There is growing evidence of the integral role of exercise following cancer treatment. Randomized trials show that exercise improves post treatment symptoms and quality of life in breast and prostate cancer patients.Regular exercise reduces cancer specific and all cause mortality for patients with breast, prostate, brain or colorectal cancer. Nearly a 40-50% relative risk reduction in cancer specific death and 20-50% relative risk reduction in all cause mortality have been reported with exercise. At present it is unknown if preclinical pleiotropic effects on tumors through aerobic exercise are clinically relevant. Before launching into further studies of exercise training on changes in tumor hypoxia and response to CRT in rectal cancer patients, it is first important to understand: 1. Whether measures of exercise exposure (i.e., exercise capacity and self-reported exercise behavior) are associated with markers of tumor hypoxia at diagnosis (prior to initiation of CRT) and 2. Whether changes in measures of exercise exposure over the course of CRT are related to changes in tumor hypoxia and response to CRT.