Training Load
Definition
Training load is a numerical estimate of how much physiological stress a training session — or a rolling window of sessions — has placed on the body. It combines two inputs that matter: the intensity of the work (usually captured as heart rate or power) and the duration for which it was held.
Most modern wearables and training platforms surface three related numbers:
- Load — the single-session stress number. Also called TRIMP (Training Impulse), TSS (Training Stress Score), or vendor-specific variants.
- ATI (Acute Training Impulse) — a rolling 7-day exponentially-weighted sum of recent session Loads. Represents current fatigue.
- CTI (Chronic Training Impulse) — a rolling 28 to 42-day exponentially-weighted sum. Represents accumulated fitness.
Why it matters to runners
Fitness rises when training stress exceeds recovery; fatigue rises in parallel. The two curves are not the same. Fitness responds slowly (weeks to months) and fades slowly. Fatigue responds quickly (days) and fades quickly. The difference — CTI minus ATI, sometimes called training stress balance or form — predicts how sharp or how tired an athlete is on any given day.
A large, rapid ATI spike above a small CTI is the classic recipe for overuse injury, illness, and the "I ran really well last Sunday but feel terrible this Wednesday" pattern. A CTI that has been climbing steadily for weeks with ATI close behind — that is well-tolerated adaptation.
Your Pacer uses these numbers descriptively rather than prescriptively. A Load of 138 in a week when CTI was 62 gets observed and interpreted in the weekly letter. The adjustment shows up as a lighter Wednesday the following week, not as an alert or alarm.
How it's computed
The core formula, simplified, is a product of duration and an intensity multiplier. The multiplier depends on heart rate relative to threshold, power relative to FTP, or pace relative to VDOT — each vendor chooses their own.
- Banister TRIMP — the original formulation, based on duration times a heart-rate-reserve multiplier weighted by sex.
- TSS (Coggan) — power-based, normalized to a one-hour-at-threshold benchmark. Common on cycling platforms.
- Vendor variants — Garmin Training Load, COROS Load, Polar Cardio Load, TrainingPeaks TSS. Values are not directly comparable across platforms — trends within one platform are.
Because the units are arbitrary and differ by vendor, the absolute Load number matters less than the ratio of ATI to CTI and the change in CTI over weeks.
Related terms
- HRV — the recovery side of the load / recovery balance.
- Periodization — planned CTI progression across base, build, peak, taper.
- Polarized Training — the same CTI can be built with more or less interference cost depending on intensity distribution.
Further reading
- Banister & Calvert, Planning for Future Performance: Implications for Long-Term Training, Canadian Journal of Applied Sport Sciences (1980). The original TRIMP paper.
- Coggan & Allen, Training and Racing with a Power Meter. The TSS formulation used across cycling.
- Foster et al., A New Approach to Monitoring Exercise Training, Journal of Strength and Conditioning Research (2001). Session-RPE basis.