This study is being conducted because low lean muscle mass is prevalent among childhood cancer survivors. Lean muscle is the non-fatty muscle tissue that makes up part of the body's lean body mass. Low lean muscle mass is associated with loss of overall body strength, declining mobility and eventually, loss of independence. Among childhood cancer survivors, low lean muscle mass may contribute to reduced physical functioning and a sense of fatigue with exertion, limiting ability to participate in adequate physical activity. Loss of strength and a sense of fatigue with repeated movement make it difficult to participate in daily activities.