We aimed to compare the LSI of the RTD with that of the peak torque and to determine the inter-limb difference in the peak torque and RTD after ACLR with STG grafts. We investigated the associations of the IKDC-SKF score with the LSI of the RTD and peak torque. Our findings showed that the RTD200 and peak torque were significantly lower in the involved limb than in the uninvolved limb. The LSI of the peak torque showed a significant correlation with the IKDC-SKF score, but there was no correlation with the RTD100 or RTD200. These results partially support our hypothesis.
The LSI of the RTD100 was significantly higher than that of the RTD200 and peak torque. No significant difference was found in the RTD100 between the involved and uninvolved limbs. The present results indicate that the RTD100 of the involved limb is comparable to that of the uninvolved limb, supporting our hypothesis. The voluntary RTD100 of the quadriceps is significantly correlated with the non-voluntary RTD induced by electrical stimulation (19); thus, the RTD100 is considered to reflect neural drive actions such as the firing frequency effects of motor units (18). The recovery of neural function after ACLR may manifest differently depending on the type of tendon graft. The central activation ratio (CAR) measures the potential muscle exertion ability using electrical stimulation during MVIC (33–35) and is used to assess the neural drive of the quadriceps femoris after ACLR. A recent systematic review revealed that the CAR in the involved limb after ACLR with BTB grafts was lower than that in the uninvolved limb (35). By contrast, the CAR in the involved limb after ACLR with STG grafts was higher than that in the uninvolved limb (35). One possible reason for the different results between the studies was that post-operative pain affected the neural drive. Anterior knee pain after ACLR has been determined in 48% of patients receiving BTB grafts and in 20% of those receiving STG grafts at 6 months after ACLR (21). Another study showed a positive result for anterior knee pain in 73% of patients with BTB grafts and 35% of patients with STG grafts at 8 months after ACLR (21, 23). Regarding the RTD100 after ACLR, the LSI at 6 months after ACLR with BTB grafts was reported to be 49% (14), and the LSI at 11 months after ACLR with BTB or STG grafts was 72% (16). In our study, the RTD100 was 95.9%, which was higher than that in previous studies, indicating that the RTD100 recovered after ACLR with STG grafts. Therefore, post-operative pain due to differences in the graft type may have affected neural drive recovery after ACLR.
The RTD200 and peak torque were significantly lower in the involved limb than in the uninvolved limb. No difference was found in the LSI of the peak torque or RTD200, indicating that the RTD200 is an index for detecting between-limb differences as well as the peak torque. The RTD200 is affected by structural factors with musculotendinous stiffness (20). A previous study reported structural changes, such as an increased cross-sectional area and decreased stiffness in the patellar tendon after ACLR with BTB grafts (22). In an animal study, the duration of patellar tendon stiffness recovery was approximately 1 year (36, 37). The LSI of the RTD200 at 6 months after ACLR with BTB grafts was 43% (14); even at 4 years after ACLR with BTB or STG grafts, it was up to 78% (12). In our study, the LSI of the RTD200 was 86.0%, which is higher than that reported previously (12, 14). Therefore, the RTD200 of the quadriceps after ACLR with STG grafts may be less likely to decrease than that after ACLR with BTB grafts. However, the RTD200 is affected by structural factors and musculotendinous stiffness (20); thus, the RTD200 after ACLR with BTB grafts was lower than that after ACLR with STG grafts. Additional studies should be conducted to clarify the effects of the graft type on RTD200 recovery after ACLR.
The LSI of the peak torque was positively correlated with the IKDC-SKF score. Thus, the IKDC-SKF score was not significantly correlated with the LSI of the RTD100 or RTD200. These results do not support our hypothesis that the LSI of the RTD and peak torque is significantly correlated with the IKDC-SKF score. These study results are similar to those of other studies showing that the IKDC-SKF score was significantly correlated with the LSI of the peak torque (38, 39). However, the LSI of the RTD100 and RTD200 was not correlated with the IKDC-SKF score. At 3 months after ACLR, the RTD of the involved limb showed a significant positive correlation with the IKDC-SKF score (13). In contrast, there was no correlation between the LSI of the RTD and IKDC-SKF score, even at 4 years after ACLR (12). The cause may be the difference in the IKDC-SKF scores; the IKDC-SKF score was 66.5 points at 3 months after ACLR (13) and 86.8 points at 4 years after ACLR (12). The IKDC-SKF score in this study was 86.3 points; thus, a high IKDC-SKF score may also influence the results of any correlation between the LSI of the RTD and the IKDC-SKF score. No significant correlation was found between the LSI of the RTD and the IKDC-SKF score in the present study, but the peak torque could reflect subjective knee function after ACLR with STG grafts. The peak torque after ACLR was reported to be related to the kinetic asymmetry between the involved and uninvolved leg during landing (40–42). However, previous studies of the RTD after ACLR have been limited to the association with the kinetics of walking and running, such that its usefulness as a functional screening tool remains unproven (7, 43). Therefore, the peak torque of the quadriceps should be considered a more useful index to assess functional recovery after ACLR.
Some limitations of this study should be addressed. First, the participants were limited to young female athletes. Age and sex differences might affect knee extension torque (44, 45); thus, the present results might apply to young female athletes only. Second, all the participants in this study had undergone ACLR with STG grafts. Different types of ACLR might lead to different results. Finally, knee function in the present study was assessed based on the subjective knee score and was not a result of functional dynamic tasks.