The Odstock Two Channel Stimulator



The old O2CHS is being phased out and replaced with an updated version, the O2CHS II. It is a direct derivative of the ODFSIII and the O2CHS, drawing on the extensive clinical experience of these devices. While the original O2CHS was used by over 200 people and answered a clinical need, it was found to be deficient in certain areas. Firstly the lay out of the clinician's controls meant that it was less than intuitive to use. These have now been arranged in a logical order. Secondly, it lacked the updated functions in the ODFSIII that the clinicians had become accustomed to. These include a pause switch for halting the devices output, a choice of asymmetrical or symmetrical biphasic outputs, a linear pulse width control of output (3-350m s front panel control) and current set internally (20-100mA). These functions have been incorporated in the device and in addition, a better power supply design gives a more stable output as battery power declines and an a low battery warning is also given. The device is packaged in a lower profile box with rounded corners making it more cosmetically and ergonomically acceptable. A comprehensive instruction manual is provided that gives details of algorithms for many applications an their indications for use. These algorithms can be quickly set up and adjusted in the clinical setting without the need of technical support. A mandatory one and a half-day training course is provided for all clinicians wishing to use the O2CHSII. The device is CE marked and protected by a patent.


As with its predecessor, the O2CHSII is controlled using either one or two footswitches although hand switches can also be used. The two channels can work independently, each with its own foot switch or synchronised together using just one switch. Channel 1s configuration is very similar to the ODFSIII. Following either release or pressure on the footswitch, stimulation begins with a ramping of the amplitude. The rate of the ramp can be adjusted from between 0 and 4 seconds. The stimulation then remains at a fixed amplitude until either there is a change in the footswitch state (adaptive timing) or after a fixed time (fixed timing) chosen between 0.5 and 6 seconds. In adaptive timing, stimulation will end after a chosen time (0.5 to 6 seconds) unless this function is disabled, enabling the output to remain until the foot switch state changes. This later function can be useful to maintain quadriceps or gluteal muscles for as long as weight remains on the switch. An extension can be added to the stimulation period. This is a fixed time (0-1.5s) that extends the stimulation time past the change in foot-switch state. This was original added to provide an eccentric contraction of anterior tibialis after heel strike, lowering the foot to the ground but is useful in timing the contraction of other muscle groups. Finally, there is a descending ramp (0 to 4 seconds). The second channel has the same configuration except the initial rising ramp can be preceded by a delay (0 to 2 seconds). This means the second channel can be timed to occur at other times in the gait cycle than at a foot switch state change. In most applications channel 1 is used for common peroneal stimulation to produce dorsiflexion while channel 2 stimulates another muscle. 



Below is described a selection on applications or "algorithms" that are used in our clinic. Many other applications are also possible.

Clinical experience

133 patients are currently using the O2CHS or the O2CHSII. 61 have multiple sclerosis, 35 have a stroke, 12 spinal cord injury, 9 cerebral palsy and 16 have other pathologies. The most common application is bilateral dropped foot used by 56 patients followed by dropped foot with hamstrings 37 and dropped foot and gluteal muscles 13. 14 use the device for dropped foot and calf, 8 for dropped foot and quadriceps and 3 use it for bilateral gluteal stimulation.

Bilateral dropped foot following multiple sclerosis

Twenty-five people used the O2CHS for bilateral common peroneal stimulation of which 18 had initially used the ODFS for correction of a single dropped foot prior to developing bilateral dropped foot. Of the 25, 18 had sufficient data recorded for an initial and follow up assessment at an average of 4.6 (1.5 -10 months) after beginning use of the O2CHS. Their average age was 50 (34-62) years and their time since diagnosis, 9.6 (1-21) years. Seven additional subjects are not included in this audit because they were unable to complete the 10m walk without use of the stimulator. 6 of all 25 users had discontinued use of the device, 2 within 6 weeks because of insufficient help from the device and a third user after 18 months for the same reason. One user stopped after 6 months due to a skin reaction to the electrodes while another user stopped after 12 months use because it was too much effort to set up and use the device. No reason is recorded for the sixth subject. There was a mean increase in walking speed of 48% p<0.01 when the device was used after 4.6 months compared with unaided walking speed at the start of treatment. There was a reduction PCI of 17% (p<0.025) in the same period. There was a trend towards a training effect (carryover effect) in walking speed as unaided walking speed increased by 13% p=0.086

Case study of Bilateral gluteal stimulation for correction of trandalanberg gate.

A 56-year old man with an incomplete SCI lesion at C3/4/5 following a RTA was referred to our clinic, 15 months post injury. He was a partial walker, able to walk a few meters with supervision but chiefly depended on the wheel chair for mobility. He had general weakness but his main concern was lack of hip extension and abduction, effecting both sides but to a greater extent on the left. This caused his hip to drop when weight was taken through it leading to instability while walking and a noticeable sway. Initially a single channel ODFS was used, stimulating the hip extensors and abductors when weight was placed on the footswitch situated under the left heel. After 6 weeks daily use of the stimulator the patient was able to demonstrate some active hip extension. The device was used at home for a further 3 months and in this time the patient experimented stimulating the gluteal muscles on the right side and felt that this was also beneficial. It was therefore decided to set up a bilateral device. His walking had now improved to the extent so that it was possible to perform a 10m walking speed and PCI tests. The measurements showed a 21% increase in walking speed and a reduction of 27% in PCI when the device was used and his walking was noticeably steadier.

At a follow up appointment 3 months later walking speed and PCI had not changed significantly, speed increasing by 22% with a reduction of 23% in PCI when the device is used. However, after a further 6 months daily use, his unassisted walking speed had increased by 71% with a reduction of 28% in PCI compared with when the bi-lateral gluteal stimulator was used for the first time. While gait was more secure when the device was used, walking speed was only 4% faster when it was used indicating a significant training effect from the device. While gait is still slow and the wheelchair must still be considered his main form of mobility, limited community walking is now possible.


The 02CHS II is the same price as the O2CHS and costs £379. As an option, the device can be supplied without the double footswitch and footswitch lead (used when both a heal and toe switch is required under the same foot - for example dropped foot correction and calf stimulation) at a reduced price of £330. Please see the section on courses in this newsletter for forthcomming courses. Course will also be advertised on our web page

Paul Taylor

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