Comparison of muscle activation and knee function between injured and non-injured sides post-ACL injury

Ela Yuliana (Universitas Negeri Yogyakarta)
Yasep Setiakarnawijaya (Faculty of Health and Sport Sciences, Universitas Negeri Jakarta, Indonesia)
Wara Kushartanti (Faculty of Medicine, Universitas Negeri Yogyakarta, Indonesia)
Rachmah Laksmi Ambardini (Faculty of Medicine, Universitas Negeri Yogyakarta, Indonesia)
Muhammad Ikhwan Zein (Faculty of Medicine, Universitas Negeri Yogyakarta, Indonesia)
Dewi Nurhidayah (Faculty of Sport Sciences, Universitas Cendrawasih, Indonesia)

Physical Therapy Journal of Indonesia
Vol. 7 No. 2 (2026): Inpress July-December 2026

Background: Decreased knee function after anterior cruciate ligament (ACL) injury is associated with decreased sports participation and even re-injury. This study aimed to assess quadriceps and hamstring activation and knee function by comparing the injured and uninjured sides following ACL injury.Methods: A sample of 19 people, consisting of 15 people from the surgery group and 4 people from the non-surgery group. Knee functional assessments were performed on the injured and uninjured sides, including assessment of quadriceps and hamstring muscle activation using electromyography (EMG), and knee functional tests using single and triple hop tests. Differences in knee function between the injured and uninjured sides were identified using Paired Samples T-Tests, and non-normally distributed data were analyzed using the Wilcoxon test with a significance level of 5%. Meanwhile, a limb symmetry index (LSI) score of ≥90% was used to categorize adequate and symmetrical function.Results: In the surgical group, there were no significant differences between the injured and uninjured sides in quadriceps (P= 0.887), hamstring (P= 0.574), single hop test (P= 0.064), except for the triple hop test (P= 0.020). In the non-surgical group, there were no significant differences between the injured and uninjured sides in quadriceps (P= 0.715), hamstring (P= 0.225), triple hop test (P= 0.107), except for the single hop test (P= 0.008). Meanwhile, LSI values showed varying results. The surgical group had LSI values for quadriceps (98.77%) and hamstring (95.31%) muscle activation, single hop test (82.58%), and triple hop test (83.29%). The non-operative group had LSI values for quadriceps muscle activation (88.62%), hamstring (85.43%), single hop test (74.07%), and triple hop test (65.13%).Conclusion: ACL injury causes a dynamic decline in knee function in both operative and non-operative groups, although it shows adequate and balanced function for quadriceps and hamstring muscle activation, but only in the operative group, and for the non-operative group, quadriceps and hamstring muscle activation are not yet functionally adequate and symmetrical.