Physical inactivity contributes to increased risk for hypertension, coronary heart disease, diabetes, various cancers, and depression. Research shows small increases in leisure time physical activity (LTPA) level among the least active populations result in larger improvements in overall health than any increase in LTPA among more active populations. People with disabilities (PwD) are less likely to meet physical activity (PA) guideline recommendations than their counterparts in the general population (39.2% vs 53.8%). People with mobility disabilities, such as those with amputations, are less active than those with other disabilities. Amputees, however, are largely absent from physical activity-related and disability-related research. One step toward improving LTPA participation among amputees is understanding motivations to be active and the experiences influencing those motivations. Using Self-Determination Theory (SDT) as a framework, this dissertation employed a sequential explanatory mixed methods approach to integrate fitness app intervention data with interpretative phenomenological analysis (IPA) findings. The quantitative component evaluated an app-based intervention with a waitlist control experimental design. Outcomes of motivations and PA level (Aim 1) were evaluated using linear mixed effect models. Amotivation, extrinsic motivation, and intrinsic motivation were evaluated as separate outcomes. Changes in amotivation and total activity level were significant during the intervention; there were no significant changes in extrinsic or intrinsic motivation. Amotivation (complete disinterest in LTPA) increased in both groups, but the increase was greater in the waitlist control group, suggesting use of the app staved off amotivation even though it did not contribute to increases in intrinsic motivation. Total activity increased in the waitlist control group only. Moderation was tested using SDT constructs of general causality orientation, a personality trait that represents a person’s belief about behavioral change and reasons to change (Aim 2). Amotivation is moderated by general causality orientation. Results from intervention analyses, including attrition analysis, were used to develop interview guides and participant inclusion criteria for the qualitative phase. In-depth interviews with amputees (Aim 3) explored motivations to be active and embodied PA experiences. IPA resulted in the development of six superordinate themes. Data from both the intervention and interviews were integrated to develop a deeper understanding of amputees’ experiences with motivations to be active (Aim 4). Participants identified barriers and facilitators to PA engagement that were unrelated to and unaffected by motivation to be active. These experiences disrupted the association between motivation and participation which added context to the intervention findings in which changes in intrinsic motivation over time did not parallel changes in PA over the same intervention period. Public health implications and suggestions for future research are discussed.