They allow the addition of more metabolic energy than is available without them. But, it's very difficult to quantify muscle strenght as each individual's strenght will vary. I tend to equate operating leg-driven vehicles like bicycles as merely forms of walking or running.
Motors represent a form of slavery.
It's used to indicate that a product or service is provided which a master, e.g., the beneficiary, feels he or she is not able to obtain without another's unwilling cooperation. The concept does not hold an inherent goodness or badness. The concept does imply mutal dependance, such that the master and the slave are equally captives of an institution. That's a low output, low enough a person of average health should be able to keep it up all day long. She doesn't need the help of a slave.