Microstim Redesign - An opportunity for improvement

The Microstim that we have been using for the last 8 years has proved to a popular stimulator for 2 channel muscle exercise. However, the design was based on the materials and ideas available at the time and our experience and user feedback suggest that it was time to make some improvements. There were also changes in some regulations that we needed to comply with which meant that we had to do some work on the ‘innards’.

Inputs to the Redesign

In our daily work with patients we quickly get to hear the likes and dislikes with regard to our equipment. The clinical staff also have their own feelings and we discuss this feedback at our regular departmental meetings. In addition we welcome remarks and criticism from colleagues and customers outside of Salisbury and feedback from the technicians here who have to deal with problems over the telephone.

The major concerns that we had to consider were:

• Size – the box is quite ‘chunky’ due to allowance being made for other connectors and options to be fitted; these were never used. At the time the range of enclosures which had a suitable battery compartment was quite limited.
• Front panel controls – The output level knobs are attached to controls with a metal shaft and this shaft needed some careful machining before the front panel or knobs could be fitted. The mode control was sometimes confused for another output adjustment and could be ‘twiddled’ by patients. The mode setting was also not terribly clear.
• Battery compartment – the wired in connector was awkward to use one-handed by our Stroke patients and damage to the connecting wires could occur if the battery were tipped out or yanked.
• Electrode output sockets – You may have seen the new ODFS III with shrouded connectors where no bare metal is showing. This is a safety requirement to prevent persons pushing their electrode connections into unsuitable electricity supplies. 
• Software support – the original author of the software has emigrated to Australia so changes could not be made easily to the output waveforms
• Internal circuit changes – avoid the use of some components which require critical adjustment to get the best performance.

There were also some features we wanted to add

• Different modes. Principally we wanted to develop a mode where two groups of muscle could be exercised without a release period in between. We have called this an ‘overlapping’ mode and initially we are using it for the correction of subluxed shoulder.
• Smoother output controls so that the stimulation came on earlier in the range and was more gradual.

The ‘new’ Microstim 2v2

We have addressed all the features outlined above within the limitations of a standard box and off the shelf components. The new case is slimmer with a better battery compartment that incorporates spring contacts instead of the wired clip. The mode switch on the front panel has push buttons to increase and decrease the mode with a clear display. The electrode sockets are in the side of the case instead of the rear. A belt clip could be fitted but we considered that an exercise stimulator would usually be used when the person is sitting. A label on the top of the box shows which socket is which and lists the function of all the modes. When the MS2v2

is turned on there is a long beep and then a number of short beeps equal to the mode set. Note that mode zero has zero beeps!

Mode Function

0. 40Hz Alternate long ramp
1. 40Hz Simultaneous long ramp
2. 20Hz Continuous short ramp
3. 20Hz Alternate short ramp
4. 20Hz Simultaneous short ramp
5. 40Hz Continuous short ramp
6. 40Hz Alternate short ramp
7. 40Hz Simultaneous short ramp
8. 40Hz Overlapping short ramp
9. 20Hz Overlapping short ramp

Long ramp = 6 seconds, Short ramp = 2 seconds

The software has been completely re-written to permit the overlapping output mode mentioned above. The previously unused modes (8 & 9) are now used for overlapping and the 10Hz settings have been replaced with 40Hz Alternating Simultaneous modes with a long six second ramps making them suitable for patient with high tone. The other modes use 2 second ramps, which are suitable for general use. If you require a stimulator with 10Hz modes or any other variation, please contact use and we can produce a "special" for you. The output controls are ‘smoother’ and manufacture is easier because components don’t have to be adjusted.

Shoulder subluxation

When using the old Microstim in its alternating mode to stimulate the deltoid and supraspinatus muscles, the humerus would rise and full with each contraction. This constant movement, while exercising the muscle effectively, may not have encouraged shortening of the muscles and therefore better long term alignment of the capsule. If two channels are used, the first exercising the middle fibres of deltoid with supraspinatus and the second exercising the anterior and posterior fibres of deltoid are alternated but with overlapping contraction periods, the humerus will be lifted and then maintained in position for the duration of the exercise period.

Conclusions

We are pleased with the way the MS2v2 has developed and the initial reaction from clinicians and patients has been favourable. Costs of manufacture are not much different to the previous version so there will be no increase in the sale price. The rewriting of the software should enable us to offer customised versions on an individual or small batch basis.

Steve Crook, Paul Taylor, Stacey Finn

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