The shoulder wall 27 is planar with a pair of openings 28 provided through the laterally extending shoulder wall 27 to allow each of a pair of elongate arms 29 of the moveable pressure application wheel 15 to pass through as explained further below.

[0176] A guide rebate 30 with a latching rebate 31 is provided into an outer surface on two opposed sides of the upper housing portion 21. The guide rebate 30 extends substantially vertically over the upper housing portion 21.

[0177] The latching rebate 31 is provided at an upper part of each guide rebate 30 extending circumferentially from the guide rebate 30 but in communication with the guide rebate 30 as shown in FIG. 3 at least such that rotation of the pressure application wheel 15 can latch the pressure application wheel 15 in the raised position and rotation in an opposite direction is required to release the pressure application wheel 15 from the latched position, freeing it for movement toward and away from the patient.

[0178] As mentioned above, the electrode holder 19 of the illustrated embodiment is mounted relative to the lower housing portion 21 of the electrode housing case 11 by one or more resiliently deformable bands 20 to allow small movements of the electrode holder 19 (and electrode 14) within the electrode housing case 11 (translation in the X and Y directions and rotation about the Z axis) but which bias the electrode holder 19 (and the electrode 14) into a home or reference position when the pressure is removed or lessened.

[0179] The bands 20 of the illustrated embodiment attach to the electrode holder 19 and the electrode housing case 11 extending through one or more guides 32 on the electrode holder and through one or more openings 33 provided in the electrode housing case 11.

[0180] The electrode holder 19 has a flat upper surface which abuts the pressure transfer unit 16 when the movable pressure application wheel 15 is in the lowered position.

[0181] The moveable pressure application wheel 15 is mounted relative to the electrode housing case 11, allowing rotation of the moveable pressure application wheel 15 as well as linear movement of the moveable pressure application wheel 15 toward and away from the patient.

[0182] In the illustrated preferred embodiment, the moveable pressure application wheel 15 is configured as an upper annular ring 34 located outside the upper housing portion 22 and a spaced apart lower annular receiver 35 to receive the pressure transfer unit 16, located within the void 25 in the lower housing portion 21, separated by a pair of elongate arms 29. In the illustrated embodiment, the lower annular receiver 35 and the pressure transfer unit 16 are provided as two components.

[0183] Each of the elongate arms 29 as shown is L-shaped, having a longer upper portion and a lower, shorter, perpendicular portion 36. In the preferred form, the longer upper portion extends from the upper annular ring 34 of the moveable pressure application wheel 15 and the lower, shorter, perpendicular portion 36 links a lower end of the longer arm to the lower annular receiver 35.

[0184] The inner side of the elongate arm 29 and/or upper annular ring 34 of the illustrated embodiment is provided with a latching tongue (obscured in Figures). The latching tongue extends parallel to the shoulder wall 27 between the upper housing portion 22 and the lower housing portion 21 of the housing case 11. The latching tongue is dimensioned (height) to be received in or at least partially within the latching rebate 31 and is dimensioned (length) to be received in or at least partially within the guide rebate 30 to guide movement of the moveable pressure application wheel 15 up and down and allow latching in the raised position via rotation of the moveable pressure application wheel 15 when the latching tongue is aligned with the latching rebate 31.

[0185] The pressure transfer unit 16 applies pressure on the electrode in the Z direction (towards the skin) whilst allowing a relatively small amount of translation in the X and/or Y directions and also rotation about one or more of the X axis and/or Y axis and/or Z axis. Accordingly, in the preferred embodiment, the pressure transfer unit 16 has an at least partially arcuate abutment portion 37 to abut the electrode holder 19.

[0186] The pressure transfer unit is preferably a ball transfer unit, in which a solid ball freely rotates within the pressure transfer unit assembly, provided at a lower end of the moveable pressure application wheel 15, preferably within the electrode housing case.

[0187] In the illustrated embodiment, the pressure transfer unit 16 is a separate unit to the moveable pressure application wheel 15 and is attached relative thereto.

[0188] The abutment portion 37 of embodiment shown is a solid ball that can freely rotate within the pressure transfer unit assembly (i.e. a ball transfer unit is used), but may take an alternative form such as for example a closed bag or containment portion containing a fluid such as a liquid or gel for example.

[0189] The actuator 17 of the pressure adjustment mechanism is a bolt or similar with a portion provided at least partially within the bore 26 in the upper housing portion 22.

[0190] The resiliently deformable adjustor spring 18 is interposed between the actuator 17 and an upper part of the lower annular receiver 35. The adjustor spring 18 is partially confined within the bore 26 of the upper housing portion 22 to cause compression of the adjustor to change in a linear direction toward and away from the patient when the actuator 17 is wound up and down to increase and decrease the pressure applied to the pressure transfer unit 16 respectively.

[0191] The electrode down mode is best illustrated in FIG. 3. After donning of the prosthesis, the pressure application wheel 15 is typically rotated out of the latching position and then moves downwardly, under the influence of the resilient adjuster, toward the patient's body to cause the electrode 14 to abut the skin of the patient in the target area. In electrode down mode, when the electrode is pressed against the skin by the pressure application structure, the pressure applied by the electrode 14 onto the patient can be adjusted using the pressure adjustment mechanism by winding the actuator 17 in the appropriate direction.

[0192] The electrode up configuration is best illustrated in FIG. 4. Before doffing of the prosthesis, the pressure application wheel 15 is lifted and rotated into the latching rebate 31. This will typically release the electrode 14 which can then move back into the reference position if it has moved away from that position during use. The pressure adjustment mechanism may be relaxed prior to lifting the pressure application wheel 15. In electrode up mode, the orientation of the housing case 11 can be adjusted as necessary using the mounting portions 12.

[0193] The adjustable electrode assembly of the present invention therefore provides a simple and adjustable mechanism that provides partial decoupling of prosthetic socket and electrode, to reduce or eliminate socket movement artefacts, and also provides for adjustment of the pressure applied to an electromyographic electrode and the orientation of the electrode relative to the socket, and thereby the residual limb, to further reduce motion artefacts, improve electrode contact and allow better control of a prosthesis as a result.

[0194] Example motion artefacts are illustrated in FIGS. 8 to 10 showing motion artefacts using three electrode conditions, namely an ‘ideal secured to skin’ condition (FIG. 8); a ‘conventional housing’ (FIG. 9); and using the adjustable housing of the preferred embodiment (FIG. 10). Of particular note is the reduction in motion artefacts when using the adjustable housing (in FIG. 10) versus those seen during use of the conventional housing (FIG. 9).

[0195] The effectiveness of the embodiments illustrated in FIGS. 1 to 7 relies, at least in part, on a biasing element being of low stiffness so that, in the electrode down mode, the electrode moves with the skin of the user, not with the socket. There is therefore, a trade-off between reliably returning the electrode to its home position in the electrode up mode and ensuring it moves with the skin in the electrode down mode.

[0196] Further, the total height of the housing illustrated in FIGS. 1 to 7 (circa 50 mm) would not easily fit under clothing. FIGS. 11A to 23D show an assembly which has a lower profile than that illustrated in FIGS. 1 to 7. In many of the configurations illustrated in FIGS. 11A to 23D, a pair of resilient biasing arrangements are provided, a home return resilient biasing arrangement to bias the electrode (or the electrode holder into the home position and a pressure application resilient biasing arrangement to bias the moveable pressure application structure into the down (and/or up) position. The first resilient biasing arrangement and the second resilient biasing arrangement are preferably independent of one another.

[0197] FIGS. 11A to 11C show a low-profile design with the ball-unit integrated with the electrode. In the low-profile design illustrated in these Figures, the ball-unit 110, which provides the electrode (housed in the electrode holder 111) with 5 degrees of freedom of movement, is integrated with the electrode (or the electrode holder). This helps to apply the force centrally to the electrode.

[0198] The pressure application plate 114 is mounted relative to the electrode housing case 112 via a 4-arm mechanism 113 that ensures that the pressure application plate 114 remains parallel to the base of the housing case 112. The pressure application plate 114 is pulled down by the looped elastic element 115 shown in the Figures, which has adjustable pre-tension, in this case through the lengthening or slackening of the looped elastic element 115 using the bobbin attached to the arm 113 via member 116 via the crank arm 117. This is one example only: the elastic element could be any arrangement of elastic material or a conventional spring.

[0199] This arrangement shown forms a bi-stable mechanism, which provides a simple way of locking in the electrode up position. FIGS. 11A and 11B show the electrode up and electrode down positions respectively, illustrating the bi-stable action.

[0200] The angle between the bi-stable crank arm 117 and side-arm 113 supporting the plate 114 is shown as 90 degrees. This can be varied to produce different plate down-force versus displacement characteristics.

[0201] A home position biasing system consisting of elastic cords 118 returns the electrode to its home position when the pressure plate 114 is lifted. Alternative biasing arrangements are described below.

[0202] FIGS. 12A and 12B show a low-profile design similar to that illustrated in FIG. 11A to 11C with a flexible bag 120 replacing the ball-unit 110 (the plate 114 has been removed in FIG. 12B to show the structures beneath). The flexible fluid-filled bag 120 that provides the electrode with 5 degrees of freedom of movement when the electrode is pressed against the skin (electrode down mode). If the flexible bag 120 is attached to both the electrode holder 111 and the plate 114, in the electrode up mode, the biasing cords 118 only needs to centre the electrode as the bag 120 lifts it off the skin.

[0203] The elastic cords 118 return the electrode to its home position when the pressure plate 114 is lifted, in a similar way to the embodiment illustrated in FIGS. 11A to 11C. Alternative biasing arrangements are described below.

[0204] FIGS. 13A to 13D show a low-profile design with the looped elastic element of FIGS. 11A to 12B replaced by a torsion spring 130. An adjustable torsion spring 130 is an example of an alternative way of spring loading the arms 113 (albeit indirectly) so that the plate 114 pushes the electrode against the skin. This configuration lacks the advantage of being bi-stable and, hence, a catch 131 is provided to hold the plate 114 in the up position, as illustrated in FIGS. 13C and 13D. Alternative biasing arrangements are described below.

[0205] Alternative biasing arrangements to those illustrated in FIGS. 11A to 13D could be used to return the electrode to its home position when in the electrode up mode. The biasing arrangements include one or more of using fixed elastic bias cords, a means of slackening the cord as the pressure application plate descends, routing the bias cords via the electrode case (as shown in FIGS. 14A to 14D) and/or the pressure application plate (as shown in FIGS. 13A to 13D), foam or similar (as shown in FIGS. 17A and 17B) to centralise the electrode as it is returned to its home position, sprung arms to tension stiff biasing cords and a means for the user to independently control the biasing cords.

[0206] FIGS. 14A to 14D illustrate the use of fixed vertical elastic bias cords 140 and lateral elastic bias cords 141. This arrangement of elastic biasing cords is similar to the configuration illustrated in FIGS. 11A to 11C but the ball-unit 110 remains in contact with the plate 114 when it is lifted off the skin. This configuration also has improved geometry for centralising the electrode in its home position.

[0207] The geometry of the elastic biasing cords could be chosen to ensure that the ball-unit 110 is not in contact with the plate 114 when it is lifted off the skin. In the electrode up mode, the ball may be either held against the plate or there may be a gap between the ball and plate.

[0208] It should also be noted that the elastic biasing cords could be replaced by conventional springs.

[0209] FIGS. 15A to 15C show a single loop of elastic bias cord 150 that slackens as the electrode is lowered onto the skin from the position in FIG. 15A to that shown in FIG. 15B. This arrangement uses a single loop of elastic bias cord 150, which slackens when the electrode is pressed downwardly against the skin. This happens because the segments of cord on the left (from plate to left most guide) shorten as the plate 114 moves down. This overcomes the trade-off between reliably returning the electrode to its home position in the electrode up mode and ensuring it moves with the skin in the electrode down mode mentioned above.

[0210] In the embodiment illustrated in FIGS. 16A and 16B (plate removed for clarity), the single loop of elastic bias cord 150 passes via the plate 114 to lift the electrode as the plate 114. This embodiment is similar to the arrangement of FIGS. 15A to 15C, except the path followed by the cord 150 also passes through guides on the moving plate 114. This means that, as the plate 114 is lifted, the electrode is also lifted. This may provide better biasing in both vertical and lateral directions and thereby reduce the sensitivity of the home location to socket orientation.

[0211] In the embodiment illustrated in FIGS. 17A and 17B, compliant foam 170 is used to centre the electrode as it is lifted off the skin. This configuration is similar to the arrangement illustrated in FIGS. 16A and 16B but, in the electrode up mode, a compliant foam 170 with tapered mating surfaces is used to centralise the electrode, thereby avoiding the need for lateral biasing cords. When in the electrode down mode, if the foam 170 contacts the electrode, this design may suffer from the trade-off problem mentioned above.

[0212] In FIGS. 18A to 21D, the main feature is the pair of sprung arms 180 that are pulled up by the elastic elements 181. These tension the stiff biasing cords 182 that return the electrode to its home position. When the plate 114 moves down to push the electrode against the skin, it also pushes the sprung arms down providing enough slack length in the stiff biasing cords to allow the electrode to move with the skin, not the socket.

[0213] FIG. 18A and FIG. 18B shows an arrangement with biasing arms 180 sprung using biasing cords 181 acting to tension stiff biasing cords 182 that hold the electrode in its home position.

[0214] FIG. 19A and FIG. 19B shows a similar arrangement with sprung biasing arms acting to tension stiff biasing cords that hold the electrode in its home position.

[0215] FIG. 20A and FIG. 20B shows bias arms with 4 high support bands return the electrode to its home position, and a compliant foam portion 170.

[0216] FIGS. 21A to 21D shows bias arms with corner cords high to low to return the electrode to its home position.

[0217] In the configuration illustrated in FIGS. 22A to 22F, the user operates the bias cords directly. A bar 220 means by which the user can independently control the biasing cords 221 is provided; tensioning them for electrode up mode and releasing them for electrode down mode. This is in addition to a means of lifting or lowering the electrode, here the bi-stable arrangement. Although two separate user controls are less preferred, this configuration could be used. One way to avoid two separate controls would be to allow the plate to remain in contact with the ball-unit (no bi-stable action); so that tensioning the bias cords lifts the electrode against the down-force of the pressure plate.

[0218] FIG. 22A shows the bi-stable lock engaged and the bias cord bar 220 in electrode up position. FIG. 22B shows the bi-stable lock disengaged, but the cord bar 220 still in electrode up position. In FIG. 22C, the cord bar 220 has been moved to electrode down position. FIG. 22D shows the electrode pushed against the skin.

[0219] FIGS. 23A to 23D is a detailed isometric view of the engagement between the electrode housing and the mounting portions in the embodiment illustrated in FIG. 23A.

[0220] In this embodiment, studs 230 protrude from the socket 231, passing through oversize holes 232 in clamping tabs 233 which allow lateral movement of the electrode housing body 112 from the centred position. These holes 232 are in the clamping tabs 233 which clamp down the main electrode housing case 112 when the knurled knob 235 is tightened down onto an intermediate washer 234. The mating surfaces between tabs 233 and electrode housing case 112 are slightly serrated as illustrated to ensure high friction, and allow the electrode housing case 112 to move in an arc prior to being clamped. Overall, this configuration gives 5 mm of X and Y translation (dependent on the size of the holes 232 in the tabs 233), and 20 degrees of Z rotation (depending on the extent of the mating surfaces).

[0221] The one or more embodiments are described above by way of example only. Many variations are possible without departing from the scope of protection afforded by the appended claims.