The Science Behind
Eventyr Performance

Every training adaptation begins with the same biological principle: apply a stimulus, recover, and emerge stronger. This cycle - called supercompensation - was formally described by Yakovlev (1977) and remains the cornerstone of modern periodization theory.

The four phases are:

1. Training stress: A session disrupts homeostasis - muscles fatigue, glycogen depletes, microtrauma accumulates.
2. Recovery: The body repairs and rebuilds, targeting the specific systems stressed.
3. Supercompensation: Fitness rises above the pre-training baseline - this is the adaptation window.
4. Detraining: If no new stimulus arrives, fitness returns to baseline within 10–14 days.

Critically, adaptation only occurs during recovery - not during the session itself. An athlete who never recovers does not improve; they degrade. The art of coaching is managing the stress-recovery balance across days, weeks, and seasons.

Two sessions of equal duration can have vastly different physiological impacts. A 60-minute low-intensity paddle and a 60-minute race-intensity interval session are not the same stress on the body. Any useful training model must account for both duration andintensity.

Heart rate is the most practical and validated proxy for internal training intensity. It responds to exercise intensity, environmental conditions, and fatigue - capturing factors that external load metrics (speed, watts) cannot. For sports like windsurfing where power meters are impractical, heart rate is the gold standard.

The challenge is combining duration and heart rate into a single, meaningful number. That is precisely what Eric Banister solved in 1975.

TRIMP (Training Impulse) was introduced by Banister et al. (1975) and refined by Morton et al. (1990). It weights each minute of exercise by the relative heart rate intensity - giving high-intensity minutes far more load value than easy ones, which reflects the non-linear physiological cost of hard effort.

The exponential term e^(1.92 × ΔHR) is the key innovation. It captures the well-established physiological fact that blood lactate - and therefore metabolic stress - rises exponentially above the lactate threshold. A session at 90% of max HR is not three times harder than one at 30% - it is far more than that.

When heart rate data is unavailable (GPS-only sessions), Eventyr falls back to a conservative estimate of 50 TRIMP per hour, equivalent to moderate aerobic effort. This keeps the model populated even for incomplete data sessions.

The Performance Management Chart (PMC) was formalised by Banister (1991) and popularised for endurance sport by Dr Andrew Coggan in the early 2000s. It models fitness and fatigue as two competing exponential moving averages of daily training load, and derives form as the difference between them.

The 42-day and 7-day time constants are not arbitrary. They approximate the biological half-lives of fitness gains and fatigue dissipation, validated across endurance sports in research by Banister, Coggan, and others. A longer window (42 days) captures the slow accumulation of aerobic adaptations; a shorter window (7 days) captures how quickly fatigue accumulates and resolves.

TSB is not simply a measure of how tired the athlete is. It describes the balance between fitness and fatigue - and crucially, the ideal TSB for training is different from the ideal TSB for competition (Bosquet et al., 2007; Mujika & Padilla, 2003).

Eventyr uses a six-zone model that reflects this nuance:

The Acute:Chronic Workload Ratio (ACWR) was described by Gabbett (2016) in The British Journal of Sports Medicine and has since become one of the most widely adopted injury and illness prediction metrics in professional sport.

The ratio compares how hard the athlete is currently working (ATL) relative to what their body is conditioned for (CTL). A ratio close to 1.0 means the current load matches fitness - sustainable and productive. A ratio significantly above 1.0 means the athlete is working well beyond their conditioned capacity - a physiological red flag.

Eventyr displays ACWR as a live metric and applies colour coding that matches the risk zones above. When ACWR exceeds 1.3, the platform generates a coach warning with specific recommended actions.

The European College of Sport Science and American College of Sports Medicine published a joint consensus statement (Meeusen et al., 2013) defining the progression from productive overreaching to pathological overtraining:

1. Functional overreaching (FOR)
   Short-term performance decrement with full recovery in days to weeks. This is normal, intentional, and the mechanism behind training camp and loading blocks.
2. Non-functional overreaching (NFOR)
   Sustained performance decline despite rest. Recovery takes weeks to months. Often associated with mood disturbance, elevated resting HR, and frequent illness.
3. Overtraining Syndrome (OTS)
   Chronic, severe performance decline with systemic hormonal disruption (elevated cortisol, suppressed testosterone). Recovery takes months to years. OTS is a medical diagnosis requiring physician involvement.

Immune suppression and illness risk

Exercise immunology research (Gleeson, 2006; Mackinnon, 2000) has established the "open window" hypothesis: in the hours immediately following a hard training session, immune function is transiently suppressed. White blood cell activity decreases, salivary IgA (the first line of defence against respiratory infections) drops, and susceptibility to upper respiratory tract infections rises sharply.

When this window occurs repeatedly without adequate recovery - as happens when ACWR exceeds 1.5 or TSB stays below −30 - the immune system remains chronically suppressed. Athletes in this state are significantly more likely to fall ill, which itself forces training interruption and compounds fitness loss.

The following principles distil the research above into actionable coaching practice: