Behind the Strength: How In-Line Dynamometry Guides Knee Rehab

rangeptmontana
Jun 27
2 min read

Written by Sports Physical Therapist, Brenna McGuinness, DPT

In rehab, we care not just about how you feel — but how your body is actually functioning. One tool we use to measure recovery is an in-line dynamometer. It’s quick, objective, and gives us key insights that help guide your return to activity.

What Is an In-Line Dynamometer?

An in-line dynamometer is a small device that measures how much force a muscle produces during an isometric contraction (a push or pull without movement). For knee extension, we typically:

Position you seated with your leg bent.
Attach the dynamometer just above your ankle, securing it to a fixed anchor.
Ask you to push forward (as if straightening your knee) without moving.
Record the peak force output.

Why It Matters

After injury or surgery — especially ACL reconstruction or patellar tendon repair — strength loss in the quadriceps is common and persistent. But it’s not just about how strong one leg is. We also look at:

TTBW (Torque to Body Weight ratio): This is your peak force output divided by your body weight. It helps us gauge whether your leg is producing enough force for your body size. It’s especially useful for setting objective benchmarks in return-to-sport testing.
Force Asymmetry (%): This compares your injured leg to your non-injured leg. We calculate how far off the weaker side is. Most return-to-sport criteria recommend a side-to-side difference of less than 10% before progressing to full activity.

Who Is It For?

Post-op patients (ACL, meniscus, total knee, patellar tendon)
Athletes returning to sport
Active adults or older individuals recovering from injury or deconditioning
Anyone with chronic knee pain or weakness

What It’s Used For in Rehab

Tracking strength over time
Identifying side-to-side deficits (asymmetries)
Making safer return-to-sport decisions
Setting performance goals with objective data
Building confidence with visible progress

When it comes to strength, feeling better doesn’t always mean being ready. The in-line dynamometer gives us the data to support smart, safe rehab — and helps you return to life or sport with confidence.