Depression often co-exists with other serious medical illnesses such as heart disease, stroke, cancer, HIV/AIDS, asthma, arthritis, diabetes, and Parkinson's disease. Studies have shown that people who have depression in addition to another serious medical illness tend to have more severe symptoms of both depression and the medical illness, more difficulty adapting to their medical condition, and more medical costs than those who do not have co-existing depression. Research has yielded increasing evidence that treating the depression can also help improve the outcome of treating the co-occurring illness. Depression is also a common secondary condition. It is often times a comorbidity of other chronic illnesses and diseases such as diabetes, heart disease, obesity, osteoporosis, spinal cord injuries, multiple sclerosis, stroke, Parkinson's disease, and arthritis. Depression can also worsen or exacerbate an individual's current condition.

Within the past decade, researchers have been working towards bridging the gap between mental health and exercise. They want to connect and use the psychological benefits from exercise to help treat disorders such as depression. Since depression is associated with several symptoms that affect people's mood such as feelings of sadness, emptiness, fatigue, hopelessness, loss of interest in activities, and decreased energy, exercise is thought to have potential to counteract those symptoms.

Individuals with depression typically lead a less active lifestyle with a reduction in cardio-respiratory fitness in comparison to the general population. We know that a lack of physical activity is associated with an unhealthy body composition and increased risk for chronic health conditions.  However, there is a growing body of evidence supporting exercise as a beneficial approach to treating depression.  In mild to moderate cases of depression, exercise effects are seen as comparable to antidepressant medication and in more severe cases of depression it is seen as an important complementary therapy to traditional treatment methods. Research has reported the greatest psychological effects from exercise to be, “increased feelings of mastery or self-efficacy, distraction, a reduction in negative patterns, and social interaction.”