Measuring What Matters
External load, internal load, TSS, TRIMP, RPE, heart rate, power, and the acute:chronic workload ratio. What each metric measures, and where each falls short.
Source: PhD Thesis, Chapter 2 · §2.5–§2.7 · Deakin University Repository →
This page is adapted from the Literature Review of Leighton Wells' doctoral thesis: Triathlon Coaching Practices — Optimising Training Load Processes and Communication. Read the full thesis →
External load: what the athlete does
External load (EL) refers to the objective, physical work performed by the athlete — distance, duration, speed, power, and elevation.101 It is any variable measure of physical activity that excludes internal biological responses.68 In triathlon, common EL variables include distance swum, cycled, and run, along with cycling power output and running pace.102
Sport scientists have developed composite EL metrics to capture the combined effect of training variables. In cycling, Training Stress Score (TSS) — developed by Coggan103 — quantifies the session load relative to the athlete's Functional Threshold Power (FTP). Similar approaches include running TSS (rTSS), and swimming stress score (ssTSS).104
However, EL alone does not reveal how the athlete's body responds to the stimulus. Two athletes performing identical external work may experience vastly different physiological responses.68
TSS, distance, and power are what the coach prescribes — but they only tell half the story. The same session can be a recovery ride for one athlete and overload for another. External load metrics are essential but insufficient on their own.
Internal load: how the body responds
Internal load (IL) refers to the measurable biological response to the external stimulus.68 This includes physiological measures such as heart rate, blood lactate, and oxygen consumption,105 as well as subjective measures such as Rating of Perceived Exertion (RPE).106
Session-RPE, developed by Foster,107 multiplies the athlete's RPE rating (typically on a CR-10 scale) by session duration in minutes to produce a single session load value. This has become one of the most widely used internal load measures in sport.108
TRIMP (Training Impulse), introduced by Banister,109 uses heart rate data to quantify internal load across a session. Several TRIMP variants exist including Banister's TRIMP, Edwards' TRIMP, and Lucia's TRIMP.110
IL also includes response variables such as subjective wellbeing, sleep quality, mood, and perceived recovery — measures that indicate how the athlete is coping with accumulated load.48
Session-RPE is simple, free, and remarkably valid. Every coach should be collecting this data. TRIMP variants add another layer via heart rate, but the key insight is this: internal load measures reveal whether the external prescription is being tolerated — or tolerated poorly.
The acute:chronic workload ratio debate
The Acute:Chronic Workload Ratio (ACWR) was proposed as a predictor of injury risk, comparing short-term (acute, ~7-day) load to longer-term (chronic, ~28-day) load.111 The concept, popularised by Gabbett,67 proposes a "sweet spot" where the ratio indicates acceptable training progressions.
However, this metric has been subject to significant criticism. Impellizzeri et al.112 and others have questioned the mathematical validity of the coupled ratio approach and its mechanistic basis. The mathematical properties of the rolling average ratio were shown to create spurious correlations.113
Despite these criticisms, the ACWR concept has influenced coaching practice and remains a feature of many TMS platforms. Coaches should be aware of its limitations and avoid using it as a sole decision-making tool.114
ACWR is probably the most recognisable derived training load metric in coaching software — and also one of the most contested in the literature. The takeaway: use it as one signal among many, not as an injury prediction tool. Understanding its mathematical limitations is essential.
The overtraining continuum: when adaptation becomes maladaptation
The relationship between training load and the athlete's response exists on a continuum. Functional overreaching (FOR) is a deliberate, short-term intensification that produces temporary performance decrements followed by supercompensation — this is a planned coaching strategy.56
When overreaching extends beyond what the athlete can recover from, it becomes non-functional overreaching (NFOR), characterised by stagnated or declining performance that takes weeks to months to resolve.57 If unaddressed, this can progress to overtraining syndrome (OTS) — a systemic, multi-organ response with potentially career-altering consequences.58
The Training Response Continuum
Training
Normal adaptation. Performance maintained or improving.
FOR
Functional overreaching. Planned. Recovery: days–weeks.
NFOR
Non-functional. Unplanned. Recovery: weeks–months.
OTS
Overtraining syndrome. Systemic. Recovery: months–years.
The line between productive overreaching (FOR) and damaging overreaching (NFOR) is the single most important boundary a coach must manage. The challenge: there is no objective marker that distinguishes them in real time. It is only identifiable retrospectively — which is why proactive monitoring and communication are so critical.
"External load alone does not reveal how the athlete's body responds to the stimulus. Two athletes performing identical external work may experience vastly different physiological responses."
— Impellizzeri, Marcora & Coutts, 2019
Key takeaways
No single metric tells the full story
External load shows what was done. Internal load shows how it was tolerated. Neither alone is sufficient — the interaction between the two is where meaningful coaching insight lives.
Session-RPE is simple, valid, and underused
Asking athletes how hard a session felt, multiplied by duration, produces a remarkably valid internal load measure. Every coach should be collecting this data.
ACWR: useful concept, contested execution
The acute:chronic workload ratio influenced coaching practice but has significant mathematical and mechanistic limitations. Use it as one signal, not a prediction engine.
This page draws on the Literature Review of Leighton Wells' doctoral thesis and the work of the researchers cited below, whose contributions have shaped the fields of training load measurement, monitoring, and applied sport science.
- 48
- 67
- 68
- 101
- 102
- 103
- 104
- 105
- 106
- 107
- 108
- 109Planning future performance: Implications for a systems model. Can J Appl Sport Sci. 1980;5:170–6. ↩
- 110
- 111
- 112
- 113
- 114