What effect would adding additional substrate, in the presence of a noncompetitive inhibitor, have on the enzymatic reaction?

no effect will occur.

Explanation:

non-competitive enzyme do conformational changes in active site of enzyme by attaching to the other side rather than active side which prevent the substrate to attach to the active site, in results it stops the enzyme to work while competitive inhibitors compete with substrate for active site so increase substrate in competitive inhibitor condition can overcome the its effect.

it also supports my hypothesis. if you rest first you can squeeze the clothespin more times. from the data i collected the average of the eight people concluded exercise makes your muscles more fatigued and less able to squeeze the clothespin compared to resting before squeezing.