Get a captains of crush gripper that is tough for you.
Go outside and grab the gripper and close it with one hand.
Walk while holding the gripper closed until your grip gives out, then stop.
Turn around and swtich hands, you other hand should give out right as you return to your starting spot.
You will return with fresh air in your lungs, rubber hands, and maybe some pain in your hands/wrists/forearms.
This has worked for me several times at work.