Study: KINVENT K-Pull a Reliable Tool for Measuring Quadriceps Strength After ACL Reconstruction

A recent study found the KINVENT K-Pull is not only a reliable alternative to the HUMAC NORM Cybex when it comes to rehabilitating reconstructed ACLs – it’s also faster, less costly, more practical, and more accessible to the average physical therapist.

Each year, tens of thousands of anterior cruciate ligament (ACL) reconstruction surgeries are performed in the United States, alone. After surgery, the ACL rehabilitation process can be arduous: once the worst of the inflammation subsides, patients must not only work to regain their balance and knee’s range of motion – they must also build back the strength they lost in their injured leg.

While training muscles including the calves and hamstrings is important, strengthening the quadriceps is key to successfully rehabilitating the injury and mitigating the risk of reinjury. Physical therapists (PTs) use the recovery of quadriceps strength to make decisions about a patient’s readiness to return to sport and resume other activities, like running.

When it comes to measuring quadriceps strength and tracking progress, isokinetic and isometric electromagnetic dynamometry (ED) is considered the “gold standard” testing method. Handheld, “push-type” dynamometers (HHD) may be used as an alternative. Unfortunately, both methods have significant drawbacks (outlined below).

Recently, researchers sought to find a reliable alternative to ED and HHD to measure the quadriceps strength of ACL-reconstructed patients. The resulting study, published in The Knee, set out to determine whether inline dynamometry using a “pull-type” dynamometer could serve as that reliable alternative.

The Pitfalls Assessing Quadriceps Strength with ED & HHD

Quadriceps strength – expressed as peak knee extensor torque – is most commonly measured at 60° knee flexion. The preferred method of measuring ACL strength after reconstruction is with isometric ED with the knee at 60° flexion.

And while this form of testing is highly accurate—it “can detect small, clinically relevant changes (<10%) in strength,” according to the study—it requires a large, stationary machine like the HUMAC NORM Cybex. These machines, which can cost tens of thousands of dollars, are not commonly found in standard PT clinics.

Handheld, “push-type” dynamometers – which are a staple PT tool – may be used as an alternative to ED in a pinch. But, while these dynamometers are much more practical and accessible to the average PT, the measurements they produce are insufficiently reliable in ACL-reconstruction rehabilitation; they grossly underestimate quadriceps strength by nearly 30% at 60° knee flexion, according to the study.

Prior to the study, it was determined that “pull-type” dynamometers measure the quadriceps strength of healthy individuals more accurately than “push-type” dynamometers. Because of this, researchers hypothesized the superior reliability of “pull-type” dynamometry would apply when used to measure the quadriceps strength of ACL-reconstructed individuals – and would be a viable alternative to ED and HHD testing.

Study Method

50 healthy and 52 ACL-reconstructed individuals participated in the study. Inline dynamometry using the KINVENT KForce Link (the predecessor to the K-Pull) and isometric ED using the HUMAC NORM Cybex was performed to measure the quadriceps strength of healthy participants’ dominant legs and ACL-reconstructed participants’ injured legs. All testing was performed at 60° knee flexion.

Each participant completed three trials per testing method. During each trial, the participant was instructed to extend their knee with maximal effort for five seconds to measure peak force. Data from the tests performed with the KForce Link was collected via Bluetooth and sent to KINVENT’s K-Force App (now called the KINVENT Physio App). Data from the tests performed using the HUMAC NORM Cybex was collected by the machine’s integrated software and displayed on its connected computer interface.   

The Results

The researchers’ hypothesis that inline dynamometry reliably measures quadriceps strength in ACL-reconstructed individuals was supported. The study concluded: “inline dynamometry is a reliable and economical alternative to electromechanical dynamometry for the assessment of quadriceps strength following ACL-reconstruction.”

Beyond reliability, the study found that, compared to isometric ED, inline dynamometry is:

• Faster
  • Testing on the K-Pull was completed in 3 minutes, 43 seconds
  • Testing on the HUMAC NORM was completed in 6 minutes, 20 seconds
• More portable
  • The HUMAC NORM is a hulking, stationary machine that requires installation and a power source
  • The K-Pull weighs 1.3 lbs., is rechargeable, and may be used anywhere
• More cost effective
  • The HUMAC NORM costs tens of thousands of dollars
  • The K-Pull costs $1,390 (plus KINVENT Physio App subscription, starting at $350 annually)

To learn more, read the full study.

Schedule a Demo

In addition to helping physical therapists advance patients through ACL reconstruction rehabilitation, the K-Pull may be used to test endurance and assess muscular symmetry. It’s one of eight Bluetooth-enabled KINVENT devices connected to the KINVENT Physio App.

KINVENT’s suite of biofeedback devices has revolutionized the way physical therapists, sports medicine practitioners and others deliver care to their patients. KINVENT device flexibility, accuracy and efficiency, and the app’s advanced reporting capabilities help doctors provide superior care to patients which, ultimately, drives improved health outcomes. Contact our KINVENT product experts to learn more.