Changes in passive hamstring stiffness after primary anterior cruciate ligament reconstruction: A prospective study with comparison of healthy controls

Background: The mechanical properties of knee flexors muscles contribute to reducing anterior cruciate ligament loading. This case-control study evaluated the passive knee flexors stiffness after primary anterior cruciate ligament reconstruction with comparison of healthy controls. Methods: After anterior cruciate ligament reconstruction, 88 participants (24.5 [8.6] years, 56,8% males) had two isokinetic tests at 4 and 8 postoperative months with measurement of the passive resistive torque of knee flexors and extensors/flexors strength. In the control group, 44 participants (24.5 [4.3] years, 56,8% males) had one visit with the same procedures. Passive knee flexors stiffness was calculated as the slope of the passive torqueangle curve on the last 10 of knee extension (Nm/). We investigated the impact of timing and type of surgery (autograft and combined meniscus repair) and persistent knee extension deficits on knee flexors stiffness. Findings: At 4 and 8 postoperative months, passive knee flexors stiffness was lower on the operated limb than on the non-operated limb (P < 0.001) but both limbs had significant lower values than controls (P < 0.001). Stiffness was positively correlated with knee flexors strength (P < 0.010), and knee flexors stiffness at 4 months was lower in individuals who underwent surgery <6 months from injury (P = 0.040). Knee extension deficit or the type of surgery did not have a significant influence on knee flexors stiffness. Interpretation: Similarly to neuromuscular factors that are traditionally altered after anterior cruciate ligament reconstruction, evaluating passive knee flexors stiffness changes over time could provide supplementary insights into postoperative muscle recovery.