Knee Rehab, Day 1: Getting Started with mTrigger Biofeedback

DISCLAIMER: mTrigger, LLC makes every effort to ensure published content accurately reflects current best evidence and clinical practice. All materials provided are intended for informational purposes only and should not be considered a substitute for the care of a health care professional. mTrigger, LLC does not assume any responsibility for how information obtained from mTrigger, LLC is used.

“Tighten your quad.” “Squeeze your thigh.” “Flatten your leg on the table.”

No matter how you put it, it sounds simple. However, after a knee injury or surgery, that familiar muscle contraction can suddenly feel surprisingly difficult. The patient is trying, the clinician is cueing, but the muscle barely seems to respond.

Early rehabilition involves retraining the brain to activate the quadriceps effectively. mTrigger biofeedback supports this process by making muscle activity visible, helping patients connect their effort to contract with the muscles response. 

Where to Begin

The first session should start with one exercise, a clear goal, and immediate feedback. 

For the knee, start with quadriceps activation.

Pain and swelling can interfere with a patient’s ability to voluntarily activate the quadriceps—a response known as arthrogenic muscle inhibition. A patient may be putting in considerable effort while still struggling to recruit the muscle effectively. Research strongly supports that postoperative knee pain and swelling greatly reduce quadriceps muscle activation and strength. 

That first visit provides an opportunity to help patients recognize what a quadricps contraction feels like and practice reproducing it repeatedly. mTrigger biofeedback makes muscle activity visible, helping patients see the results of their efforts during an exercise that can otherwise be difficult to feel.

mTrigger uses surface electromyography, or sEMG, to detect electrical activity associated with muscle activation through electrodes placed on the skin. That activity appears in the mTrigger app, allowing patients to watch the signal change as they contract and relax.

Unlike electrical stimulation, biofeedback does NOT create the contraction for the patient. It requires the patient to practice volitional control of muscle activation while the device provides information about the activity it detects.

Step 1: Set Up

Begin with the patient comfortably positioned for a quad set, following their rehabilitation precautions and surgical guidelines. 

Connect the electrode cables to Channel 1, turn on the device, and connect via Bluetooth to the mTrigger app. A single-channel setup keeps the first session focused on one task: recognizing and controlling quadriceps activity. 

Now, clean the skin and place the mTrigger sensing electrodes over the VMO or VMO + rectus using the mTrigger electrode placement guide. Avoid incisions, dressings, and irritated skin.

Step 2: Perform the Exercise

Start with a simple cue:

“Tighten the front of your thigh and watch the feedback rise. Now relax and let it settle.”

During each repetition, allow the patient to practice while watching the response on the screen. Ask what they feel when the feedback rises, helping them connect the visible muscle activity with the sensation of a successful contraction.
Watch the patient as well as the app. Look for quadriceps tightening and notice substitutions such as lifting the hip, squeezing the glutes, or pressing the heel down without clearly tightening the quad. If a cue does not produce the intended response, change the wording or positioning rather than simply asking the patient to try harder. A higher reading is only useful when it accompanies the intended movement.

Make sure the patient performs multiple sets! As the contraction becomes more consistent, have the patient perform a repetition without watching the screen, then check the feedback. This gives them an opportunity to recognize and reproduce the contraction with less reliance on visual input.

In settings, the clinician can select the contract time, rest interval, and number of repetitions based on the patient’s tolerance and protocol. If contractions fade or symptoms increase, shorten the effort, provide more rest, or reassess the setup. The goal is repeatable, quality, reps!

Step 3: Progress when Ready

As muscle activation improves and the rehabilitation protocol allows, biofeedback can help patients carry quadriceps activation into a straight leg raise or short arc quad exercise. These are possible progressions—not requirements for the first visit—and should follow the patient’s clinical presentation and surgical guidelines.

For a straight leg raise, cue the patient to “Tighten your thigh first, keep your knee straight, then lift your leg off the table.” Use mTrigger feedback to encourage increased quadriceps activation before lifting, then for maintaining activation while raising and lowering the leg to help keep the knee straight throughout the movement. Watch for an extension lag, where the knee bends or loses its available straight position during the lift. mTrigger is perfect for encouraging sufficient quad activation to perform this exercise correctly. 

If the patient cannot maintain knee extension, return to quad sets or use clinician-assisted practice rather than continuing unassisted repetitions with a lag. Continue to utilize mTrigger to maximize muscle activation and improve neuromuscular control.

For a short or long arc quad, use a seated position and the knee-extension range permitted by the patient’s protocol. Cue the patient to “Tighten the front of your thigh, straighten your knee through the allowed range, then lower slowly.” Use mTrigger biofeedback to encourage increased quadriceps activation as the patient straightens the knee, then to sustain active control through any prescribed hold and the slow lowering (eccentric) phase, recognizing that the signal will naturally vary throughout the movement.

When changing exercises, recheck the mTrigger goal since muscle position and task demands will change the desired MVC goal. The aim during early rehabilitation is to pair purposeful muscle activation with controlled movement.

Step 4: Session Wrap Up

On day 1, success may be small but meaningful. If the patient says, “I can see it—and now I know what I’m trying to feel,” that is huge. 

Day 1 is about helping the patient recognize a quadriceps contraction, understand how to produce it, and begin repeating it with greater control. They may still need cues, rest breaks, and adjustments to their goal, but they should leave with a clearer understanding of what a successful effort looks and feels like—and how to practice it on their own at home.

This matters because early rehabilitation involves relearning how to activate the muscle as well as rebuilding strength. mTrigger makes that practice more purposeful by providing immediate feedback, helping patients develop the awareness and control they will build on throughout recovery.

Troubleshooting

If the feedback seems inconsistent, check electrode contact, skin preparation, cable connections, and positioning. Make sure the device is not resting on another electronic device or conductive surface.

Some signal fluctuation is expected. An unexpected reading is a reason to check the setup and observe the movement—not automatically a reason to tell the patient to squeeze harder.

Summary

A successful first session gives patients a clear starting point and a goal they understand. With mTrigger, even small successes can become visible milestones, helping patients leave knowing what to practice and why each repetition matters.

mTrigger for Shoulder Rehab

Advanced Applications of mTrigger Biofeedback

References 

1. Sonnery-Cottet B, Saithna A, Quelard B, et al. Arthrogenic muscle inhibition after ACL reconstruction: a scoping review of the efficacy of interventions. Br J Sports Med. 2019;53(5):289-298. doi:10.1136/bjsports-2017-098401
2. Frerichs C, DeHope J, Chamberlain M, et al. The Effects of Verbal Cues on Electromyographic Activity During a Quadriceps Setting Exercise. J Athl Train. 2025;60(2):119-124. doi:10.4085/1062-6050-0118.23
   
3. Kirnap M, Calis M, Turgut AO, Halici M, Tuncel M. The efficacy of EMG-biofeedback training on quadriceps muscle strength in patients after arthroscopic meniscectomy. N Z Med J. 2005;118(1224):U1704. Published 2005 Oct 28.
4. Xie YJ, Wang S, Gong QJ, et al. Effects of electromyography biofeedback for patients after knee surgery: A systematic review and meta-analysis. J Biomech. 2021;120:110386. doi:10.1016/j.jbiomech.2021.110386
5. Morales-Sánchez V, Falcó C, Hernández-Mendo A, Reigal RE. Efficacy of Electromyographic Biofeedback in Muscle Recovery after Meniscectomy in Soccer Players. Sensors (Basel). 2022;22(11):4024. Published 2022 May 26. doi:10.3390/s22114024
6. Hopkins JT, Ingersoll CD. Arthrogenic Muscle inhibition: A Limiting Factor in Joint Rehabilitation. J Sport Rehabil. 2000;9(2):135-159. doi:10.1123/JSR.9.2.135 

DISCLAIMER: mTrigger, LLC makes every effort to ensure published content accurately reflects current best evidence and clinical practice. All materials provided are intended for informational purposes only and should not be considered a substitute for the care of a health care professional. mTrigger, LLC does not assume any responsibility for how information obtained from mTrigger, LLC is used.