Before starting training for a race or project we should determine what are the demands of that event to identify the key factors we need to improve in order to perform and to build a training plan for it.

Before entering into analyzing an event is important to understand that to perform a task (doing X race in that time, climbing that summit…) there are several systems that will be activated at different levels, those systems are not working separately (such as a car or a machine) but interacting as a complex system. Those are the systems we will be targeting to modify (change its biology) to improve our output (the external load).

System	Primary Function	Main Organs / Components	Measurements
Cardiovascular	Deliver O₂/nutrients and remove CO₂/metabolites	Heart, blood vessels, hemoglobin	VO₂max, HR, HRV, hematocrit, hemoglobin, echocardiography
Respiratory	Gas exchange: O₂ uptake & CO₂ removal	Lungs, diaphragm, airways, alveoli	Spirometry (FEV1/FVC), VE, ventilatory thresholds (VT1/VT2), MIP/MEP
Metabolic / Bioenergetic	ATP production (fat/CHO oxidation, glycolysis)	Mitochondria, enzymes, glycogen stores	RER, substrate oxidation, lactate curve, CK/LDH, glycogen ultrasound
Neurological / Neuromuscular	Motor control, coordination, fatigue management	Brain, spinal cord, motor units, NMJ	EMG, TMS (central fatigue), RFD, MVIC, Myoton
Muscular / Mechanical	Force generation, elasticity, structural resistance	Skeletal muscle, tendons, fascia	Strength tests, jump tests, ultrasound architecture, force plates
Endocrine / Hormonal	Regulate energy, stress, adaptation	Hormones: cortisol, T, insulin, thyroid hormones	Cortisol/T ratio, T3/T4, insulin, HOMA-IR, catecholamines
Thermoregulatory	Maintain core temperature, heat dissipation	Sweat glands, skin, hypothalamus	Core temp, sweat rate, Na+ concentration, skin temp, HR drift
Immune	Protect from infection, repair tissues	WBCs, cytokines, mucosal immunity	Leukocyte count, CRP, IgA saliva, IL-6/TNF-α
Psychological / Cognitive	Perceived effort, motivation, pacing, stress	Prefrontal cortex, limbic system, pain modulation circuits	RPE, cognitive tests, mood scales (POMS), HRV, EEG (research)
Gastrointestinal	Nutrient/fluid absorption, GI tolerance	Stomach, intestine, microbiota	GI symptoms scoring, CHO absorption tests, H₂ breath test
Renal / Fluid-Electrolyte	Fluid balance, sodium regulation	Kidneys, vasopressin system	Creatinine/urea, osmolarity, serum Na+, body weight change
Cognitive-Motor / Economy	Efficient movement with low energy cost	CNS–muscle coordination, stiffness, biomechanics	Running economy test (O₂ cost), 3D video analysis, GCT, oscillation

Event load: We will be assessing the external and estimating the internal load of the event.

Category	External Load (Event Characteristics)	Internal Load (Athlete Response)	Main Systems Involved	Typical Measures for Analysis
Duration	Time of event (minutes → multi-day)	Accumulated fatigue, metabolic depletion	Metabolic, cardiovascular, endocrine	Lactate drift, HR drift, CHO/fat oxidation, cortisol
Intensity	Pace / power requirement	HR zones, VO₂ demand, RPE	Cardiovascular, respiratory, metabolic	HR, VO₂, ventilatory thresholds, RPE
Elevation Gain/Loss	Total climb/descent, grade steepness	Eccentric damage, force requirements	Muscular/ mechanical, neuromuscular	Force plates, CK, Myoton, strength asymmetries
Altitude	Start/finish altitude, max altitude	Hypoxia stress, ventilation changes	Cardiovascular, respiratory	SpO₂, VE, hematocrit, HR at altitude
Technicality	Rocks, snow, exposure, scrambling	Cognitive load, stability, eccentric load	Neuromuscular, cognitive-motor	Economy tests, GCT, stiffness, EMG
Temperature	Heat, cold, amplitude changes	Thermoregulation stress, dehydration	Thermoregulatory, renal	Core temp, sweat rate, Na+ loss, osmolarity
Weather / Environmental	Wind, storms, humidity	Energy expenditure variability	Thermoregulatory, psychological	WBGT, wind-chill, hydration markers
Nutrition Logistics	Aid frequency, carry capacity	GI distress risk, energy availability	GI system, metabolic	CHO tolerance, GI symptom scale
Skills	Downhill speed, climbing, ropework	Motor pattern fatigue, coordination	Neuromuscular, cognitive	Movement economy, fatigue index
Decision-Making	Navigation, pacing choices	Cognitive fatigue, emotional regulation	Psychological/cognitive	Decision tests, HRV, RPE variability
Risk and Exposure	Avalanches, crevasses, night, isolation	Stress response, hormonal load	Endocrine, psychological	Cortisol, mood scales, reaction time
Other factors	Sleep deprivation, carry weight…	Cognitive decay, immune load	Cognitive, immune, hormonal	Sleep metrics, IgA, CRP, HRV

Here you can download a table to assess the Demands of the Event

Since events in trail running are not steady (different uphills, downhills, inclines, weather… during the same event) We should analyze the demands of the event by sections, that way we will be able also to identify the key moments of the event where we need to perform the best and what is the state we will be at that moment, if we need to train more for durability, repeatability, etc.

Step 1: Fill External Load

Example:

• Duration: 4:30h (2h30 uphill, 1h downhill, 1h flat…)
• Intensity: average 167ppm, 175 at uphills (av. 30′)…
• Elevation: 5,300 m, distance and incline of every uphill and downhill
• Terrain: from 1 to 3km runnable, 3-15 rocky granite…
• Temperature: 20º start, 12º in summits, 25º last part…
• etc.

Step 2: Infer Internal Load

• High thermoregulatory strain
• 6300kcal total, in uphills 90%CHO, in downhills 60%…GI load due to heat
• Neuromuscular load from long downhills
• etc.

Step 3: Identify Systems Under Stress

• Thermoregulatory
• GI
• Neuromuscular
• Metabolic (CHO oxidation high)
• etc.

Step 4: Choose how to monitor the change you want to do.

• Vo2 / HR at race pace GAP
• Core temp, sweat rate
• GI tolerance tests
• Downhill eccentric fatigue
• Lactate drift test
• etc.

Then start planning the training for it.