Does Aerobic Training Increase Motor Cortex Cerebral Blood Flow and Motor Performance in Healthy Older Adults?

Age-related motor slowing can compromise personal and public safety as older adults navigate their home environment and community (e.g., driving), and age-related increases in reaction time remain a ubiquitous finding in spite of extensive efforts to understand the physiological antecedents of motor slowing. Simple motor reaction time can increase more than 40% between the ages of 18 and 94 [11]. However, recent findings suggest that regular exercise and physical activity (i.e., running) may transfer to improved outcomes related to motor performance on activities of daily living [32]. One potential explanation for this beneficial transfer is related to physiological changes in the central nervous system following aerobic training. For example, animal models show increased motor cortex metabolic capacity [28] and vascular changes indicative of increased blood flow (BF) following prolonged training [12,41]. Both findings are suggestive of improved motor cortex health. However, it is still unknown if aerobic training leads to increased BF to the motor cortex in humans. Determining this relationship is thus crucial for designing interventions aimed at reducing age-related motor slowing. Another knowledge gap concerns the relationship between motor cortex BF and behavioral measures related to motor performance. Specifically, does increased BF improve reaction time during motor task execution? Previous findings demonstrate acute increases in motor cortex BF with contralateral limb movement [22] but there are no studies exploring whether increased BF via aerobic training results in superior behavioral outcomes related to motor output. Horwitz et al., (2000) reported an inverse relationship with reaction time and BF to ipsilateral cerebellum during a finger movement task but no fitness measures were considered [18]. Taken together, evidence from both human and animal studies suggest that aerobic training should lead to increased BF via changes in local vasculature, however, whether these increases relate to motor performance is still unknown. The overall goals for this proposal are to: 1) test whether aerobic training results in a sustained increase in BF to the primary motor cortex and 2) determine if changes in motor cortex BF following aerobic training are associated with changes in driving performance and upper limb reaction time. In this proposal we will focus our efforts on two specific aims: Specific Aim 1: Determine if a 3-month aerobic training intervention increases BF to the primary motor cortex in healthy older adults. We will use arterial spin labeling (ASL) MRI to test the hypothesis that a 3- month aerobic training intervention will increase motor cortex BF. ASL-MRI provides us with the unique ability to noninvasively measure cerebral BF in vivo [34,46] and will be used to quantify pre- and post-intervention measures of BF in 30 previously sedentary older adults. Since BF also contributes to the blood oxygen level dependent (BOLD) response, we will also use a finger tapping functional MRI motor task to determine how changes in resting BF following aerobic training impact motor cortex BOLD magnitude during a functional task. We hypothesize that increases in motor cortex BF following aerobic training will lead to increases in BOLD. Specific Aim 2: Determine if changes in BF following a 3-month aerobic training intervention are associated with changes in reaction time in healthy older adults. In this aim we will use a driving simulator and the human motor assessment panel (HuMAP) to test the hypothesis that increases in BF following aerobic training will be inversely related to reaction time. A computerized driving simulation task will be used to measure reaction time related to driving performance (e.g., brake onset distance), and the HuMAP, an automated measurement apparatus that uses light-emitting diodes to detect movement, to assess upper limb reaction time. The manner at which healthier living may promote brain health has become a central theme in the study of aging. Aerobic exercise is of specific interest to this study because it represents a lifestyle variable that is accessible to most people, proven to reduce cardiovascular disease, and has shown promise in reducing the risk of the leading cause of dementia. Results from this study will lead to future interventions tailored to increase BF to cortical regions essential for higher-level cognitive processes.