We hypothesized that the weight ratio of knee strength is a factor related to the ACL-RSI cutoff for a return to sports at 2 years after reconstruction. Differences between groups were seen in the weight ratio of knee flexion strength and the HQ ratio. The present results supported our hypothesis. Knee flexion strength at different angular velocities in uninvolved limbs as well as involved limbs should be measured and postoperative rehabilitation to increase these strengths should be planned. In addition, the HQ ratio of uninvolved limb as well as involved limb should be calculated, with plans to strengthen the force of knee flexion against knee extension.
The weight ratio of knee flexion strength and HQ ratio is the factor related to the ACL-RSI cutoff for a return to sports at 2 years after reconstruction. In this study, the following muscle strength variables were significantly larger in ACL-RSI ≥60 than in ACL-RSI <60 at 6 months after reconstruction: knee flexion strength (180°/s) of the involved limb, knee flexion strength (60°/s) of the uninvolved limb, and HQ ratio (60°/s) of the uninvolved limb. Performances such as sprint times and jump distances are affected by hamstring function 19,49. During jump landings and cutting, the anterior shear and rotational forces of the tibia are controlled by hamstring function 33,34. Excessive strain in the graft is suppressed by the hamstring 33. For these reasons, knee flexion strength and HQ ratio were significantly higher in the group with better psychological readiness.
Aizawa et al. reported no significant association between ACL-RSI score and knee flexion/extension strength LSI in patients 6 months after reconstruction 1. Müller et al. reported no significant association between ACL-RSI score and knee flexion/extension force LSI or HQ ratio LSI in patients at 6 months after reconstruction 43. O'Connor et al. 45 analyzed the relationship between ACL-RSI score and the weight ratio of knee extension/flexion strength of the involved limbs in patients 9 months after reconstruction. In that study, only the weight ratio of knee flexion strength for the involved limb correlated significantly with ACL-RSI score, and all correlation coefficients including this variable were less than 0.2, indicating a weak association 45. In that previous study, the weight ratio of knee extension/flexion strength and LSI were compared between an ACL-RSI ≥90 group and an ACL-RSI <75 group, and the only significant difference seen between groups was for the weight ratio of knee flexion strength 45. However, the effect size was concluded to be 0.15, with no meaningful difference found 45. In the present study, the flexion strength variable, rather than knee extension, showed a significant difference between groups, with effect sizes in the 0.30–0.50 range. These results appear to support the findings of O'Connor et al. Previous studies did not analyze uninvolved limb knee HQ ratios or flexion weight ratios.
Regarding leg anterior reach distance, LSI was shown to be a factor related to the ACL-RSI cutoff for a return to sports at 2 years after reconstruction. In this study, the LSI of leg anterior reach distance was significantly larger in the ACL-RSI ≥60 group than in the ACL-RSI <60 group at 6 months. In post-reconstruction patients, lower limb strength and neuromuscular control are required for the task of reaching one lower limb forward while standing on the other leg 18,42,48. In a healthy netball player, leg anterior reach distance and knee rotation moment during one-leg landing show a negative correlation 8. Leg anterior reach distance asymmetry is associated with the timing of return to sports after reconstruction 16. For these reasons, the LSI of leg anterior reach distance was significantly larger in the group with better psychological readiness.
Some individuals who meet the criteria for returning to sports after reconstruction have shown a significantly shorter leg anterior reach distance on the operated lower limb than on the non-operated side 47. Leg anterior reach distance LSI was significantly associated with the Knee injury and Osteoarthritis Outcome Score (KOOS)-Symptom (r = 0.30) and KOOS-Sport (r = 0.30) in patients 6 months after reconstruction 50. The present results partially supported the findings from those previous studies. However, previous studies have not analyzed the relationship between ACL-RSI score and leg anterior reach distance, and the present study thus provides new information on this relationship.
For SLH, lateral and medial SLH variables were shown to be factors related to the ACL-RSI cutoff for a return to sports at 2 years after reconstruction. In this study, the following SLH variables were significantly higher in the group with ACL-RSI ≥60 than in the group with ACL-RSI <60 at 6 months: lateral SLH distance of the involved limb, LSI of lateral SLH distance, medial SLH distance of the involved limb, LSI of medial SLH distance. Insufficient jump landing and balance are subjective factors in injury-related fear 46. Kinetics such as knee valgus associated with re-injury show a worse pattern in lateral SLH than in anterior SLH 53. For these reasons, lateral SLH distance and LSI may have been significantly smaller in the group with poorer psychological readiness.
Aizawa et al. identified lateral SLH distance LSI as a factor in ACL-RSI score among patients 6 months after reconstruction by simple regression analysis (β coefficient = 0.58, P=0.031) 1. The present study supported some of the findings from that study. Müller et al. showed that anterior SLH distance LSI was weakly associated with ACL-RSI score in patients 6 months after reconstruction (Pearson’s r = 0.36, P=0.023) 43. Webster et al. revealed by simple regression analysis that the LSI of anterior SLH distance is a factor in ACL-RSI score for patients at 12 months after reconstruction (β coefficient = 0.50, P=0.001) 61. In the present study, no difference was observed between groups in the LSI of anterior SLH distance, and our results did not support the findings of previous studies. In this study and past investigations, age and postoperative period of subjects differed, which may be one reason for differences in the relationship between the LSI of anterior SLH distance and ACL-RSI score.
The present study showed some limitations that merit consideration. First, in this study, an ACL-RSI score of 60 at 6 months after reconstruction, which is related to the return to sports by 2 years after reconstruction, was used as the cutoff 51. Müller et al. reported a cutoff score for ACL-RSI of 51.3 associated with a return to pre-injury level sports in patients at 6 months after reconstruction 43. The ACL-RSI cut-off at 6 months after surgery will differ depending on when the outcome of returning to sports after reconstruction is judged. Second, limits exist to generalizing the results of this study to patients with significantly different characteristics, such as age and sex, surgical procedures, different sports, and postoperative days before returning to the sport 30,32. Third, in this study, multiple variables differed significantly between groups. However, for variables with low effect size and power, type 2 errors must be considered.