# Neuromuscular Excitation Pattern in Expert Indoor Skydivers
## 4 Discussion
### Neuromuscular Activation  
As this is the first study exploring the neuromuscular excitation during indoor static skydiving, any comparison with previous data is not possible. A description of the neuromuscular excitation will be discussed from distinct standpoints, encompassing the anatomical region to which the muscle belongs, an in-depth analysis of the principal muscles engaged for the different postures, and co-excitations detected through the cross-correlation analysis. The NeLo index, newly proposed in the present study, is intended to complement traditional metrics of neuromuscular excitation. Typically, neuromuscular excitation is assessed using various frequency and time domain indices [18; 19], such RMS. For tasks involving several muscles, this approach generates a number of RMS values equal to the number of muscles under investigation, providing a detailed description of each single muscle excitation. However, such an approach does not offer a measure of overall excitation, which represents fundamental information for coaches and athletes in a discipline characterized by the excitation of synergistic muscles during demanding static tasks, like climbing or gymnastic rings. The development of an index indicative of overall excitation, such NeLo, allows coaches and athletes to optimize training procedures by focusing on a specific group of muscles. In this context, NeLo identified SUPINE and PRONE as the least demanding postures, not only from a cardiac perspective but also from a neuromuscular standpoint (Figure 3a). These two lying postures were shown to be equally relaxing. Indeed, the comparison between NeLo values obtained during SUPINE and PRONE did not show significant differences in overall excitation. The absence of significant differences was also observed when the contribution of muscles of different craniocaudal anatomical regions was compared (Figure 3b), nor when muscles have been merged in the transverse plane (front and back side of the body; Figure 3c). This latter finding was unexpected. While in the SUPINE posture, the skydivers adopt a relaxed position leaning against the airstream with their back; in PRONE, they have to face the wind, assuming an arched position with backward extension. Considering that, outside the tunnel, this position necessitates the activation of back extensors muscles [20], the absence of a higher excitation in the back muscles while assuming the arched position suggests a compensatory mechanism that, inside the tunnel, can be attributed to the airstream pressure exerted by the aerodynamic force acting on the body surface. Consequently, the muscular effort sustained by the skydiver in PRONE appears comparable to the SUPINE posture, likely aiming at modulating muscular excitation solely for minor adjustments. Such a scenario seems plausible particularly among expert skydivers, like the instructors recruited for the present study. Indeed, these two postures are usually introduced to beginners at different stages, with PRONE being presented first due to its lower difficulty. For beginners, the assessment of muscular excitation in these two postures is likely to reveal differences that may be attributed to detrimental co-contractions that are physiological when performing unfamiliar tasks. Differently for professional skydivers, these postures showed to be equivalent in terms of muscle engagement. This suggests that expert skydivers are able to modulate the neuromuscular control to minimize the effort even during tasks of different difficulties. Similar neuromuscular adaptation was also reported in other sports wherein a high level of specialization is reached in response to long-term training, resulting in a lower level of muscle activity [21]. NeLo increased considerably in the other two investigated conditions, SIT and HD, resulting in the most challenging postures. One possible explanation of this observation may be attributed to the different body surfaces exposed to the airstream compared to the lying postures.