## 9 Adaptation to the Repeated Perturbation Cycles  
The findings from the studies reported in the preceding sections were obtained mostly under steady-state conditions, i.e., when the initial uncontrolled reactions to the onset of the platform translation have vanished. Usually, the first few translation cycles are omitted from the analysis. Yet, these initial cycles and associated reactions and their changes as a function of the successive cycles can tell something about the way subjects comply with the condition and, in perspective, whether the problems in balancing behaviour affect more the transient phase or the steady-state behaviour or how pathologies affect these initial reactions. Adaptation implies a gradual shift from a reactive response to the abrupt onset of the perturbation series and from conscious control of the balancing behaviour to a more automatic processing, including modulation of the reflex responses triggered by the continuous stimulation (see [87] for the neck responses in seated subjects). Hence, understanding the adaptation phenomenon adds to our capacity of better exploiting this challenging perturbation procedure in order to understand the mechanisms and processes whereby the appropriate balancing behaviour is achieved and maintained in order to produce a more suitable behaviour.  
The strength of the reflex response triggered by a certain ankle rotation during a series of horizontal translations is modulated appropriately by its repetition [88]. Even more so, the very same responses elicited by the translating platform itself should be liable to modulation by the continuous perturbation cycles. By using a sequence of discrete impulsive perturbations, [PERSON] et al. [89] had already suggested that adaptation was due to a generalized habituation in the postural control system. Others have shown that event related cortical potentials connected to the postural stimulus diminish over time with the successive cycles [90]. Of course, at the onset of the sequence of perturbations, a combined postural and startle response ensues [91; 92].  
Taken together, the studies show that adaptation occurs when a subject's balance is challenged by repeated perturbations. Under these circumstances, the sensorimotor processes underlying the adaptation to the repeated perturbations would require the collection of information about the amplitude velocity and frequency of the displacement of the support surface and of the instantaneous and continuously varying position of the body's CoM relative to the platform. This information would improve the performance by reducing redundant muscle activity (the initial co-contraction of the antagonist leg muscles) and enhancing the intersegmental coordination [33; 41]. The reduction in the muscle activity concerns both the reflex responses and the anticipatory activities, a sign of coordinated modulation of responses occurring at different times of the perturbation cycle. Interestingly, the reduced muscle bursts of activity do not modify their time relationship with the perturbation cycle [93]. This indicates that the adaptation modulates so nicely with the neuronal excitability and muscle activity that it need not modify the time windows of their activation to comply with the changes in the body's mechanics. As a consequence, the head back-and-forth oscillation is diminished as well [41; 94]. The speed of the adaptation process is susceptible to fatigue, even if the basic task of controlling the CoM of the body is substantially maintained [95; 96; 97].