Anticipatory and compensatory postural adjustments in response to loading perturbation of unknown magnitude.

In response to sudden postural perturbations, the posture control system uses anticipatory and compensatory postural adjustments (APAs and CPAs) to maintain balance and equilibrium. APAs strengthen as the perturbation magnitude increases, while CPAs remain constant because APAs make the necessary adjustments. However, the magnitude of a postural perturbation cannot always be fully known. This research focused on postural adjustments in response to perturbations with unknown magnitude. Participants caught falling sandbags of three weights on a tray held in their hands. Participants were told about the weight used for the upcoming trial in the KNOWN condition and not told in the UNKNOWN condition. Surface electromyography (sEMG) of the lumbar muscles and displacement of the center of pressure (COP) were recorded synchronously. The results showed that APAs and CPAs were stronger in the UNKNOWN condition than in the KNOWN condition. Meanwhile, in the UNKNOWN condition, the activity of the lumbar muscles and displacements of the COP showed no difference between weight levels. The lumbar erector spinae (LES) and lumbar multifidus (LMF) activated earlier in the UNKNOWN condition than for the heaviest weight in the KNOWN condition. The outcome of this study indicates that APAs and CPAs of lumbar muscles and displacements of the COP are affected by the knowledge of postural perturbations. The central nervous system (CNS) coped with load perturbations of unknown magnitude with redundancy response strategy, based on the maximum assumption of perturbation magnitude.