Decrease in gait variability while counting backward: a marker of "magnet effect"?

Counting backward (CB) and walking are both rhythmic tasks. An improvement of CB performance has been reported while walking, and has been interpreted as a "magnet effect" which is the tendency of biological oscillators to attract each other. The objective of this study was to compare the coefficient of variation (CoV) of stride time (ST) and the number of enumerated figures while single- and dual-tasking between older adults who increased and decreased their CoV of ST while CB. The number of enumerated figures and the CoV of ST under single-task (i.e., CB while sitting or walking alone) and dual-task (i.e., CB while walking) were measured among 100 community-dwelling older subjects (mean, 69.8 +/- 0.07 years). Subjects were separated into two groups according to the dual-task-related changes in CoV of ST (i.e., either above or below the mean value of CoV of ST under single-task). Seventeen participants decreased their CoV of ST while CB compared to usual walking (2.6 +/- 1.6% vs. 2.0 +/- 1.3%, P < 0.001), while 83 increased their CoV of ST (1.7 +/- 0.6% vs 3.4 +/- 2.3%, P < 0.001). The subjects who decreased their CoV of ST had a tendency to enumerate more figures while walking compared to sitting (20.9 +/- 6.3 vs 19.4 +/- 4.7, P = 0.046) unlike those who increased their CoV of ST (20.3 +/- 5.0 vs 21.8 +/- 6.0 while sitting, P = 0.001). We found that most of subjects had worse gait and CB performance while dual-tasking. Conversely, a limited number of subjects improved significantly their gait performance and simultaneously had a tendency to improve their CB performance while walking compared to sitting. This behavior was observed only among subjects with the highest gait variability and could be interpreted as an implicit strategy based on the "magnet effect".