In the field of obesity research, physical activity and activity energy expenditure were viewed for many years simply as one side of the energy balance equation. Interest in these domains was driven predominantly by the challenge to explain why obese individuals were in positive energy balance, and how much movement would be needed to offset levels of energy intake. An array of objective and subjective measurement approaches have been devised to capture movement in its various guises; sometimes with the intention to also measure energy expenditure. Misalignment of assessment tool and outcome measure can lead to misinterpretation of the extent to which physical activity level contributes to weight gain, and the particular therapeutic benefit of exercise for treating comorbidities. Despite considerable research effort, there remains some confusion in what physical activity messages and interventions are most valuable for combatting obesity and comorbidities. Revisiting the basics of human movement measurement may be needed before we can fully appreciate the interface between physical activity and obesity.