Testing Strength in ACL Reconstructed Patients: Is Symmetry the Answer?

This is a summary of a recently published article in Medicine and Science of Sport and Exercise entitled “Quadriceps Strength Predicts Self-Reported Function Post ACL Reconstruction”.

Dr. Pietrosimone’s co-authors on this paper include Adam Lepley, Matthew Harkey, Brittney Luc- Harkey, Troy Blackburn, Phillip Gribble, Jeffrey Spang and David Sohn.

Why Did We Perform this Study

quadriceps-musclesQuadriceps muscle weakness is common following anterior cruciate ligament (ACL) injury and ACL reconstruction. This muscle weakness often persists many years after someone has undergone knee surgery and returned to participation in physical activity. Quadriceps weakness often leads to greater disability in those with ACL reconstruction, and there is evidence that quadriceps dysfunction may negatively impact gait mechanics that can increase the risk of developing knee osteoarthritis. Therefore, it is critical for patients who have undergone ACL reconstruction to maximize quadriceps strength following surgery and maintain optimal quadriceps strength throughout their lifetime. Unfortunately, it remains unclear how much quadriceps strength is needed to function at a high level following ACL reconstruction. Furthermore, there are no established best practice guidelines for quantifying quadriceps strength in patients with an ACL reconstructions. Traditionally, many clinicians and researchers have compared quadriceps strength of the injured limb to that of the uninjured limb; thereby trying to maximize the strength symmetry between limbs after ACL injury. Conversely, we have previously reported that greater overall quadriceps strength, normalized to the body mass of the individual, is strongly associated with self-reported disability in those with an ACL reconstruction. Therefore, in this study, we wanted to determine the best method for using quadriceps strength to predict self-reported function following ACL reconstruction. We felt that this study would help us develop valuable clinical cuff-off scores that could be used to guide strengthening regimens for patients with an ACL reconstruction.

What we did in this study


We tested isometric quadriceps strength at 90-deg of knee flexion. Most individuals following ACL injury and reconstruction can be tested in this position at multiple time points. Also, testing in this manner may be more easily conducted with a range of strength testing instrumentation in clinical and research settings.

We recruited 96 individuals with a primary ACL reconstruction on only one limb (62 females and 34 males; 21.7±3.85 years old; 37.04 ± 36.7 [range 6-161] months post ACLR). We tested the maximal strength quadriceps strength on each limb in a random order using a dynamometer. Each participant performed maximal voluntary isometric contractions in a seated position with their knees flexed to 90° (See Figure 1). On the same day participants completed the subjective section of the International Knee Documentation Committee Index questionnaire in order to determine the magnitude of self-reported disability for each participant. We considered anyone scoring over 90% on the International Knee Documentation Committee Index to be at a high level of function (we termed these individuals “High Functioners”). Next we created Receiver Operating Characteristic Curves to determine how well quadriceps strength predicted who would be a High Functioner. After determining the quadriceps strength cut-off scores that maximized the sensitivity and specificity for predicting the High Functioner status, we calculated odds ratios to determine how well our cut-off scores were at determining a High Functioner status.

What we found and how it Impacts Healthcare

Overall, regardless of quantification method, quadriceps strength was able to significantly predict which individuals would be High Functioners. Interestingly we found that overall quadriceps strength, normalized to body mass ,demonstrated a higher accuracy for predicting who would be a High Functioner, compared to quadriceps strength symmetry. Specifically, those who were able to produce overall quadriceps strength that exceeded 3.10 Newton-meters per kilogram of body mass demonstrated 8.15 times higher odds of being a High Functioner. Therefore, a 150lb individual (68.18 kg) would need to generate 211.36 Newton-meters of torque with their quadriceps in order meet the prescribed cut-off score. We found that individuals who achieved a strength symmetry value of 96.5% (injured limb strength/ uninjured limb strength) would have a 2.78 times higher odds of being a High Functioner. Therefore, an individual who can produce 250 Newton-meters of torque with the uninjured limb would need to produce 241.25 Newton-meters of torque on the ACL reconstructed limb to achieve this cut-off value.

Overall Strength vs Strength Symmetry

Strength symmetry has traditionally been used to determine which participants are able to generate enough strength to return to participation in physical activity. While improving strength symmetry is still important for ACL reconstructed patients, achieving symmetry alone may not be sufficient for optimizing therapeutic outcomes. It is possible that some people may demonstrate symmetrical quadriceps strength, yet they may not exhibit enough strength relative to the size of their body. Our data suggests that maximizing overall quadriceps strength to support the size of the person is critical in determining the functional status for that individual. Bilateral quadriceps weakness may be of particular concern in individuals who have a history of bilateral ACL injury. Rehabilitation goals for ACL reconstructed individuals may need to be amended to include the development of strength relative to a patient’s size rather than the contralateral limb. Bilateral strengthening may be critical for many patients in order to achieve the desired ratio on the ACL reconstructed limb (3.10 Newton-meters/ kilograms of body mass), while still marinating strength symmetry between limbs. It may also be necessary to address the maintenance of a healthy body weight, or when prudent, to lose excess non-lean body weight which will also improve the ratio of quadriceps strength to body mass. Future ACL rehabilitation guidelines should consider testing patients’ overall strength relative to their body mass in addition to strength symmetry outcomes.