Here's a mad fact: lifting weights burns very little calories. A sets of heavy Rippetoe style, ultra epic, religious, no-homo squats burns less calories than mastrubating for 20 minutes does.
In order to determine how much calories are needed during an exercise such as squats we first have to determine how much work we are doing during, let's say, 100kg/220 lbs squat.
Work = Force x Displacement
Displacement - during a regular squat perfomed by an average person the weight moves about 50 cm up and 50 cm down. This means that the total displacement is 1 m.
Force = applied pressure
F = Mass x G (gravity)
# Earth's gravity causes an acceleration of 9.81 m/s per second
So, during a 100 kg squat the force is:
F = 100 x 9.81 = 981 N; this means that the work is:
W = 981 N x 1 m = 981 J
(1 J = 0.24 cal )
(1 J = 0.24 cal )
So the work during one 100 kg squat is 235.44 cal or about 0.235 kcal.
However we also have to find the effieciency of the human body during all this.
The efficiency of human muscle has been measured (in the context of rowing and cycling) at 18% to 26%. The efficiency is defined as the ratio of mechanical work output to the total metabolic cost, as can be calculated from oxygen consumption. This low efficiency is the result of about 40% efficiency of generating ATP from food energy, losses in converting energy from ATP into mechanical work inside the muscle, and mechanical losses inside the body.
From Wikipedia
Now, let's calculate the average efficiency:
18 + 26 = 44; 44/2 = 22
So, the average efficiency of the human muscle structures is 22%. This means that the human body loses 78% of the energy during generation of force. Said in simple terms in order to perform work which requieres 1 kcal we need to consume 4.5 kcal.
Let's find out how much calories we need in order to burn 0.235 kcal given the fact that the average efficiency of the body is 22%.
0.235 / 0.22 = 1.07 kcal
This means that one 100kg squat burns 1 kcal.
In other words to burn 1 apple or 150 kcal you need to perform 150 squats with 100kg.
Enjoy.
P.S. Rippetoe's workout is about 2-3 exercises x 15 reps = 45 reps = about 50 calories or less burned during the movements themselves. No wonder his athletes look like this.