How much force can a person pull on a rope?

[Brion Toss]

Hi Kevin,
Most hitches will weaken rope about 20%; I don't know about webbing. Possibly the failure was sequential ó rapidly so ó as otherwise one would expect the doubled portion to be smallest. Hitch aside, there were in any event likely some localized deformations in the webbing.
As for the max force, a typical hauler can readily pull a sustained load of 75 pounds, and the limiting factors are height and traction. That's why a team of relatively light people, pound for pound, will usually beat a team of relatively heavy people in a tug o'war: they can put more feet on the ground, thus more effective traction.
The other important factor here is impact loading, something that a well-coordinated heaving gang is [i]designed[i] to apply. You might have doubled the sustained purchase at peak loads, something that would have been enough to overcome block friction and materials elasticity.
I think the lesson here is the need for a suitable safety factor in all hauling applications, which is to say a usably precise knowledge of what the loads will be. Clearly, that way-strong-seeming gear wasn't nearly strong enough.
Fair leads,
Brion Toss