The science of training load: ATL, CTL, and TSB in plain English

If you’ve ever wondered whether you’re training too hard, not enough, or just right, you’re not alone. Coaches and athletes have relied on a small set of ideas for decades to answer that question. The good news: they have clear names and a simple relationship. Once you get them, you can use them to plan and adjust your training—or let software do it for you. ## Acute Training Load (ATL): your short-term fatigue **Acute Training Load**, or ATL, is a measure of how much stress you’ve put on your body recently—usually over the last week or so. Think of it as “recent training stress.” Every workout adds to it. Hard sessions push it up; easy days or rest bring it down. ATL answers: *How banged up am I right now?* If your ATL is high, you’re carrying a lot of short-term fatigue. That’s not inherently bad—it’s the result of hard training—but it’s something to watch. Athletes often let ATL rise during a build phase and then let it drop before a race so they feel fresh. ## Chronic Training Load (CTL): your fitness **Chronic Training Load**, or CTL, looks at a longer window—typically weeks to months. It’s a rolling average of your training load and is often called “fitness.” Unlike ATL, CTL changes slowly. A single hard week might spike your ATL, but your CTL will only nudge up a bit. CTL answers: *How fit am I over time?* Raising CTL is how you get fitter: consistent training gradually increases it. When you take time off, CTL drifts down. So in practice, ATL is “how tired I am lately” and CTL is “how fit I’ve become.” ## Training Stress Balance (TSB): freshness vs. fatigue **Training Stress Balance**, or TSB, is the simple idea that ties them together: **TSB = CTL − ATL** When CTL is higher than ATL, TSB is positive: you’re relatively fresh compared to your recent load. When ATL is higher than CTL, TSB is negative: you’re carrying more short-term fatigue than your longer-term fitness might suggest. So: - **Positive TSB** — You’re in a good place to perform or race; fatigue is under control. - **Negative TSB** — You’re accumulating fatigue; useful in a training block, but not ideal for key races. - **Near zero** — A balanced state: you’re training but not digging a deep hole. Athletes use TSB to time rest, tune taper, and avoid overtraining. The goal isn’t to keep TSB always positive—you often want to go negative during hard blocks—but to bring it back up when it matters. Many coaches aim for a slightly positive or near-zero TSB on race day so you’re sharp without being overcooked. During a build phase, it’s normal to sit in negative TSB for a while; the key is to plan recovery so you don’t stay there too long. ## How Zorv automates this across all your devices The catch with ATL, CTL, and TSB is that they only work if the numbers come from *all* your training. If you run with a Garmin, cycle with a Wahoo, and log strength in another app, each system only sees a slice. Your “training load” in one place is incomplete. Zorv fixes that by unifying your data from Garmin, Whoop, Strava, Apple Watch, and more into one timeline. We compute ATL, CTL, and TSB from that single view—so the same science applies whether the workout came from your watch, your bike computer, or a manual log. You get one set of numbers that reflect everything you did, and your AI coach can use them to give you smarter recommendations. No more mental math, no more spreadsheets, no more fragmented dashboards. Just one timeline and the metrics that actually describe your load. When you’re ready to train with the full picture, Zorv is built for it.