Why Do We Test the “Good” Leg Before ACL Surgery?

You're preparing for ACL surgery on one knee—so why is your physical therapist testing the other leg?

Because your “good” leg can give us extremely valuable information about where we want the reconstructed knee to eventually return.

We Need a Baseline

Before surgery, we may measure the uninvolved leg's:

• Quadriceps strength

• Hamstring strength

• Single-leg hop distance, when appropriate

• Triple-hop distance, when appropriate

• Other strength or performance measures

These numbers create a snapshot of your physical capacity before surgery.

Months later, we can compare the reconstructed leg against those original values.

Why Not Just Compare Your Legs Later?

A common method during ACL rehabilitation is the Limb Symmetry Index (LSI).

For example, if your reconstructed leg produces 90% of the performance of your uninvolved leg, you may have a 90% LSI.

But there is a potential problem.

During months of rehabilitation, the “good” leg can also lose strength and athletic capacity.

Imagine your reconstructed leg improves—but your other leg simultaneously gets weaker.

Your legs may look more symmetrical even though you haven't fully returned to your previous level of performance.

A Fixed Target Instead of a Moving Target

One strategy highlighted in ACL rehabilitation research is Estimated Pre-Injury Capacity (EPIC).

Instead of comparing the reconstructed leg only with the other leg at the same postoperative time point, EPIC uses the uninvolved leg's preoperative performance as a fixed benchmark.

That gives us another question to ask later in rehabilitation:

“How close is the reconstructed leg to the level of performance we measured before surgery?”

Why This Is Valuable

Testing before surgery gives your rehabilitation team objective information that may remain useful at:

• 6 months

• 9 months

• 12 months

• Return-to-sport testing

ACL rehabilitation is a long process.

The more useful information we collect at the beginning, the better we can understand your progress later.

So when your physical therapist tests your “good” leg before ACL surgery, we're not ignoring the injured knee.

We're establishing a target for where we eventually want that knee to go.

Preparing for ACL surgery? Petroski Physio uses objective strength and performance testing to establish your pre-surgery baseline and help guide your rehabilitation. Contact us to schedule an ACL prehab evaluation.