A method of forming a fluid-filled chamber for incorporating into an article of footwear, the method comprising: positioning a first polymer sheet on a substantially first flat surface of a tool ; positioning a second polymer sheet on the first polymer sheet ; moving a second substantially flat surface of the tool into contact with the second polymer sheet ; maintaining a gap between the first polymer sheet and the second polymer sheet at a predetermined area ; heating one of the two substantially flat surfaces of the tool to heat one of the first polymer sheet and the second polymer sheet ; joining the first polymer sheet and the second polymer sheet together at locations outside of the predetermined area to define a peripheral bond of the chamber ; maintaining the gap between the first polymer sheet and the second polymer sheet includes adding a tensile element defining the predetermined area between the first polymer sheet and the second polymer sheet ; adding the tensile element defining the predetermined area between the first polymer sheet and the second polymer sheet includes positioning a first tensile layer of the tensile element in contact with the first polymer sheet and positioning a second tensile layer of the tensile element in contact with the second polymer sheet ; heating one of the two substantially flat surfaces of the tool bonds one of the first polymer sheet and the second polymer sheet to a respective one of the first tensile layer and the second tensile layer; moving the second substantially flat surface of the tool away from the second polymer sheet after joining the first polymer sheet and the second polymer sheet ; positioning the second polymer sheet on the substantially first flat surface of the tool ; moving the second substantially flat surface of the tool into contact with the first polymer sheet ; and heating the one of the two substantially flat surfaces of the tool to heat the other of the first polymer sheet and the second polymer sheet . positioning a first polymer sheet on a substantially first flat surface of a tool ; positioning a second polymer sheet on the first polymer sheet ; moving a second substantially flat surface of the tool into contact with the second polymer sheet ; maintaining a gap between the first polymer sheet and the second polymer sheet at a predetermined area ; heating one of the two substantially flat surfaces of the tool to heat one of the first polymer sheet and the second polymer sheet ; joining the first polymer sheet and the second polymer sheet together at locations outside of the predetermined area to define a peripheral bond of the chamber ; maintaining the gap between the first polymer sheet and the second polymer sheet includes adding a tensile element defining the predetermined area between the first polymer sheet and the second polymer sheet ; adding the tensile element defining the predetermined area between the first polymer sheet and the second polymer sheet includes positioning a first tensile layer of the tensile element in contact with the first polymer sheet and positioning a second tensile layer of the tensile element in contact with the second polymer sheet ; heating one of the two substantially flat surfaces of the tool bonds one of the first polymer sheet and the second polymer sheet to a respective one of the first tensile layer and the second tensile layer; moving the second substantially flat surface of the tool away from the second polymer sheet after joining the first polymer sheet and the second polymer sheet ; positioning the second polymer sheet on the substantially first flat surface of the tool ; moving the second substantially flat surface of the tool into contact with the first polymer sheet ; and heating the one of the two substantially flat surfaces of the tool to heat the other of the first polymer sheet and the second polymer sheet . <EOS>