## 4 Discussion
### Kinetic and Kinematic Data  
The jumping-based metrics used in the present study (e.g., the RSI, GCT, impulse, jumping height) were chosen on the basis that they exhibited high levels of consistency, thereby making them ideal candidates for evaluating jumping-based performances [12]. Statistically significant between-group differences were evident for the unilateral jumping RSI and GCT, especially for the injured/non-dominant limbs. Evidence has shown that knee extensor muscle strength can be compromised in the uninjured limb following knee injury [35] and that muscle strength is associated with RSI performances during unilateral DJ performances [36]. Intriguingly, no statistically significant differences were evident for peak isometric torque between groups, which highlights the importance of incorporating functionally relevant tests as these may invoke greater overall contributions from the knee joint. Similarly, there was a lack of evidence for between-group differences regarding unilateral jumping impulse or peak ground reaction force in the present study, which likely implies similar motor coordination strategies during the take-off phase [37]. Our results echoed the findings from other studies that implemented unilateral jumping [38] and stair descent [39] but contradicted those by [PERSON] et al. [40], who showed 11% greater peak vertical GRF in individuals with PFP. It is plausible that methodological differences in the experimental setup may, at least partially, account for some of the differences given that [PERSON] et al. [40] used a 0.31 m box for their DJ trials, whereas we used a 0.25 m box. It is known that box height has a proportional effect on the magnitude of the ground reaction forces such that greater box heights elicit higher ground reaction forces [41]. The RSI metric, which comprised ground contact time (GCT) relative to flight time (a surrogate for jumping height), showed discernible between-group differences. Such a finding was intriguing given that disparities were evident for both the involved and uninvolved limbs compared to healthy controls. Accordingly, this result may indicate that the individuals in our sample might have employed a different motor control strategy during the initial landing and subsequent take-off, which is reliant on the stretch-shortening cycle and is known to be different in those with functional knee deficits [40; 42]. More specifically, the lower RSI values of the PFP group can largely be explained by the longer contact duration during the initial landing (i.e., CGT) which may indicate a compensation strategy to minimise the vertical impulse generation to reduce knee loading [11] and correlates with the finding that people with PFP have lower rates of the force development of the muscles around the knee [43]. Although the RSI has been identified as a better marker of rehabilitation progression in those with ACL reconstruction [44; 42], to the best of our knowledge, there are no previous studies reporting RSI in people with PFP, especially in terms of its potential value as a diagnostic metric. Studies focusing on the diagnostic accuracy of various clinical tests used to assess PFP showed that functional tests such as squatting, stair climbing and kneeling exhibited the highest sensitivity (0.84-0.91) but demonstrated limited specificity (0.43-0.50) [7; 45]. Our study provides partial evidence that the RSI may serve as a potential diagnostic metric when evaluating those with PFP as part of a comprehensive testing battery given its sensitivity (0.85) and specificity (0.72). Moreover, GCT could also serve as potential surrogate given that its sensitivity (0.91) and specificity (0.59) scores tend to outweigh the majority of other functional diagnostic markers typically used in the evaluation of PFP patients (e.g., pain during stair climbing, pain during kneeling) (see Figure 2) [8].