Understanding Your mTrigger Signal: Part 1 — Making Sense of the Numbers

Understanding sEMG amplitude, individualized activation targets, and why a bigger number is not always better.

You place the electrodes on your patient’s quadriceps, open the mTrigger app, ask for a contraction, and the activation meter starts moving.

350 μV. 600 μV. 900 μV.

But what do those numbers actually mean? Is 900 better than 600? Should your patient be trying to reach 1,000 μV? And if the number is lower at the next visit, does that mean they have gotten weaker?

Surface electromyography (sEMG) gives rehabilitation professionals valuable insight into muscle activity that would otherwise be difficult to see. But to use that information effectively, it is important to understand what the signal represents—and what it does not.

The goal of mTrigger isn’t simply to produce the biggest number possible. The goal is to use real-time information about muscle activity to improve awareness, recruitment, coordination, movement quality, and ultimately function.

What Is mTrigger Actually Measuring?

When the nervous system tells a muscle to contract, motor neurons activate muscle fibers. This produces electrical activity that can be detected at the skin using surface electrodes.

mTrigger detects this electrical activity and displays its amplitude in microvolts (μV). [1,2]

In general, increasing muscular effort often results in increasing sEMG amplitude because additional motor units are recruited and/or their firing behavior changes. However, the relationship is much more complicated than:

Higher microvolts = stronger muscle.

Research on surface EMG consistently cautions against interpreting EMG amplitude as a direct measurement of muscle force, strength, or neural drive. The signal recorded at the skin is influenced not only by the activity occurring within the muscle but also by anatomy, electrode location, the tissues between the muscle and electrode, and characteristics of the movement being performed. [1–3]

That means a reading of 800 μV does not mean a muscle is “twice as strong” as it was when it produced 400 μV.

Instead, think of the mTrigger signal as a window into how much detectable electrical activity is occurring beneath the electrodes during that specific task and setup.

So, What Is a “Good” mTrigger Number?

There isn’t one.

A common question from new mTrigger users is, “What number should my patient be getting?”

The better question is:

What level of activation is appropriate for this patient, during this exercise, for the goal we are trying to achieve?

A healthy athlete performing a resisted knee extension may generate a very different signal than a patient attempting their first quadriceps set after knee surgery. The same patient may also generate different values during a quad set, straight-leg raise, squat, and step-down.

Electrode location can also substantially influence the amplitude recorded from a muscle. Research has demonstrated that moving electrodes along different regions of the same muscle can change the detected sEMG signal, particularly in larger or more complex muscles. [2,4,5]

This is why mTrigger’s MVC goal is best treated as an individualized activation target rather than a universal benchmark.

As discussed in our Setting Your MVC Goal article, clinicians can establish a starting target by observing several representative contractions or by using the MVC setup protocol. From there, the goal can be adjusted so that it is challenging, achievable, and appropriate for the exercise being trained.

Don’t Chase the Highest Number

This may be the most important concept for new mTrigger users.

More muscle activity is not always the goal.

Consider a patient working on a step-down. Their quadriceps activation increases dramatically—but they also lose control of the movement, shift their trunk, and push aggressively through the opposite leg.

The higher number did not necessarily represent better rehabilitation.

Likewise, if mTrigger is being used to decrease an unwanted compensation, reduce excessive muscle activity, or teach relaxation, a lower signal may actually represent success. The existing mTrigger inhibition and compensation protocol uses this exact principle: instead of asking the patient to drive the meter higher, feedback can teach them to reduce activity in an overactive or compensating muscle.

The signal should always be interpreted in the context of the movement you are trying to create.

Putting the Number in Context

At this point, the most useful shift is to stop treating the mTrigger value like a score and start treating it like feedback. The number becomes meaningful when it helps you answer a clinical question: Can the patient find the target muscle? Can they reproduce the contraction? Can they activate it without relying on a compensating strategy? Part 2 builds on this foundation by looking at the signal during repeated exercise, across visits, and as the patient progresses into more demanding movement.

Related mTrigger Resources

Setting Your MVC Goal

Inhibition and Compensation with Biofeedback

References

1. Vigotsky AD, Halperin I, Lehman GJ, Trajano GS, Vieira TM. Interpreting Signal Amplitudes in Surface Electromyography Studies in Sport and Rehabilitation Sciences. Front Physiol. 2018;8:985. doi:10.3389/fphys.2017.00985.
2. Disselhorst-Klug C, Schmitz-Rode T, Rau G. Surface electromyography and muscle force: limits in sEMG-force relationship and new approaches for applications. Clin Biomech (Bristol, Avon). 2009;24(3):225-235. doi:10.1016/j.clinbiomech.2008.08.003.
3. Farina D, Holobar A, Merletti R, Enoka RM. Decoding the neural drive to muscles from the surface electromyogram. Clin Neurophysiol. 2010;121(10):1616-1623. doi:10.1016/j.clinph.2009.10.040.
4. Burden A. How should we normalize electromyograms obtained from healthy participants? What we have learned from over 25 years of research. J Electromyogr Kinesiol. 2010;20(6):1023-1035. doi:10.1016/j.jelekin.2010.07.004.
5. Merletti R, Cerone GL. Tutorial. Surface EMG detection, conditioning, and pre-processing: Best practices. J Electromyogr Kinesiol. 2020;54:102440. doi:10.1016/j.jelekin.2020.102440.