Insulin sensitivity is how efficiently cells respond to insulin's signal to take up glucose. High sensitivity = small insulin doses produce big responses.
Mechanism: A function of fitness, muscle mass, sleep, body composition, and dietary pattern. Improves rapidly with exercise (within a single session) and slowly with sustained lifestyle change.
Use case: The most leveraged metabolic state to optimize. Resistance training is the single biggest mover. A post-meal walk improves it acutely.
Caveats: Acute insulin sensitivity (one workout) and chronic sensitivity (months of training) are different. Train for both.