Training Load Explained: Reading Your Acute-to-Chronic Ratio

Training Load answers a plain question: how does your recent activity compare with what you are used to? A week of long hill walks after a quiet month pushes it up; a holiday on a sun lounger pulls it down. My Bio Age shows the answer as a ratio, such as 1.07×, with a coloured zone bar. Here is where the idea comes from, what the research does and doesn't support, and how to read your own number.

The idea behind an acute-to-chronic ratio

In a widely cited 2016 review, Tim Gabbett popularised comparing a short “acute” window, typically the latest week, with a “chronic” average of roughly the previous three to six weeks. Drawing on studies of team-sport athletes, he argued that rapid jumps in load were likely behind many non-contact injuries, while hard training built up gradually might help protect against them.

Impellizzeri and colleagues (2020) challenged this. They noted that no study had properly tested whether changing the ratio changes injury rates, that ratios have statistical quirks which add noise, and that the ratio isn't consistently linked to injury risk in one direction. In their view, the evidence doesn't support using it to steer training to prevent injury.

So treat Training Load as a descriptive prompt about change in your routine, not an injury predictor. The broader principle is less contested: the Physical Activity Guidelines for Americans link injury risk to the size of the jump from your usual activity to a new level, and advise increasing activity gradually.

How My Bio Age calculates Training Load

The calculation uses active energy, the calories burned through movement and exercise rather than at rest, which Apple Watch records automatically according to Apple's HealthKit documentation.

• Acute: your average daily active energy over the last 7 days.
• Chronic: your average daily active energy over the last 28 days.
• Ratio: the first divided by the second, shown as, say, 1.07× and limited to 0.3–2.2×.

At 1.0×, your past week matched your four-week average; at 1.3×, it ran about 30% above it. The calculation also compares your HRV over the last 7 days with the last 21, and without at least one HRV reading in the past week there is no ratio, however complete your activity data. So a dash on the Training Load card can mean missing HRV as well as missing activity.

Reading the zones

• Above 1.4×: Overreaching (red). Well above your recent norm.
• 1.2–1.4×: Building (yellow). A step up from your usual level.
• 0.8–1.2×: Optimal (green). Your week resembles your month.
• 0.6–0.8×: Deloading (yellow). A lighter week than usual.
• Below 0.6×: Under-training (red). Much less recorded activity than usual.

These are the app's categories, not validated cut-offs for you, and they describe arithmetic rather than fitness. A planned rest week, a holiday or a few days off with a cold will read as Deloading or Under-training, which may be exactly the right choice. Equally, green isn't proof that your plan suits you.

The strain signals underneath

The detail view adds a Strain Signals card with four supporting percentages:

• HRV Trend compares your 7-day and 21-day HRV averages, with an arrow when they differ by about 10% or more and “Stable” otherwise. Its percentage shows how similar the two averages are, so it drops when HRV moves in either direction, up as well as down.
• Activity Pattern reflects how regularly you were active over 14 days, with long runs of quiet days counting against it. It measures consistency, not volume.
• Temp Stability reflects how much your wrist temperature varied across the last seven nights.
• Oxygen Health reflects how often your blood oxygen readings fell below 95% over the last seven days.

These add context; the ratio itself comes from active energy alone. Wrist temperature and blood oxygen aren't available on every Apple Watch. Check that readings exist before reading anything into these two rows: without wrist-temperature readings, Temp Stability shows a full score, and without blood-oxygen readings, Oxygen Health shows zero.

Training Load and the weekly Index

Training Load is one of the 12 core inputs to the weekly MyBioAge Index™. For each recent complete Monday-to-Sunday week, the app averages the daily ratios and scores the result by how close it sits to 1.0: the nearer, the better, so even a week towards either edge of the green band can count slightly against you, and a week well above or below your usual level counts against you more. Weeks with less complete data count for less, and because this is a weekly average alongside the other core inputs, a single spike won't define your Index. See how Apple Health metrics become the MyBioAge Index™.

What active energy can miss

• Effort that burns few calories. Heavy lifting or skill work can feel demanding yet register less active energy than the effort suggests. As Gabbett's review describes, sports scientists often track how hard a session felt (session RPE), not only what a device records.
• Sessions without the watch. A match where watches aren't allowed, or a workout while it was charging, makes the week look lighter.
• Days you didn't wear it. Each window is averaged over every day, so an unrecorded day counts as zero and lowers your recent average. Today counts as one of the seven days even before it's over, so the ratio tends to read a little lower early in the day.
• Estimates, not measurements. A 2017 Stanford study found that wrist-worn devices estimated energy expenditure far less accurately than heart rate, though Apple Watch had the lowest error of the seven tested. Apple says personal details such as height and weight feed its calorie estimates, so keep them up to date in Health.

For more on the input itself, read using the Recovery dashboard alongside your Activity rings.

Reading it alongside Recovery

Training Load is most useful next to your Recovery score and how you feel. A Building week with steady Recovery and good sleep tells a different story from the same ratio with poor sleep and heavy legs. Our guides to Recovery Capacity and the dashboard go further.

Download My Bio Age free on the App Store to compare your recent Apple Health activity with your own usual load, alongside Recovery when enough data is available. The iPhone app is completely free, with no in-app purchases, subscription or account required. Training Load and your other scores are calculated privately on your iPhone, where your health data stays.

My Bio Age offers wellness information, not medical advice, and Training Load is not an injury assessment or exercise clearance. If you have pain or an injury that isn't settling, speak to a doctor or physiotherapist. If you get chest pain, unusual breathlessness or feel faint while active, stop; the NHS advises calling emergency services for sudden chest pain that does not go away. Our Methods & Sources page explains the Index in more detail.