in Heyn et al.’s121 meta‐analysis of cognitively impaired elders, it does not appear that increases in aerobic capacity are required for cognitive benefits to accrue, as was previously thought. Cognitive benefits seen after non‐aerobic exercise also cast doubt on a central role for aerobic capacity.2 Exercise does increase cerebral blood flow acutely, as do cognitive tasks, and increased frontal cortex capillary density has been observed in animal models after exercise training, suggesting a possible common pathway for improved brain oxygenation and, thereby, function. Many of the most promising mechanisms in animal models have yet to be confirmed in human studies, however. Further exploration of the potential mechanisms of benefit in this crucial domain of health and function is needed, including studies linking changes in cerebral blood flow, depressive symptoms, self‐efficacy, physical fitness, or burden of chronic disease and disability to cognitive changes associated with exercise participation.
Recent studies have suggested a potentially more important role for resistance exercise in cognition that was previously thought. For example, a meta‐analysis of randomised controlled trials by Kelly122 of various exercise modalities in healthy adults found that aerobic exercise had no significant benefit in any cognitive domain, whereas resistance training was associated with improvements in reasoning, and Tai Chi with attention and processing speed. In adults with mild cognitive impairment (MCI), resistance exercise but not aerobic training improved executive function compared to a stretching control group in the EXCEL trial.123 In the SMART study,124 high‐intensity resistance training for 6 months improved global and cognitive function in older adults with MCI compared to sham exercise (calisthenics), a benefit that persisted over 18 months. In this same study, changes in lower extremity strength mediated 64% of the benefit in executive function in the training group,24 whereas gains in aerobic capacity were unrelated. Such findings again suggest a pathway of anabolic adaptation linking brain and musculoskeletal adaptations in this cohort, although the precise morphological and functional connectivity adaptations underlying cognitive benefit remain to be fully elucidated.125
Considerations regarding exercise for frail individuals with cognitive impairment
The novel cognitive frailty construct was first defined some years ago and is characterised by the presence of both physical frailty and potentially reversible cognitive impairment in the absence of dementia.126 The role of physical exercise interventions on cognitive function in older adults has been previously studied112,127; however, the large variability in measured cognitive outcomes and methodological aspects of studies makes it difficult to draw conclusions about the effects of physical exercise on cognition in older adults. A recent randomised controlled trial (RCT) examined the effects of four months of low‐intensity high‐speed resistance training (i.e., muscle power training) in older adults with MCI and found that the intervention improved cognitive function and physical performance.128 Similarly, it has been shown that four weeks of high‐speed resistance training combined with walking and balance exercises improved gait ability, balance, and muscle strength and reduced the incidence of falls in frail patients with dementia after long‐term physical restraint during nursing care.4 In agreement with other studies, there is a strong link between cognition and muscle strength, and physical frailty is associated with worse cognitive performance in multiple domains such as verbal memory, processing speed, and working memory in older people.126,129 Therefore, an individualised multicomponent exercise training with special emphasis on high‐speed resistance training may be a cornerstone for frail individuals with cognitive impairment to improve their physical and cognitive function simultaneously.5
Effects of exercise interventions on mobility and frailty syndromes
Beginning in 1990,130 with the first report of high‐intensity resistance training in nonagenarians, it has been shown that exercise interventions inclusive of robust resistance training increase muscle strength in frail older individuals.17,131‐133 As expected, most of the studies reported improved strength using resistance training programmes132,133 or multicomponent exercise interventions, including resistance training.17,131 Nevertheless, some studies did not find significant changes in muscle strength in this population, which may be related to the use of home‐based exercise interventions,134 few weight‐bearing exercises,135 or very low workloads106 that may not have provided sufficient stimuli for inducing strength gains. The use of scales of perceived exertion rather than strength testing to guide progression of loads during resistance training in frail older people is another factor that may limit the stimulus and consequently reduce the magnitude of adaptations.136
Exercise interventions also reduce the incidence of falls in older adults with physical frailty.17,133,135,137 Most studies have used multicomponent exercise programmes including the combination of resistance training, balance and/or gait retraining,17,135 and (less commonly) only resistance exercises133 or an alternative exercise intervention such as Tai Chi.137
Several trials have also investigated the effects of exercise interventions on gait ability in frail older people, with conflicting results. Whereas some studies showed improvements in gait ability after the physical training period,17,132,138,139 others found no improvement.17 Interestingly, the bulk of studies that demonstrated enhancements in gait ability utilised multicomponent exercise programmes,4,17,138‐140 while others used only resistance exercises133 or a combination of aerobic training and yoga.141
Based on the evidence that multicomponent exercise interventions are more effective in improving most, if not all, of the frailty syndrome hallmarks (i.e., poor balance, reduced muscle strength, poor gait ability, and increased incidence of falls), it is recommendable that this type of intervention, which includes resistance training, gait retraining, and balance exercises, among others (i.e., occupational therapy) be prescribed to prevent frailty syndrome in the elderly, as well as in people with pre‐frailty.5,6,136,142 Recently, the Vivifrail Project, an EU‐funded project that is part of the Erasmus+ programme, focuses on providing training and design material to promote and prescribe such physical exercise in older adults143‐145 (www.vivifrail.com).
In addition, because of the strong associations between functional capacity test performance and muscle power output or rate of force development in the healthy elderly,6,92,95 explosive resistance training has emerged as an essential intervention to improve functional capacity in older adults, including those who are frail.5,6 Indeed, in a 12‐week multicomponent exercise programme including explosive resistance training applied in institutionalised frail nonagenarians, improvements were observed in muscle cross‐sectional area, muscle fat‐infiltration, maximal strength, muscle power output, balance, gait, and sit‐to‐stand ability, along with a reduction in the incidence of falls.4,17 Therefore, explosive resistance training should always be considered in exercise interventions to improve frail older individuals' functional capacity.
One potential adverse event related to muscle power training is injury to tendons/cartilage, particularly of the rotator cuff and knee, where degenerative tears are commonplace,104 or exacerbation of abdominal/inguinal hernias.146 Interestingly, a systematic review of the effects of resistance training in frail older adults reported only one case of shoulder pain related to resistance training intervention out of 20 studies and 2544 subjects.147 Notwithstanding, to prevent injuries that could interrupt the exercise programme and its benefits, screening for such problems is critical, care must be taken in the workload and volume progression, and heavy and repetitive workloads, as well as unfavourable positioning (such as an overhead or military press in rotator cuff disease), should be avoided.
Disease prevention through exercise
Both healthy and chronically ill older adults are candidates for preventive strategies that will lessen the burden of comorbidity, disability, and premature death caused by incident disease. Physical activity patterns may be influenced by ageing