Abstract:
Embodiments relate generally to devices, methods and stimulus units for use in measuring neuromuscular function. One particular embodiment relates to a device comprising a substrate, wherein the substrate has a base portion and at least one limb coupled to the base portion. The device further comprises at least two stimulation electrodes operably associated with the substrate for providing a stimulus to a body part. The device further comprises at least two sensing electrodes operably associated with the substrate for sensing an electrical potential in the body part. The at least two sensing electrodes are spaced from the at least two stimulation electrodes. The device further comprises an elongate member coupled to one of the base portion and the at least one limb and having indicia for indicating separation of the at least two stimulation electrodes and the at least two sensing electrodes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/672,853, filed Apr. 20, 2005 and U.S. Provisional Patent Application No. 60/774,646, filed Feb. 21, 2006, the entire contents of both of which are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     Embodiments relate to a device, method and stimulus unit for testing neuromuscular function. In particular, embodiments employ one or more stimulation and sensing electrodes disposed on a substrate for placement on a body part of a test subject.  
       BACKGROUND  
       [0003]     When performing nerve conduction testing, such as testing for carpal tunnel syndrome, for example, a series of stimuli are provided to a part of the body adjacent to the nerve desired to be tested and the response of the body to each stimulus is measured. Such responses may include a muscle response, in the form of a compound muscle action potential (CMAP), and a nerve response, in the form of a sensory nerve action potential (SNAP).  
         [0004]     A relevant parameter in determining whether a subject may be experiencing carpal tunnel syndrome, or other forms of systemic or entrapment neuropathies, is the nerve conduction velocity of the stimulated nerve. Nerve conduction velocity is determined by measuring the distance between the stimulation site and detection site on the stimulated body part and then observing the time elapsed between stimulus of the nerve and detection of the SNAP evoked in response to the stimulus.  
         [0005]     Typically, a medical technologist performing the nerve conduction testing will take a measuring tape and place it along the body part to estimate the distance between the stimulating electrode and the sensing electrode, once the electrodes are in place. Alternatively, the technologist may measure a fixed distance and then place the stimulating and sensing electrodes accordingly. Such manual measurement methods are prone to error and can be cumbersome, requiring the physician to locate the measuring tape and position it against the subjects body, while attempting to keep the patient still, in order to take the distance measurement.  
         [0006]     It is desired to address or ameliorate one or more of the shortcomings or disadvantages of existing nerve conduction testing techniques, equipment or arrangements, or to at least provide a useful alternative thereto.  
       SUMMARY  
       [0007]     Embodiments relate generally to devices, methods and stimulus units for use in measuring neuromuscular function. One particular embodiment relates to a device comprising a substrate, wherein the substrate has a base portion and at least one limb coupled to the base portion. The device further comprises at least two stimulation electrodes operably associated with the substrate for providing a stimulus to a body part. The device further comprises at least two sensing electrodes operably associated with the substrate for sensing an electrical potential in the body part. The at least two sensing electrodes are spaced from the at least two stimulation electrodes. The device further comprises an elongate member coupled to one of the base portion and the at least one limb and having indicia for indicating separation of the at least two stimulation electrodes and the at least two sensing electrodes.  
         [0008]     The indicia may be selected from the group consisting of: magnetic indicia; electrical indicia; optical indicia; and mechanical indicia. The substrate may be flexible to conform to a shape of the body part when positioned over the body part. The body part may be a hand and wrist area of an arm. Alternatively, the body part may be a leg and ankle area.  
         [0009]     The stimulation and sensing electrodes may each have a layer of conductive gel disposed on portions thereof. The stimulation and sensing electrodes may be covered by a protective material wherein, in use of the device, the protection material is removed before placement of the stimulation and sensing electrodes on the body part. The at least two sensing electrodes may be positioned distal to at least two stimulating electrodes along the body part. The at least two sensing electrodes may be positioned proximal to at least two stimulating electrodes along the body part.  
         [0010]     The at least two sensing electrodes may comprise a first electrode pair, wherein one of the sensing electrodes in the pair is an active electrode, and the other of the sensing electrodes in the pair is a reference electrode. The device may further comprises a scanner for reading the indicia on the distance measurement member. The substrate and/or the distance measurement member may comprise a unique identifier of the device. The unique identifier may be encoded on one of the distance measurement member and the substrate. The unique identifier may be machine-readable.  
         [0011]     The device may further comprise a coupling unit for electrically coupling a current stimulation controller to the at least two stimulating electrodes. The coupling unit may be supportingly connectable to the base portion of the substrate. The coupling unit may be connectable to the base portion by conductive connectors. The coupling unit may comprise a temperature sensor, which may comprise am infrared optical sensor.  
         [0012]     Another embodiment relates to a stimulus unit for use in measuring neuromuscular function, comprising: a substrate, the substrate having a base portion and at least one limb coupled to the base portion; at least two stimulation electrodes operably associated with the substrate for providing a stimulus to a body part; and at least two sensing electrodes operably associated with the substrate for sensing an electrical potential of the body part, wherein the at least two sensing electrodes are spaced from the at least two stimulation electrodes and wherein the at least one limb has at least one extensible section for permitting adjustment of the spacing between the at least two sensing electrodes and the at least two stimulation electrodes.  
         [0013]     Another embodiment relates to a device for use in measuring neuromuscular function, comprising: a substrate, the substrate having a base portion and at least one limb coupled to the base portion; at least two stimulation electrodes operably associated with the substrate for providing a stimulus to a body part; at least two sensing electrodes operably associated with the substrate for sensing an electrical potential in the body part in response to the stimulus, wherein the at least two sensing electrodes are spaced from the at least two stimulation electrodes; and distance measurement means fixed relative to the substrate for measuring separation of the at least two stimulation electrodes and the at least two sensing electrodes.  
         [0014]     Another embodiment relates to a stimulus unit for use in measuring neuromuscular function, comprising: a substrate, the substrate having a base portion and two limbs coupled to the base portion; at least two stimulation electrodes operably associated with the base portion for providing a stimulus to a body part; and at least one sensing electrode operably associated with each limb for sensing an electrical potential in the body part in response to the stimulus, wherein the at least two stimulation electrodes are spaced from the sensing electrodes.  
         [0015]     Another embodiment relates to a method of measuring a separation between at least one distal electrode and at least one proximal electrode, the at least one distal electrode having an elongate member fixed relative to the at least one distal electrode, indicia being formed on or in the elongate member, the method comprising: affixing the at least one proximal electrode to a proximal part of a body; affixing the at least one distal electrode to a distal part of the body; securing a scanner relative to the at least one proximal electrode; and drawing a free end of the elongate member past the scanner to expose the indicia to the scanner and thereby measure the separation between the at least one distal electrode and the at least one proximal electrode.  
         [0016]     Another embodiment relates to a method of measuring a separation between at least one distal electrode and at least one proximal electrode, the at least one distal electrode being coupled to the at least one proximal electrode using an extensible electromechanical distance measurement sensor, the method comprising: affixing the at least one proximal electrode to a proximal part of a body; extending the at least one distal electrode relative to the at least one proximal electrode by extending the electromechanical distance measurement sensor; and determining the separation based on an output of the extended electromechanical distance measurement. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     Embodiments are described below in further detail, by way of example only, with reference to the accompanying drawings, in which:  
         [0018]      FIG. 1  is a block diagram of a system for automatic nerve conduction testing, according to one embodiment;  
         [0019]      FIG. 2  is a diagram illustrating connection of a coupling unit to a stimulus unit when the stimulus unit is attached to a wrist and hand area on an arm;  
         [0020]      FIG. 3  is a perspective view of the coupling unit and stimulus unit of  FIG. 2 , shown connected and showing insertion of a distance measurement member into the coupling unit;  
         [0021]      FIG. 4  is a schematic representation of a stimulus unit according to one embodiment;  
         [0022]      FIG. 5  is a schematic representation of a stimulus unit according to another embodiment;  
         [0023]      FIG. 6  is a schematic representation of a stimulus unit according to another embodiment;  
         [0024]      FIG. 7  is a schematic representation of a stimulus unit according to another embodiment;  
         [0025]      FIG. 8A  is a plan view of a stimulus unit according to another embodiment;  
         [0026]      FIG. 8B  is a bottom view of the stimulus unit of  FIG. 8A ;  
         [0027]      FIG. 9A  is a plan view of a stimulus unit according to another embodiment;  
         [0028]      FIG. 9B  is a bottom view of the stimulus unit of  FIG. 9A ;  
         [0029]      FIG. 10A  is a plan view of a stimulus unit according to another embodiment;  
         [0030]      FIG. 10B  is a bottom view of the stimulus unit of  FIG. 10A ;  
         [0031]      FIG. 11  is an illustration of a distance measurement member according to one embodiment;  
         [0032]      FIG. 12  is an illustration of a distance measurement member according to another embodiment;  
         [0033]      FIG. 13  is an illustration of a distance measurement member according to another embodiment;  
         [0034]      FIG. 14  is a flow chart of a method of nerve conduction testing; and  
         [0035]      FIG. 15  is an example cross-section of a base portion of a stimulus unit. 
     
    
     DETAILED DESCRIPTION  
       [0036]     Embodiments of the invention can be used to apply an automatic nerve conduction test for systemic or entrapment neuropathies, such as Carpal Tunnel Syndrome. During the test, a series of impulse stimuli are applied to a subject&#39;s body part adjacent a nerve or nerve group. The responses to the stimuli are analyzed to detect the evoked action potentials (for example, CMAP for a motor nerve test and SNAP for a sensory nerve test), and to measure the onset latency and peak amplitude of the responses. Other measureable parameters of interest include peak latency, duration and the integrated area under the response curve between onset and the peak amplitude.  
         [0037]     Referring to  FIG. 1 , there is shown a system  100  for performing automatic nerve conduction testing. System  100  comprises a control module  110  that interfaces with a stimulus unit  130  via a stimulus and data acquisition module  120  to provide stimuli to a body part and detect evoked responses, such as CMAP and SNAP responses, to the stimuli. Other evoked responses that may be detected inculde F-wave, A-wave and H-reflex responses. Control module  110  may be in the form of a computer device, such as a laptop, desktop personal computer or a handheld computing device.  
         [0038]     Control module  110  comprises a processor  114  and memory  112 . Control module  110  has a user interface  116  associated therewith that communicates with processor  114  to enable a user to interface with system  100  during, before or after the testing. The memory  112  stores computer program instructions for execution by processor  114  during performance of the automatic nerve conduction testing. Memory  112  also stores a first-in-first-out stack of sampled response waveforms (traces) for analysis by processor  114 . Processor  114  controls stimulus and data acquisition module  120 , which in turn controls the output of stimulus unit  130 .  
         [0039]     Stimulus unit  130  has two or more stimulus electrodes (for example, S 1 , S 2 , S 3  and S 4  shown in  FIG. 4 ) for contacting the skin of the body adjacent a nerve that is desired to be tested and also has two or more sensing electrodes (for example, E 1 , E 2 , E 3  and E 4  shown in  FIG. 4 ) for sensing the action potentials, such as CMAP and SNAP, on the skin at body part locations spaced from the stimulus sites. According to one embodiment, the stimulus unit  130  may be used to stimulate more than one nerve grouping at the same time. For example, stimulus unit  130  may be used to stimulate the median and ulnar nerve groupings in the hand simultaneously and separately detect the responses to that stimulation. Alternatively, stimulus unit  130  may detect responses to stimulus of only a single nerve grouping. Examples of embodiments of stimulus unit  130  are shown in FIGS.  4  to  7 ,  8 A,  8 B,  9 A,  9 B,  10 A and  10 B.  
         [0040]     Stimulus and data acquisition module  120  has one or more controllers (not shown) for receiving and interpreting commands from processor  114 , for conditioning response signals received from stimulus unit  130  and providing such conditioned response signals to processor  114  for analysis according to the stored computer program instructions in memory  112 . Example commands received at stimulus and data acquisition module  120  from processor  114  include stimulus intensity setting commands and operational commands, such as start or stop commands. Additionally, if stimulus unit  130  is configured to provide (or cooperate with stimulus and data acquisition module  120  to provide) a temperature measurement or a measurement of the distance between the stimulation and detection points, such measurements may be provided by stimulus and data acquisition module  120  to processor  114  in response to an appropriate command received at stimulus and data acquisition module  120 .  
         [0041]     The task of processor  114  is to establish the neuromuscular function testing protocol to be administered via stimulus unit  130  and to analyze each stimulus-response waveform passed from the signal detection and processing framework (i.e. stimulus unit  130  and stimulus and data acquisition module  120 ).  
         [0042]     Referring also to  FIGS. 2 and 3 , stimulus unit  130  is shown in further detail, in use on a wrist and hand area of a person&#39;s arm. Stimulus unit  130  connects electrically with stimulus and data acquisition module  120  via a coupling unit  220 , which couples directly to stimulus unit  130  to provide a stimulus current and to receive the evoked action potentials in response.  
         [0043]     Coupling unit  220  forms part of stimulus and data acquisition module  120 . Coupling unit  220  may be a dumb unit, in that it does not contain a controller exercising specific control functions, in which case another part of an underside stimulus and data acquisition module  120  located away from coupling unit  220 , and in communication therewith via cable  225 , performs the stimulation control and signal processing functions. Alternatively, coupling unit  220  may include a controller for performing stimulus control and/or received signal processing functions.  
         [0044]     Coupling unit  220  couples to stimulus unit  130  by one or more connectors to position coupling unit  220  in a fixed location relative to stimulus unit  130 . The connectors shown in  FIG. 2  are snap connectors, with receiving parts  250  located on an underside of coupling unit  220  and projecting parts  252  located on an upper surface of stimulus unit  130 . These connecting parts may be formed of conductive material, such as a conductive metal, for enabling a current stimulus to be provided from coupling unit  220  to stimulus unit  130  via the one or more connectors. Example conductive metals include nickel-plated brass or stainless steel. Instead of snap connectors, other forms of conductive connector may be employed.  
         [0045]     In one embodiment, snap connector parts  250 ,  252  are not used for providing current stimulus signals, but are instead used to close a circuit (with a conductor extending between the two projecting parts  252 ) to provide an indication to stimulus and data acquisition module  120  that coupling unit  220  is connected to stimulus unit  130 . In a further alternative embodiment, one or more non-conductive connecting parts may be used to form a connector connecting coupling unit  220  to stimulus unit  130 .  
         [0046]     Stimulus unit  130  has an output connector  270  located on an end of a connector limb  272  for providing evoked response signals detected by the one or more sensing electrodes to stimulus and data acquisition module  120 , via coupling unit  220 . Output connector  270  is releasably received in a socket  222  formed in coupling unit  220 . Socket  222  has a structure formed for receipt of output connector  270  and for forming electrical connections with each of the conductors (which are, in turn, connected to the sensing electrodes) along connector limb  272 . Connector limb  272  resembles a flexible ribbon cable. If the current stimulus wave-forms are not provided to stimulus unit  130  by the physical connection of connecting parts  250 ,  252 , then they may be provided by conductors connected to the stimulating electrodes via connector  270 .  
         [0047]     Stimulus unit  130  has a base portion  230 , with at least one limb  232  extending therefrom, in addition to connector limb  272 . Limb  232  has at least one sensing electrode positioned on the limb  232  for placement at any desired site for detection of CMAP or SNAP (or both) responses, depending on the type of testing that is to be conducted. One or more stimulus electrodes, together with a ground electrode (GND), are located in or adjacent base portion  230 . Limb  232  extends distally of wrist crease  212  and crosses at least part of the palm  214 . As shown in  FIG. 3 , limb  232  has two sensing electrodes  234 ,  236  located toward a distal end of limb  232 . Optionally, a third sensing electrode  238  may be located more proximally on limb  232 , intermediate base portion  230  and distal sensing electrodes  234 ,  236 , for sensing a CMAP response from the hypothenar area.  
         [0048]     Stimulus unit  130  is formed mostly of flexible materials for placement on anatomical structures and for generally conforming to the shape of such anatomical structures. For example, base portion  230  is intended to be positioned proximally of a wrist crease  212  so is to extend at least partially along and around part of a forearm  210 . Certain parts of stimulus unit  130  (for example, those around the electrodes) have an adhesive substance, such as a foam adhesive layer, on a underside thereof, for affixing the stimulus unit to the relevant anatomical structures prior to testing. Flexible circuitry extends through stimulus unit  130  between the electrodes and connectors. Thus, stimulus unit  130  can be used with anatomical structures of varying shapes and sizes due to its flexibility and ability to conform and adhere to anatomical structures, as required.  
         [0049]     Stimulus unit  130  employs a substrate of a flexible material, such as a medical grade polyester film (or other materials having similar properties). The substrate may be about 3 to 8 thousandths of an inch thick, for example. Where adhesive is required to affix a part of the stimulus unit  130  to an anatomical structure, this adhesive may be provided on a layer of medical grade adhesive foam of about 1/32 of an inch thickness. The foam is adhered to an insulation layer on the substrate on one side with a relatively strong adhesive and has an adhesive of relatively less strength for removable attachment to the test subject. The electrodes may comprise a silver or silver chloride layer formed on the substrate. The substrate also has flexible circuit tracings formed thereon for constituting the conductors between electrodes and the input and/or output connector. Such circuit tracings may comprise silver and a dielectric layer. An example of the layers of stimulus unit  130  is shown and described in further detail in relation to  FIG. 15 .  
         [0050]     Prior to affixation to the body part, stimulus unit  130  may have backing sheets on those part of stimulus unit  130  that have an adhesive substance on their undersides for adhesion to the skin. Each such backing sheet is removed immediately prior to adhesion of the relevant part of stimulus unit  130  to the corresponding anatomical structures. For the sensing, stimulus and ground electrodes, an area of conductive gel, such as hydrogel, is interposed between the electrode and the skin surface (instead of the adhesive foam), for facilitating conductivity of electrical signals between the electrodes and the skin.  
         [0051]     Stimulus unit  130  is a generally flat device, as viewed from the user&#39;s perspective, prior to affixation to the test subject. However, stimulus unit  130  does have several layers, as described above. In use of stimulus unit  130 , and with the backing sheets removed, the adhesive foam parts and electrodes are positioned to lie against the skin. These skin contact surfaces may be conveniently referred to as being formed on the underside of the stimulus unit  130 . Printed labeling, including affixation instructions, may be provided on the side of stimulus unit  130  that does not contact the skin.  
         [0052]     Coupling unit  220  has a temperature sensor  260 , such as an infrared temperature sensor, positioned on a lower surface of coupling unit  220  that is to be positioned to face the body part when coupled to stimulus unit  130 . Temperature sensor  260  is used to detect the temperature of the skin prior to and/or during the testing. If temperature sensor  260  is used to take a temperature measurement prior to initiation of the testing, it can be placed over the palmar region or other anatomical structure, as appropriate, prior to connection of coupling unit  220  to stimulus unit  130 . Alternatively, the temperature measurement may be obtained after connection of coupling unit  220  to stimulus unit  130 , provided that stimulus unit  130  has an appropriate opening  262  to allow temperature sensor  260  to directly sense the skin temperature.  
         [0053]     Coupling unit  220  also has a slot  240  formed in a housing of coupling unit  220  for receiving a distance measurement strip  280 . Slot  240  extends all the way through coupling unit  220  so that the distance measurement strip  280  can be drawn though slot  240  in order to perform the distance measurement function, as described herein. In the embodiment shown in  FIG. 3 , scanners  290 , such as optical scanners, are used to scan indicia located on distance measurement strip  280  between a free end  284  and a fixed end  282 , which is attached to limb  232  in the vicinity of a sensing electrode.  
         [0054]     Fixed end  282  may be attached to limb  232  by an adhesive or a mechanical connection, for example. Fixed end  282  may be attached to limb  232  in such a way that allows the distance measurement strip to be manually torn off or otherwise removed once it has been used.  
         [0055]     Example distance measurement strips having different forms of indicia are shown in FIGS.  11  to  13 . For the embodiment shown in  FIG. 3 , the indicia on distance measurement strip  280  are optically readable indicia that can be read by scanners  290  as the distance measurement strip  280  and the indicia thereon passes by the scanners  290  when distance measurement strip  280  is drawn through slot  240  in coupling unit  220 .  
         [0056]     Scanners  290  are located within the housing of coupling unit  220  and are positioned to sense indicia on the distance measurement strip  280  and to provide output signals to stimulus and data acquisition modules  120  via cable  225 . The electrical signals corresponding to the scanned optical indicia are processed within stimulus and data acquisition module  120  to determine the distance between the stimulus electrodes, which are in a fixed position relative to optical scanners  290 , and sensing electrodes located on a distal extremity of the body part, such as a finger, the size and length of which will depend on the physical characteristics of the test subject.  
         [0057]     The distance measurement calculation is performed by stimulus and data acquisition module  120 , taking into account the point along distance measurement strip  280  at which scanners  290  are positioned when distance measurement strip  280  comes to rest, the known distance between scanners  290  and the stimulating electrodes when coupling unit  220  is connected to stimulus unit  130  and the known distance between the point at which fixed end  282  is connected to limb  232  and the sensing electrodes  234 ,  236  located on limb  232 .  
         [0058]     Depending on the type and/or configuration of the indicia on distance measurements strip  280 , only one scanner  290  may be necessary. For example, if the indicia comprise gray scale indications, such as is shown in  FIG. 12 , only one optical scanner is required. However if the indicia comprised offset quadrature indicia, such as is shown in  FIG. 11 , two scanners are required to be able to determine the distance based on such indicia. Alternatively, the pair of quadrature scanners  290  may be offset and the indicia aligned with no offset.  
         [0059]     In alternative embodiments, indicia other than optically readable indicia may be formed in, positioned on or otherwise fixed in relation to distance measurement strip  280  for enabling determination of the distance between the sensing electrodes and stimulation electrodes. Mechanical markings or formations may be applied to distance measurement strip  280 , for example, in the form of crenulations along one edge or deformations in part of the strip. Alternatively, electrical or magnetic indicia may be formed in, or in relation to, distance measurement strip  280  for sensing by corresponding sensors in coupling unit  220 . Whether the indicia is optical, mechanical, electrical, magnetic, a combination of two or more of these or any other machine-readable form, the indicia are, at least according to such embodiments, using an appropriate sensing means positioned within coupling unit  220  for generating electrical signals for transmission to a signal processor within stimulus and data acquisition module  120  via cable  225 .  
         [0060]     In other alternative embodiments, the distance measurement strip  280  may be provided with human readable indicia for alignment with a fixed visible alignment marker on coupling unit  220  or a part of base portion  230 , so that a person may readily determine from the human readable indicia and the alignment marker the distance between the sensor electrodes and the stimulus electrodes. Alternatively, instead of distance measurement strip  280  being fixed at a location near the sensor electrodes and having its free end extend across base portion  230 , distance measurement strip  280  may be fixed at a location on or adjacent base portion  230  and extending toward the sensing electrodes for alignment of human readable indicia on the strip with an alignment marker positioned at a particular location on limb  232  adjacent to the sensing electrodes. For such embodiments using human readable indicia, the distance measurement determined with reference to the alignment marker would need to be input into control module  110  via user interface  116 .  
         [0061]     In a further alternative embodiment using human readable indicia, coupling unit  220  may be provided with an extensible measuring strip that retractably extends from coupling unit  220  for visual comparison with an alignment marker positioned adjacent one or more of the SNAP sensing electrodes  234 ,  236 . In an alternative of such an embodiment, the retractable strip may use machine-readable indicia to determine the distance according to indicia that can be read from the strip by a scanner within coupling unit  220  when a free end of the retractable strip is positioned at the alignment marker.  
         [0062]     Particular embodiments of further optical distance measurement methods may include use of stereoscopic optical sensors, triangulation of a marker light (where the marker is attached at or adjacent the sensing electrodes and the optical sensor is located in the coupling unit  220 ) and optical pattern recognition techniques. In a further embodiment, an acoustic time-of-flight calculation may be performed in relation to a marker source attached at or adjacent the sensing electrodes, with the acoustic sensor located in the coupling unit  220 . Embodiments employing electrical distance measurement may include sensing a deformation of a wire loop having a modified self-inductance depending on its position along the distance measurement strip or along an extensible section in limb  232 .  
         [0063]     Electromechanical embodiments may use transducers, such as strain gauges, potentiometers or linear variable differential transformers (LVDT). Such embodiments may use structure embedded within distance measurement strip  280  or an extensible section in limb  232  and corresponding sensing structure and circuitry within coupling unit  220 . Specific mechanical distance measurement embodiments may employ a form of tape measure built into coupling unit  220 , with sensors to determine the position or rotation of the tape wheel within coupling unit  220 , and or human readable indicia visible on the tape as it is extended from the coupling unit  220 .  
         [0064]     In certain embodiments, stimulus unit  130  may be employed with only a simple mating connector to connect to connector  270  in place of coupling unit  220 . For such an embodiment, as there is no necessity to connect coupling unit  220  to stimulus unit  130 , connector projections  252  are not required. Also, without a temperature sensor  260 , opening  262  in stimulus unit  130  is not required.  
         [0065]     The embodiment of stimulus unit  130  shown in  FIGS. 2 and 3  has a base portion  230 , a connector limb  272  and a distally extending limb  232  connected to, and extending away from, the base portion  230 . Connector limb  272  is connected to, and extends away from, a proximal part of base portion  230 . The base portion  230  is used to position the stimulation electrodes adjacent the nerve bundle desired to be stimulated during the testing, while the limb  232  extends distally to position the sensing electrodes in the desired locations for sensing SNAP and/or CMAP evoked responses. The connector limb  272  is used to couple to the stimulus and data acquisition module  120  and provide output signals corresponding to the electrical signals coupled to the conductors exposed by connector  270 .  
         [0066]     The base portion  230 , distally extending limb  232  and connector limb  272  or a basic configuration of the stimulus unit  130 . Variations of such a basic configuration form further embodiments, as described below. For example, stimulus unit  130  may have more than one distally projecting limb  232 . Further, connector limb  272  may extend from a different part of the base portion  230 , depending on whether the stimulus unit is for right hand or left hand testing, for example. While the precise shape and configuration of base portion  230  may vary, the features and functions of base portion  230  according to the basic configuration described above are common to all embodiments.  
         [0067]     Referring also now to  FIG. 4 , one particular embodiment of stimulus unit  130  is shown schematically, as located on a person&#39;s right hand for performing median and ulnar nerve conduction testing,  
         [0068]     Base portion  230 , as shown in  FIG. 4 , has two stimulation electrode pairs S 1 , S 2  and S 3 , S 4  formed in the substrate. The first stimulation electrode pair S 1 , S 2  is to be positioned over the median nerve running centrally through the wrist, while the second electrode pair S 3 , S 4  is to be positioned over the ulnar nerve. In the examples shown in  FIG. 4 , a distal edge of base portion  230  is approximately aligned with the wrist crease  212  and the base portion  230  is fixed in position by adhesion with the skin. In this position, a ground electrode GND is positioned distally of the stimulation electrodes but proximally of the sensing electrodes and generally toward a distal edge or area of base portion  230 .  
         [0069]     Stimulus unit  130 , as shown in  FIG. 4 , has a first limb  232  extending distally from base portion  230  for attachment to the fourth digit (ring finger) on the right hand. Fixed end  282  of distance measurement strip  280  is affixed to limb  232  adjacent, but proximal of, sensing electrode  234 . Free end  284  of distance measurement strip  280  extends proximally from fixed end  282  for passing through slot  240 , when coupling unit  220  is connected to the stimulus unit  130 .  
         [0070]     Stimulus unit  130 , as shown in  FIG. 4 , has a second limb  432  connected to, and extending distally from, base portion  230 . Second limb  432  has first and second sensing electrodes E 1 , E 2  formed in respective first and second attachment portions  434 ,  436  having adhesive on an underside thereof for holding the sensing electrodes E 1 , E 2  on to the skin at desired locations. Sensing electrode E 1  is positioned approximately over the middle of the thenar area, while sensing electrode E 2  is wrapped around a distal joint of the thumb.  
         [0071]     The first and second limbs  232 ,  432  each have a respective extensible portion  412 ,  414  for accommodating size differences among hands by allowing lesser of greater extension of the extensible portions  412 ,  414 , depending on hand size. Extensible portions  412 ,  414  may be simply formed of a somewhat flattened coil or loop in the respective limb.  
         [0072]     The stimulus unit  130  shown in  FIG. 4  has a connector  270  with a plurality of connecting conductors  274  located at an end of connector limb  272 . Connecting conductors  274  communicate with conductors formed in the substrate of stimulus unit  130  and extending through the limbs  232 ,  432  and base portion  230 . Connecting conductors  274  connect with corresponding conductors in socket  222  of coupling unit  220 .  
         [0073]     Referring now to  FIG. 5 , there is shown a further embodiment of a stimulus unit, designated by reference numeral  500 . Stimulus unit  500  is intended for use in nerve conduction testing of the sural nerve in a human leg. Stimulus unit  500  has a base portion  530  for location over the sural nerve on a lower part of a right leg, as shown in  FIG. 5 .  
         [0074]     Connected to base portion  530  is a connector limb  572  having a connector  570  on an end thereof and connector conductors  574  exposed within connector  570 . Connector  570  is received in a socket  222  of coupling unit  220 . Similar to base portion  230 , base portion  530  has snap projections  552  for connecting to corresponding recesses in coupling unit  220 .  
         [0075]     Base portion  530  has a reference stimulation electrode S 2  formed in the substrate and an array  516  of active stimulation electrodes (S 1   a , S 1   b , S 1   c , S 1   d , S 1   e ) formed distally of S 2  in the substrate. The array  516  is used to selectively provide stimuli to different locations within an area covered by the array  516 .  
         [0076]     The substrate of stimulus unit  500  further comprises a distally extending limb  504  connected to, and integrally formed with, base portion  530 . Limb  504  has an extensible portion  514  formed therein for allowing adjustment of the distance between the sensing and stimulus electrodes to account for different leg sizes. A distal end portion  540  is formed at a distal end of limb  504  and comprises sensing electrodes E 1 , E 2 . A ground electrode GND is also formed in limb  504 , intermediate distal end portion  540  and the extensible portion  514 .  
         [0077]     Distal end portion  540  has attachment portions  536 ,  538  for securing electrodes E 1 , E 2  to the skin of the ankle just below, and on either side of, the lateral malleolus  512 . Ground electrode GND is attached to the skin using an adhesive attachment portion  534 .  
         [0078]     Distance measurement strip  280  is connected at fixed end  282  to a part of distal end portion  540  adjacent attachment portion  538 . Distance measurement strip  280  extends proximally toward base portion  530  so that free end  284  can be passed through slot  240  of coupling unit  220  for measurement of the distance between the sensing electrodes E 1 , E 2  and the stimulation electrodes S 2 , S 1   a  to S 1   e.    
         [0079]     As shown in  FIG. 5 , opening  562  in base portion  530  is located between projecting connector parts  552 . In such a configuration, the coupling unit  220  has a temperature sensor  260  positioned in between recessed connecting parts  250  to correspond with the configuration of base portion  530 . Such a modified coupling unit  220  may also be used with the stimulus unit shown  FIG. 4 , with opening  262  being positioned in between projecting connector parts  252 .  
         [0080]     It should be noted that stimulus unit  500  is one specific embodiment of the more general embodiment of stimulus unit  130  described above. Thus, while stimulus unit  500  is of a different shape and configuration to that shown in  FIG. 3 , for example, it is formed in a similar manner, using similar materials and is used in a similar way.  
         [0081]      FIG. 6  shows a further embodiment of a stimulus unit, designated by reference numeral  600 . Stimulus unit  600  is formed of similar materials and operates in a similar way to the stimulus unit embodiments shown in FIGS.  2  to  5 , except that it has a different electrode configuration and a modified extensible portion  614 .  
         [0082]     Stimulus unit  600  has a first limb  632  and a second limb  622 , both of which extend distally from a distal edge or part of base portion  630 . First limb  632  has a first sensing electrode E 1  formed in a part of the substrate that is positioned to generally overlie a thenar muscle. Electrode E 1  is held on to the thenar area by an adhesive-backed attachment portion  634 .  
         [0083]     Extensible portion  614  is formed distally of attachment portion  634  in limb  632 . Extensible portion  614 , as shown in  FIG. 6 , is formed of a plurality of loops formed in the plane of the substrate in a snaking, s-shape. Distally of extensible portion  614 , first limb  632  branches into a first branch  641  and a second branch  645 . First branch  641  has sensing electrodes E 5 , E 6  formed in attachment portions  642 ,  644 , which attach the electrodes E 5 , E 6  to appropriate locations on the fifth digit (little finger). The second branch  645  comprises sensing electrodes E 3 , E 4  located in attachment portions  646 ,  648  for attaching the electrodes E 3 , E 4  to appropriate locations on the fourth digit (ring finger). Sensing electrode pair E 5 , E 6  can be used to sense evoked SNAP responses resulting from stimulation of the ulnar nerve by stimulation electrodes S 3 , S 4  positioned over the ulnar nerve.  
         [0084]     Sensing electrode pair E 3 , E 4  can be used to sense evoked SNAP responses for both ulnar and median nerves, in response to stimulus from the ulnar stimulus pair S 3 , S 4  or median stimulus pair S 1 , S 2 . Sensing electrode E 1  is used to detect CMAP responses to stimulus from the median stimulating electrode pair S 1 , S 2 .  
         [0085]     Second limb  622  has a sensing electrode E 2  positioned toward a distal end of limb  622  and attached to a hypothenar area of the hand by adhesive attachment portion  624 . Electrode E 2  is positioned to sense evoked CMAP responses resulting from stimulus of the ulnar nerve by stimulation electrode pair S 3 , S 4 .  
         [0086]     Stimulus electrodes S 1  to S 4 , together with a ground electrode GND are formed in the substrate in base portion  630 . The connecting projection parts  652  are also formed in base portion  630  for connecting to coupling unit  220 , either as a purely mechanical connection or as electrically conductive connectors for supplying stimulus to the stimulus electrodes S 1  to S 4 .  
         [0087]     A connector limb  672  extends laterally from base portion  630  and has a connector (not shown) on an end thereof for connecting to socket  222  of coupling unit  220  to provide the detected evoked signals back to the stimulus and data acquisition module  120 .  
         [0088]     As shown in  FIG. 6 , a distance measurement strip  680  is connected to first limb  632 , at a connection portion  640  thereof. Fixed end  682  is connected to connection portion  640 , while free end  684  of distance measurement strip  680 , extends proximally toward base portion  630 , for insertion into slot  640  of coupling unit  220 , when coupling unit  220  is attached to base portion  630 . As with other embodiments employing a distance measurement strip, distance measurement strip  680  has some form of indicia formed on or in the strip along at least part of its length for reading by a scanner  290  in coupling unit  220  or for comparison with an alignment marker on base portion  630 .  
         [0089]     Referring now to  FIG. 7 , a schematic representation of a further embodiment of a stimulus unit is shown, designated by reference numeral  700 . Stimulus unit  700  is identical to stimulus unit  600 , except that the first limb  732  of stimulus unit  700  has three branches, rather than two. Stimulus unit  700  is formed of similar materials and operates in a similar way to the stimulus unit embodiments shown in FIGS.  2  to  5 , except that it has a different electrode configuration and a modified extensible portion  714 .  
         [0090]     Stimulus unit  700  has a first limb  732  and a second limb  722 , both of which extend distally from a distal edge or part of base portion  730 . First limb  732  has a first sensing electrode E 1  formed in a part of the substrate that is positioned to generally overlie a thenar muscle. Electrode E 1  is held on to the thenar area by an adhesive-backed attachment portion  734 .  
         [0091]     Extensible portion  714  is formed distally of attachment portion  734  in limb  732 . Extensible portion  714 , as shown in  FIG. 7 , is formed of a plurality of loops formed in the plane of the substrate in a snaking, s-shape. Distally of extensible portion  714 , first limb  732  branches into a first branch  741  and a second branch  745 . First branch  741  has sensing electrodes E 5 , E 6  formed in attachment portions  742 ,  744 , which attach the electrodes E 5 , E 6  to appropriate locations on the fifth digit (little finger). The second branch  745  comprises sensing electrodes E 3 , E 4  located in attachment portions  646 ,  648  for attaching the electrodes E 3 , E 4  to appropriate locations on the fourth digit (ring finger). Sensing electrode pair E 5 , E 6  can be used to sense evoked SNAP responses resulting from stimulation of the ulnar nerve by stimulation electrodes S 3 , S 4  positioned over the ulnar nerve. The third branch  755  has sensing electrodes E 7 , E 8  located in attachment portions  756 ,  758  for positioning electrodes E 7 , E 8  at appropriate locations around the third digit (middle finger).  
         [0092]     Sensing electrode pair E 3 , E 4  can be used to sense evoked SNAP responses for both ulnar and median nerves, in response to stimulus from the ulnar stimulus pair S 3 , S 4  or median stimulus pair S 1 , S 2 . Sensing electrode E 1  is used to detect CMAP responses to stimulus from the median stimulating electrode pair S 1 , S 2 .  
         [0093]     Second limb  722  has a sensing electrode E 2  positioned toward a distal end of limb  722  and attached to a hypothenar area of the hand by adhesive attachment portion  724 . Electrode E 2  is positioned to sense evoked CMAP responses resulting from stimulus of the ulnar nerve by stimulation electrode pair S 3 , S 4 .  
         [0094]     Stimulus electrodes S 1  to S 4 , together with a ground electrode GND are formed in the substrate in base portion  730 . The connecting projection parts  752  are also formed in base portion  730  for connecting to coupling unit  220 , either as a purely mechanical connection or as electrically conductive connectors for supplying stimulus to the stimulus electrodes S 1  to S 4 .  
         [0095]     A connector limb  772  extends laterally from base portion  730  and has a connector (not shown) on an end thereof for connecting to socket  222  of coupling unit  220  to provide the detected evoked signals back to the stimulus and data acquisition module  120 .  
         [0096]     As shown in  FIG. 7 , a distance measurement strip  780  is connected to first limb  732 , at a connection portion  740  thereof. Fixed end  782  is connected to connection portion  740 , while free end  784  of distance measurement strip  780 , extends proximally toward base portion  730 , for insertion into slot  740  of coupling unit  220 , when coupling unit  220  is attached to base portion  730 . As with other embodiments employing a distance measurement strip, distance measurement strip  780  has some form of indicia formed on or in the strip along at least part of its length for reading by a scanner  290  in coupling unit  220  or for comparison with an alignment marker on base portion  730 .  
         [0097]      FIGS. 8A and 8B  show respective plan and bottom views of a further embodiment of a stimulus unit, designated by a reference numeral  800 . Stimulus unit  800  is for placement on a right hand for stimulation of the median and ulnar nerves in a manner similar to that described in relation to  FIG. 4 . Stimulus unit  800  has a first limb  832  extending distally from a generally central part of a distal edge or portion of base portion  830 . Stimulus unit  800  also has a second limb  822  extending distally toward a thenar area, when placed on a hand. Second limb  822  has an adhesive attachment portion  824  for attaching a sensing electrode E 1  over a part of the thenar area. Stimulus unit  800  is similar to stimulus unit embodiments  600  and  700 , in that it has two distally extending limbs, one of which has only a CMAP sensing electrode and the other of which has both CMAP and SNAP sensing electrodes separated by an extensible portion  814 .  
         [0098]     A connection portion  840  is formed at a distal end of extensible portion  814 , but proximally of a first branch  845  which supports sensing electrodes E 3 , E 4  formed at adhesive attachment portions  846 ,  848  for attaching electrodes E 3 , E 4  to a fourth digit (ring finger). Connection portion  840  is of a sufficient dimension to enable attachment of a fixed end of a distance measurement strip, such as anyone of those shown and described in relation to previous embodiments or in relation to  FIGS. 11, 12  or  13 .  
         [0099]     Extensible portion  814  is formed so as to have a plurality of loop portions extending in a snaking pattern in the same plane as that of the rest of the substrate. Proximal of extensible portion  814  on first limb  832  but distal of base portion  830 , a sensing electrode E 5  is located, within adhesive attachment portion  834 . Sensing electrode E 5  is positioned so as to be able to overlie a hypothenar area of the right hand.  
         [0100]     As shown in  FIG. 8B , an adhesive attachment portion  876  is provided along a part of connector limb  872 , somewhat adjacent base portion  830 , for assisting secure attachment of stimulus unit  800  to the wrist. Further, an additional adhesive attachment portion  831  is provided on opposite side of base portion  830  to that of attachment portion  876 . Attachment portion  831  serves to provide additional surface area for adhesive attachment of stimulus unit  800  to the wrist area.  
         [0101]     Connector limb  872 , is formed of a greater length than other embodiments, as it is designed to wrap around the wrist so that connector  870  can connect to socket  222  and coupling unit  220  from the right side (as viewed in plan view). As with other embodiments described herein, stimulus unit  800  has projecting connector parts  852  on base portion  830  for connecting to coupling unit  220 . Specifically, the stimulating electrodes make up one adhesive section. At least one sensing electrode makes up another separate adhesive section and the separate adhesive sections are connected by and extensible non-adhesive section. In this way the stimulating and sensing electrodes sections can be place a variable distance apart to accommodate different sizes and varying anatomy.  
         [0102]     Referring now to  FIGS. 9A and 9B , a further stimulus unit embodiment is shown, designated by reference numeral  900 . Stimulus unit  900  is similar to stimulus unit  800 , except that it is only designed for testing of the ulnar nerve. Consequently, stimulus unit  900  only has a single pair of stimulating electrodes S 1 , S 2  positioned on base portion  930 . Stimulus unit  900  has a single limb  932  projecting distally from a ground electrode GND and adhesive attachment portion  931  formed immediately distally of base portion  930 .  
         [0103]     Stimulus unit  900  has an extensible portion  914  formed in limb  932 , intermediate a first sensing electrode E 1  for overlying a hypothenar area and distal sensing electrodes E 2 , E 3  for attachment to a fifth digit (little finger). Sensing electrode E 1  is attached to the hypothenar area by adhesive attachment portion  934 , while sensing electrodes E 2 , E 3  are attached to the fifth digit (little finger) by adhesive attachment portions  946 ,  948  respectively.  
         [0104]     Stimulus unit  900  has a connection portion  940  for receiving in a connecting fashion the fixed end of a distance measurement strip, such as any one of those shown and described in relation to other embodiments or as shown and described in relation to FIGS.  11  to  13 . Connection portion  940  is formed in limb  932  distal of extensible portion  914  but proximal of a branch  950  from which electrodes E 2 , E 3  extend laterally.  
         [0105]     Like stimulus unit  800 , stimulus unit  900  has projecting connection parts  952  for connecting the base portion  930  to coupling unit  220 . Further, a connector limb  972  extends from base portion  930  and has a connector  970  on an end thereof for receipt in socket  222  of coupling unit  220 .  
         [0106]     Referring now to  FIGS. 10A and 10B , a further stimulus unit embodiment is shown, designated by reference numeral  1000 . Stimulus unit  1000  is almost identical to stimulus unit  900 , except that it is intended for stimulus of only the median nerve. Stimulus unit  1000  has a single pair of stimulating electrodes S 1 , S 2  positioned on base portion  1030 . Stimulus unit  1000  has with a single limb  1032  projecting distally from a ground electrode GND and adhesive attachment portion  1031  formed immediately distally of base portion  1030 .  
         [0107]     Stimulus unit  1000  has an extensible portion  1014  formed in limb  1032 , intermediate a first sensing electrode E 1  for overlying a thenar area and distal sensing electrodes E 2 , E 3  for attachment to a third digit (middle finger) or optionally the fourth digit (ring finger). Sensing electrode E 1  is attached to the thenar area by adhesive attachment portion  1034 , while sensing electrodes E 2 , E 3  are attached to the third or fourth digit (middle or ring finger) by adhesive attachment portions  1046 ,  1048  respectively.  
         [0108]     Stimulus unit  1000  has a connection portion  1040  for receiving in a connecting fashion the fixed end of a distance measurement strip, such as any one of those shown and described in relation to other embodiments or as shown and described in relation to FIGS.  11  to  13 . Connection portion  1040  is formed in limb  1032  distal of extensible portion  1014  but proximal of a branch  1050  from which electrodes E 2 , E 3  extend laterally.  
         [0109]     Turning now to  FIG. 11 , there is shown a representation of one embodiment of a distance measurement strip, designated by reference numeral  1180 . Distance measurement strip  1180  has a fixed end  1182  for fixation to connection portion  1040 ,  940 ,  840 ,  740 ,  640 ,  540  or another point adjacent to the distal sensing electrodes. On an opposite end of distance measurement strip  1180  is a free end  1184  for insertion into, and passage through, slot  240  of coupling unit  220 .  
         [0110]     Intermediate fixed end  1182  and free end  1184 , quadrature indicia  1186  are printed or otherwise placed on a surface of distance measurement strip  1180  for scanning by scanners  290 . The quadrature pattern of indicia  1186  is used by control module  110  to determine the relative amount of progress of distance measurement strip  1180  through slot  240 , together with the known separations of other parts of stimulus unit  130  (or  500 ,  600 ,  700 ,  800 ,  900  or  1000 ) and the predetermined physical relationship of coupling unit  220  to the base portion of the stimulus unit.  
         [0111]     In addition to the distance measurement indicia  1186 , identifying indicia  1188  is provided on a part of distance measurement strip  1180  toward free end  1184 . This further indicia specifies a unique identifier of the stimulus unit, such as a serial number or other form of unique identifier for tracking the use of the stimulus unit to ensure that it was used once only. The identifying indicia  1188  may also indicate a type of the stimulus unit (e.g. right median, left sural) and/or a use-by date (because the conductive gel tends to dry over time). Unique identifier indicia  1188  may be encoded, for example, in the form of a barcode or other machine-readable code so that it can be read into stimulus and data acquisition module  120  via scanners  290  and subsequently recorded into memory  112 .  
         [0112]     Referring now to  FIG. 12 , there is shown a further embodiment of a distance measurement strip, designated by reference numeral  1280 . Distance measurement strip  1280  has a fixed end  1282  for use in the manner described above in relation to fixed end  1182 . Similarly, an opposite free end  1284  is provided on the elongate strip.  
         [0113]     Distance measurement strip  1280  has distance related indicia  1286  printed or otherwise placed on a portion thereof toward fixed end  1282 , while indicia specifying a unique identifier is provided on distance measurement strip  1280  more toward free end  1284 . Distance measurement related indicia  1286  and the identifier related indicia  1288  employ a gray scale for determining the distance or unique identifier. Calibration indicia  1289  are also provided proximate free end  1284  for calibration of the light intensity signals returned to scanner  290  from light impinging on the gray scale indicia.  
         [0114]     Distance measurement indicia  1286  may comprise a strip of continuously darkening gradations corresponding to the distance of travel of distance measurement strip  1280  through slot  240 . The light intensity signals thus returned by scanner  290  (only one scanner  290  is required for sensing gray scale intensity) may be interpreted by stimulus and data acquisition module  120  or processor  114  to determine the distance that corresponds to the gray scale position at which distance measurement strip  1280  comes to rest in front of scanner  290 . Identification indicia  1288  may use gray blocks to encode a unique identifier.  
         [0115]     Referring now to  FIG. 13 , there is shown a further embodiment of a distance measurement strip, designated by reference numeral  1380 . Like the embodiments of  FIGS. 11 and 12 , distance measurement strip  1380  has a fixed end  1382  for connection to a distal part of stimulus unit  130  and an opposite free end  1384  for receipt in slot  240 . Distance measurement indicia  1386  comprises a series of equally spaced, equal width bars of about two millimeters in width. Identification indicia  1388  may include a numeric identifier, such a serial number, together with an encoded version of the numeric identifier. The encoded identifier may be encoded in the form of a barcode or gray scale, for example.  
         [0116]     For each of the distance measurement strip embodiments shown in FIGS.  11  to  13 , the length of the elongate strip will depend on the form and configuration of the stimulus unit  130  and the position on the distally extending limb to which it is attached. However, embodiments of the distance measurement strip may have a length in the order of 20 to 40 centimeters or in the vicinity of 30 to 35 centimeters.  
         [0117]     Other embodiments of the distance measurement strip may use indicia that can be sensed by a scanner  290  that is not purely optical in nature. Further, according to alternative embodiments, the identification indicia  1188 ,  1288  or  1388  may be formed on a part of stimulus unit  130  other than the distance measurement strip.  
         [0118]     Referring now to  FIG. 14 , there is shown a flow chart of a method of nerve conduction testing that involves measuring the separation between at least one distal electrode on a limb of the stimulus unit  130  (or  500 ,  600 ,  700 ,  800 ,  900  or  1000 ) and one of the proximal stimulation electrodes on the base portion. The method is designated by reference numeral  1400  and begins at step  1410 , at which the base portion  230 ,  530 ,  630 ,  730 ,  830 ,  930 , 1030  is attached to a proximal part of the body, such as the forearm proximal of the wrist crease, or a lower leg proximal of the ankle. The base portion is attached in the desired position by peeling a backing sheet from the stimulus unit  130 ,  500 ,  600 ,  700 ,  800 ,  900 ,  1000  from the base portion so that the adhesive attachment portion on the underside of the base portion is exposed and can be adhered to the body.  
         [0119]     At step  1420 , the distal sensing electrodes are attached in the appropriate locations using the adhesive attachment portions surrounding each sensing electrode, with the backing sheets removed. In step  1430 , coupling unit  220  is connected to the base portion using the corresponding connecting parts  250  on coupling unit  220  and connecting parts  252 ,  552 ,  652 ,  752 ,  852 ,  952 ,  1052  on the base portion of the stimulus unit.  
         [0120]     At step  1440 , the distance measurement strip  280 ,  680 ,  780 ,  1180 ,  1280 ,  1380  is inserted into slot  240  of coupling unit  220  and pulled and/or pushed therethrough so that the indicia on the distance measurement strip is read by one or more scanners  290 . The signals generated by scanners  290  in response to the passage or final rest position of the distance measurement strip are transmitted from coupling unit  220  to a controller within stimulus and data acquisition module  120  and then onto processor  114  for processing to determine the separation of the stimulus and sensing electrodes.  
         [0121]     At step  1450 , the nerve conduction testing is carried out using the stimulus and sensing electrodes on stimulus unit  130 ,  500 ,  600 ,  700 ,  800 ,  900 ,  1000  and taking into account the determined separation of the stimulus and sensing electrodes as necessary.  
         [0122]     It should be noted that, while the sensing electrodes are generally described herein as being distally positioned and the stimulation electrodes are described as being more proximally positioned, these positions represent nerve conduction testing in an antidromic orientation. It should be understood, however, that the relative functions of the sensing and stimulating electrodes may be reversed to an orthodromic orientation. In an orthodromic orientation, the stimulus may be applied at the fingers and/or thenar and/or hypothenar areas and the evoked response sensing may occur at the wrist, for example.  
         [0123]     Referring now to  FIG. 15 , there is shown an example side cross-section of a base portion of a substrate according to an illustrative embodiment of a stimulus unit. The illustrative substrate is designated by reference numeral  1500  and is shown prior to application to a body part.  
         [0124]     Substrate  1500  has a base layer  1510 , which forms the top (or upper or outer) layer facing away from the body part. This base layer  1510  is formed of medical grade polyester or a similar material and has sufficient rigidity to form the base for flexible circuitry and enable subsequent conductive and insulative layers to be formed thereon, while having sufficient flexibility to enable the entire substrate  1500  to bend to generally conform to the shape of the body part to which it is to be affixed.  
         [0125]     Electrodes  1520  are formed on base layer  1510 , either directly or on a thin priming or separation layer (not shown) coating the underside of base layer  1510 . Electrodes  1520  are electrically coupled to external connectors via conductors  1530  in the form of flexible circuit tracings formed on base layer  1510 . As with electrodes  1520 , conductors  1530  may be directly formed on base layer  1510  or may be separated therefrom by a priming or separation layer.  
         [0126]     Portions of substrate  1500  that are not to be exposed to the body part (such as conductors  1520 ) are covered by an insulation layer  1535 . This insulation layer  1535  covers conductors  1530  for electrodes  1520 , which in the example cross-section are stimulating electrodes. Electrodes  1520  have a layer of conductive gel  1540  formed around them for facilitating conduction between electrodes  1520  and the skin of the body part on which the substrate  1500  is positioned.  
         [0127]     For portions of substrate  1500  that are not covered by conductive gel  1540 , but that surround the electrodes  1520  and conductive gel  1540 , a double-sided adhesive layer  1550  is formed over the insulation layer  1535 . Adhesive layer  1550  may be a foam (or other) material impregnated or coated with one or more adhesive substances or it may be a layer of the adhesive substance itself.  
         [0128]     The adhesive layer  1550  and conductive gel  1540  is covered by a protective backing sheet or layer  1560  so that the adhesive and conductive qualities of the adhesive layer  1550  and conductive gel  1540  are preserved until application of substrate  1500  to the body part. The total thickness of substrate  1500  may be in the order of 0.7 to 1.5 millimeters, approximately.  
         [0129]     The embodiment shown in  FIG. 15  is not to scale, is for purposes of illustration only and some variations or modifications may be made, depending on the specific requirements of the stimulus unit embodiment and methods of forming it. Although the entire stimulus unit cross-section shown in  FIG. 15  is described as the substrate and designated by reference numeral  1500 , base layer  1510  may also be considered to be (or be part of a substrate, with electrodes  1520  and conductors  1530  being formed on the substrate.  
         [0130]     While the stimulus unit embodiments shown and described herein generally show a unitary substrate including one or more limbs and a base portion, each of the areas or portions of the stimulus unit having sensing or stimulation electrodes may be formed on a separate substrate. For example, distal sensing electrodes positioned around a finger may be formed on a substrate distinct from the substrate on which the proximal stimulation electrodes are formed. In such embodiments of the stimulus unit, as conductors  1530  cannot be formed to cross between substrates, the separate substrates must be either electrically coupled to each other (for example, by connectors) or have separate connectors for interfacing with coupling unit  220 . Such embodiments may be useful where, for example, the extensible portion in one of the limbs is formed as a strain gauge or other electromechanical sensor to indicate the degree of extension of the limb and thereby provide a measurement of the separation of the separate substrates and their respective electrodes. Such embodiments therefore do not require a distance measurement strip.  
         [0131]     Reference herein to a limb is not intended to include a reference to a human limb, such as an arm or leg. Rather, it is a reference to a part of a stimulus unit embodiment.