Brief Summary: This clinical trial tests how well a couple-based physical activity (PA) intervention, Mates in Motion, works in patients undergoing hematopoietic cell transplantation (HCT) and their care giving partners to be more physically active. Mates in Motion provides training in communication skills and behavior change techniques to help HCT patient-caregiver dyads support one another in adoption and maintenance of PA. This study may help researchers determine if a couple-based PA intervention improves PA, physical endurance and global physical health among patient-care giver dyads undergoing HCT.

Detailed Description: PRIMARY OBJECTIVES: I. To determine feasibility of recruitment, adherence, and retention of a couple-based physical activity (PA) intervention in a pilot randomized controlled trial (RCT). II. Determine acceptability of the intervention (multiple dimensions of treatment satisfaction). III. Describe changes in PA, physical endurance, and global physical health from baseline to follow-up as a function of randomized condition. OUTLINE: Patient-caregiver dyads are randomized to 1 of 2 arms. ARM I: Dyads participate in the Mates in Motion program consisting of weekly sessions to train couples in the use of communal coping strategies to support one another in achieving PA goals and skill building focus on instruction and practice in effective communication, with emphases on adaptive speaking, responsive listening, and joint decision-making and problem-solving around PA over 8 weeks. Patient-partner dyads receive weekly step-count goals, complete walk-tests and questionnaires and wear an Actigraph device and Garmin activity tracker on study. ARM II: Patient-partner dyads wear an Actigraph device, compete walk-tests and questionnaires on study. Dyads receive usual care on study. Dyads receive a Garmin activity tracker at the end of the study.