## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**ACWR** & **Comparison** & \
\begin{tabular}{c} **Mean** \\ **Difference** \\ \end{tabular} &
\begin{tabular}{c} **Standard** \\ **Error** \\ \end{tabular} & \(p\)**-Value** & **Cohen’s d** \\ \hline \multirow{2}{*}{Total Distance (TD)} & Underload vs. Sweet spot & \(-\)0.214 & 0.244 & 1 \\  & Underload vs. Danger zone & 0.58 & 0.261 & 0.079 \\  & Sweet spot vs. Danger zone & 0.366 & 0.109 & 0.002 * & 0.37 \\ \hline \multirow{2}{*}{Speed \(>\) 19.8 km/h (HS19)} & Underload vs. Sweet spot & \(-\)0.286 & 0.171 & 0.283 \\  & Underload vs. Danger zone & 0.612 & 0.189 & 0.004 * & 0.28 \\  & Sweet spot vs. Danger zone & 0.327 & 0.101 & 0.004 * & 0.33 \\ \hline \multirow{2}{*}{Speed \(>\) 25.2 km/h (HS25)} & Underload vs. Sweet spot & 0.091 & 0.088 & 0.906 \\  & Underload vs. Danger zone & 0.184 & 0.115 & 0.327 \\  & Sweet spot vs. Danger zone & 0.275 & 0.104 & 0.025 * & 0.28 \\ \hline \multirow{2}{*}{Mechanical work (MW)} & Underload vs. Sweet spot & \(-\)0.045 & 0.15 & 1 \\  & Underload vs. Danger zone & 0.363 & 0.167 & 0.092 \\ \cline{1-1}  & Sweet spot vs. Danger zone & 0.318 & 0.097 & 0.003 * & 0.32 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Z-score test contact time. definition of vertical stiffness. Indeed, by definition [47; 48; 49; 50], low stiffness values describe a run that does not have a \"bouncing gait\" and, therefore, is energetically ineffective [31; 51]. Low vertical stiffness combined with low flight time and high contact time suggests that energy absorption is not being efficiently redirected into propulsion. This can reduce the running economy and increase joint strain. Instead, high stiffness values are typical of those having an energetically effective run and who manage to transform a large percentage of potential elastic energy acquired during the landing phase into the flight phase for the next step. Another possible explanation may be related to fatigue. Fatigue-related reductions in vertical stiffness, as reported in studies using both wearable resistance and standardized run assessments, further validate the importance of monitoring stiffness as an indicator of biomechanical and neuromuscular status during training [44; 46]. The correlation between temporal parameter values and fatigue presents a crucial insight for coaches and practitioners seeking practical implications in training. As fatigue sets in, there is a notable reduction in vertical stiffness during a moderate intensity run, impacting an athlete's ability to efficiently convert potential elastic energy into propulsion [52; 53]. [PERSON] et al. [54] highlighted how running parameters like contact time (CT) and vertical stiffness (K) can be notably influenced by short-term neuromuscular fatigue when performing repeated high intensity runs. Furthermore, [PERSON] et al. [30] observed that increasing levels of fatigue--particularly at higher speeds--often lead to lengthened contact times, suggesting a shift in neuromuscular coordination and a decline in running efficiency. [PERSON] et al. [55] documented that, in acute fatigue states, there are discernible rises in contact time, as well as a drop in vertical stiffness, indicating an adaptive response to neuromuscular constraints. Similarly, [PERSON] et al. [56] noted that an increase in knee flexion angle at initial contact, associated with lower vertical stiffness, could serve as an alternate strategy to dampen impact forces. This understanding underscores the importance of monitoring fatigue levels during training sessions [57]. To address this, coaches can integrate accelerometer/inertial sensor metrics, which offer heightened sensitivity to external load variables. Future research should concentrate on the longitudinal assessment of vertical stiffness to better understand how it changes during different training and competitive phases. Examining how vertical stiffness adapts to various workloads could reveal important insights into biomechanical and neuromuscular adaptation in football players.