Kris Stallenberg 90-min Z3/4 (Sweet Spot) Pushing #09 The Grinder is a test of mental fortitude and muscular endurance. This continuous 70-minute block simulates a long, sustained climb or a hard solo effort, forcing your body to become more efficient at utilizing fuel. By progressively increasing and then decreasing the intensity, you'll improve your ability to handle changes in pace without cracking. bike