This chapter presents an overview of basic principles of exercise metabolism and reviews acute and adaptive responses to exercise. It describes the factors that limit exercise capacity and contribute to the deconditioned state of people with neurologic disability. The main factors contributing to individual differences in exercise capacity are sex, age, and level of physical activity. A major obstacle to documenting exercise capacity of individuals with neurologic impairments has been the lack of testing protocols that can accommodate motor and balance disturbances. The chapter also describes biomechanical and metabolic factors in relation to the energy expenditure of normal gait and that of individuals with neurologic conditions. Factors that help to explain differences in the economy of movement in healthy populations include age, walking environment, and walking speed. For many individuals with neurologic conditions, the co-existence of neuromuscular and cardiovascular impairments increases the complexity of the physiologic adaptations to training.