Stiff S10 Clutch What Is Wrong With My Transmission?

What is wrong with my transmission? - stiff s10 clutch

I have a 1990 Chevy S10. its 5-speed manual transmission. Add all my things, if the truck can not. if the vehicle is running with is none of my equipment to be used, and the clutch pedal is stiff. what can be the most likely cause of the problem? Thanks