First, one has to be mindful that crucifixion included various types of impaling punishments, which may have taken place on a single vertical stake, an actual tree, or what modern persons would commonly think of as a “cross”. Crucifixion was intended to be a slow, painful, public form of death to act as a deterrent to similar activities by others. Often, the impaling or nailing was done so that death would take place by agonizing asphyxiation. This would be accomplished by positioning the body with the arms out from or over the head in a manner requiring the individual to “pull up” each time a breath is taken. Obviously, one would eventually tire to the point of no longer being able to pull up to breath. Based on modern medical analysis, death could have occurred due to various reasons from asphyxiation itself to cardiac issues or even more complex conditions such as acidosis. Such deaths were commonly documented to occur anywhere from hours to days after initial placement.

Considering that the entire process was intended to be a public punishment and not merely an execution, it is not surprising that some who were crucified had to “carry their cross”. However, it is widely accepted that this was in most documented cases not the entire cross, but rather merely the “cross-beam” upon which the upper limbs were impaled. While not as heavy as what some might think of as an entire cross, this would certainly begin to tire the person before they even reached the destination to be hung and struggle for each breath.

Fallow, Thomas Macall (1911). “Cross and Crucifixion”. In Chisholm, Hugh (ed.). Encyclopedia Britannica. Cambridge University Press. p. 506.

Ball, DA (1989). "The crucifixion and death of a man called Jesus". Journal of the Mississippi State Medical Association. P. 77–83.