## 1 Introduction  
Maintaining a standing position is a task that requires proper neuromuscular coordination, the goal of which is to keep the center of mass (CoM) in the correct relationship with the base of support (BoS) [1, 2]. The action of external forces is one of the factors that cause a disturbance in balance, forcing the body to take compensatory action to prevent a fall. Depending on the severity of the perturbation, the body must apply an appropriate strategy, and thus the sequence of activation of the muscles responsible for restoring balance must change. It is possible to regain stability by (1) adjusting the CoM relative to the BoS or (2) adjusting the BoS to the CoM [3].  
In the first case, recovery can be achieved in two ways, depending on the severity and direction of the perturbation. In response to small, low-frequency perturbations, the body behaves as an inverted pendulum, which is mostly controlled by the ankle joint muscles. In response to higher, high-frequency disturbance, the body operates like a double inverted pendulum, in which the corrective action is carried out by the hip and trunk muscles [4]. When perturbations are extensive, the projection of the CoM exceeds the BoS area. At this point, two other strategies are used to recover the balance. The first is the loaded side-step (LSS) strategy, which involves taking a side step with the limb initially passively loaded due to the perturbation. The second option is the unloaded crossover step (UCS) strategy, during which a side-step is performed with the limb primarily passively unloaded. In this strategy, the limb performing the step crosses with the supporting leg [5, 6].  
As mentioned by [PERSON] and [PERSON] [7], postural control is a fundamental motor task that must adapt quickly to a dynamically-changing environment. This is mainly noticeable when unexpected perturbations occur. According to [PERSON] et al. [8], perturbations can be (1) mechanical stimuli (sudden perturbation like pulling, pushing, and tugging or continuous motorized movement of support surface); (2) sensory stimuli (visual, vestibular, and proprioceptive); or a (3) combination of such perturbations [8; 9; 10]. The direction of the application of the disturbance also seems to be an important factor. [PERSON] et al. [11] emphasize that the response to mediolateral perturbation is more challenging as compared to anterior-posterior ones. It is worth noting that it is much easier to find papers in which the reactions of the body (both during gait and standing) [12] to perturbations taking place in the sagittal plane are analyzed. A considerably smaller number of authors undertake to evaluate the responses to perturbations occurring in the frontal plane. Since perturbations in the frontal plane induce an asymmetrical response, it has been observed that the postural control is dominated by the limb loading/unloading mechanism (aforementioned LSS and UCS strategy), which is predominantly controlled by the hip abductors and adductors [13; 14; 15]. It is worth mentioning that [PERSON] and co-workers [4; 16; 17] have emphasized that almost 95% of the anterior-posterior sway happens around the ankle and the hip axis, due to the reduction of the high number of degrees of freedom by compressing them into two muscular synergies termed the ankle and hip. Therefore, it seems interesting to see if the same mechanism occurs when the body responds to mediolateral perturbations that do not lead to a step strategy.  
Therefore, the purpose of this paper was to review all previous papers investigating how healthy young adults deal with mediolateral perturbations in a standing position, including reviewing the methodology, outcomes, and sequence of responses in these studies. To this end, four sequences were established showing the responses, successively of the ankle, knee, pelvis, CoP, and CoM, as a function of time against destabilizing stimuli in the frontal plane. It will allow systematizing responses to the mediolateral equilibrium perturbations occurring in the daily environment.