Aerobic exercise, cardiorespiratory fitness, and the human hippocampus

Abstract

The hippocampus is particularly susceptible to neurodegeneration. Physical activity, specifically increasing cardiorespiratory fitness via aerobic exercise, shows promise as a potential method for mitigating hippocampal decline in humans. Numerous studies have now investigated associations between the structure and function of the hippocampus and engagement in physical activity. Still, there remains continued debate and confusion about the relationship between physical activity and the human hippocampus. In this review, we describe the current state of the physical activity and exercise literature as it pertains to the structure and function of the human hippocampus, focusing on four magnetic resonance imaging measures: volume, diffusion tensor imaging, resting-state functional connectivity, and perfusion. We conclude that, despite significant heterogeneity in study methods, populations of interest, and scope, there are consistent positive findings, suggesting a promising role for physical activity in promoting hippocampal structure and function throughout the lifespan.

Keywords: cardiorespiratory fitness; exercise; magnetic resonance imaging; perfusion; white matter.

Figure 1.

Conceptual illustration representing the diversity of study designs and physical activity related measurements employed in the studies included in this review. Note. Exercise dose is defined as intensity (ranging from light to moderate-to-vigorous physical activity [MVPA]), duration (e.g. minutes per bout or week of exercise), and frequency (e.g. number of exercise bouts per week).

Figure 2.

Conceptual illustration representing the neuroimaging measurements focused on in this review and how they can be viewed with respect to the hippocampus. Note. This is a simplified conceptual scheme rather than a theoretical or mechanistic model.

Figure 3.

Heat map illustration of the number of analyses outlined in this review, assessing hippocampal integrity (y-axis) using magnetic resonance imaging (MRI) measures of morphology (GM Volume), white matter integrity (WMI), white matter volume (WM Volume), white matter hyperintensities (WMH), functional connectivity, and perfusion to examine associations with physical activity measurements. Note. Physical activity study designs (x-axis) include: (1) cross-sectional physical activity questionnaires (Q[C]); (2) longitudinal physical activity questionnaires (Q[L]); (3) accelerometry (AM); (4) cross-sectional cardiorespiratory fitness (CRF[C]); (5) longitudinal cardiorespiratory fitness (CRF[L]), including only observational studies and not randomized clinical trials; (6) interventions, including randomized clinical trials, training studies, and detraining studies, specifically measuring outcomes pre- and post-intervention (INT); and (7) interventions using the relative change in cardiorespiratory fitness as the main outcome (INT[ΔCRF]). The observations included in the heat maps are based on individual analyses within studies, as numerous studies conducted multiple analyses using various metrics of hippocampal integrity and physical activity. a) The number of analyses (n = 205) assessing any of the four magnetic resonance imaging measures reviewed in this study with relation to physical activity. b) The number of analyses (n = 124) assessing hippocampal morphology using total volume, left and right hippocampal seeds, anterior and posterior seeds, and subfields, examined in association with measures of physical activity. c) Number of analyses assessing (n = 58) white matter connections to the hippocampus using total volume, measures of white matter integrity, including fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AxD), radial diffusivity (RD), and myelin water fraction (MWF), and white matter hyperintensities (WMH), examined in association with measures of physical activity.