###### Abstract  
In proprioceptive training, unstable devices produce multidirectional perturbations that must be counterbalanced by the postural control systems and core-muscle activation. We investigated whether different sizes and shapes of three gymnastic balls could affect core-muscle activation and postural balance when performing the same exercise. **Methods:** Eleven young healthy subjects were assessed on the balls, assuming two body postures (bipedal seated and unipedal seated) and performing a dynamic exercise. Two balls were spherical with different diameters, and one was ovoid. Postural balance and muscle activation were assessed through center of pressure (CoP)-related parameters and surface electromyography. **Results:** Statistical analysis showed a significant effect of the gymnastic balls (\(p<0.001\)) and the body postures (\(p<0.001\)) for the CoP-related parameters, with the ovoid shape and the bipedal sitting representing the easiest conditions. Core-muscle activation was affected only by body postures, with a higher activation in the unipedal sitting (\(p<0.01\)). In the dynamic exercise, significant differences were only detected for the CoP-related parameters (\(p<0.001\)). **Conclusions:** The shapes and sizes of the gymnastic balls produced different degrees of destabilization under the same body posture but left the core-muscle activation unaltered. In the dynamic exercise, the conformation of the balls did not represent the main determinant in producing destabilizing effects.  
balance; unstable device; center of pressure; core stability +
Footnote †: journal: Applied Sciences  
platform that quantifies the center of pressure (COP) displacements [10, 11] and the surface electromyography to assess the core-muscle activation [12, 13]. In more detail, [PERSON] and colleagues studied different core stability exercises measuring the COP mean velocity during their execution. Their study gave useful information for the prescription of exercises of increasing difficulty based on the assumption that the higher the COP mean velocity, the more destabilizing the exercise [10]. Similarly, [PERSON] and colleagues documented significant improvements in seniors' COP-related parameter scores after running a Fitball exercise program [11]. Additionally, in a group of 23 older adults, [PERSON] and colleagues found an improvement of COP-related parameters after attending a balance training program using wobble boards [14]. Unlike the COP-related parameters, core-muscle activation seems to show controversial responses to the destabilizing exercises. For instance, no effects on core-muscle activation were detected when performing upper-body strength exercises while sitting on a labile rather than stable surface [13]. In fact, [PERSON] and colleagues observed a higher paraspinal muscle activation when performing deadlifts with the feet on a stable surface with respect to the unstable counterpart [15]. Conversely, an increased activation of the rectus abdominis and external oblique muscles was observed during a prone bridge with the feet on a Swiss ball when compared to a stable surface [12]. These previous investigations focused on the destabilization produced by unstable devices when compared to stable conditions. Nevertheless, this approach does not consider two variables that could modulate the amount of destabilization induced by an exercise: the size and the shape of the unstable devices employed. Indeed, the surface of the ball in contact with the ground could depend on these two variables, influencing the destabilization level. Therefore, the aim of the present study was twofold. First, we aimed to investigate whether three gymnastic balls different in size and shape could affect the COP-related parameters of the same exercises. Second, we aimed to determine how specific muscles responsible for core stabilization responded to the destabilization produced by the gymnastic balls.