Fitness-Fatigue Form (TSB) Calculator

About this calculator

The Banister impulse-response model splits an athlete's training load into two competing effects: CTL (Chronic Training Load, often called “fitness”), a long, roughly 42-day exponentially weighted average of daily training stress, and ATL (Acute Training Load, “fatigue”), a short, roughly 7-day exponentially weighted average of the same stress. Subtracting the two, TSB = CTL − ATL, gives Training Stress Balance, commonly just called “form.”

A strongly negative TSB means recent training has outpaced adaptation and the athlete is carrying heavy fatigue, while a positive TSB signals freshness — often engineered deliberately in the taper before a race. This framework, popularized by platforms like TrainingPeaks, underlies periodization planning for cyclists, runners, and other endurance athletes.