Online adjustments of leg movements in healthy young and old.

Online movement adjustments are crucial for daily life. This is especially true for leg movements in relation to gait, where failed adjustments can lead to falls, especially in elderly. However, most research has focused on reach adjustments following changes in target location. This arm research reports two categories of online adjustments (see Gaveau et al., Neuropsychologia 55:25-40, 2014 for review). Small, frequently undetected, target location shifts invoke fast, automatic adjustments, usually without awareness. In contrast, large target location shifts can lead to slow, voluntary adjustments. These fast and slow adjustments presumably rely on different neural networks, with a possible role for subcortical pathways for the fast responses. Do leg movement adjustments also fall into these two categories? We review the literature on leg movement adjustments and show that it is indeed possible to discern fast and slow adjustments. More specifically, we provide an overview of studies showing adjustments during step preparation, initiation, unobstructed, and obstructed gait. Fast adjustments were found both during stepping and gait. In the extreme case, even step adjustments appear to be further modifiable online, e.g., when avoiding obstacles during tripping. In older adults, movement adjustments are generally slower and of smaller magnitude, consistent with a greater risk of falling. However, fast responses seem less affected by aging, consistent with the idea of independent parallel mechanisms controlling movement adjustments (Gomi, Curr Opin Neurobiol 18:558-567, 2008). Finally, putative neural pathways are discussed.