Originally Posted by
kyselad
Carrying furniture != riding bike. I believe the point was that weight doesn't influence efficiency if you're getting extra momentum for your extra effort. A big boy on a bike is harder to stop than a little one. For the furniture, that extra momentum ain't helping you since it's working in the wrong direction (i.e. down via gravity). Still, I agree muscles don't provide equal returns at certain loads.
Yes, carrying furniture != riding a bike, but neither does riding a bike equal riding an electric bike.
Muscles are complex things. Obviously if you assume that all the energy used by the muscles goes into the momentum of the bike (minus frictional losses) then you will get no loss of efficiency, but that is assuming the conclusion (since we know that newton's laws are conservative from the get go). Carrying furniture is just an extreme example of the fact that all the energy used by the muscles does not get turned into mechanical energy such as momentum (in fact, unless the furniture is going up stairs then none of the energy gets converted into mechanical energy).
The real point is that if your body is getting hot, tired, sweaty, then there is a loss of efficiency, which must be accounted for. In the case of carrying furniture the loss is as high as 100%. Who knows what it is in the case of cycling. However, considering the fact that I sweat at least as much when cycling as I do when carrying furniture I bet it's probably the dominant source of inefficiency and I bet it somehow depends on the weight of the bike.