Arbitrary Units (AU)

Formula

Example

60-minute strength session at sRPE 7 = 420 AU. 45-minute easy run at sRPE 4 = 180 AU. 30-minute mobility at sRPE 3 = 90 AU. Total day: 690 AU.

How Afitpilot Uses This

AU is the headline number on the session card, bigger than tonnage. Tonnage stays visible as a secondary, strength-specific row. We sum AU across the week (see Weekly AU) and split by modality. Historical sessions without an actual duration are backfilled using planned duration as a stand-in and flagged with an asterisk so coaches don't over-trust them.

AU benchmarks across sports

Known Limitations

• •Two sessions with identical AU can be physiologically very different. A 30-minute sprint workout at sRPE 8 (240 AU) is not the same stimulus as a 60-minute steady-state ride at sRPE 4 (240 AU), even though AU treats them as equal.
• •Estimated AU (planned duration in place of actual) is only as good as how well athletes followed the planned time. A session that ran 50% over plan is mis-scored under estimation.
• •AU does not capture proximity to failure for strength work, RIR and per-exercise RPE still matter for that. AU complements those metrics rather than replacing them.
• •Coach commentary that talked about tonnage trends has not all been re-pointed at AU yet, some legacy summary surfaces still emphasise tonnage.

What We're Improving

• Shipped

  Universal Internal-Load Currency (sRPE → AU)

  Move from tonnage-as-primary to a universal session-load number using Foster's sRPE × duration method. AU is now the headline load metric on every session card and rolls up weekly with a 3-bucket modality split (Strength / Endurance / Mixed). Tonnage stays visible as a strength-specific secondary.

• Future

  Power / HR Endurance External Load (TSS, TRIMP)

  Today, endurance external load falls back to sRPE × duration because we don't capture power data or HR thresholds. Adding a power-meter ingest pipeline (TSS) and a stored HRmax / HRrest profile (HR-zone TRIMP) would let endurance sessions report objective external load alongside the subjective AU number. Out of scope for the load-monitoring v1 ship.

Science Context

AU was introduced by Foster et al. (2001) as a way to translate sRPE into a quantitative load score. The choice of "arbitrary units" was deliberate, Foster wanted to discourage cross-cohort comparison and emphasise within-athlete trends. Per Impellizzeri's 2020 review, AU is now considered the most generalisable internal-load metric across sport types, and is the foundation on which downstream constructs (acute load, chronic load, monotony, strain) are built.

Related Terms

• Tonnage (Load)The total weight moved in a session, the simplest measure of mechanical stress placed on the body. Tonnage captures "how heavy was today?" in a single number.
• One-Rep Max (1RM)The heaviest weight you can lift for a single repetition with proper form. The 1RM is the reference point that almost every strength program is built around, training percentages, plate jumps, and meet attempts all anchor to this single number.
• Estimated 1-Rep Max (e1RM)A mathematical estimate of your true 1RM (one-rep max), the heaviest weight you could lift for a single repetition, derived from a submaximal set. It lets us track strength progress without you ever needing to actually max out.
• Session RPE (sRPE)A single 1-10 effort rating for an entire training session, recorded after the session ends. Unlike per-exercise RPE (which captures individual sets), sRPE rolls the whole session into one number, your subjective answer to "how hard was today, all-in?".
• TRIMP (Training Impulse)An HR-based internal load metric that multiplies session duration by an HR-derived intensity weighting. TRIMP is the heart-rate counterpart to sRPE-based AU: same idea (volume × intensity = load), different intensity signal. It's the standard load metric in endurance sport, running, cycling, rowing, cross-country skiing.