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Strength & Load

Arbitrary Units (AU)

Also known as: Session Load, Foster Load, Internal Training Load

The primary internal-load number for a session: sRPE multiplied by actual duration in minutes. AU produces one comparable load score across strength, endurance, mobility, and skill sessions — something tonnage cannot do because it only exists for sessions that lift external weight.

Formula

sessionAU = sRPE x actualDurationMin [Foster sRPE method]

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

Who / ContextValueNote
Recreational lifter (3 sessions/week)300-500 AU per session1,000-1,800 AU/week is sustainable indefinitely
Endurance athlete (training block)500-900 AU per sessionLong rides/runs push AU high via duration, not just sRPE
Hybrid athlete2,500-4,500 AU/weekStrength + cardio composes well in AU; tonnage cannot do this
Foster's original cohortsRPE x min, threshold ~6,000 AU/weekAbove which monotony and strain warnings start to apply in his framework

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.