Mentally represented motor actions in normal aging II. The influence of the gravito-inertial context on the duration of overt and covert arm movements.

Here, we address the question of whether normal aging influences action representation by comparing the ability of 14 young (age: 23.6 +/- 2.1 years) and 14 older (age: 70.1 +/- 4.5 years) adults to mentally simulate arm movements under a varying dynamic context. We conducted two experiments in which we experimentally manipulated the gravity and inertial components of arm dynamics: (i) unloaded and loaded vertical arm movements, rotation around the shoulder joint, (ii) unloaded and loaded horizontal arm movements, rotations around the shoulder and elbow joints, in two directions (inertial anisotropy phenomenon). The main findings indicated that imagery ability was equivalent between the two groups of age for the unloaded arm movements, but better for the young than the older group, for the loaded arm movements. For the horizontal movements, we found better imagery ability for the young than the older adults for both movement directions and loads. Finally, young and old adults showed low (<8%)-temporal variability for both overt and covert arm movements in all conditions. Our findings showed a specific decline of action representation in the aging brain and suggest that internal models of action become imprecise with advance in age. This is not exact to say that there is a severe impairment of motor prediction in old adults as they can mentally represent their arm movements with high-temporal consistency. Finally, we propose that motor imagery could be used as a therapeutic tool for motor rehabilitation in aged adults.