In this study, the scores of the RSI during SVCJs and single-leg hop tests, except for the SHD, were significantly lower on the involved limb than on the uninvolved limb in post-ACLR athletes, and they showed the largest effect size for the difference in the RSI during SVCJs. No significant difference was found in the LSIs of single-leg hop tests between post-ACLR and healthy athletes. By contrast, the RSI during SVCJs was significantly lower in post-ACLR athletes than in healthy athletes, and the effect size was large. Among post-ACLR athletes, less than 30% of those with LSIs above 90% in the single-leg hop tests had LSIs above 90% in the RSI during SVCJs. These results support our hypotheses, except for the difference in the LSIs between the groups in the single-leg hop tests.
Gokeler et al. reported a significant difference in SHD measurements between the involved and uninvolved limbs of post-ACLR athletes . Herrinton et al. reported similar findings for the THD and CHD measurements with a moderate effect size . These studies have reported a duration of 6.7–7.8 months since reconstruction in post-ACLR athletes. Our study contradicts these findings and reports a mean duration of 11.6 (6.7–31.2) months. The LSIs of the single-leg hop tests that measure distance increased as duration increased after reconstruction . The longer duration after reconstruction in our study may have influenced the difference observed in the results between the present study and previous studies. The LSIs of the single-leg hop tests showed that post-ACLR athletes who had undergone reconstruction approximately 11 months earlier recovered to the same level as healthy athletes.
In this study, the RSI during SVCJs was significantly lower on the involved limb than on the uninvolved limb of post-ACLR athletes and had a large effect size. Their corresponding LSIs were significantly lower than that of healthy athletes, and the effect size was large. Myer et al. reported that the mean LSI of the jump height of post-ACLR athletes was 89% . Similarly, the mean jump height and RSI during SVCJs in the present study was 86% and 81%, respectively. The study findings show that greater lower limb asymmetry can be detected by calculating the RSI with the jump height and contact time. In many previous studies that have assessed jump with landing and leaping for healthy athletes, the contact time and RSI have been used as parameters of jump performance [30, 31]. The RSI for continuous vertical jumps in post-ACLR athletes is not known. Our study results therefore provide new insights into the RSI during SVCJs in post-ACLR athletes who are determining the appropriate timing to return to sport.
Less than 30% of the post-ACLR athletes with an LSI of 90% or above in the single-leg hop tests had an LSI over 90% in the RSI during SVCJs. The results of this study showed that the symmetry of single-leg forward hop performance was restored in post-ACLR athletes more than 6 months after surgery, but asymmetry remained in their reactive continuous vertical jump performance. According to a study that analyzed the LSIs of the jump height during SVCJs, SHD, and THD in post-ACLR athletes 54 weeks after reconstruction, the LSI of the jump height during SVCJs was the lowest, which is similar to our study findings . The present results support our hypothesis. For post-ACLR athletes at the phase of determining the timing of their return to sport, the RSI during SVCJs was shown to detect lower limb asymmetry more easily than the single-leg hop tests.
In our study, a side-to-side difference was found in the single-leg hop tests and RSI during SVCJs in both post-ACLR and healthy athletes. In healthy athletes, the LSI for each test ranged from 91.9–96.5%. Several studies have reported no or small side-to-side differences in single-leg hop tests in healthy athletes [33, 34]. A difference was found in the methods of LSI calculations, and various definitions were used to categorize the lower limbs between the studies. Some studies mostly analyzed the differences and ratios between the dominant and non-dominant limbs separately in healthy athletes [33, 35, 36]. Our study divided the lower limbs of healthy athletes into poor-performing and well-performing limbs. Therefore, we observed a greater difference when comparing the dominant and non-dominant limbs. In our study, the LSI was calculated using results of the poor-performing limb as the numerator and results of the well-performing limb as the denominator, which was effective in identifying the asymmetry in the limbs.
Single-leg hop tests are a forward jumping test, but SVCJs are continuous jumps in the vertical direction. Vertical jumps are frequently required in sports such as basketball, handball, and volleyball, which record a high incidence of ACL injuries [37–39]. A pre-return to sport vertical jump performance assessment may be useful for post-ACLR athletes participating in these sporting events. However, vertical jumping tasks are less frequently used than horizontal jumping tasks such as the single-leg hop tests in assessing post-ACLR athletes . To ensure return to sport in post-ACLR athletes participating in sports that require frequent vertical jumps, evaluations related to vertical jumps would need to be included in the return to sport criteria.
A recent systematic review identified the need for modification of return to sport criteria for adequate decision making regarding the timing of return to sport in post-ACLR athletes [40, 41]. Even if a test is able to accurately capture an athlete’s recovery status, generalization is difficult if it is not feasible in a clinical setting. Recent advances in technology have made it possible for relatively inexpensive devices such as mat switches  and smartphone applications  to accurately measure flight time and RSI for vertical jumps involving landing and leaping. The RSI during SVCJs employed in this study has the following advantages as a functional assessment of post-ACLR athletes. First, similar to the single-leg hop tests, this is a single-legged task; therefore, the asymmetry of the lower limb function can be confirmed. Second, SVCJs can be performed in a relatively space-saving manner. The results of this study indicate that the RSI during SVCJs may serve as a new indicator for detecting lower extremity asymmetry in post-ACLR athletes at the phase of determining the timing of their return to sport.
This study has several limitations. First, although sex influences the effect of lower body explosiveness on jump landings as assessed by the RSI , it was not included in the analysis. Approximately 70% of the study participants were female. Second, the surgical technique for inclusion was not limited. Third, because the study included post-ACLR athletes who were within 6 months to 2 years after surgery, the return to sport status varied, but it was not analyzed separately for timing or return to sport status. Finally, we only used the OptojumpTM system for the RSI during SVCJs measurements. In RSI measurements, fixed bias may occur due to differences in measurement equipment . Therefore, careful attention is needed when comparing the data from this study with studies that have used different measuring instruments.