The term life force is used a number of times during the series. Does life force simply mean chakra? If so, then do non-ninjas (such as Teuchi, the owner of Ichiraku Ramen) not have life force? Is there a canonical definition for this term?
The way I see it, Life Force is a completely different energy from chakra.
- Chakra can be depleted and restored (even though if you deplete it too much, you die). When Life Force gets used in a technique, the user's vitality usually takes a hit, as well as his life-span. (Nagato was left completely drained, and never recovered from using the Gedo Mazo in battle).
- Life Force was seen to be transferred and infused in other individuals to revive them. Both "super healing" and actual bringing back from the dead was seen. Mere "chakra" does not have this property.
- Life Force seem to have some sort of connection with the Yang (or physical) energy your body produces, and mixed with Yin to form chakra.
Not to mention humans once lived without chakra, and it was given to them by the God Tree, the term "Life Force" indicates that without it there can be no life, hence the two are different.
So what is Life Force?
The term is very hazy, it wasn't explicitly explained or discussed in the series, but from what I get, it's a completely different energy, closely related to the Yang energy, and it determines your vitality and life-span.
Here are my thoughts:
"...chakra is a form of life energy all living individuals naturally produce to some degree"-Naruto wiki
Every living thing has charkra. You need to have good chakra control to use ninjustu/genjustu. For example, Lee has charkra, which is why he can walk on water and up trees, but he can't use it "externally as ninjutsu or genjutsu" -Naruto wiki.
I don't believe the term "life force" is defined specifically in the series. Charkra is the only form of life energy I think is mentioned, and running out of charkra kills you. So, I would think charkra is life energy, but that disagrees with the definition.
Chakra is created by taking the energy from ones cells and controlling it with ones will,And Yang chakra is the chakra of the body which governs Vitality(life force).Madara stated that the only thing remaing if hashirama was the life force in his cells and since he had his cells if made him stronger and he could use wood style. So life force/energy(from the cells)chakra is the same since all they do is bring it to their stomach with their minds and manipulate it