A Pictorial History of Insertable EMG Sensors

By John D. Perry, PhD, Inventor

The combination blood-flow sensor  ("photoplethysmograph")  and muscle activity sensor ("electromyograph") was first developed to investigate the mechanisms of sexual arousal.   Along the lower edge of the sensor barrel a single longitudinal silver EMG electrode is visible.  (Two other long silver electrodes are located at 90 and 180 degrees behind the sensor body.)  Above the electrode, the dark circle of a photocell aimed at the vaginal wall is visible.  To the left of the photocell, the white base of a miniature incandescent lamp is visible.  A five-pin DIN plug was molded into the base.  An insertion stop, right, prevents the sensor from going too far into the vagina, and a retaining bulb, left, helps to prevent it from falling out during contractions.

The manufacturing simplicity of circular electrodes was explored with this prototype device, made and tested in 1975.  Unfortunately, the device proved relatively insensitive to pelvic muscle contractions.  In a 1991 article, Binnie et al came to the conclusion that circular electrodes were not suitable for EMG work, while longitudinal electrodes provided virtually identical information to the traditional inserted needle electrodes.  The "sub" shape proved unstable, as the device was expelled like a missile on contraction. 

The circular electrodes soon gave way to longitudinal ones, shown here in a clear acrylic sensor which reveals their construction [1976]. Two of the three electrodes are visible here.  The retaining bulb has been made larger, and the insertion stop is also bigger.  This sensor was made in a mold, carefully created by mating two small doorknobs and a central cylinder.  "Hobby" epoxies, used to make trinkets and novelties, were used for the construction.  Later, a dentist suggested using pink denture acrylics

The first 50 vaginal EMG sensors were hand-made, in plaster molds, on a kitchen counter between 1976 and 1981. Most users were research psychologists involved in biofeedback.  Note the irregularities on the color and the poor seal of the electrodes.  This one was made for Elizabeth Noble, PT, about 1977, and was used for several years before being replaced (to take it out of sight and circulation!) with a brand new sensor (next below). 

In 1981 the inventor was approached by William Farrall, a manufacturer of specialized scientific instruments.   Farrall began to make highly polished, refined sensors in his Nebraska factory.   Unfortunately, a high failure rate (over 50%) due to surface imperfections caused the price to rise to $250, and eventually $450 a decade later.  More than a thousand of these sensors were hand-made by one woman who worked in Farrall's factory.  These sensors were designed to be cold-sterilized in "Cidex" and used forever. 

After refining the vaginal sensor (above), Farrall starting making anal sensors as well (right).  This allowed treatment of female children considered too young for vaginal placements, and of men of all ages.  The growing awareness of the problem of post-prostate surgery incontinence, and of the treatability of fecal incontinence and functional constipation ("anismus") led to a high demand for these devices starting in the mid 1980s.

The Evolution of the Single-User Sensor

The use of insertable sensors to treat pelvic muscle disorders had barely gotten of the scientific ground when the first epidemics of genital herpes created (totally unfounded) fears of the use of sterilized sensors, and the need for a "single-user" sensor became obvious. 

In 1984 the inventor met William Benton, founder of Verimed, Inc. who said "I can make that for you to sell for $19.95!".  We formed EMG Sensor Corporation as a joint venture, and in late 1985 two large crates containing 2,000 sterilized, individually-wrapped sensors arrived in Philadelphia.  Final quality check found several "defective" sensors, so 100 bags were opened and tested -- with a 20 percent failure rate.  Upon investigation, it was learned that excessive solvent glue had penetrated the pressure-fit internal connections and had often "insulated" the bare wires inside.  The first batch could not be sold.  [Later, they would be used as decorations for a Christmas Tree.]

Eventually, the manufacturing problems were solved -- at great expense to Verimed -- and the Single-User Sensor reached the market -- at $149.95, more than seven times the original target.  The three plastic parts had to be extensively modified by hand before assembly.  Wires were spot-welded to the electrodes for better reliability.  The Single-User Sensor was immediately a success, and many thousands were sold in the US and Europe.  In an attempt to lower the consumer cost, the original molds were purchased from Verimed, but production problems continued.  A plea for help went out.

In December 1992 Self-Regulation Systems of Redmond, Washington, agreed to invest the $50,000+ needed to create an entirely new mold (one which would not require expensive hand-finishing work).  After the AHCPR issued its favorable Guideline on Incontinence in March, 1992, the demand for sensors increased dramatically.  Many clinicians could not obtain any insertable sensors for several months, and began to substitute surface patch electrodes instead.   Finally in April, 1994 SRS announced production of their new single-user sensors -- priced at $59.95 (Quantity 5). 

Experimental Designs

Over the years many attempts have been made to create  a replacement for the original sterilizable sensors made by Farrall Instruments from 1981 to 1993.  A retired jeweler and instrument maker helped design and hand-crafted several experimental sensors. This one (left) includes a central catheter hole for use with a deep vaginal pressure balloon to measure inter-abdominal pressure (based on a paper by Molly Doroghty.)  

German researcher Dr. Paul Enck wanted a longer, narrower anal EMG sensor to use with obese fecal incontinence research subjects; several of these "T-Bar" sensors (right) were made in 1988 for his research.   These experimental designs were shown at the 1989 AAPB convention.  Five years later Thought Technology, Ltd., introduced a "T-Bar" sensor of very similar design.  

Last revised on May 04, 2005

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