Delivery and Extraction: What Injury Teaches Us About Recovery
Recovery is often discussed in terms of individual modalities, and less often in terms of the principles that determine whether a modality is likely to be useful. This article presents models and principles that can be applied with limited technology, together with the audit process we use to decide what to do with a patient or an athlete. My aim is that nobody finishes it thinking that it sounds sensible but that they cannot afford what we do. I am the clinic director at Parker Performance Institute, where we see special operators, professional and recreational athletes, and a large number of youth athletes, who have experienced traumatic brain injury or concussion. Predominantly we work with tactical athletes: special operators who are deployed or back from deployment and need to be ready to go out again. Much of our work focuses on the energy crisis that follows injury to an area of the brain, because a cascade of events interferes with integration and with many processes that are also affected by training load and by the recovery modalities we apply.
When an athlete trains, the limiters are either the delivery of energy or the extraction of energy at the target zone. The model I work from is not mine. It draws on contemporary science, and in particular on a recovery wheel by Robin Thorpe and Robert Allen, with eight segments that I treat as spokes. I concentrate on the spoke that I think receives the least meaningful attention and is somewhat misunderstood, which is cognitive load. Underneath all of it runs one question: when we create an intervention, are we changing the whole system, or are we shifting one variable around inside it? I try to be clear about which of what we do produces a sustainable change and which is a novel stimulus or something we are still researching.
The Reservoir and the Battery
Recovery, or the energy level of a person, is often thought of as a receptacle. Every stressor fills the container with accumulated load, and recovery empties it and returns the person to baseline. Allostatic load raises the level and the strain on the whole system, and only when the capacity of the reservoir is exceeded do we see symptoms that show someone is overloaded or overreached. I take this version of the model from Andrew Hauser, an athletic trainer in Arizona, as a heuristic and not a peer-reviewed framework. The difficulty is that an overflow cannot be seen coming, and we usually do not know that someone is overloaded until symptoms of breakdown appear, so we want to understand the level of stress entering the system across all of its dimensions.
Traditionally the reserves of the body have been looked at as a battery, and I think this fails in several ways. A battery does not improve its capacity by being discharged and in fact it degrades, so the analogy gives the wrong signal about why we load, train and prepare people. A vessel works better, because when we fill a cup the response can be an increase in capacity over time. The purpose of adaptation is to make the reservoir bigger, and the battery model does not allow for that.
The Recovery Wheel and What We Measure
The Thorpe and Allen framework, which I use to separate drivers from symptoms, is presented in their 2025 textbook. It places four drivers of fatigue at the centre (metabolic, structural, central nervous system and cognitive), related to a ring of symptoms that an athlete may report and an outer ring of interventions. When we target a symptom, for example muscle soreness, with massage or compression, which are good interventions, we are not reaching the driver of what is happening, and the more fundamental the driver, the more long-term impact an intervention has.
We put these domains into a common language that the team can work through, and we measure them in practice. For sleep we use a plethysmography ring with sleep indexing and a self-rating questionnaire, which is not as well validated as a sleep study but gives enough actionable information, because we are looking for trends and opportunities and not for pathology. For the autonomic and breathing domain we use resting heart rate, heart rate variability and the Compass 31 questionnaire. For the sensory and vestibular-ocular domain we use oculomotor evaluation and near point of convergence. None of this is testing for the sake of testing.
Operator Syndrome and the Continuum
The second framework comes from the operator population. Frueh and colleagues (2020) described operator syndrome on the basis of professional consultations with more than 50 special operators and their partners, as a set of interrelated domains and not as a severity score: traumatic brain injury, chronic pain, cognition, endocrine status, sleep, substance use, relationships and transition out of service. It is a framework of dysfunction, but it maps well onto recovery frameworks, and its domains resemble the spokes of the wheel. Injury and performance sit at opposite ends of one continuum, and everybody lies somewhere along it.
The authors describe operator syndrome as the natural consequence of a very high allostatic load, to which people who compete at the highest level are also exposed. Breakdown is a series of doses and not a single event, which is why single tests often show little and why we use clusters of tests and trend them over time. A brain injury disrupts the endocrine axis, which disrupts sleep, and poor sleep amplifies pain and mood changes, so that sleep gets worse. The domains that break down are also those from which we can rebuild, and the lowest-hanging fruit is sleep, although both the environment and the physiological conditions for it must be set up.
The axis runs from adaptive through compensating and compromised to depleted. If a domain is overloaded and the person can no longer adapt appropriately, they begin to compensate, and if compensation continues for too long they become compromised and then depleted. Relative energy deficiency in sport follows the same process (Mountjoy et al., 2023). The model tells us what the dimensions are but not what to do, and for that we need to understand principles.
"Recovery, or the energy level of a person, is often thought of as a receptacle. Every stressor fills the container with accumulated load, and recovery empties it and returns the person to baseline. Allostatic load raises the level and the strain on the whole system, and only when the capacity of the reservoir is exceeded do we see symptoms that show someone is overloaded or overreached."
Oxygen Delivery Multiplied by Extraction
One of the principles I took from time spent with Andrew Hauser is a return to the physiology of energy balance. Oxygen consumption equals cardiac output multiplied by the arteriovenous oxygen difference, so what is pumped around and what is extracted are the two sides of the equation. Any device or modality shifts one side, either delivery, meaning the pump and the plumbing, or the ability of the target tissue to use what it is sent. That ability is the limiter for the athlete and should drive the intervention.
Compression, heat and most passive modalities act principally on delivery, by manipulating blood flow, cardiovascular output and vessel diameter. Metabolic conditioning, blood flow restriction training, high-intensity interval training and altitude act more on extraction. Breathing and respiratory training acts on the autonomic nervous system, which crosses over both and is the key governor of the whole system. Cold exposure acts on delivery through local vasoconstriction, but it can also blunt adaptation signals (Roberts et al., 2015), so the timing of interventions matters. The trend is to target delivery, because it is easier to manipulate and the athlete feels something, whereas extraction is a harder sell.
The same principles apply to fatigue. During sustained work, by-products of muscle metabolism build up in the interstitial space and activate the group III and IV afferents, which project to the central nervous system and have an inhibitory effect on central drive (Blain et al., 2016). The metabolic state of the tissue therefore influences the ability of the brain to compute the value of continuing to exert effort against the urge to quit. Whether to recover or not then becomes a practitioner-based decision with a cost on both sides, since recovery may dampen the training effect, whereas without it the athlete may be less available for a high-quality output at the next session.
Perception of Effort and Interoception
Rating of perceived exertion is a useful instrument, and my view is that what the athlete reports is a readout from the anterior cingulate cortex, where the cost of continued effort is computed at any given moment (Williamson et al., 2001). Perception of effort is modulated by self-talk (Blanchfield et al., 2014), by load history, by training state and by the autonomic context of the athlete (Pageaux, 2016). A tired athlete reports a much higher perceived load than a fresh athlete because the brain recalibrates the cost of doing the same work.
This is why I place weight on interoception. In a virtual reality study, participants cycled against the same resistance while a headset showed them going uphill or downhill (Mouatt et al., 2023). The workload never changed, but perceived exertion varied considerably, and the participants whose perception shifted the most had poor interoceptive accuracy, whereas those with better accuracy gave more weight to what they felt than to what they were shown. It was a small study and it demonstrates a mechanism more than it gives a recommendation, but an athlete who fades in the second half may not have a fitness problem. Poor interoceptive accuracy also makes an athlete more dependent on the readouts of wearables, and quantifying metrics, which we do a great deal in the clinic, may displace the instrument that the athlete carries. There has to be a connection with the athlete that explains why they perceive what they perceive, alongside what the data tell us.
Brain Injury as an Energy Crisis
An injury, for me, is a failure to balance the ledger: demand rises with activity or games, delivery of nutrients and fuel to the target tissue drops, and we finish with a deficit. The difficulty with the brain is that it is very hard to unload, since a limb can be offloaded or a training session skipped, but cognitive demand arrives whether or not we programme it. When something happens to the brain, whether an injury, a blow to the head or repetitive heading of the ball, the cell membranes are disrupted, sodium and calcium enter the cells and potassium leaves. This makes it harder to restore homeostasis, and hyperglycolysis follows, since more fuel must be mobilised to produce the same output, while cerebral blood flow decreases. The mismatch between raised demand and reduced blood flow is a transient energy crisis (Giza and Hovda, 2014), in which both delivery and extraction are compromised.
The autonomic picture is less consistent, but reviews of autonomic dysfunction after brain injury describe altered autonomic function at rest and under challenge (Mercier et al., 2022; Talbert et al., 2024). In the studies I looked at, about two thirds of people with a brain injury showed reduced parasympathetic involvement, and about 80% showed a poor response to a challenge such as physical exertion or orthostatic stress. This is where graded exertion becomes useful: we use heart rate monitored graded exercise tests, and the point of degradation gives a sense of maximum capacity and becomes the ceiling of the dose as we work people forward.
Cognitive Load and Brain Endurance Training
Mental fatigue slows reaction time, worsens decision making, increases errors and injury risk, and lowers endurance and technical precision (Marcora, Staiano and Manning, 2009; Smith et al., 2016; Van Cutsem et al., 2017). It does not necessarily show up in maximal force output, so measuring only force-related parameters gives no insight into the mental side of fatigue. We have started to look at brain endurance training, and reports in the literature include improvements in endurance, repeated sprint performance and agility in professional football players (Staiano et al., 2022), and in passing and shooting when the players were tested fatigued (Staiano et al., 2025). Dosing and transfer to a specific sport remain open questions.
Auditing an Intervention
When we look at what we do, we audit it. If we intend to improve heart rate variability, or another outcome, we ask whether the intervention began to empty the reservoir so that the athlete is ready to go again, whether it grew the reservoir and made the athlete more adaptive, or whether it merely changed something for a day while nothing changed in the long term. A fourth question is whether it improved the athlete's ability to read their own state or replaced it.
"A tired athlete reports a much higher perceived load than a fresh athlete because the brain recalibrates the cost of doing the same work."
Blood Flow Restriction as a Recovery Tool
The passive blood flow restriction cuff applied at rest has been promoted as a recovery tool, typically at 70 to 80% of limb occlusion pressure, and the benefit appears to come from the release and not from the squeeze. After release, reactive hyperemia enters the limb and the endothelial cells detect the luminal shear, which leads to nitric oxide-mediated vasodilation. We think that this overshoot of delivery is beneficial. Caution is needed in people with a history of clotting, hypertension or rhabdomyolysis (Patterson et al., 2019). I do not know whether the evidence is yet sufficient to say that working under occlusion improves force restoration the next day, but the trend seems to suggest that it does (Howard et al., 2026a, 2026b).
Proprioception, Interoception and Breathing
The last piece concerns how we train proprioception and interoception. Young people who relied more on ankle proprioceptive signals in a standing test with vibration applied to the soleus were more likely to develop low back pain over the following two years (Claeys et al., 2015). The diaphragm also matters. As the nervous system fatigues, respiration receives priority, and blood is shunted from the spinal and peripheral musculature back to the diaphragm (Harms et al., 1997; Sheel, Boushel and Dempsey, 2018). Muscle spindles are highly vascular structures, so a small reduction in the delivery of blood degrades the signal they send. Paraspinal and core work combined with breathing has considerable value over about eight weeks, in line with the finding that eight weeks of inspiratory muscle training changes proprioceptive use and low back pain (Janssens et al., 2015).
Summary and Sequencing
Recovery is an energy problem, and it is a delivery versus extraction problem, not a single number from a readout. Preparation is the size of the glass, and the question is how much the capacity for work can be increased before the demands of the season begin to drain it. The sequencing I use is as follows. Breath work comes first, shifting the autonomic context. Light aerobic work comes second, because it supports delivery in a parasympathetic state. Passive modalities come last, so that adaptation signalling can proceed uninterrupted. In each case I ask what we are trying to do, what would make an intervention more or less effective, and where the athlete is on the continuum.
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Frequently Asked Questions
Q: Why is recovery better described as a reservoir than as a battery?
A battery reports its own status, which is subjective when individuals rate themselves, and its capacity does not improve by being discharged but degrades. A reservoir, or vessel, can respond to being filled with an increase in capacity over time, which fits the purpose of adaptation: to make the reservoir bigger. In this model every stressor adds allostatic load, and symptoms of overload or overreaching appear only when the capacity is exceeded. The reservoir model is presented as a heuristic and not as a peer-reviewed framework, and it is useful mainly because it directs attention to the stress entering the system across all of its dimensions.
Q: What do delivery and extraction mean when choosing a recovery modality?
Oxygen consumption equals cardiac output multiplied by the arteriovenous oxygen difference, so a modality acts either on delivery, meaning blood flow and the vessels, or on the ability of the target tissue to use what it is sent. Compression, heat and most passive modalities act mainly on delivery, whereas metabolic conditioning, blood flow restriction training, high-intensity interval training and altitude act more on extraction. Breathing and respiratory training acts on the autonomic nervous system, which crosses over both. Cold exposure can blunt adaptation signals (Roberts et al., 2015), so timing matters, and the approach described here places breath work first, light aerobic work second and passive modalities last.
Q: Why is brain injury described as an energy crisis?
After a brain injury the cell membranes are disrupted, sodium and calcium enter the cells and potassium leaves, which makes it harder to restore homeostasis. Hyperglycolysis follows while cerebral blood flow decreases, and this mismatch between raised demand and reduced blood flow is a transient energy crisis (Giza and Hovda, 2014). Reviews also describe altered autonomic function at rest and under challenge after brain injury (Mercier et al., 2022; Talbert et al., 2024). In the studies presented here, about two thirds of people showed reduced parasympathetic involvement and about 80% showed a poor response to a challenge, which is one reason graded exertion tests are used to set the ceiling of the dose.
Q: What can an athlete's perception of effort tell us, and how does it relate to wearable data?
Perceived exertion can be understood as a readout from the anterior cingulate cortex of the cost of continued effort at a given moment (Williamson et al., 2001), and it is modulated by self-talk, load history, training state and autonomic context (Blanchfield et al., 2014; Pageaux, 2016). In a virtual reality cycling study with an unchanged workload, perceived exertion varied with the visual cues, and the participants who shifted the most had poorer interoceptive accuracy (Mouatt et al., 2023). Poor interoceptive accuracy may make an athlete more dependent on wearable readouts, and quantifying metrics may displace the athlete's own internal reading. For this reason the data are best interpreted together with a conversation about why the athlete perceives what they perceive.
Q: How is passive blood flow restriction used as a recovery tool, and what precautions apply?
The passive cuff is applied at rest, typically at 70 to 80% of limb occlusion pressure, and the benefit appears to come from the release more than from the squeeze. After release, reactive hyperemia enters the limb and the endothelial cells detect the luminal shear, which leads to nitric oxide-mediated vasodilation, an overshoot of delivery that Kirk considers beneficial. Caution is needed in people with a history of clotting, hypertension or rhabdomyolysis (Patterson et al., 2019). Whether working under occlusion improves force restoration the next day is not yet established, although recent studies suggest that it may (Howard et al., 2026a, 2026b).
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Dr. Nicky Kirk is the Clinic Director at Parker Performance Institute, where he leads a multidisciplinary approach to sports chiropractic care, injury rehabilitation, and athletic performance. With expertise at the intersection of chiropractic medicine and high-performance sport, Dr. Kirk works with athletes across a range of competitive levels, applying evidence-based assessment and treatment strategies to optimize movement, reduce injury risk, and accelerate return to play.
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