# Neuromuscular Excitation Pattern in Expert Indoor Skydivers
## 4 Discussion
### Preliminary Considerations on the HR Response  
Compared to the resting value determined in the supine position outside the wind tunnel (REST OUT), HR exhibited a marked rise across all the postures within the tunnel. This finding is coherent with the cardiac acceleration related to different postures and any increase in physical activity [13; 14]. Indeed, postural changes might be accompanied by a reduction in blood venous return to the heart chambers due to the gravitational effect (blood pooling). The consequent drop in arterial blood pressure evokes a positive chronotropic effect driven by a withdrawal of the parasympathetic branch and a rise in sympathetic arm activity. Nonetheless, a positive chronotropic effect occurs to increase the cardiac output and meet the rising metabolic demands of the exercising muscles when the level of physical activity increases [13; 14]. It is to be noted that the higher HR value observed in the supine position inside compared to outside the tunnel cannot be attributed to postural \begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & & **AD** & **PD** & **PM** & **LD** & **Rabd** & **ES** & **RF** & **BF** \\ \hline \multirow{3}{*}{**AD**} & r & \(-0.025\) & **0.828** & \(-0.220\) & 0.437 & \(-0.291\) & \(-0.385\) & 0.417 \\  & p & 0.949 & * **0.006** & 0.569 & 0.239 & 0.448 & 0.306 & 0.264 \\  & N & 9 & 9 & 9 & 9 & 9 & 9 & 9 \\ \hline \multirow{3}{*}{**PD**} & r & & \(-0.267\) & 0.592 & \(-0.321\) & 0.377 & \(-0.452\) & 0.546 \\  & p & & 0.487 & 0.093 & 0.400 & 0.318 & 0.222 & 0.129 \\  & N & & 9 & 9 & 9 & 9 & 9 & 9 \\ \hline \multirow{3}{*}{**PM**} & r & & \(-0.414\) & 0.625 & \(-0.458\) & 0.116 & 0.349 \\  & p & & 0.268 & 0.072 & 0.215 & 0.767 & 0.358 \\  & N & & 9 & 9 & 9 & 9 & 9 \\ \hline \multirow{3}{*}{**LD**} & r & & & \(-0.593\) & **0.880** & \(-0.481\) & 0.009 \\  & P & & & 0.092 & * **0.002** & 0.190 & 0.982 \\  & N & & & 9 & 9 & 9 & 9 \\ \hline \multirow{3}{*}{**Rabd**} & r & & & \(-0.411\) & 0.555 & \(-0.076\) \\  & P & & & 0.272 & 0.121 & 0.846 \\  & N & & & 9 & 9 & 9 \\ \hline \multirow{3}{*}{**ES**} & r & & & & \(-0.319\) & \(-0.379\) \\  & P & & & & 0.402 & 0.314 \\  & N & & & & 9 & 9 \\ \hline \multirow{3}{*}{**RF**} & r & & & & & \(-0.289\) \\  & P & & & & & 0.451 \\  & N & & & & & 9 \\ \hline \end{tabular}
\end{table} 
Table 1: Pearson product moment correlations determined in PRONE. Bold numbers indicate a significative positive correlation. Anterior deltoid (AD), posterior deltoid (PD), pectoralis major (PM), latissimus dorsi (LD), rectus abdominis (Rabd), erect spinae (ES), rectus femoris (RF), and biceps femoris (BF). The significance level was denoted as *_p_ < 0.05.  
adjustments. Rather, it likely indicates a sympathetic activation, aimed at meeting the higher energy demand required by the postural muscles involved in the maintenance of the target posture [14]. When comparing the different postures inside the tunnel, the impact of the gravitational effects and of the changes in neuromuscular activation on the HR response are expected to exert distinct influences, depending on which posture is adopted. Specifically, the heightened neuromuscular activation required to maintain the posture against the airstream is expected to have a positive chronotropic impact in both SIT and HD. Conversely, the impact of gravity on the heart pump is supposed to yield opposite effects between SIT and HD. In SIT, a rise in HR is expected to address the reduced venous return due to lower limbs blood pooling. Conversely, the reversed position in HD would increase the blood pressure on baroreceptors located in the aortic arc and carotids; this would likely evoke a baroreflex negative feedback, an increase in vagal activity and a sympathetic withdrawal, thus decreasing the HR [15; 16; 17]. The lack of differences between the HR determined in HD and SIT, in our study, indicates a predominant role of the neuromuscular activation over the baroreflex negative feedback.