Stim2Go from Pajunk is a compact 5 channel unit with some unique features
In a recent article on LinkedIn, Start with the goal, not the settings, I suggested five questions to work through before touching a single setting on the Stim2Go from Pajunk. The fourth was how the stimulation should be triggered, and one of the answers was a movement trigger with an angle threshold. In this situation, the stimulation is triggered because the client moved a limb, and you decide how much movement it takes.
Stim2Go has a number of ways in which you can generate a stimulation sequence. These are:
Based on time. Where once the sequence starts it turns muscles on and off as desired for a period of time.
Based on manual triggering. Where the sequence waits until a key is pressed in the App.
Based on motion. Where the movement amount and direction triggers stimulation of selected muscles.
Based on motion with a threshold. This is where the stimulation is triggered once a limb is moved through a defined angle.
I want to spend this whole article on the benefit of motion triggered stimulation. On most stimulators a threshold is a technical setting, there to stop the device firing by accident. On a device like the Stim2Go I think it is one of the most clinically important values on the screen, because it decides how much of the movement your client has to produce before any help arrives. Get it right and the stimulation becomes something the client earns. In one direction the stimulation supports them; in the other, nothing happens at all.
What the threshold actually measures
It helps to be precise about what the Stim2Go measures, because it is not what some people assume. The unit contains an inertial sensor, a three-axis accelerometer and gyroscope much like the ones in a smartphone. Strapped to a limb segment, it tracks the angle of that segment as it moves. When we use the Stim2Go for FES Cycling it is strapped to the right thigh and the movement of the leg by a passive-active bike is used to trigger the stimulation based on the changing thigh angle. In a movement-triggered programme with a threshold, you tell the unit both an angle and a direction, and the stimulation sequence fires when the segment passes that angle.
So the trigger is movement, not muscle activity. The Stim2Go does not read EMG and has no EMG input. It is watching the forearm lift, or the thigh tilt, or the trunk lean forward, and waiting for it to get to the desired position.
The instructions for use set a floor. These threshold modes are only for people with an active range of motion, defined as a voluntary movement that changes the limb's angle by at least 10 degrees. Below that, the sensor cannot reliably tell an attempt from a twitch or a shuffle in the chair.
For those clients the manual trigger is the right tool, and here the clinician is "the sensor". You watch for the start of the attempt, or feel for the muscle tightening under your hand, and press the trigger key in the app at that moment. The stimulation stays paired with the client's effort even though they cannot yet fire it themselves. I set out the practical side of both modes for sit-to-stand in Triggered Stimulation for Sit-to-Stand on our sister site, fescycling.com.
Why it matters that the stimulation waits
The case for triggered stimulation over stimulation on a timer has been building for more than twenty years, mostly in the arm after stroke. It is a good case, though not a closed one.
A 2019 meta-analysis of EMG-triggered stimulation (Monte-Silva and colleagues, 26 studies) found a robust short-term effect on impairment but no evidence yet on activity and participation. Another 2019 analysis of 48 randomised trials (Yang and colleagues) found that electrical stimulation helped arm function but could not separate triggered from cyclic stimulation. Knutson's group in Cleveland found an advantage for triggered stimulation on dexterity in 2016, then in a larger multisite trial published this year found it on impairment scores instead, not dexterity. The trials point the same way more often than not, and the size of the effect depends on which ones you read.
The study I find most telling tested the timing question directly. Biasiucci and colleagues (2018) gave 27 people with chronic stroke ten sessions of hand-opening FES alongside their usual therapy. Both groups wore the same equipment. In one group the stimulation was triggered by a brain-computer interface that detected each attempt to open the hand; in the other the same stimulation arrived at random. The contingent group improved by 6.6 points on the Fugl-Meyer upper limb scale against 2.1 for the random group, and kept most of it six to twelve months later. Same equipment, same muscles, stimulation in both groups. The difference was whether the stimulation was in response to an attempt.
That trial used brain signals rather than a movement sensor, and 27 people is a small trial. But it isolates the thing a threshold is for.
KEY POINT: Stimulation that answers the client's own attempt has better support than stimulation on a clock, though the trials do not agree on how much better or for which outcomes. The threshold is the setting that decides what counts as an attempt.
Set it too low and the stimulation does the work
Here is the problem with a threshold set at the first flicker of movement. The client barely starts, the stimulation finishes the job, and the movement looks good. It is easy to take that as success.
There is a reasonable body of work suggesting it is not. Lotze and colleagues (2003) compared half an hour of voluntary wrist movement with the same movements produced passively by a motor. The voluntary training produced better performance and larger changes in the motor cortex. That was in healthy volunteers, in a single session, so I would not lean on it too hard, but it fits what clinicians already believe about active versus passive practice.
The more uncomfortable finding comes from rehabilitation robotics. Engineers at the University of California, Irvine, building a robot to assist arm movement after stroke, found that when the device would complete a movement, people simply reduced their own effort and let it move them passively (Wolbrecht and colleagues, 2008). They called this slacking. It is not really laziness. The nervous system is "economical", and when an outside source will supply the force it stops supplying its own. Their answer was a controller that gave slightly less help than the person needed, which kept them working actively.
A movement-triggered stimulator is a much simpler device than a robot, but the same economy applies. If a small, effortless movement buys a full stimulated contraction, a small, effortless movement is what you will get.
Set it too high and there is nothing to learn from
The opposite mistake is just as easy to make with good intentions. Set the threshold at what the client could do on their best day, and most attempts fall short. No stimulation, therefore no completed movement and no feedback from a movement that worked. Ten failed attempts teach the client mainly that the device does not respond to them.
Guadagnoli and Lee (2004) set out a framework for this, which they called the challenge point. The idea is that learning depends on the information a person gets from their own performance, and that information is greatest when the difficulty of the task is matched to their ability. Too easy a task means that there is little to learn, but too hard and the failures carry no useful information. It is a framework rather than a trial, and it came from motor learning in healthy people, but it describes the threshold problem neatly.
How I would set it
I am a bioengineer, not a physiotherapist, but I know the Stim2Go very well. With that said, this is how I would approach setting it up when you want to use motion triggering with a threshold.
Measure the movement first, with the stimulation off. Strap the unit on, open the programme, and ask for the movement several times while you watch the angle the app reports. You want to know what the client can do on their own today, not what they did last week.
Make sure it's the segment you want to move. The sensor responds to anything that tilts it. If you want shoulder flexion and the unit is on the upper arm, a client who leans their trunk back may reach the angle without using the shoulder at all. Watch for the compensation as well as the number.
Set it so most attempts reach it, and none reach it without a clear effort. That is my working rule rather than a published target, and you can use your judgement.
Then let the client feel the difference. A threshold that is reachable with effort turns the stimulation into a positive response. They try it and enjoy a feeling of success.
KEY POINT: A good threshold sits at the edge of what the client can do today and is reachable with a clear effort on most attempts, but never reachable by accident. Measure it with the stimulation off, and watch how the client reaches it as well as whether they do.
Moving the threshold as the client improves
A threshold is not set once. As the client clears it reliably, raise it, so they have to produce more of the movement before the help arrives. On a tired day, or after a setback, lower it without treating that as failure.
The Irvine group tested this principle in a small randomised trial with their robot, giving 26 people with chronic stroke slightly less assistance than they needed across 24 sessions (Reinkensmeyer and colleagues, 2012). They improved more than a group doing conventional tabletop therapy, though the main result was a trend rather than a clear difference, and some of the gains faded at follow-up. Modest, but it points the right way.
Mounting the Stim2Go on a limb makes threshold triggering possible
A client with an ASIA C and C5/C6 spinal cord injury had experienced a subluxed shoulder with some scapular involvement. We set up FES Cycling at the first visit without difficulty but were more cautious with respect to his shoulder stability. We set up a two channel "auto-repeat" stimulation programme for deltoid and supraspinatus using an 8 second sequence to run for 20 minutes. Deltoid was set to contract for 0 to 50% of the sequence and the supraspinatus for 50 to 100% of the sequence. A biphasic rectangular pulse of 300 µs at 50 Hz was used with sufficient current to elicit a visible tetanic contraction. This programme was used for the first few weeks of training to encourage greater shoulder stability and muscle strength. Next, a third channel was introduced to stimulate the rhomboid muscles, which were set to contract simultaneously with supraspinatus. Note that initially, no motion-triggered stimulation was used.
After a few weeks of conditioning training, we further extended the programme to be motion-triggered with a threshold. For this phase of training, we attached Stim2Go to the upper arm and observed the range of motion possible for the client with forward reaching of the arm. We set the threshold for triggering accordingly and we included a further channel of stimulation at the triceps. Now, as the client reaches forward with his arm, once the trigger point angle is reached, this fires the triceps to further assist with arm extension and then the shoulder stabilisation muscles.
Raising the threshold is one step in a longer sequence of handing the movement back to the client, from timed stimulation through manual triggering, movement triggering and higher thresholds, to less stimulation and eventually none.
What the evidence does not yet show
I would like to comment about the formal research evidence. No formal trial has tested the Stim2Go's threshold modes for functional outcomes. I'm comfortable with this because I can see, in a practical sense, that the process works. Outcomes will, of course, depend in part on the number and quality of the repetitions performed. More than that, I have not found a trial of any FES system that compared a progressively raised threshold with a fixed one. What exists is good evidence that stimulation should answer the client's attempt, decent evidence that people do less when a device will do it for them, and a well-regarded framework for matching difficulty to ability. The argument in this article is built from those pieces. I think it is sound, but of course it is still an argument rather than a result. We want you to try the approach for yourselves.
What the client sees on the screen
There is one more reason I like thinking about the threshold this way, and it is not in any trial. It changes the conversation with the client.
A client on timed "auto-repeat" stimulation is having something done to them. A client on a well-set threshold can be told, and can see on the screen, that the stimulation arrives because they tried. A few weeks later they can be shown that the bar has moved and they are still clearing it. For someone who has spent months being moved by other people, the evidence that their own effort is making something happen can matter as much as the contraction itself.
What I would suggest
If you are using a Stim2Go, or thinking about it, three things are worth trying with your next client:
Before running a movement-triggered programme, spend two minutes measuring what the client can do with the stimulation off, and set the threshold from that rather than from the programme template.
Write the number down, alongside the outcome measure you chose for the goal, so that next week you can tell whether it should move.
If the client cannot yet change the limb's angle by 10 degrees, use an auto-repeat programme for strength conditioning or try the manual trigger and fire it from what you see and feel, rather than falling back on a timer.
At Anatomical Concepts we supply the Stim2Go in the UK and run hands-on training in setting it up for functional work. We are as happy to talk through whether stimulation suits a particular client as to talk about the device, and sometimes the answer is that something else should come first.
The choice of programme, and whether stimulation is right for a given person at all, stays with the treating clinician and the client.
If you would like to talk through a client, a caseload or a training session, get in touch. I am always glad to have the conversation.
Further reading
Biasiucci A, Leeb R, Iturrate I, Perdikis S, Al-Khodairy A, Corbet T, et al. Brain-actuated functional electrical stimulation elicits lasting arm motor recovery after stroke. Nature Communications 2018;9(1):2421. https://doi.org/10.1038/s41467-018-04673-z
Lotze M, Braun C, Birbaumer N, Anders S, Cohen LG. Motor learning elicited by voluntary drive. Brain 2003;126(4):866 to 872. https://doi.org/10.1093/brain/awg079
Wolbrecht ET, Chan V, Reinkensmeyer DJ, Bobrow JE. Optimizing compliant, model-based robotic assistance to promote neurorehabilitation. IEEE Transactions on Neural Systems and Rehabilitation Engineering 2008;16(3):286 to 297. https://doi.org/10.1109/TNSRE.2008.918389
Reinkensmeyer DJ, Wolbrecht ET, Chan V, Chou C, Cramer SC, Bobrow JE. Comparison of three-dimensional, assist-as-needed robotic arm/hand movement training provided with Pneu-WREX to conventional tabletop therapy after chronic stroke. American Journal of Physical Medicine and Rehabilitation 2012;91(11 Suppl 3):S232 to S241. https://doi.org/10.1097/PHM.0b013e31826bce79
Guadagnoli MA, Lee TD. Challenge point: a framework for conceptualizing the effects of various practice conditions in motor learning. Journal of Motor Behavior 2004;36(2):212 to 224. https://doi.org/10.3200/JMBR.36.2.212-224
Monte-Silva K, Piscitelli D, Norouzi-Gheidari N, Batalla MAP, Archambault P, Levin MF. Electromyogram-related neuromuscular electrical stimulation for restoring wrist and hand movement in poststroke hemiplegia: a systematic review and meta-analysis. Neurorehabilitation and Neural Repair 2019;33(2):96 to 111. https://doi.org/10.1177/1545968319826053
Yang JD, Liao CD, Huang SW, Tam KW, Liou TH, Lee YH, et al. Effectiveness of electrical stimulation therapy in improving arm function after stroke: a systematic review and a meta-analysis of randomised controlled trials. Clinical Rehabilitation 2019;33(8):1286 to 1297. https://doi.org/10.1177/0269215519839165
Knutson JS, Gunzler DD, Wilson RD, Chae J. Contralaterally controlled functional electrical stimulation improves hand dexterity in chronic hemiparesis: a randomized trial. Stroke 2016;47(10):2596 to 2602. https://doi.org/10.1161/STROKEAHA.116.013791
Knutson JS, Friedl AS, Hansen KM, Harley MY, Hogan SD, Cunningham DA, et al. Contralaterally controlled functional electrical stimulation for upper extremity recovery following stroke: a multisite randomized controlled trial. Stroke 2026;57(2):338 to 348. https://doi.org/10.1161/STROKEAHA.125.052891
PAJUNK. Stim2Go Instructions for Use, version 1.8.1.
Related articles on our site
Standing Up Again: How Responsive Electrical Stimulation Can Support Sit-to-Stand Practice
One Device, Many Exercises: Getting More From a Single Stimulator
Articles on this site are researched and drafted with the help of AI tools, then checked, edited and approved by me. Every citation is verified against the original source before publication.