The study is designed to investigate the effects of passive heat therapy on blood vessel health in 40 people (48 enrolled with attrition rate of 20%). Following an extensive set of vascular function tests, participants will engage in either a passive heat therapy intervention for 60 minutes, 4 times a week for 8 weeks, or a placebo intervention at a lower temperature. Seven of the 8 weeks of intervention will occur in the home setting. Vascular function tests will be repeated after the 8 weeks to determine if chronic passive heat stress improved vascular health.

The experimental arm (HEAT) participants undergo an 8-week intervention with 4 weekly passive heat therapy sessions. The aim of each session is to raise the participants' core temperature by at least 1°C by resting under heating blankets set at approximately 42°C. The control arm (SHAM) participants undergo the same procedure as above except the temperature setting of the blankets is lower at approximately 34°C.

Screening, baseline assessments and the first 4 sessions will be conducted in the investigator's SCI lab to familiarize the participant with how to apply the heating blankets and ensure safety. After the 4th session, participants will receive a heating blanket for use at home in the remaining 7 intervention weeks. Cardiac MRI and peripheral vascular assessments are performed at baseline and within two weeks after completing the intervention. Weekly sessions (4x per week) in weeks 2 to 8 will be performed at home and secure VA-approved online video conferencing software will be used by study staff to remotely supervise the sessions and collect data.

Inclusion Criteria:

1. Participant demonstrates understanding of the study and has provided an appropriately signed and dated informed consent.
2. Participants will be male or female, 18-60 years of age, at the time of Visit 0.
3. Female participants must be non-lactating. Female patients are eligible only if they have a negative pregnancy test throughout the study period (or are postmenopausal). Postmenopausal women taking hormone replacement will be included if they have been on a stable dose for =6 months.
4. Stable SCI (AIS A or B) of longer than 1-year duration.
5. Use of medications for clinical management of medical problems directly related to SCI is not considered an exclusion criterion. Status such as detrusor instability, muscle spasms, and other common SCI problems (other than those that could alter vascular responsiveness) may be allowed for those SCI participants who have been on stable doses for =6 months. Results from participants taking such medications will be analyzed to determine possible medication-related differences from those on no medications.

Exclusion Criteria:

1. Participants who smoke, or:
2. Uses daily administration of anti-inflammatory medications (stable doses of NSAIDs or statins are acceptable; steroids are exclusionary)
3. Uses daily administration of vasoactive medications (e.g., alpha antagonists or agonists or phosphodiesterase inhibitors).
4. Has a current pressure ulcer or skin breakdown.
5. Has an active, uncontrolled, autoimmune or inflammatory disorder
6. Has no history of or current alcohol or substance use disorder
7. Has a history of heat related illness (e.g., heat stroke)
8. Has a history of any illness that, in the opinion of the study investigator, might confound the results of the study or poses an additional risk to the participant by their participation in the study. This includes, but is not limited to, a clinically relevant medical or surgical history.
9. Is unlikely to cooperate with the requirements of the study.