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.

Inclusion Criteria:

1. Histologically confirmed gastrointestinal malignancy
2. Plan for chemoradiation
3. At least 18 years old
4. Karnofsky performance status of at least 70% at study entry
5. Ability to read and understand English
6. Signed informed consent prior to initiation of study-related procedures
7. All women of childbearing potential must have a negative serum pregnancy test prior to study entry

Exclusion Criteria:

1. Significant cardiac disease (i.e., left ventricular ejection fraction of <50%, unstable angina, placement of cardiac stents and myocardial infarction within previous 6 months)
2. Contraindications to a cardiopulmonary exercise test as recommended by the American Thoracic Society: (a) acute myocardial infarction (3-5 days), (b) unstable angina, (c) uncontrolled arrhythmias causing symptoms or hemodynamic compromise, (d) syncope, (e) acute endocarditis, (f) acute myocarditis or pericarditis, (g) uncontrolled heart failure, (h) acute pulmonary embolus or pulmonary infarction, (i) thrombosis of lower extremities, (j) suspected dissecting aneurysm, (k) uncontrolled asthma, (l) pulmonary edema, (m) room air desaturation at rest ≤85%, (n) respiratory failure, (o) acute noncardiopulmonary disorder that may affect exercise performance or be aggravated by exercise, (p) mental impairment leading to inability to cooperate, and (q) extensive bone metastases
3. KPS<70%
4. Females who are pregnant