What do Native Americans use buffalo sinew for?

Answer 1

they used it for sewing. Much in the same way that the western culture uses thread to create and repair, Native people used (and still do) sinew, which is tendon, for its durability in sewing.

Answer 2

When taken from its greasy outer sheath and pounded into separate fibres, sinew can be "corded" by twisting and adding new sections as the work proceeds. Sinew is never more than about 18 inches long but cording it can produce any lengths that might be needed.

Individual strands of sinew could be used for sewing glass beads to tanned leather clothes and moccasins, or for sewing the seams. Corded sinew would make a bowstring or lashings for weapons and tools.

Layers of sinew soaked in hide glue were attached to the entire outer face of some short wood or horn bows, making them extremely powerful and unlikely to crack.

Sinew was used to bind arrowheads and feathers to arrows, often in combination with hide or horn glue. The sinew would shrink as it dried, creating a very tight and strong bond.