## 4 Discussion
The decrease in the peak angle could be explained by the decrease in joint moments (details in Figure 4). There were significant differences in peak joint moments at the hip and knee extensor (P1 vs. P4: hip (\(p\) = 0.03), knee (\(p\) < 0.001)). A similar relationship was illustrated by [PERSON] et al. [19] between professional and amateur badminton players; that is, professional players with greater muscle strength and better performance showed higher knee and ankle joint moments. Additionally, a shorter time was taken to flex to the peak angle, with significant decreases in time shown at the knee and ankle. This may be caused by impaired control due to fatigue. This can also be explained in this study by the decrease in joint moments and power. In addition to the angles, the durations of the four sub-stance phases (II-V) changed during the process of the fatigue protocol. The shorter durations of II and III indicate that less time was taken to reach PF2 and PAK, respectively; that is, due to the fatiguing process, the participant placed his foot flat and flexed his knee more quickly. This is consistent with the increased ankle plantar flexor and knee flexor moments in the present study (Figure 4a), and with the opinion that fatigue reduces the capacity of muscles to generate force. The most significant increase was found in the IV phase (pre-drive-off), increasing from \(27.25\pm 10.69\%\) stance to \(43.38\pm 15.24\%\) stance. Significantly decreased hip and knee peak power in this phase provided sufficient support for this change. Less power was generated for players for the drive-off. [PERSON] et al. [10] indicated that a hop style lunge generates higher peak vertical force during loading. In this study, three participants used the hop style at the final stage of the fatigue protocol. The change of lunge style may be a strategy to generate more power for driving-off and returning to the starting position [14]. Furthermore, considering the time continuity of biomechanical variables of the lunge motion, one-dimensional statistical parametric mapping SPM (1D) was used to analyze the biomechanical waveform data in the sagittal plane. Most of the joint angle, moment, and power waveforms decreased consistently during the process of the fatigue protocol (among P1, P2, P3, and P4). Significant differences were found in the hip and knee joint angles, mainly in the pre-drive-off phase (IV) (most \(p\) values were less than 0.001). The results support the hypothesis that the fatigue effects were different in the five periods of the lunge. Moreover, these results support the view that lunge characteristics change due to fatigue. Taking into account the joint moments, both the hip and knee joints showed a significant response to the fatigue protocol. Significant differences were found between P1 and P3 in part of the period of IV, and between P1 and P4 in IV. The hip moments had a larger effective scope. However, there were only significant differences in the hip power between P1 and P4. For knee power, the significant differences were seen between P1 and P3, and also between P1 and P4, with a larger effective scope of the period. This indicates that the significant decrease in joint power occurred earlier and was mostly evident at the knee; that is, the fatigue responses manifested mostly at the knee joint. This result is consistent with a previous epidemiology study that reviewed musculoskeletal injuries among Malaysian badminton players, which reported that the majority of injuries sustained by players were due to overuse, primarily of the knee [7]. Another study [28] suggested that the rapidly changing eccentric/concentric work of the quadriceps in the varying degree of knee flexion was probably associated with patellar tendon injury. This may be why patellar tendonopathy is the most common injury of lower limbs among badminton players, and this result also supports our third hypothesis. Coaches and players should pay more attention to the training of the knee, and particularly the knee extensors.