The UNCODE Study seeks to better identify the neurological causes of muscle weakness associated with advancing age. The main study consists of 4 laboratory-based testing sessions that involve testing muscle strength and physical and cognitive function as well as a battery of tests to 1) quantify brain excitability (using non-invasive transcranial magnetic stimulation), 2) modulate brain excitability (using non-invasive transcranial direct current stimulation), 3) spinal motor nerve firing characteristics, and 4) brain structure and function characteristics based on magnetic resonance images of the brain. Additionally, three option sub-studies are also available for enrollment. The first is a genetics sub-study where a cheek swab will be used to examine associations between certain genes and the physiological and functional measures obtained from the main study. The other two sub-studies are interventions. The first sub-study is a progressive resistance exercise training study where study participants will undergo 12-weeks of exercise training (3x/wk) and at the completion of the exercise training the measures obtained in the main study will be re-assessed. The other sub-study is a mental imagery sub-study where subjects are randomly assigned to perform a mental imagery training program consisting of imaging strong muscle contractions and mobility tasks (5x/wk) or to serve as a control (i.e., to not modify lifestyle) for 6-weeks. At the completion of the respective intervention period the measures obtained in the main study will be re-assessed.