## 2 Materials and Methods
### Test Protocol  
Prior to the vertical ground reaction force (vGRF) measurements during double-leg and single-leg hops, a history was taken, and all of the studied participants underwent an examination involving a bilateral manual Lachman test and pivot shift test, performed in all of the studied participants by the same orthopedic surgeon. The body height and mass were measured.  
The measurement of vGRF during the double-leg and single-leg vertical hops was performed consecutively by using the MTD-balance system (MTD Systems, Neunburg [PERSON]. Wald, Germany), containing two force plates for right and left legs separately. The method of vGRF measurement using the MTD-balance system was based on the study by [PERSON] (2011) [18]. The procedure was very similar to that of [PERSON] and [PERSON] (2010), and [PERSON] et al. (2013) [20; 21]. All of the vGRF measurements in the study were performed by one well-experienced examiner.  
The intra-rater and inter-rater test-retest reliability of the test protocol using the MTD-balance system was presented in our previous study, and indicated an excellent intra-rater reliability for double-leg vertical hop landing (ICC = 0.84-0.96) and single-leg vertical hop landing (ICC = 0.95-0.96). The inter-rater reliability was considered good for double-leg vertical hop landing (ICC = 0.67-0.69), and excellent for single-leg vertical hop landing (ICC = 0.94-0.95) [27]. We also performed vGRF measurements using the presented protocol in clinical practice as one of the functional benchmarks to examine patients after ACLR undergoing physical therapy in our physiotherapy center [33].  
The participants were asked to abstain from unaccustomed strenuous exercise for at least 24 h before the measurements, and to avoid eating a heavy breakfast in the morning before the measurements and within two hours of the test. They were dressed in a comfortable sport outfit and sport shoes.  
The measurements were preceded by a warm-up on a cycloergometer. The participants were instructed by the examiner on the technique of double-leg and single-leg vertical hops. Each participant performed a few trials. The examiner used verbal \"start\" and \"stop\" commands to start and end the participant's continuous hoping. Each vertical hop was performed in the upright position. The protocol did not allow for countermovement, and arm movement during the hops was restricted.  
The force plates were adjusted to indicate a mass of 0 kg before a participant stepped onto it. At first, the vGRF values during double-leg vertical hops were measured. At the beginning of the measurement, the examined participant placed his right foot on the middle of the right force plate and left foot on the middle of the left plate (Figure 2a). Then, he performed 10 continuous double-leg vertical hops (Figure 2b). Next, the vGRF values during the single-leg vertical hops were measured, starting with the uninvolved leg in ACL-reconstructed patients and with the dominant limb in the control group. The examined participant placed the foot of the studied leg in the middle of the force plate (Figure 3a) and performed 10 continuous single-leg vertical hops (Figure 3b). The contralateral leg was flexed 90\({}^{\circ}\) at the knee joint. Measurement was then performed for the contralateral leg in the same way [18; 27].  
Figure 2: (**a**) The starting position for the vertical ground reaction force (vGRF) measurements during double-leg vertical hops; (**b**) the double-leg vertical hop.