## 4 Discussion  
With the current interest in core stability and postural balance, a broad range of unstable devices and exercises are emerging in the field of functional training [18,19]. However, the destabilizing effect produced by different unstable devices on the same exercise lacks objectivation. Therefore, the main purpose of this study was to investigate whether three gymnastic balls different in shape and size could influence CoP-related parameters and core-muscle activation under the same postural condition or exercise. Indeed, understanding the effects of different unstable devices allows one to better modulate training progression over time, sensitizing trainers and therapists on the choice of the proper destabilizing device. CoP-related parameters resulted in being more sensitive than core-muscle activation in assessing the destabilization level induced by the three gymnastic balls. Both Unit Path and Area95 highlighted a significantly higher instability when the postural task was performed on the Gym53 and Gym65 with respect to the Eggball. We suppose that the ovoid shape of the Eggball could have minimized the multidirectional displacements that subjects suffered while maintaining postures in Gym53 and Gym65. Based on the assumption that the higher the CoP mean velocity, the more destabilizing the exercise [10], the development of training protocols of increasing difficulty has to focus not only on the type of exercise but also on the device to be employed. In our case, the Eggball should be suggested for employment in the early stages of rehabilitation or training, rather than the Gym53 or the Gym65. Furthermore, the postural balance control showed greater CoP displacements and a greater velocity under the US with respect to the BS posture. A widely accepted assumption is that, biomechanically speaking, the degree of stability is proportional to the size of the base of support, and it is maximized in any direction when the line of gravity is furthest inside the edge of the base of support [20]. In our case, the feet on the ground and the ball on which the subject was seated established the base of support. During the US posture, the nondominant lower limb was raised, leading to the base of support's restriction. Thus, the Unit Path and Area95 results confirmed the abovementioned assumption when unstable devices contributed to determining the base of support. Our results showed nonsignificant differences between the three gymnastic balls (i.e., Gym53, Gym65, and Eggball) on the core-muscle activation for the same body posture (i.e., BS and US). Thus, core-muscle activation was independent of the shape and size of the gymnastic ball that was used. The easy postural task that was proposed with a foothold always on the stable ground could have accounted for the unchanged core-muscle activation. Otherwise, we can speculate that core muscles equally contributed to the trunk stabilization while concurrent muscle coactivations could have occurred in order to face the different perturbations induced by the gymnastic balls. Specifically, global and local muscles (e.g., lower limbs and spinal muscles) could have counterbalanced the body sway induced by the gymnastic balls. Researchers contended that data on the activation of core muscles during tasks performed on unstable surfaces [21] or in a seated position (in addition to a standing one) [22] are needed. As far as we are concerned, our study is the first to investigate muscle activation while performing the same exercise with different gymnastic balls. Indeed, previous studies focused on the effect of core-muscle activation in different exercises or when comparing labile versus stable surfaces [15,21]. The significantly different activation of core muscles between the BS and US conditions that we found was in line with previous findings [23]. Even though the muscles that were assessed were different (lumbar multifidus spinae, thoracic multifidus spinae, lumbar erector spinae, thoracic erector spinae, and gluteus maximus), [PERSON] et al.