8.3 Internal Load, Autonomic Readiness (HRV), Biomarkers, and PROMs

8.3 Internal Load, Autonomic Readiness (HRV), Biomarkers, and PROMs

While tracking systems and force plates quantify external load (the physical work completed by the athlete independent of their internal state), biological adaptation, tissue remodeling, and overtraining syndrome are governed by internal load—the relative physiological, metabolic, autonomic, and psychological strain imposed by that external stimulus. Within Domain 3, Task 3B, the sport scientist must integrate perceptual, cardiorespiratory, autonomic, biochemical, and psychometric monitoring tools into a cohesive athlete-readiness decision matrix.

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Coupling External Load with Internal Load

The Internal:External Efficiency Index

Two athletes completing the exact same external work ($6{,}500\text{ m}$ total distance and $650\text{ m}$ high-speed running) can experience vastly different internal strain depending on their $\dot{V}\text{O}_{2\max}$, muscle glycogen stores, hydration state, ambient heat/humidity, or accumulated fatigue. By dividing an external output metric by a concurrent internal cost metric during standardized drills, practitioners compute an External:Internal Efficiency Index (e.g., $\frac{\text{Mean Velocity (m}\cdot\text{min}^{-1}\text{)}}{\text{Mean Heart Rate (bpm)}}$ or $\frac{\text{Total Distance (m)}}{\text{sRPE Load (AU)}}$). A rising efficiency ratio across a mesocycle indicates positive cardiorespiratory adaptation (more mechanical work per beat of heart rate or unit of perceived effort), whereas a falling ratio flags acute fatigue, heat strain, or glycogen depletion.

Session Rating of Perceived Exertion (sRPE) & Differential RPE (dRPE)

Introduced by Foster et al., Session Rating of Perceived Exertion ($\text{sRPE}$) quantifies global internal training load by multiplying the athlete's post-session rating on the modified $0\text{–}10$ Category-Ratio ($\text{CR-10}$) scale by the total session duration in minutes (including warm-up, tactical drills, and conditioning):

$\text{sRPE Load (Arbitrary Units, AU)} = \text{CR-10 RPE} \times \text{Session Duration (minutes)}$

From daily $\text{sRPE}$ loads over a 7-day microcycle, two secondary load-distribution indices are calculated:

$\text{Training Monotony} = \frac{\text{Mean Daily Load}_{\text{7d}}}{\text{SD}_{\text{7d}}}, \qquad \text{Training Strain} = \text{Total Weekly Load}_{\text{7d}} \times \text{Training Monotony}$

When every day of the week has a similar moderate-to-high load, the weekly standard deviation ($\text{SD}_{\text{7d}}$) shrinks, driving Training Monotony $> 2.0$ and causing Training Strain to spike—a classic precursor to non-functional overreaching and illness.

• Strict Timing Standardization ($15\text{–}30\text{ min}$ Post-Session): Athletes should wait $15\text{ to }30\text{ minutes}$ after the session ends (or answer within a standardized post-session window via a private digital app to prevent peer conformity) before rating $\text{sRPE}$. Asking immediately at the final whistle introduces severe recency bias: finishing practice with a brutal $3\text{-minute}$ conditioning shuttle inflates the global rating, whereas ending with a $15\text{-minute}$ walk-through and static stretch artificially depresses it.
• Differential RPE ($\text{dRPE}$): Because a single gestalt score cannot distinguish between cardiopulmonary and musculoskeletal strain, $\text{dRPE}$ separates perception into central breathlessness/cardiorespiratory exertion ($\text{RPE-B}$) and peripheral lower-limb muscular exertion ($\text{RPE-L}$) (typically collected on the finer $0\text{–}100$ Borg centiMax CR-100 scale to avoid rounding clustering).

Heart-Rate-Derived Internal Load (TRIMP Models) & Submaximal HR

To capture continuous objective cardiovascular strain during endurance and team sports, three primary Training Impulse ($\text{TRIMP}$) models are utilized:

1. Banister's Continuous TRIMP: Multiplies exercise duration ($t$ in minutes), fractional Heart Rate Reserve ($\Delta\text{HR}_{\text{ratio}} = \frac{\text{HR}_{\text{ex}} - \text{HR}_{\text{rest}}}{\text{HR}_{\max} - \text{HR}_{\text{rest}}}$), and an exponential sex-specific weighting factor $y$ ($y = 0.64 e^{1.92 \Delta\text{HR}_{\text{ratio}}}$ for males; $y = 0.86 e^{1.67 \Delta\text{HR}_{\text{ratio}}}$ for females) that models the non-linear blood-lactate curve. Limitation: Relies on mean session $\text{HR}$, smoothing out intermittent high-intensity intervals.
2. Edwards's Summated Heart Rate Zones ($\text{SHRZ}$): Bins time ($t_i$ in minutes) spent in five arbitrary $\% \text{HR}_{\max}$ zones and multiplies each by a linear integer weight: $\text{Edwards's TRIMP} = 1(t_{50\text{–}60\%}) + 2(t_{60\text{–}70\%}) + 3(t_{70\text{–}80\%}) + 4(t_{80\text{–}90\%}) + 5(t_{90\text{–}100\%})$
3. Lucia's Physiological TRIMP: Anchors three zones to individual laboratory gas-exchange thresholds: Zone 1 ($< \text{VT}_1$, weight $= 1$), Zone 2 ($\text{VT}_1\text{ to }\text{VT}_2/\text{RCP}$, weight $= 2$), and Zone 3 ($> \text{VT}_2$, weight $= 3$), providing superior physiological individualization over fixed $\% \text{HR}_{\max}$ bands.
4. Submaximal Exercise Heart Rate ($\text{HR}_{\text{ex}}$) and Heart Rate Recovery ($\text{HRR}_{60\text{s}}$): Embedding a standardized $4\text{-minute}$ submaximal shuttle run ($\sim 12\text{ km}\cdot\text{h}^{-1}$, eliciting $75\text{–}85\%\ \text{HR}_{\max}$) into the warm-up followed by $60\text{ s}$ of quiet standing provides a highly sensitive aerobic marker. A chronic $4\text{–}8\text{ bpm}$ drop in $\text{HR}_{\text{ex}}$ paired with faster $\text{HRR}_{60\text{s}}$ reflects positive plasma volume expansion and stroke volume adaptation—unless accompanied by elevated submaximal $\text{RPE}$, suppressed maximal heart rate, and heavy legs, which signals parasympathetic overreaching!

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Autonomic Nervous System Monitoring via Heart Rate Variability (HRV)

Why Time-Domain $\text{lnRMSSD}$ Is the Applied Gold Standard

Heart Rate Variability ($\text{HRV}$) measures the millisecond beat-to-beat variation between consecutive normal-to-normal cardiac $R\text{-}R$ intervals governed by the Sinoatrial ($\text{SA}$) node. For daily athlete monitoring, the metric of choice is the Root Mean Square of Successive Differences ($\text{RMSSD}$) transformed via the natural logarithm ($\text{lnRMSSD}$, often scaled by $\times 20$ to produce a $0\text{–}100$ score) to correct positive skewness.

Why do sport scientists use time-domain $\text{lnRMSSD}$ rather than frequency-domain spectral power metrics like the Low-Frequency to High-Frequency ($\text{LF/HF}$) ratio? Spectral $\text{HF}$ power and $\text{LF/HF}$ are heavily confounded by spontaneous respiratory rate variations (requiring strict metronome-paced breathing over $5\text{-minute}$ laboratory recordings). In contrast, beat-to-beat $\text{RMSSD}$ directly isolates vagal (parasympathetic, acetylcholine-mediated) modulation and remains valid and reliable during ultra-short $60\text{-second}$ waking recordings (preceded by a $60\text{-second}$ stabilization period) under natural breathing.

Standardized Collection, Rolling 7-Day Mean ($\text{lnRMSSD}_{\text{7d}}$), and $\text{CV}\%$

• Collection Protocol: Captured immediately upon waking at a consistent time (supine or seated, bladder emptied, prior to caffeine, food, or orthostatic stressors) via an ECG chest strap or validated photoplethysmography ($\text{PPG}$) sensor, or sampled during nocturnal slow-wave sleep ($\text{SWS}$).
• Rolling 7-Day Mean and Coefficient of Variation ($\text{lnRMSSD}_{\text{CV}}$): Because single-day $\text{HRV}$ fluctuates with acute stressors, practitioners track the 7-day rolling mean ($\text{lnRMSSD}_{\text{7d}}$) alongside the 7-day Coefficient of Variation ($\text{lnRMSSD}_{\text{CV}} = \frac{\text{SD}_{\text{7d}}}{\text{Mean}_{\text{7d}}} \times 100$) relative to the athlete's individual baseline Smallest Worthwhile Change ($\text{SWC} = 0.5 \times \text{SD}_{\text{baseline}}$):
  • Positive Adaptation: Stable or increasing $\text{lnRMSSD}_{\text{7d}}$ paired with a low $\text{lnRMSSD}_{\text{CV}}$ ($< 5\text{–}7\%$, indicating minimal day-to-day homeostatic perturbation).
  • Acute Sympathetic Stress / Unstable Recovery: A drop in $\text{lnRMSSD}_{\text{7d}}$ below the lower $\text{SWC}$ band paired with an elevated $\text{lnRMSSD}_{\text{CV}}$ ($> 10\text{–}12\%$), reflecting large day-to-day autonomic swings as the athlete struggles to recover between sessions.
  • The Parasympathetic Hyperactivity / Saturation Trap: An abnormally high $\text{lnRMSSD}$ (or acetylcholine receptor saturation at very low resting heart rates $< 42\text{ bpm}$) is not always a sign of peak readiness. When elevated $\text{lnRMSSD}$ and bradycardia coincide with high perceived fatigue, mood disturbance, and suppressed peak heart rate, it reflects parasympathetic overreaching—proving that $\text{HRV}$ must always be interpreted alongside subjective wellness and performance metrics.

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Biochemical, Endocrine, and Patient-Reported Outcome Measures (PROMs)

Blood & Salivary Biomarkers and Pre-Analytical Standardization

When ordering periodic venous, capillary, or salivary panels, pre-analytical errors frequently invalidate data unless strict sampling rules are enforced:

1. Circadian Rhythm & Fasting: Cortisol and Testosterone follow a steep diurnal rhythm, peaking within $30\text{–}60\text{ minutes}$ of morning waking (Cortisol Awakening Response) and declining through the day: cortisol roughly halves (or more) by late afternoon, while testosterone falls less, often by about $20\text{–}25\%$. Comparing an athlete's $08{:}00$ baseline draw against a $15{:}30$ afternoon draw creates a false-positive drop in the Free Testosterone : Cortisol ($\text{T:C}$) ratio! All endocrine and metabolic draws must occur at a standardized morning time ($07{:}00\text{–}09{:}00$) after an $8\text{–}12\text{ h}$ overnight fast, stabilized seated/supine posture ($10\text{–}15\text{ min}$), and $\ge 24\text{–}48\text{ h}$ post-exercise washout.
2. Creatine Kinase ($\text{CK}$) & Blood Urea Nitrogen ($\text{BUN}$): $\text{CK}$ leaks across damaged sarcolemmal membranes after eccentric loading or physical collisions, peaking $24\text{–}72\text{ hours}$ post-exercise. Because baseline $\text{CK}$ varies wildly by muscle mass, biological sex, ethnicity, and the Repeated Bout Effect, athletes must be evaluated against individualized Bayesian/z-score baselines rather than hospital reference ranges. $\text{BUN}$ reflects hepatic deamination of amino acids during glycogen-depleted endurance protein catabolism (confounded by high dietary protein intake and dehydration).
3. Ferritin and the Acute-Phase Inflammation Confounder: Serum Ferritin reflects total body iron stores essential for hemoglobin synthesis and oxidative enzymes. However, ferritin is an acute-phase reactant: during systemic inflammation, infection, or the $24\text{–}72\text{ hours}$ following heavy muscle-damaging exercise, elevated Interleukin-6 ($\text{IL-6}$) and C-Reactive Protein ($\text{CRP}$) cause the liver to sequester iron and artificially spike serum ferritin for several days—masking true iron deficiency unless $\text{CRP}$ and transferrin saturation are co-measured!
4. Salivary Immunoglobulin A ($\text{sIgA}$): Quantifies mucosal immune competence; a $> 30\text{–}40\%$ decline in individual $\text{sIgA}$ secretion rate during congested travel or intensification blocks signals an "open window" of elevated Upper Respiratory Tract Infection ($\text{URTI}$) risk.

Psychometric Superiority of Patient-Reported Outcome Measures (PROMs)

In a landmark systematic review of 56 studies, Saw et al. (2016) demonstrated that subjective self-report wellness and stress-recovery questionnaires ($\text{PROMs}$) respond with superior sensitivity and consistency to both acute increases and decreases in training load compared to objective resting blood, hormone, heart rate, or $\dot{V}\text{O}_2$ markers.

• Validated Multi-Item Questionnaires: The Profile of Mood States ($\text{POMS}$, 65 items) tracks tension, depression, anger, vigor, fatigue, and confusion (healthy athletes display the classic "Iceberg Profile" with elevated Vigor and low negative mood states; overtraining flattens or inverts the iceberg). The Recovery-Stress Questionnaire for Athletes ($\text{RESTQ-Sport}$, 76 items), Daily Analysis of Life Demands for Athletes ($\text{DALDA}$), and Multi-Component Training Distress Scale ($\text{MTDS}$) capture biopsychosocial stress and recovery balance. Because long instruments induce questionnaire fatigue and habituated clicking if given daily, they are best deployed weekly or at phase transitions.
• Brief Daily Custom Wellness Scales: For daily morning monitoring, $4\text{–}6\text{ item}$ Likert scales ($1\text{–}5$ or $1\text{–}7$ points) evaluating sleep quality, muscle soreness, general fatigue, psychological stress, and mood maintain high athlete compliance, provided practitioners educate athletes on how scores guide supportive recovery rather than punitive roster deselection.

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Worked Calculation Example: Internal Load & 7-Day HRV Interpretation

Scenario: A midfielder completes a $90\text{-minute}$ practice. Twenty minutes post-session, she reports a global $\text{CR-10 RPE} = 7$. Her continuous heart rate telemetry across the $90\text{ minutes}$ recorded time in $\% \text{HR}_{\max}$ zones as follows: Zone 1 ($50\text{–}60\%$): $10\text{ min}$; Zone 2 ($60\text{–}70\%$): $20\text{ min}$; Zone 3 ($70\text{–}80\%$): $30\text{ min}$; Zone 4 ($80\text{–}90\%$): $20\text{ min}$; Zone 5 ($90\text{–}100\%$): $10\text{ min}$. In addition, her historical baseline $\text{lnRMSSD}$ has a mean of $4.20$ with a standard deviation ($\text{SD}_{\text{baseline}}$) of $0.24$, while her current 7-day rolling mean ($\text{lnRMSSD}_{\text{7d}}$) is $3.98$ with an elevated $\text{lnRMSSD}_{\text{CV}}$ of $11.8\%$ (baseline $\text{CV} = 4.5\%$). Calculate her $\text{sRPE}$ Load, Edwards's $\text{TRIMP}$, and evaluate her autonomic status relative to her Smallest Worthwhile Change ($\text{SWC} = 0.5 \times \text{SD}_{\text{baseline}}$).

1. Calculate $\text{sRPE}$ Load: $\text{sRPE Load} = 7 \times 90\text{ min} = 630\text{ AU}$
2. Calculate Edwards's Summated Heart Rate Zones ($\text{TRIMP}$): $\text{Edwards's TRIMP} = (1 \times 10) + (2 \times 20) + (3 \times 30) + (4 \times 20) + (5 \times 10) = 10 + 40 + 90 + 80 + 50 = 270\text{ AU}$
3. Evaluate 7-Day $\text{lnRMSSD}$ against $\text{SWC}$: $\text{SWC} = 0.5 \times 0.24 = 0.12 \implies \text{Normal Baseline Band} = 4.20 \pm 0.12 = [4.08,\; 4.32]$ Because her current $\text{lnRMSSD}_{\text{7d}}$ ($3.98$) has fallen below the lower $\text{SWC}$ threshold ($4.08$) and her $\text{lnRMSSD}_{\text{CV}}$ has spiked to $11.8\%$, she is experiencing meaningful vagal withdrawal and unstable day-to-day autonomic recovery.