## 4 Discussion  
During NiTi root canal instrumentation, the influence of pecking amplitude on torque and force generation remains largely unexplored [27]. The current study employed a custom automated instrumentation and torque/force sensing device, which enabled standardized experimental conditions that minimized biases typically associated with manual procedures, leading to highly reproducible results. Using the Reciproc Blue instrument, the  
Figure 4: Canal-centering ratios after Reciproc Blue instrumentation with different pecking amplitudes. Group S: small-amplitude group (1 s apically and 0.5 s coronally); Group M: medium-amplitude group (2 s apically and 1 s coronally); Group L: large-amplitude group (4 s apically and 2 s coronally). Values indicate the mean and standard deviation (\(n=10\)). Values with different uppercase letters are significantly different between different pecking amplitudes (\(p<0.05\)). Values with different lowercase letters are significantly different between different measuring points (\(p<0.05\)). Figure 5: (**A**) Time to fracture (s) and (**B**) length of fractured segment (mm) of Reciproc Blue instruments measured using dynamic cyclic fatigue test at different pecking amplitudes. Group 1 mm: 1 mm amplitude group; Group 3 mm: 3 mm amplitude group. Values indicate mean and standard deviation (\(n=10\)). * \(p<0.05\). (**C**,**D**) Scanning electron microscopy images of fractured surface of (**C**) Reciproc Blue of Group 1 mm and (**D**) Group 3 mm, showing numerous dimples spread across entire surface. Arrows indicate the origin of the crack. Bars = 100 μm. results demonstrated that a longer pecking amplitude increases the upward and downward forces, decreases the canal-centering ratios, and extends the time to fracture. Therefore, the null hypothesis was rejected. This study focused on reciprocating motion, which is an extensively investigated kinematic mode that may reduce the risk of instrument fracture and canal deviation [20]. Reciprocating motion improves the cyclic fatigue resistance of NiTi instruments over continuous rotation by alternating loading and unloading, reducing stress [18]. Additionally, it improves the canal-centering ability, mainly because balanced movement promotes more even force distribution [20]. However, the optimal pecking amplitude for reciprocating motion remains poorly understood, despite the importance of appropriate operator manipulation for preventing atrogenic errors by minimizing stress generation and canal deviation [8]. In this study, this issue was investigated by employing the Reciproc Blue instrument as a representative reciprocating NiTi instrument with high flexibility [21] and cyclic fatigue resistance [21; 22; 23]. The downward force affects the safety of NiTi instrumentation because an excessive downward force may lead to the tight binding of the instrument to the canal wall, causing torsional stress and fracture [34; 35; 36]. This study demonstrated that Group L produced a larger maximum downward force compared with Group S, likely owing to the longer contact time caused by the larger amplitude of the instrument's pressing motion. Thus, a larger amplitude may cause a sudden increase in the downward force, contributing to the catastrophic failure of NiTi instruments. The upward force during instrumentation, referred to as the screw-in force, is generated by the engagement of the cutting flutes on the rotating instrument, creating an apically directed force [36]. This force is associated with instrument fracture caused by sudden excessive torsional stress [35]. In this study, Group L exhibited significantly higher cumulative upward force compared with the other groups. However, the maximum upward force values were not significantly different among groups. Because the cumulative force represents the total force exerted during instrumentation, Group L may have been subjected to greater overall stress compared with other groups owing to the screw-in force.