I guess it depends on what manner you apply a force to achieve the work.
Here, brain power is inversely related to the difficulty in which the work is achieved. Meaning, if the work is achieved easily an in a timely manner (low difficulty), a large amount of brain power can be assumed employed. But, if the work is achieved rather sloppily and with much physical laboring (high difficulty), then a lack of brain power can be assumed employed. Either way it is a rough approximation and current efforts have been directed at pondering more insightful questions.