## 4 Discussion  
This retrospective study investigated the effects of COVID-19 infection on neuromuscular activation pattern of the knee stabilizer muscle in professional volleyball female players. The athletes enrolled had only mild COVID-19 symptoms, thus, our study mainly focused on the consequent athletic condition impairment. To the best of our knowledge, this is the first study investigating the effects of COVID-19 on neuromuscular system in a cluster of athletes at high risk of ACL injury. We found a significant delay in the muscular activation time of VM, RF, MH, and LH at the return-to-training compared with their own values before COVID-19 infection. Moreover, in the athletes who reported fever there was a significant reduction of the phase angle at the BIA. Neuromuscular activation patterns and the rate of recruitment of thigh muscle fibers, particularly quadriceps and hamstrings, play a key role in providing dynamic stability and reducing the risk of injury [7,48]. MH and LH are a dynamic ACL agonist, with various tendon insertions that can be used selectively to control the limb during functional task [49, 50, 51]. Thus, a delay in hamstring activation does not contrast the anterior displacement of the tibia, increasing anterior tibial shear force and magnify ACL loading during sport specific task [16]. The VM and RF through their indirect connection to the patellar ligament, which runs in the center of the two femoral condyles, have a stabilizing moment in both vars and valgus conditions [17]. Therefore, a delay in quadriceps muscle activation represents a dynamic instability of the knee during the initial phase of the cutting movements, from the pre-contact to the acceptance of weight phase [17]. Moreover, the late activation of VM and an unbalanced mediolateral quadriceps musculature recruitment pattern increases the apparent valgus dynamic position of the knee, increasing ACL load at landing after a jump [46,52]. The electrophysiological changes documented in COVID-19 athletes could increase the ACL injury risk. Moreover, according to [PERSON] et al. [53] electromyography plays a key role in COVID-19 to evidence myopathic and neuropathic changes, although performing complete electrophysiological examination, especially in patients with worst conditions [26], was extremely challenging during the pandemic. In this context, [PERSON] and colleagues [54], used quantitative EMG studies and found myopathic changes in 11 (55%) of 20 patients examined, complaining long term fatigue after COVID-19 infection, The authors \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **T0** & **T1** & \(p\)**Values T1-T0** \\ \hline BCM (kg) & 32.9 \(\pm\) 4.5 & 31.3 \(\pm\) 3.8 & 0.007 * \\ Fat Mass \% & 25.8 \(\pm\) 4.1 & 24.5 \(\pm\) 3.5 & 0.093 \\ Fat Mass Weight (kg) & 19.8 \(\pm\) 4.4 & 18.4 \(\pm\) 4.0 & 0.009 * \\ Free Fat Mass Weight (kg) & 56.5 \(\pm\) 3.6 & 55.8 \(\pm\) 4.1 & 0.386 \\ Muscle Mass (kg) & 41.2 \(\pm\) 3.1 & 39.4 \(\pm\) 2.6 & 0.003 * \\ ECM (kg) & 22.4 \(\pm\) 1.98 & 23.4 \(\pm\) 2.4 & 0.035 * \\ ECM/BCM ratio & 0.6 \(\pm\) 0.2 & 0.7 \(\pm\) 0.2 & 0.002 * \\ Phase Angle (°) & 7.8 \(\pm\) 0.7 & 7.0 \(\pm\) 0.4 & 0.003 * \\ Total Body Water (kg) & 41.4 \(\pm\) 2.4 & 38.5 \(\pm\) 4.2 & 0.025 * \\ Intracellular Water (kg) & 24.8 \(\pm\) 3.3 & 22.9 \(\pm\) 3.2 & 0.007 * \\ Extracellular Water (kg) & 16.4 \(\pm\) 1.4 & 17.1 \(\pm\) 1.7 & 0.051 \\ O\({}_{2}\) consumption (mL/min) & 272 \(\pm\) 26 & 262 \(\pm\) 69.6 & 0.952 \\ Basal metabolic rate (kcal) & 1878 \(\pm\) 146 & 1780 \(\pm\) 126 & 0.004 * \\ \hline \hline \end{tabular}  
* Continuous variables are expressed as means \(\pm\) standard deviations. Statistical analysis performed to assess intra-group differences was paired sample \(t\)-test;
* = \(p\) < 0.05. Abbreviations: BCM = body cellular mass; ECM = extracellular mass.  
\end{table} 
Table 2: Bioelectrical impedance analysis of female volleyball athletes (n = 10) before COVID-19 infection (T0) and at the return-to-training day (T1).