When you walk you are lifting weight, body weight.
Force of gravity is F=mass*acceleration = Newtons = body wight * g = kg*9.8m/s^2.
Even if you are walking slowly you are lifting one foot off the ground and your whole body comes up slightly as you move forward.
If you have a 50 pond pack as you step momentarily your body weight plus pack weifht is on one foot/leg as the loaded knee flexes.
That is why using two walking sticks when carrying a hevy pack takes load of the legs ad puts some of it on your arms.
You burn more calories standing still with a pack than without. Your muscles are costantly flexing to keep you upright.
Analytically it would require working out a work integral across your body for the walking motion. Work = ʃ Eˑdl.