A rising limb symmetry score does not always mean the surgical knee got stronger. Moran (2022) tracked 144 athletes from 6 to 9 months after ACL reconstruction. Nearly a quarter of them gained more than 10 percent extension strength in the healthy leg during that window, not the surgical one. That group already carried a higher symmetry score at 6 months, before most of that healthy-leg gain had even happened. A clinician reading a rising percentage as surgical-limb progress should check which leg actually moved.
What did Moran (2022) actually measure?
The study followed 144 patients after ACL reconstruction, retesting quadriceps extension strength at 6 months and again at 9 months (Moran 2022). Thirty-five patients, about 24 percent of the group, gained more than 10 percent extension strength in the uninjured leg over that period. That subgroup's limb symmetry index at the 6-month mark averaged 67.3 percent, against 59.4 percent for the rest of the cohort. Surgical-leg strength was similar between the two groups at both time points. The higher symmetry score belonged to the group whose healthy leg had further to climb, not to the group whose surgical leg had recovered more.
Why would the healthy leg's strength change the symmetry score?
Limb symmetry index is a ratio: surgical-leg strength divided by healthy-leg strength. Every ratio has two numbers that can move. A clinic that retests only the surgical leg, and assumes the healthy leg held still, is measuring half the equation. Rice and McNair (2010) describe why the surgical leg lags in the first place. Joint injury reflexively inhibits the quadriceps through neural pathways. That inhibition operates independent of how hard the patient tries. It can keep surgical-leg strength gains slow and uneven for months. The uninjured leg, still absorbing more of the patient's daily load, keeps adapting on its own schedule the whole time.
Does this mean strength symmetry testing should be trusted less?
Not thrown out. Weighed correctly. Undheim (2015) reviewed 39 studies on isokinetic strength testing after ACL reconstruction. The review found these measures have not been validated as reliable predictors of a successful return to sport. Testing protocols also vary widely across the literature. Wellsandt (2017) found a related version of the same problem from a different angle. Patients cleared more often against a moving healthy-leg baseline than against their own pre-injury capacity. In that cohort, 57.1 percent passed a 90 percent limb symmetry threshold. Only 28.6 percent met 90 percent of their own estimated pre-injury strength. Two different baselines, two different pass rates, same patient.
How does this change Monday morning?
Retest both legs at every strength-symmetry checkpoint, not just the surgical one. A symmetry percentage without a same-day healthy-leg number attached cannot tell a clinician which side moved. Log both raw values in the chart, not only the ratio. That way a jump in the percentage can be traced back to its source before it goes into a return-to-sport decision. Strength testing and functional progressions bill under CPT 97530 (Therapeutic Activities) or CPT 97112 (Neuromuscular Re-education). Pairing that with a reactive measure adds a data point that is not expressed as a ratio between two moving legs. QuickBoard's Agility Rating captures bilateral reaction time and choice accuracy under full body weight, and bills separately under CPT 97750 (Physical Performance Test).
Takeaways
- Moran (2022) found that patients whose healthy leg later gained over 10 percent extension strength already carried a higher symmetry score at 6 months, while the surgical leg performed about the same as everyone else's.
- Limb symmetry index is a ratio, and the healthy leg is not a fixed number.
- Rice and McNair (2010) found that arthrogenic muscle inhibition slows surgical-leg strength gains independent of patient effort.
- Wellsandt (2017) found 57.1 percent of patients passed a 90 percent symmetry threshold, but only 28.6 percent met 90 percent of their own pre-injury strength.
- Log both legs' raw strength numbers at every checkpoint, not just the ratio.
A symmetry score is a snapshot of two moving numbers, not one. See Ortho Rehab and ACL Return to Sport for the full clinical use case, the published Research library for the QuickBoard studies behind the Agility Rating, and How QuickBoard Works for the reactive assessment model.
Citations
- Moran 2022
- Wellsandt 2017
- Rice 2010
- Undheim 2015
See QuickBoard research for full citation details.