## 4 Discussion
### Ground Reaction Force and Extension Performance  
The results demonstrated that both fast-velocity and moderate-velocity flywheel training effectively enhanced lower-limb PEF and, to a certain degree, PCF. Under fast-velocity conditions, the PEF showed significant improvement at multiple post-training time points. In contrast, the PCF exhibited a transient elevation 8 min after exercise. No meaningful changes were observed in the slow-velocity condition, suggesting limited efficacy of slow-velocity flywheel training in eliciting short-term explosive neuromuscular adaptations.  
From a biomechanical standpoint, the increase in PEF is primarily attributed to enhanced concentric force generation. This phenomenon is closely linked to elevated neuromuscular drive under high-speed mechanical stimuli. It has been demonstrated that high-inertia flywheel training can recruit a greater number of motor units, particularly those associated with type II fast-twitch muscle fibers, thereby improving the rapid force production capacity [26]. Supporting this, [PERSON] et al. reported that moderate-to-high inertia flywheel training significantly enhanced explosive lower-limb performance, especially vertical jump and ground reaction force metrics, within 4 to 8 min after training [27].  
The PCF reflects the lower limb's ability to absorb landing impact and recycle elastic energy, which is influenced by limb stiffness, intersegmental coordination, and the efficiency of the stretch-shortening cycle. [PERSON] et al. emphasized that both the reactive strength index and the PCF share common neuromechanical underpinnings, particularly in relation to tendon stiffness regulation and neuromuscular elasticity [28]. In this study, participants in the fast-velocity condition generated greater mechanical tension through active braking during the eccentric phase. This may have improved tendon elasticity and joint stiffness regulation, contributing to better impact attenuation and rebound capacity. However, individual variability in training background and movement execution appeared to affect the PCF response.  
Physiologically, fast-velocity flywheel training induced high-speed eccentric tension, which has been shown to elevate sarcoplasmic calcium ion concentrations and increase the phosphorylation of myosin light chains [29]. Concurrently, short-term neural adaptations can enhance the excitability of alpha motor neurons and facilitate synchronized motor unit recruitment [30]. These combined effects lead to more efficient cross-bridge cycling and enhanced muscle fiber activation, resulting in improved explosive force generation. Furthermore, [PERSON] et al. highlighted that the eccentric overload mechanism inherent in flywheel training promotes rapid neuromuscular remodeling, which serves as a foundation for PAP [31].  
Recent research has further underscored the importance of tailoring inertial loads. For instance, [PERSON] et al. found that higher-load flywheel training not only enhanced ground reaction forces and power outputs but also prolonged the time course of post-activation performance enhancement [32]. Similarly, [PERSON] et al. suggested that high-speed velocity-based training protocols offer superior acute activation of neuromuscular output when compared to slower variants, a finding that aligns with the performance trends for PEF and PCF observed in the current investigation [33].  
Importantly, despite the relatively large eccentric loads involved, the slow-velocity training condition lacked sufficient movement speed to effectively activate fast-twitch motor units or elicit efficient stretch-shortening cycle engagement. As a result, the lack of velocity-specific stimuli may have dampened PAP effects due to premature fatigue or suboptimal motor rhythm [34].