Randomized controlled trial comparing reconstruction of anterior cruciate ligament (ACL) with autologous quadrupled semitendinosus graft or with both semitendinosus and gracilis. It is intended to specially evaluate if by using only the semitendinosus (ST) tendon, the strength of the limb for hamstrings is affected different compared to using both gracilis and ST.

Data checks programmed by a third party after inclusion of participant number 10, 20, 30 and 42. This includes source data verification.

Simple size calculations estimating a difference of 50% in strength improvement between both groups at the six months follow-up visit. 80% power and 5% alpha error.

Strength measured with dynamometer, laxity with ACL laxity tester and function with 3 scores: IKDC, Tegner-Lysholm and KOOS.

Missing data will be minimized by double checking when registering data. It might be necessary to increase sample size to 50 patients of intermediate analysis show it is necessary to find an statistically significant difference.

Trial could be stopped early if one intervention shows to be better in the intermediate analysis. These analysis will be conducted after the first 16 and 32 participants have been included.

Inclusion Criteria:

• Patients with anterior cruciate ligament injury that are candidates for surgical reconstruction with hamstrings. They should be at least 16 years old.

Exclusion Criteria:

• Multiligament injuries. Re-ruptures of the ACL.