Hi Brett,

it just occurred to me that

sprint = constant + b1 (force) + b2 (velocity) + b3 (power), is equal to
sprint = constant + b1 (force) + b2 (velocity) + b3 (force:velocity), where the last term is an interaction term.

This is true unless you measured power, force, velocity independently which means there are errors associated with each measure. Even then, the above should be close. But if your power was defined by a direct FV product, it should be what it is above. That may answer some of the questions you posed too. Hope it helps.