Anticipatory postural adjustments in reach-to-grasp: effect of object mass predictability

Anticipatory postural adjustments (APAs) are thought to compensate for upcoming and predictable perturbations before they occur, e.g., a backward shift of the body center of pressure (COP) before raising the arm. When the goal of arm movements is to reach, grasp, and manipulate an object, predicting the effect of raising the arm on body COP before reach onset could incorporate the properties of the object to be lifted, as both will affect postural control during reaching and object manipulation. Alternatively, the central nervous system (CNS) might use separate APAs to compensate for the effect of arm raising from raising the arm and object. To distinguish between these two scenarios, we asked subjects to reach, grasp, and lift an object whose mass (100g, 750g, or 1400g) was either constant across trials or variable from trial to trial ('predictable' and 'unpredictable' condition, respectively). We hypothesized that object mass would affect the magnitude of APAs in the predictable condition before the onset of object lift but not before the initial arm onset. We also expected COP variability following object lift to be reduced as a result of APAs. For the unpredictable condition, we expected 'default' APAs that would minimize postural perturbation following object lift. We found that both magnitude and timing of APAs were modulated as a function of predictable object mass prior to contact, rather than at the onset of the reaching movement. Specifically, COP position moved forward with increasing object load (p<0.05) and peak COP velocity related to object contact occurred significantly early for heavier loads (p<0.05). For the random condition, the COP position and timing at all loads resembled that associated with larger predictable loads. These findings suggest that modulating COP to a future event might be more accurate when timed to temporally close events, thus potentially reducing the computational load as well as risks of prediction errors. Additionally, our results might suggest limitations in the predictive capability of the CNS in relation to 'how far in the future' it can go when predicting the consequences of planned actions.