Biofeedback after ACL Reconstruction in Ski Season

It’s officially ski season! Unfortunately, that means it’s also knee injury season. It’s no secret that anterior cruciate ligament (ACL) injuries are common during the winter skiing months – over 200,000 ACL occur annually in the United States alone.(1) Most ACL injuries are seen between 15-30 years of age, however, the snow and ice claim plenty of skiers over 30 as well.(1)

After an ACL injury, 90% typically elect to undergo surgery and have the ligament reconstructed.(1) The next nine to twelve months are largely devoted to the rehab and recovery process. At the end of it all, residual muscle imbalances, a fear of re-injury, and altered lower extremity mechanics can still persist.(1) Failure of current therapeutic approaches to restore symmetry combined with other risk factors yields a very high 30% re-tear rate.(1) Clearly this indicates that we are missing something in our rehabilitation plans! Visual EMG biofeedback is one of the tools that can help enhance the ACL recovery process by addressing some of the critical lingering deficits noted above.

EMG is a tool used to record the electrical activity of a specific muscle or group of muscles.(2) The information provided by EMG signifies to what degree that muscle is active.(2) These patterns of muscle activation are then converted into visual signals that can be observed as feedback by the clinician and athlete.(2) Using EMG biofeedback can help to improve neuromuscular training and is perhaps one of the most useful tools because of its immediate feedback component.(2)

Several key factors tend to persist after an ACL injury, all of which contribute to long term performance, knee health, and functional activity. Let’s take a minute to review what the literature says about deficits after ACL reconstruction and how biofeedback can be a helpful tool for overcoming these barriers. Then we will discuss clinical examples and application for each.

Restoring Quad Function

The recovery of the quadriceps muscle following ACL reconstruction is critical. EMG biofeedback is used to facilitate muscle activation during exercise and has been shown to help increase isometric peak torque of the quad muscle after 6 weeks of exercise with biofeedback intervention.(3–5) In fact, it was even more effective than neuromuscular electrical stimulation at recovering peak quad torque and achieving active knee extension.(3,5) When compared to the non-operative limb at 12 weeks, the addition of biofeedback to muscle strengthening exercises helped to facilitate the recovery of quadriceps muscle function following ACL reconstruction.(4,5) This recovery is critical, as some studies have shown 57% of people still have extension mechanism deficits at 12 months post op.(6) This quad deficit is something that can be measured and tracked with mTrigger as well. By using the Neuromuscular Deficit Test (or Train in dual channel mode) to perform a bilateral exercise, the system can determine the average maximum voluntary contraction level (MVC) and calculate the deficit. This is an extremely important aspect of recovery – more to come on that later.

Let’s look at some exercise examples:

Long Arc Quad
1. 1. Using single or dual channel, place the electrodes on the quadricep muscle (VMO and rectus femoris, or two on the VMO) of the involved side.
  2. Instruct patient to activate their quadricep muscle while performing a long arc quad, trying to get their leg all the way straight.
  3. As the patient performs the exercise, they are instructed to increase the muscle activation meter and to maintain that high level of isometric contraction before resting. I would recommend starting with at 5-10s isometric contraction at the top of the exercise and progressing from there.
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SLR / Quad set with game
1. 1. Using a single channel, place the electrodes on the quadricep muscle (VMO and rectus femoris or two on the VMO) of the involved side. See photo above.
  2. Perform a straight leg raise using the Muscle Ball Game for some fun, entertainment, and a good rehab challenge!

Restoring Functional Movement

Altered lower extremity mechanics often persists following an ACL reconstruction and must be an intentional focus during the rehabilitation process in order to prevent limb asymmetries.(7) It was found that when squatting, those with an ACL reconstruction exhibit decreased knee extension moments, knee flexion angles, and a tendency to offload their surgical limb.(7,8) Furthermore, deficits seen during squatting were also seen during landing and more advanced movements. It is critical to work on improving the biomechanics and muscle activation pattern of the squat so that this correct form and patterning transfers to more dynamic and sports specific skills down the road, ultimately protecting the athlete from reinjury.(8) Furthermore, these same mechanics must be transferred to single leg activities and squatting to further challenge the neuromuscular and strength systems.(6)

QUADS & GLUTE MED

Squat
1. 1. Using dual channel, place one set of electrodes on the right quadricep (VMO and rectus) and one set on left quadricep.
  2. Instruct patient to perform a squat (back squat, kettlebell squat, body weight squat) slowly and smoothly.
  3. As the patient performs the exercise, they are instructed to increase the muscle activation of their quadricep muscle then sustain that increased level of activation throughout the exertion of the exercise. Both meters should increase together to indicate equal weight and activation.
    *As most athletes tend to shift towards more of a hip activation strategy to reduce effort at the knee, it is important to train squatting patterns to sufficiently activate the quads in order to promote increased strength and performance.(9)
Step up
1. 1. 1. Using dual channel, place one set of electrodes on the quadricep (VMO and rectus) and one set on the glute medius of the involved side.
    2. Instruct patient to perform a step up with their involved leg to the desired height slowly and smoothly.
    3. As the patient performs the exercise, they are instructed to increase the muscle activation of their quadricep muscle as they step up. The meter should increase on channel 1 until they reach a fully standing position. They are also instructed to increase the activation of the glute medius throughout the excursion of the exercise until they reach a fully standing position, so the meter on channel 2 should also go up. When lowering back down, they are instructed to increase the activation of both channels right away and sustain it throughout the movement until reaching the ground.
  *Channel 1 is attached to the quad, channel 2 is attached to the glute muscle. You can see as she gets tired, there is less quad activation as she “falls” into the hip more. This is an excellent visual training tool for her!
Single Leg Squat
1. 1. Using dual channel, place one set of electrodes on the quadricep (VMO and rectus) and one set on the glute medius of the involved side.
  2. Instruct patient to perform a single leg squat to a box/bench desired height slowly and smoothly.
  3. As the patient performs the exercise, they are instructed to increase the muscle activation of their quadricep muscle as they lower down and stand back up. The meter should increase on channel 1. Simultaneously, they are instructed to increase the activation of the glute medius throughout the excursion of the exercise. The meter on channel 2 should also go up.

HAMSTRINGS

Hamstring deficits after ACL reconstruction are unfortunately too common and linked to increased risk of re-injury.(10) Partially due to graft selection and improper training, a hamstring deficit can be detrimental. The hamstring muscle “pulls back” on the lower leg bone offering further protection to the reconstructed ACL graft.(11) Therefore, regaining sufficient hamstring strength following ACL surgical is critical. A big part of this is achieving optimal loading in order to bring about specific neural, morphological, and mechanical adaptations.(11) To respect biological healing constraints and tissue capacity, optimal loading will depend on each individual, where they are in the rehab process, and their goals.(11)

Bridge/Heel Dig (low sustained intensity/isometric holds)
1. 1. Using dual channel, place one set of electrodes on the hamstrings (biceps femoris) and one set on the glutes of the involved side.
  2. Instruct patient to to dig their heels into the table/box (or a ball) and lift their hips up off the ground performing a bridge.
  3. As the patient performs the bridge, they are instructed to activate their glute muscle first (channel 2), then the hamstrings (channel 1). Note timing of activation on the feedback meter. Both meters should increase and stay activated throughout the duration of the motion. Channel 2 is attached to her glutes and should activate first. Channel 1 is attached to the hamstring and should activate just after the glutes do.
Single Leg Deadlift (motor re-patterning and functional muscle strengthening)
1. 1. Using single channel, place one set of electrodes on the hamstring (biceps femoris)
  2. Instruct patient to perform a proper single leg deadlift using a kettle bell, bar, or body weight moving slowly and smoothly.
  3. As the patient performs the exercise, they are instructed to increase the muscle activation of their hamstrings muscle as they lower down, performing the eccentric portion of the exercise. The meter on channel 1 should increase. As they return to standing, the activation meter should be maintained and ideally increase.
* The electrodes on the R leg are to show placement. They are not attached to an active channel; L leg is attached to CH1 with the same electrode placement as shown on R leg.
Deadlift
1. 1. Using dual channel, place one set of electrodes on the left hamstring and one set on the right hamstring.
  2. Instruct patient to perform a proper deadlift (kettle bell, bar, trap bar, or band) slowly and smoothly.
  3. As the patient performs the exercise, they are instructed to increase the muscle activation of their hamstrings muscle as they lower down. The meter should increase equally on both channels – cue verbally to improve activation balance as needed. As they return to standing, instruct them to again increase the activation of both hamstrings. The meter on both channels should go up.
* Listen to how verbal cueing during rep 3-4 (26 seconds in) helps to improve left sided muscle activation when coming out of the hole. All forms of biofeedback can be helpful in achieving proper muscle activation.

Next Steps

One of the most import things to remember is that you don’t stop using surface EMG after active knee extension and basic movement is established. Life doesn’t happen sitting down, you must make sure an athlete can continue to make progress and functional gains in progressively challenging tasks and positions. Using these principles and your knowledge of the literature, progress your patients through their prescribed regimen while tracking real-time activation and progress with sEMG BFB.

Summary

After an ACL injury and subsequent surgery, there are several critical factors to address in rehabilitation: quad activation, knee extensor mechanism, muscular imbalances, and altered lower extremity biomechanics. Neuromuscular and strength deficits can persist for a couple years following an ACL injury, making the use of proven mechanisms to enhance recovery critical. Surface EMG biofeedback is one of those tools that can be used not only in the beginning stages of rehab to facilitate the recovery of muscle function and strength, but throughout the entire rehab process to keep athletes on track and making functional progress.

Want to learn more about biofeedback for knee injuries?