## 4 Discussion  
Functional testing after ACL-R is a complex field and has become more common in recent years, serving as a valuable tool for assessing and monitoring patients' progress throughout the recovery process. It is still under debate as to which return-to-sport testing battery or combination of tests can provide a reliable predictive value for return-to-sport decision making. Furthermore, differences in test standardisation, measuring techniques and interpretation make it difficult to implement tests in clinical practice. Previous research has demonstrated that the SLVHT is sensitive to functional deficits at the knee joint, and it is better able to detect asymmetries when compared to horizontal hop tests, suggesting it as a useful tool for global evaluation [32]. To our knowledge, the SLVHT has not been monitored during rehabilitation over time in bigger cohorts to investigate whether it can measure progress in joint function accurately and independently over time. The main finding of this study was that three months after ACL-R, the SLVHT jump height was significantly lower in the injured leg compared to the non-injured leg. The jump height of the injured leg increased significantly from three to six months after surgery. In common rehabilitation protocols after ACL-R and also in this study, patients start with running activities, pyometric training and jumping exercises after 3 months. Consequently, a change in knee function and muscle force between these two time points could be expected. The significant increase in jump height can therefore be explained by the bigger training load and neuromuscular adaption. The SLVHT seems to display this change objectively. Return-to-sport test batteries differ throughout sports and age groups, and it is debatable whether the results might be affected by gender-specific characteristics such as body composition and other factors influencing knee movement. Our study population represented a typical group of ACL patients, with a wide age range and about one-third of them being female patients. Subgroup analysis showed that the female patients had a lower absolute jump height compared to males. This is expected, since muscle strength is generally higher in men than in women. However, the LSI did not differ between men and women. In addition, the regression analysis showed that gender was not an independent factor influencing the jump height of the injured leg. Therefore, the LSI of the vertical jump  
Figure 3: Correlation of the jump height of the single-leg vertical jump test with the injured leg with (**A**) the distance jump, (**B**) the IKDC SKF and (**C**) the KOOS value at 6 months postoperatively. height may be independent of gender, and changes throughout the rehabilitation process might not be influenced by this. We therefore hypothesise that the SLVHT may be a suitable tool for assessing rehabilitation progress for both men and women. Moreover, younger patients (under 30 years of age) had a higher jump height during the SLVHT with the non-injured leg, and the LSI values at 3 and 6 months differed between younger and older patients. This difference may reflect faster rehabilitation progress in the younger age group and quicker adaption to the training load [16]. It is commonly known that younger athletes tend to be involved in higher levels of sport than older patients [33; 34]. Hence, a better neuromuscular basis and functional status might be assumed in younger patients, and the LSI of the SLVHT seemed to reflect these differences reliably. To verify our hypothesis, regression analyses was performed, and it showed that the jump height during the SLVHT with the injured leg was determined solely by the jump height of the non-injured leg, and the LSI was independent of other potentially confounding factors, as discussed above. Rehabilitation outcomes after knee joint surgery are often recorded with subjective measurement instruments such as the IKDC SKF and KOOS-QoL questionnaires [14; 30].