How Strong Should You Be After ACL Reconstruction?

New Reference Values Across Age and gender

Strength testing is an important part of rehabilitation after anterior cruciate ligament reconstruction (ACLR). Traditionally, clinicians have often compared the reconstructed limb with the uninvolved limb using the limb symmetry index (LSI). However, achieving symmetry does not necessarily mean that an athlete has regained sufficient strength. Both limbs may be weak.

A new study by Kipp and colleagues provides reference values for quadriceps and hamstring strength across different age groups and between female and male athletes. The results are particularly interesting because the reported values—especially among female athletes—are lower than many clinicians may expect.

What did the researchers investigate?

The study included 321 athletes who were scheduled to undergo primary ACL reconstruction:

• 171 female athletes, with a mean age of 27.5 years
• 150 male athletes, with a mean age of 25.0 years

All participants wanted to return to an athletic activity following surgery.

Before ACL reconstruction, the athletes completed isokinetic strength testing of the uninvolved limb. Quadriceps and hamstring strength were measured concentrically at 60°/second using an isokinetic dynamometer.

Peak torque was normalized to body mass and reported as Newton-metres per kilogram (Nm/kg). The athletes were divided into six age groups:

• 10–14 years
• 15–19 years
• 20–29 years
• 30–39 years
• 40–49 years
• 50+ years

The researchers also calculated the conventional hamstring-to-quadriceps strength ratio.

What were the reference values?

Strength decreased with age

The study found a linear reduction in normalized strength with increasing age:

• Quadriceps peak torque decreased by approximately 0.016 Nm/kg per year.
• Hamstring peak torque decreased by approximately 0.009 Nm/kg per year.

Age explained approximately 25% of the variation in quadriceps strength and 24% of the variation in hamstring strength. This suggests that age should be considered when interpreting absolute strength after ACL reconstruction.

A 16-year-old athlete and a 50-year-old athlete should not necessarily be assessed against exactly the same reference value.

What about the differences between women and men?

The adult female groups generally demonstrated lower mean quadriceps and hamstring peak torque than the corresponding male groups.

For example, among athletes aged 20–29 years:

• Female quadriceps strength was 2.21 Nm/kg compared with 2.79 Nm/kg in males.
• Female hamstring strength was 1.06 Nm/kg compared with 1.36 Nm/kg in males.

However, an important nuance is that the sex differences in the statistical model were not statistically significant. The values should therefore not be interpreted as evidence that female athletes need less strength or should automatically be given lower rehabilitation targets.

Instead, the findings indicate that age and sex may help provide context when clinicians interpret strength results.

The hamstring-to-quadriceps ratio remained relatively stable

Despite the decline in absolute strength with age, the hamstring-to-quadriceps ratio remained relatively consistent across age groups and sexes. Mean ratios ranged from approximately 0.46 to 0.54.

This is also noteworthy because a ratio of at least 0.60 is sometimes recommended. The findings show that healthy uninvolved limbs do not necessarily achieve this traditional benchmark.

As with all ratios, the result should be interpreted alongside the athlete’s absolute quadriceps and hamstring strength. A satisfactory ratio can still occur when both muscle groups are weak.

Reference

Kipp JD, Bednarek AL, Sherry MA, Keefe MW, Chumanov ES. Isokinetic Quadriceps and Hamstring Peak Torque in Athletes Aged 10–50+: Preoperative Uninvolved Limb Reference Values for ACL Reconstruction. International Journal of Sports Physical Therapy. 2026;21(9):832–842. doi:10.26603/001c.166786