Organelo que regula el equilibrio hídrico en protistos de agua dulce.

These organelles are contractile vacuoles

Explanation:

Contractile vacuoles are intercellular pumps that are slowly filled with fresh water and eventually expel their contents outside the cell. Freshwater protists maintain the hydric equilibrium by contractile vacuoles that pump out the water of the cell and thus prevent the cell swelling induced by osmotic stress.

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.