Introduction
cognitive decline, dementia, cognitive impairment
With a rapidly growing aging population, the incidence of cognitive decline, such as mild cognitive impairment and dementia, is increasing [Accumulating evidence indicates that physical exercise combined with cognitive training may improve cognitive function for older adults with cognitive decline [Zhu et al. (2016) investigated how the training characteristics of combined cognitive and physical training influenced cognitive performance for community-based older adults. They investigated the effects of different session length (≤ 1 h vs. > 1 h), duration (≥ 16 weeks vs. < 16 weeks), and training frequency (< 5 sessions/week vs. ≥ 5 sessions/week) on cognitive function. They found that the session length and duration did not affect cognitive performance; however, the training frequency did influence the cognitive effects of combined training [Systematic reviews and meta-analyses of effects of combined cognitive and physical training have reported analyses of frequency impacts. These review studies reported that older adults who received a lower frequency (1–2 sessions/week) of combined training had better cognitive performance, denoted by the magnitude of the effect size, than those who received a higher frequency (3–5 sessions/week) [The training frequency may affect cognitive skill acquisition [Contradictory to the previous review studies and skill acquisition theory discussed above, Gheysen et al. (2018), conducting the potential moderator analysis of the efficacy of a combined physical and cognitive intervention, found frequency (≥ 3 vs. 1 session/week) did not significantly influence the effects of combined training on cognition. Although the findings of Gheysen et al. did not favor low-frequency training, there was a trend that the effect size in the low-frequency training was higher than in the high-frequency training. High-frequency training might lead to more effective short-term learning than low-frequency training [No empirical studies to date have directly compared how different frequencies of combined physical and cognitive training affect cognitive function for older adults with cognitive decline. It is critical to search for the precise amount of training that is sufficient, but not overwhelming, to alleviate cognitive decline and enhance cognitive function for older adults with cognitive decline. This study investigated the impact of training frequency on cognitive outcomes after combined physical and cognitive training for older adults with cognitive decline. We hypothesized that combined physical and cognitive training at different frequencies may lead to benefits on different cognitive functions in older adults with cognitive decline.