# Neuromuscular Excitation Pattern in Expert Indoor Skydivers
## 1 Introduction  
Indoor skydiving has witnessed a substantial growth in popularity worldwide. It developed from a training venue, specifically dedicated to military paratroopers, into a facility with safe and stable environmental conditions suitable for training sessions and competitive events of the International Aeronautical Federation [1]. During indoor skydiving competitions, practitioners must execute, while floating, a combination of maneuvers to perform coded sequences along pre-determined invisible paths inside the vertical wind tunnel, encompassing twisting and rolling movements in all directions by assuming different body postures. The posture adopted by the skydiver inside the wind tunnel determines the body surface exposed to the airstream, playing a crucial role in the success of these maneuvers, the precision of the direction, and velocity of the movements. Indeed, inside the wind tunnel, the skydiver experiences three different forces: (i) gravity, (ii) the aerodynamic force, and (iii) the force exerted by muscles to counteract the drag force acting on the body. The area of the body surface exposed to the airstream influences the aerodynamic force acting on the skydiver [2]. A fully extended supine or prone position maximizes the body's surface area exposed to the airstream, leading to a rapid ascent. In contrast, reducing the skydiver body surface exposed to the airstream by assuming more compact postures facilitates descent. Although expanding the body surface facing the airstream to gain height is intuitive, its execution against the wind tunnel airstream is considerably demanding, especially from a muscular standpoint, and necessitates a coordinated activation of multiple muscle groups. Efforts sustained by indoor skydivers expose the musculoskeletal system to a heighten risk of muscle overloads and joint injuries, resulting in cervical and lumbar back pain and shoulder dislocation [3, 4, 5, 6, 7]. During wind tunnel training, confidence is built through mastering a series of progressively challenging poses, starting with the least difficult position, lying prone (belly-to-earth), advancing to supine (belly-to-ceiling), then sitting, and finally, assuming a reversed position with the head towards the floor. To counteract the airstream and control each body posture, including an arched pose, several joints (such as the glenohumeral, spinal column, and acetabulo-femoral) are subjected to significant mechanical loads. These may contribute to muscular overloading and the development of osteoaarticular pain [5]. The level of experience of individual skydivers may likely influence the optimization of muscle groups activation, thereby reducing the occurrence of muscles overloads, injuries, and detrimental co-contractions. Only a few studies have explored skydiving features, primarily concerning the dynamic modeling of the human body during freefall, parachuting-related injuries, and balance control [2, 3, 8]. Despite the growing popularity of indoor skydiving, a comprehensive description of the muscles engaged while fluctuating inside the wind tunnel is still lacking. Considering the risks of muscular overloads and joint injuries associated with indoor skydiving [3, 4, 5, 6], an investigation into muscle activation patterns in experienced practitioners may provide important insights into the optimal neuromuscular strategy for maintaining basic positions and preventing the occurrence of discipline-related injuries. Furthermore, a detailed definition of the optimal muscular activation strategy when used as a reference would enable the identification of detrimental activations in beginners, a growing population within this discipline. Therefore, the aim of the present study was to describe the muscular engagement in experienced indoor skydivers while assuming the four fundamental postures.