Human body calories consumption estimate based on physics approximation I was looking for references online about equations that relates human kinetics and calories consumption, and it seems there are no many engineers interested in weight lifting (go figure...). I think under the right assumptions it cannot be to complicated.
If I lift a box of 30 Kg, for 0.5 meters in one seconds, following a perpendicular trajectory to earth, I can say:
WORK = FORCE * DISPLACEMENT
FORCE = m * a = 30Kg*9.8m/s^2 = 294Kg*m/s^2 = 294N
WORK = 294N * 0.5m = 147J
POWER = WORK / TIME
POWER = 147J / 1s
POWER = 147W

1W = 0.24Cal/s
147W = 35.11Cal/s  in 1 seconds = 35.11 calories

So assuming the human body is a perfect machine with no losses, is it correct to say that I just used (at least) 35.11 Calories?
 A: There has been a lot of research on this topic in sports science, mostly on aerobic sports such as running and cycling. In professional cycling in particular, understanding these numbers is crucial. As a rough rule of thumb, in cycling people have found that the efficiency of the human body at power output levels close to what is called the "functional threshold power" (conceptually it's the maximum amount of mechanical power output that can be sustained indefinitely; for elite cyclists this number is somewhere around $400\,W$) is about 23%. 
In other words, if your mechanical power output is 200W, your body will need to consume roughly $200/0.23\,\mbox{W}\approx870$W. Notice, however, that this is for a specific kind of exercise. The numbers for weight-lifting are likely to be different. My guess is that 20% will still give you a decent estimate, accurate to within a few percent. Of course, for your weight lifter you would have to account for the fact that during the process of lifting the external weight, the center of gravity of the lifter's body moves as well, and the energy required for that needs to be included in the calculation.
A: The CN Tower in Toronto holds annual stair climbs for charity.
The record for climbing the $1776$ stairs is a little under $8$ minutes.
Data for physics calculations is surprisingly hard to find, but assuming an $80$ kg climber going up $1776$ steps, each $0.18$ m high in $480$ seconds, we come out with a power output of $522$ Watts
This compares well with the functional threshold power of $400$ Watts for elite cyclists working indefinitely (not just 8 minutes) in @Pirx's answer...
