Knee Extensors Activation Device Versus Rolled Towel for Early Quadriceps Activation After ACL Reconstruction: Protocol for a Randomized Controlled Trial

BACKGROUND Rupture of the anterior cruciate ligament (ACL) and its surgical reconstruction are followed by substantial deficits in quadriceps muscle function. In the early postoperative period, reflexive inhibition of the quadriceps, known as arthrogenic muscle inhibition (AMI), leads to drastically reduced voluntary muscle activation. Quadriceps activation failure contributes to weakness, which is concerning since quadriceps strength is strongly related to knee function and stability. Without effective intervention, patients can experience persistent strength deficits, delayed functional recovery, and even altered knee biomechanics due to inadequate quadriceps activation. Notably, one study reported that by 2 weeks after ACL reconstruction, the electromyographic (EMG) activation amplitude of the vastus medialis was less than 50% of normal levels, highlighting the severity of early postoperative quadriceps inhibition.

Early rehabilitation therefore prioritizes reactivation of the quadriceps. Standard ACL rehabilitation protocols universally include quadriceps setting exercises beginning immediately after surgery. Quadriceps setting is an isometric contraction of the knee extensors (quadriceps) with the knee in extension, often performed with a rolled towel or bolster placed under the knee to provide a slight knee flexion angle and a target to press down against. This simple exercise is aimed at recruiting the quadriceps muscle and counteracting AMI as soon as possible. Clinical guidelines and expert consensus recommend initiating such isometric quadriceps contractions on post-op day 1 to restore active knee extension and prevent loss of motion. The conventional method using a rolled towel under the knee is easy and requires no special equipment; however, it provides minimal feedback to the patient. The patient or therapist typically counts repetitions, and the assessment of contraction quality (e.g. how hard the muscle is engaging) is largely subjective.

Enhancing feedback during early exercises may improve quadriceps muscle recruitment. Prior studies have shown that adjunctive biofeedback modalities can significantly improve quadriceps activation and outcomes in early ACL rehabilitation. For example, it's demonstrated that adding electromyographic biofeedback (visual EMG signal feedback) to standard exercises in the first weeks after ACL reconstruction led to greater quadriceps EMG output and better knee extension range than exercise alone. Similarly, neuromuscular electrical stimulation (NMES) is often used alongside exercise to help activate the quadriceps, with evidence that combined NMES and exercise yields stronger quadriceps than exercise alone in the early phase post-ACL. These findings underscore the importance of maximizing muscle activation through innovative rehabilitation strategies during the vulnerable immediate postoperative period.

The Knee Extensors Activation (KEA) Device was developed to address the need for objective feedback and standardization in early quadriceps rehabilitation. The KEA device is a novel, portable tool consisting of an inflatable cushion, a hand-held air pump, an integrated pressure gauge, and a digital repetition counter. The inflatable cushion (placed under the patient's knee) provides resistance during isometric contraction, similar to a rolled towel but with adjustable pressure. The built-in pressure gauge displays the pressure inside the cushion, and is equipped with sensor markers that indicate when a full contraction (sufficient pressure threshold) is achieved. When the patient contracts the quadriceps and presses the knee down on the cushion, a pressure indicator moves; reaching the end-range sensor triggers the digital counter to register a successful repetition. The counter only increments when the pressure has risen above a preset threshold (indicating an adequate contraction) and then returns to baseline, ensuring that only correct, full-effort repetitions are counted. By providing visual feedback of pressure and an automatic rep count, the device aims to engage the patient and encourage maximal activation of the quadriceps with each contraction. In doing so, it standardizes the quad set exercise and reduces reliance on the therapist's subjective judgment of effort or on the patient's inconsistent self-monitoring.

It's hypothesized that using the KEA device in the first week after ACL reconstruction will result in greater quadriceps muscle activation compared to the conventional rolled towel method. It's further hypothesized that patients using the device will report equal or better early functional outcomes, such as reduced pain and kinesiophobia, due to potentially greater confidence in knee contraction and improved knee extension ability. This paper presents the protocol for a two-arm randomized controlled trial designed to test these hypotheses. The trial's primary objective is to compare the quadriceps muscle EMG amplitude during a maximal contraction on day 5 post-surgery between patients trained with the KEA device versus those performing standard care (towel under knee). Secondary objectives include comparing patient-reported outcomes (knee function, pain, and fear of movement) between the two groups at the end of the intervention. We anticipate that the findings will provide evidence on whether an inexpensive, feedback-enhanced device can improve early ACL rehabilitation outcomes, which could have implications for standard postoperative care.