If I exercise by weightlifting, I can "burn calories". The way I understand the process is as follows: when I'm moving the weight up, the energy from chemical bonds turns into potential energy of the weight. When I'm moving it down, though, the same potential energy turns into heat inside my muscles and ligaments. The heat is then dissipated around - that's why I'm sweating. The same happens e.g. when running, only now the weight is my body.

Now assume that I'm unloading a truck, i.e., only moving weights down. I'm now just using the potential energy to heat my body. In fact, because of this additional heat, I may burn less fat than I would burn at rest to maintain my body temperature. Therefore, if I wand to lose some pounds, it seems I should prefer watching TV on my sofa to unloading a truck. This conclusion is counter-intuitive - unloading a truck seems feels like any other physical activity. So, what's going on here?