The Testosterone Question
There is a scene that plays out in tens of thousands of men’s lives every year, and it goes roughly like this. A man in his late thirties notices something. Not a dramatic collapse, not a crisis, but a drift. The mornings are harder. The ambition that used to arrive with the alarm clock now has to be summoned. Body composition is shifting in ways that diet alone cannot explain. Libido is quieter. Recovery from training takes longer than it did at thirty, but that is expected—the problem is that recovery from training takes longer than it did six months ago, and six months before that. The mirror does not lie. Neither does the spreadsheet of lifting numbers, the trend line moving slowly in the wrong direction.
The man, who is almost certainly a runner or an endurance enthusiast of some kind because those are the men most likely to be paying close attention to their fitness, goes to his physician. The bloodwork comes back. His testosterone is low-normal, or simply low. The physician, if he is thorough, suggests lifestyle changes. More sleep. Less stress. Better nutrition. Nothing he is not already doing.
What neither the man nor his physician is likely to know is that his training—the thing he has been doing precisely to stay healthy, to stay sharp, to age well—may be one of the primary mechanisms driving the hormonal decline he is trying to arrest.
The evidence for this is not ambiguous. It is published in peer-reviewed journals, replicated across independent study populations, and ignored with remarkable consistency by an endurance sports culture that has a substantial financial and psychological investment in not examining it too closely.
The Male Hormonal Architecture
To understand what goes wrong with high-volume endurance training, you need a working model of what the male endocrine system is supposed to do.
Testosterone is produced primarily in the Leydig cells of the testes under the direction of luteinizing hormone, which is secreted by the pituitary gland in pulses governed by gonadotropin-releasing hormone from the hypothalamus. This chain—hypothalamus to pituitary to gonads—is called the hypothalamic-pituitary-gonadal axis, the HPG axis, and it is exquisitely sensitive to perturbation. Testosterone in adult men circulates in total concentrations of roughly 300 to 1000 nanograms per decilitre, with a diurnal rhythm that peaks in the early morning and declines through the afternoon. Approximately 2 to 3 percent is biologically active as free testosterone, with the remainder bound to sex hormone-binding globulin and albumin. It is the free fraction that matters most for the tissue-level effects that men care about: muscle protein synthesis, erythropoiesis, libido, mood, cognition, and the metabolic machinery that keeps adipose tissue from colonising the abdomen.
From approximately age thirty, total testosterone declines at a rate of roughly 1 to 2 percent per year. This is not pathology. It is normal aging. The question is not whether this decline will occur but whether the choices a man makes—including his exercise choices—accelerate it, decelerate it, or hasten it to a degree that is clinically and functionally significant long before it should be.
Cortisol is testosterone’s principal antagonist. Produced by the adrenal cortex in response to physical stress, psychological stress, sleep deprivation, and caloric deficit, cortisol is catabolic in its predominant effects. It promotes protein breakdown, suppresses immune function, and at chronically elevated levels, directly suppresses HPG axis activity. The testis does not secrete testosterone efficiently when cortisol is chronically elevated. The hypothalamus does not pulse gonadotropin-releasing hormone normally under conditions of sustained physiological stress. The system downregulates. The man’s bloodwork comes back in the low-normal range, and nobody asks how many kilometres he ran last week.
The ratio of testosterone to cortisol—the T/C ratio—has been widely employed as an index of the body’s anabolic-to-catabolic balance. It is a proxy measure with limitations, but as a clinical signal it has shown consistent utility across decades of sports endocrinology research. A falling T/C ratio indicates that the balance is shifting toward catabolism: toward tissue breakdown rather than tissue building, toward suppressed adaptation rather than enhanced adaptation. Understanding what drives that ratio—what pushes it up versus down—is the central question of this chapter.
Hackney and the Exercise-Hypogonadal Male Condition
In the late 1980s and through the 1990s, A.C. Hackney at the University of North Carolina began documenting something that the running community did not want to hear. Male distance runners, examined at rest, showed consistently lower testosterone concentrations than sedentary controls. This was not a post-exercise effect. These men had not just finished a run. The blood samples were taken at rest, following normal recovery periods, under standardised morning conditions. The testosterone suppression was not acute and transient. It was chronic and resting.
Hackney named the phenomenon the Exercise-Hypogonadal Male Condition—EHMC—and the name itself carries the clinical weight of what it describes. Hypogonadism is the medical term for insufficient testicular function. Exercise-induced, male-specific, chronic hypogonadism. This is not a trivial finding about a minor biomarker. This is a description of a training-induced dysfunction of the reproductive axis—a system that, in men, governs not only fertility but body composition, bone density, muscle mass, cardiovascular health, and psychological wellbeing.
The mechanism Hackney identified was precise and troubling. In clinical hypogonadism caused by testicular failure, you expect to see elevated LH—the pituitary, detecting low testosterone, increases its gonadotropin output to stimulate the gonads. But in EHMC, the LH was not elevated. It was normal. The pituitary was not sounding an alarm. The signal to the testes was present, but the testes were not responding adequately. Or, in some cases, the hypothalamus itself was reducing the pulse frequency of GnRH, suppressing the entire downstream cascade before the signal even reached the pituitary. The system was adapting—downregulating the entire reproductive axis in response to the chronic energetic and metabolic demands of high-volume endurance training—in a way that did not trigger the compensatory mechanisms that would otherwise protect against hypogonadism.
This matters enormously for diagnosis and treatment. A man with exercise-induced hypogonadism, examined by a physician using standard clinical criteria, may not look hypogonadal on the LH screen. The testosterone is low. The LH is normal. The conventional clinical interpretation is often that the testosterone is just in the lower range of normal. The physician does not order a training history. Nobody asks about weekly mileage.
The Hooper Data
The definitive modern evidence for EHMC came in 2016, when Hooper and colleagues published a study in Medicine and Science in Sports and Exercise that should have reshaped the conversation about male endurance training but largely did not.
Hooper and colleagues recruited men who were running an average of 81 plus or minus 14 kilometres per week—high-volume recreational runners, the kind of man who runs five or six days a week and defines himself partly through his mileage. These men were compared with sedentary controls. Blood was drawn not once or twice but seventeen times across a four-hour window, with samples collected every fifteen minutes between 08:00 and 12:00 hours. This protocol was designed to capture the full diurnal testosterone pattern, not just a single point estimate that might miss within-session variation.
The results were unambiguous. The EHMC group showed significantly reduced testosterone concentrations at all seventeen time points compared with sedentary controls. The effect held across the entire measurement window. The statistical threshold was met at every time point, with p values at or below 0.05 throughout. This was not a finding that could be attributed to measurement noise or within-subject variability. It was a consistent, sustained suppression of the testosterone signal across the entire morning testing window.
Critically, the LH concentrations were not elevated in the EHMC group. As Hackney had observed decades earlier, there was no compensatory pituitary response. The system was suppressed centrally, or the testes were insufficiently responsive, or both—but in either case, the normal feedback mechanism that would be expected to protect against hypogonadism was not functioning. The men running 81 kilometres per week had testosterone profiles consistent with mild hypogonadism, and their pituitary glands were not raising the alarm.
The threshold of 81 kilometres per week corresponds to roughly 50 miles. The earlier version of this chapter described that volume as “a moderately ambitious recreational runner.” That was misleading. The average recreational runner logs fifteen to thirty kilometres per week. Eighty-one kilometres per week is serious competitive training—the domain of sub-three-hour marathon aspirants and club racers who structure their lives around mileage. Hackney and Lane, in a 2018 review of the endocrine effects of endurance exercise, scoped EHMC to athletes training more than seven hours per week at moderate-to-high intensity—a threshold well above what most recreational runners achieve (Hackney & Lane, 2018). The man running three hours on Sunday and four shorter sessions during the week is at the upper edge of recreational volume, not the typical case. If his total sits below fifty to sixty kilometres per week, the EHMC evidence does not clearly apply to him.
That said, for the man who does accumulate seventy to one hundred kilometres per week—and there are many such men in half-marathon and marathon culture—the Hooper data remain troubling. That man may be running himself into hormonal suppression, and the standard clinical workup is unlikely to identify the cause.
The Hooper data are complemented by a substantial convergent literature. Luigi and colleagues, publishing in Endocrine in 2012, reviewed HPG axis alterations in males subjected to chronic endurance training and documented reduced gonadotropins and testosterone levels, altered LH pulsatility, and reduced biological-to-immunological LH ratio. They characterised these as consequences of stress-related physiological adaptation and altered energy availability. They noted specifically that testosterone deficiency in endurance-trained men alters endocrine-metabolic and neuromuscular adaptations to exercise, reduces muscle strength, reduces aggressiveness in competition, and increases the risk of overtraining. The very training that is supposed to build fitness is, in high-volume endurance athletes, undermining the hormonal substrate on which fitness adaptation depends.
There is a particularly perverse quality to the EHMC feedback loop that deserves explicit attention. When testosterone is chronically suppressed, the body’s capacity for training adaptation is itself impaired. Muscle protein synthesis slows. Recovery from training sessions takes longer. The man, sensing that he is not getting fitter fast enough, concludes that he needs to train harder. He increases mileage. He adds tempo runs. He pushes the effort level. Each of these adjustments increases the cortisol burden, deepens the HPG suppression, further delays recovery, and produces an even slower rate of adaptation. The system spirals. The harder the man trains, the less he gets from the training, because the hormonal environment required to convert training stress into training adaptation is being systematically dismantled by the volume and nature of that training.
This is not hypothetical. The Luigi review specifically noted that testosterone deficiency increases the risk of overtraining syndrome—the clinical state in which accumulated training stress exceeds the body’s adaptive capacity and produces persistent performance decrements despite rest. Overtraining syndrome and EHMC share a hormonal signature: low testosterone, disrupted cortisol rhythm, impaired neuroendocrine regulation. The man who is overtrained is often the man who has been running too much for too long. The hormonal profile of overtraining and the hormonal profile of EHMC are, in meaningful respects, the same profile. The question is whether you recognise what you are looking at.
Why the HPG Axis Downregulates
The evolutionary logic of EHMC is not difficult to reconstruct. The HPG axis is an energetically expensive system. Testosterone synthesis requires cholesterol, enzymatic machinery, and dedicated cellular infrastructure in the testes. Reproduction is energetically costly. When the organism is under sustained metabolic stress—which is exactly what high-volume endurance training represents—the hypothalamus has an adaptive incentive to reduce investment in the reproductive system. Energy is being consumed at a rate that signals scarcity or danger. In an ancestral environment, that signal would typically indicate famine, predator pressure, or forced migration. In none of those contexts is reproductive investment the priority. Survival is the priority. The HPG axis downregulates. Resources are redirected.
The problem is that the modern high-volume runner is not in a survival emergency. He is a middle-income professional who goes for a run before work. His dietary intake, while perhaps imperfect, is not severely restricted. But his hypothalamus cannot distinguish between the metabolic stress signal of running 80 kilometres a week and the metabolic stress signal of pursuing large prey across a savanna in a season of scarcity. The physiological response is the same. The endocrine system suppresses the reproductive axis, and the man’s testosterone declines.
There is a second mechanism that operates in parallel. Cortisol, the primary glucocorticoid stress hormone, rises acutely during endurance exercise and remains elevated for extended periods in response to high training volumes. At chronically elevated concentrations, cortisol acts directly on the hypothalamus to suppress GnRH pulsatility, on the pituitary to reduce LH secretion, and on the Leydig cells to impair testosterone synthesis. The T/C ratio falls. The catabolic environment persists not just during training but in the hours between sessions, precisely when the anabolic environment is needed for adaptation and recovery. The man trains harder. He recovers more slowly. He adds more mileage to compensate. The cycle accelerates.
What Recreational Running Actually Does to Testosterone
The preceding sections describe the endocrine consequences of high-volume endurance training. An honest account of the evidence requires stating what happens at the volumes most recreational runners actually train.
The answer, straightforwardly, is that moderate-volume running—fifteen to forty kilometres per week, three to four sessions—does not produce EHMC. The Hackney data, the Hooper data, and the Luigi review all describe a dose-response relationship: the HPG suppression scales with training volume and intensity. At recreational volumes, testosterone may dip acutely after a hard session and recover within hours. The chronic, resting suppression that defines EHMC has not been documented at these lower volumes.
Running also has the strongest mortality evidence of any single exercise modality. Lee and colleagues, in a 2014 study of 55,137 adults followed for fifteen years, found that runners had a thirty percent lower risk of all-cause mortality and a forty-five percent lower risk of cardiovascular mortality than non-runners—benefits that persisted even at modest volumes of five to ten minutes per day (Lee et al., 2014). Saeidifard and colleagues documented a twenty-one percent reduction in all-cause mortality associated with resistance training (Saeidifard et al., 2019). Rucking has zero mortality data.
The earlier version of this chapter implied that running, at any substantial volume, was hormonally dangerous. That framing was misleading. The hormonal concern is real but volume-dependent, and the cardiovascular benefits of moderate running are among the most robustly supported findings in exercise science. The case for rucking’s hormonal advantages applies most clearly to the man who is already running sixty to eighty or more kilometres per week and experiencing the symptoms of hormonal decline—not to the man running thirty kilometres per week and feeling fine.
The Resistance Side: What Heavy Loading Does
Against this backdrop of endocrine suppression, the hormonal response to resistance-type loading looks like the photographic negative.
In 1999, Kraemer and colleagues published what remains one of the foundational studies in exercise endocrinology: a ten-week periodised heavy-resistance training programme compared across younger men with a mean age of approximately thirty years and older men with a mean age of approximately sixty-two years. The findings for both groups were clinically significant.
Younger men showed training-induced increases in free testosterone both at rest and in response to acute exercise following the programme. The programme produced not just an acute hormonal spike on training days but a resting upregulation of the androgenic environment. The testes were not being suppressed. They were being stimulated.
Older men—the population in which testosterone decline is most consequential, and for whom the question of hormonal optimisation is most urgent—showed significant increases in total testosterone in response to exercise stress, along with significant decreases in resting cortisol. Both squat strength and thigh muscle cross-sectional area increased. Kraemer and colleagues noted that even elderly men respond with an enhanced hormonal profile in the early phase of a resistance training programme, and that the likely mechanisms include both enhanced LH pulsatility and direct stimulatory effects of exercise-induced lactate on testicular cAMP production. The Leydig cells, given the right stimulus, can respond. The system is not inert. The question is what stimulus you are providing.
The contrast with high-volume running could not be more direct. Running 81 kilometres per week suppresses testosterone and fails to stimulate compensatory LH increase. Periodised resistance training elevates testosterone in both young and old men while reducing resting cortisol. The T/C ratio moves in opposite directions under these two training stimuli.
The Farmer’s Walk Problem
The Kraemer data described traditional resistance training: barbell squats, deadlifts, presses. The question for this chapter is whether loaded ambulatory exercise—rucking specifically—produces a similar hormonal stimulus, or whether it falls into the endurance-suppression pattern.
The most direct evidence comes from an unexpected source: strongman sport. In 2015, Gaviglio and colleagues published a study examining acute salivary testosterone and cortisol responses to four different training protocols in twenty-seven elite male rugby players. The protocols included high-volume resistance work, high-intensity resistance work, a repeated-sprint protocol, and strongman-type exercises. The strongman protocol included the farmer’s walk—a bilateral loaded carry in which the athlete grips heavy implements and walks for distance or time, the movement most biomechanically analogous to loaded rucking.
The farmer’s walk produced significant acute testosterone increases when analysed on an individualised basis. The p value was below 0.01. Cortisol, by contrast, declined during the resistance-based protocols. The T/C ratio improved. The strongman-type loaded carry, a movement in which you pick up heavy objects and walk with them—which is, in structural essence, exactly what rucking is—elevated testosterone rather than suppressing it.
The mechanistic pathway is consistent with what Kraemer documented. Loaded carries recruit high-threshold motor units. They demand whole-body tension: grip, forearm, upper back, thoracic extensors, core, hip stabilisers, quadriceps, calves. The neuromuscular demand is substantially different from steady-state endurance running, which recruits a relatively narrow range of motor units at submaximal intensity and does not require the sustained, multi-joint tension that loaded locomotion imposes. The lactate response to heavy loaded carries is sufficient to stimulate the testicular cAMP production mechanism that Kraemer identified. The mechanical loading of the posterior chain, particularly the erector spinae and glutes under axial load, appears to generate the kind of systemic anabolic signal that the HPG axis responds to with increased LH pulsatility.
Rucking, it should be noted, is not the farmer’s walk—and the load difference is not subtle. Strongman farmer’s walk implements typically weigh 100 to 150 kilograms per hand, producing total loads of 200 to 300 kilograms. A recreational rucker carrying 20 percent of an 80-kilogram body weight is carrying 16 kilograms. The difference is roughly five-to-ten-fold. The Gaviglio result cannot be directly imported to the rucking context without this qualification front and centre. The magnitude of the acute hormonal response is load-dependent: the pronounced testosterone spikes observed in elite rugby players using near-maximal strongman implements will not be replicated at the substantially lighter loads characteristic of recreational rucking. Whether a 16-kilogram backpack produces any measurable acute testosterone elevation remains an open empirical question that no published study has addressed. Loaded ambulatory exercise recruits the posterior chain under sustained axial load, generates the metabolic and neuromuscular stimulus that the HPG axis interprets as an anabolic signal, and appears to elevate rather than suppress testosterone. The magnitude of the effect will scale with the magnitude of the load, and the directional relationship—while not yet established by direct rucking trials—is supported by convergent mechanistic and observational evidence.
What the Military Knows About Hormones
There is a population that has been performing regular load carriage for decades and whose hormonal profiles have been examined systematically: elite military personnel.
Taylor and colleagues published a characterisation of daily anabolic hormone profiles in elite military men in 2016 that produced a striking finding. These men—with a mean age of thirty-three years, well past the peak testosterone years—had salivary testosterone concentrations that exceeded those of young, healthy male recreational weightlifters with a mean age of eighteen years, and also exceeded those of male university students with resistance training experience and a mean age of twenty-four years. The elite military men were thirteen to fifteen years older than the recreational weightlifters but had comparable or superior androgenic profiles.
The key word in that sentence is comparable. The diurnal pattern and magnitude of testosterone secretion in military men whose occupational demands include regular load carriage was nearly identical to that observed in young, recreationally weight-trained men. Not similar to recreational runners. Similar to weightlifters.
Taylor and colleagues were careful not to attribute causation, and several confounds deserve explicit attention. First, survivorship bias: military selection filters for men with above-average baseline fitness, body composition, and likely hormonal profiles. The men measured were not a random sample of the male population—they were the survivors of a selection process that preferentially retains the hormonally robust. Second, military training is not load carriage alone. These men perform structured resistance training, sprint work, occupational tasks requiring maximal effort, and have access to nutritional and recovery support that recreational ruckers do not. Third, the study did not measure hormonal profiles before and after a load carriage intervention—it measured existing profiles in men who happened to carry loads as part of a diverse occupational training regimen. The study cannot isolate load carriage as the specific hormonal driver.
The finding is consistent with the mechanistic picture assembled from the Kraemer and Gaviglio data, but the consistency is weaker than the earlier version of this chapter implied. Regular load carriage, embedded in a comprehensive training programme, is associated with maintenance of anabolic hormone profiles. Whether it causes that maintenance—or whether the men who maintain high testosterone are simply the men who survive military selection—cannot be determined from this study design.
Contrast this with what Hooper documented in high-volume runners, and the picture becomes coherent. Men running 81 kilometres per week show chronic testosterone suppression without compensatory LH increase. Military men performing regular load carriage show testosterone profiles comparable to recreational weightlifters. The divergence between these populations is not fully explained by age, genetics, or baseline fitness differences. It is consistent with, at least in part, the hormonal consequences of what they are doing with their bodies—though the observational nature of the military data limits causal attribution.
The Hybrid Hormone Zone
The argument is not that rucking is identical to weightlifting, or that the loaded carry fully replicates the hormonal stimulus of periodised resistance training. It is more precise than that, and in some respects more interesting.
Rucking is a resistance-endurance hybrid. It is sustained cardiovascular activity—zone 2 aerobic work, moderate metabolic demand, elevated but not excessive heart rate—combined with the mechanical stimulus of axial loading through the posterior chain. It is neither the pure catabolic stress of high-volume endurance training nor the purely intermittent anabolic stimulus of traditional resistance exercise. It sits in a middle zone that the endocrine system appears to respond to favorably: sufficient mechanical load to stimulate the HPG axis toward testosterone production, insufficient volume and intensity of purely cardiovascular stress to chronically suppress GnRH pulsatility.
The Looney metabolic model, published in Medicine and Science in Sports and Exercise in 2021, established that the metabolic cost of walking with a load scales nonlinearly with weight carried. A backpack of 20 percent of body weight—roughly 16 kilograms for an 80-kilogram man—elevates the metabolic demand of walking into a range consistent with moderate-intensity aerobic training. The cardiovascular system is being trained. The erector spinae, glutes, and posterior chain are under sustained mechanical load. The respiratory system is challenged but not overwhelmed. The diurnal cortisol profile is unlikely to be chronically disrupted at these intensities and volumes. Based on the convergent evidence from Kraemer, Gaviglio, and Taylor, the HPG axis is unlikely to be suppressed by this stimulus in the way that high-volume endurance running suppresses it.
This is what the convergent evidence suggests may be a hormonal sweet spot—though the word “suggests” carries real weight here because no published study has directly measured chronic testosterone or cortisol in recreational ruckers. Zone 2 cardiovascular adaptation without the chronic cortisol burden of high-volume endurance work. Posterior-chain mechanical loading without the central nervous system fatigue of heavy barbell training four days per week. The mechanistic reasoning is sound: rucking at moderate loads avoids the specific mechanisms that drive chronic testosterone suppression while preserving some of the mechanisms that acutely stimulate testosterone production in heavier loaded-carry research. But mechanistic reasoning and measured outcomes are not the same thing. The T/C ratio in recreational ruckers has not been characterised in any published study. This is inference, not established fact.
The Ageing Male: Where This Becomes Urgent
For men under thirty, these distinctions have real but limited immediate clinical significance. Testosterone production is robust, the HPG axis is resilient, and the suppressive effects of endurance training, while real, are more easily offset by the system’s baseline capacity. The young runner’s hormonal profile may be suboptimal, but it is unlikely to produce the clinical syndrome of hypogonadism in the short term.
For men over forty, the calculus changes dramatically. The baseline testosterone trajectory is already declining at 1 to 2 percent per year. The androgenic reserve that buffered against exercise-induced suppression in the twenties is being systematically reduced by normal ageing. The HPG axis becomes less resilient. Leydig cell mass decreases. LH pulsatility diminishes. The gap between optimal and suppressed testosterone narrows.
In this context, adding the chronic suppressive load of high-volume endurance training to an already-declining baseline is not a neutral choice. It is a compounding variable. The 48-year-old who runs 60 kilometres per week may be experiencing testosterone suppression that, in combination with age-related decline, produces a total androgenic deficit that would not have occurred with either factor alone. His physician, reviewing his bloodwork and noting that his testosterone is at the bottom of the normal reference range, is not wrong to say it is normal. It is normal for a 70-year-old. It is not normal for a 48-year-old who exercises aggressively and eats reasonably well and sleeps seven hours a night.
The Kraemer data on older men is therefore not just academically interesting. It is practically urgent. Older men—in the study, with a mean age of 62 years—showed significant testosterone elevation in response to exercise stress and significant cortisol reduction after ten weeks of periodised resistance training. The system remained responsive. The Leydig cells could still be stimulated. The HPG axis, presented with the right mechanical signal, was capable of an anabolic response even in the seventh decade of life. The question is what signal you are providing it.
Running 60 kilometres per week provides the wrong signal. The HPG axis reads it as chronic metabolic stress and reduces testosterone production accordingly. Carrying a loaded pack through varied terrain for an hour provides a mechanically different signal: demand on the posterior chain, moderate metabolic load, no chronic cortisol accumulation, and an acute hormonal response that the Gaviglio data—drawn from loaded carries in elite athletes—characterise as testosterone-elevating rather than testosterone-suppressing, a directional pattern that the military load carriage literature supports at more moderate intensities.
The Cardiovascular Dimension
A counterargument surfaces at this point, and it deserves a direct answer rather than avoidance. Running produces well-documented cardiovascular adaptations: reduced resting heart rate, increased stroke volume, improved VO2max, reduced arterial stiffness. If rucking is hormonally superior, is it cardiovascularly inferior? Is the man who gives up running for rucking trading hormonal health for cardiovascular health?
The answer is no, but the evidence for this requires understanding the dose-response relationship between exercise intensity and cardiovascular adaptation. The cardiovascular adaptations most strongly associated with longevity—specifically the mitochondrial adaptations, the capillary density improvements, and the left ventricular remodelling that characterise well-trained aerobic athletes—occur primarily in the moderate-intensity zone, what exercise physiologists call Zone 2. This is the zone in which blood lactate remains below approximately 2 millimoles per litre, the aerobic energy system dominates, and the training stimulus is sustainable for sixty to ninety minutes at a time.
Zone 2 training at running pace and Zone 2 training at loaded walking pace produce the same cardiovascular adaptation signal. The mitochondria do not care whether the elevated metabolic demand is being generated by moving legs quickly over ground without a load or by moving legs more slowly with twenty kilograms on the back. What they respond to is duration of elevated metabolic demand at the appropriate intensity—and rucking with a moderate load produces that metabolic demand at walking pace. The Looney metabolic modelling, validated across seven independent military datasets, demonstrated that carrying a backpack equal to 20 percent of body weight at a brisk walking pace elevates metabolic cost to a level consistent with moderate-intensity cardiovascular training in most adult men.
The man who replaces a 60-kilometre running week with three or four rucking sessions totalling comparable metabolic work is not abandoning cardiovascular training. He is performing cardiovascular training at the same physiological intensity, with the same adaptational stimulus to the aerobic energy systems, while simultaneously removing the chronic cortisol burden and HPG suppression that the running volume was producing. He is not trading one health outcome for another. He is, as the evidence indicates, getting both.
The Evidence Gap and What It Means
It is important to be precise about what the evidence does and does not establish.
No published study of any design—not a randomised controlled trial, not a cohort study, not a case series—has directly measured chronic resting testosterone, free testosterone, or cortisol in recreational ruckers. This is not a gap in one evidence stream. It is the absence of any direct evidence for the central claim of this chapter. The inferential case is built from converging evidence streams: the EHMC literature documenting chronic suppression in high-volume runners, the resistance training literature documenting testosterone elevation with loaded carries at near-maximal loads, and the military load carriage literature documenting maintenance of anabolic hormone profiles in men performing regular load carriage as part of diverse training. These streams converge on a consistent mechanistic narrative, but convergence is not equivalence with direct experimental evidence, and each stream carries its own limitations—load magnitude (Gaviglio), survivorship bias (Taylor), and volume thresholds (Hooper)—that weaken the inferential chain when applied to recreational rucking at fifteen to twenty percent body weight.
What the evidence gap means practically is that the case presented here should be understood as a strong inferential argument rather than an established experimental fact. The mechanistic pathways are known. The directional effects of each component are documented. The convergence across independent literature streams is striking. But the rucking-versus-running hormonal comparison, controlled for volume and intensity and conducted with comprehensive endocrine profiling, has not been done. This is a research gap, and it is one worth naming explicitly because the argument here is strong enough to act on without waiting for the confirmatory RCT, but honest enough to acknowledge that the confirmatory RCT has not been conducted.
The standard for action in practical exercise prescription is not the same as the standard for regulatory approval of a pharmaceutical agent. A man in his mid-forties who is running sixty kilometres per week and experiencing declining testosterone, declining recovery capacity, and declining mood does not need to wait for the definitive trial. The mechanistic evidence is sufficient to support a reasonable change in training approach. That change—reducing running volume, substituting loaded walking—is not associated with known harm and is consistent with a substantial and convergent body of evidence pointing toward a more favorable hormonal outcome.
What the Man on the Trail Understands Without Knowing It
Return to the man on the trail from the prologue. His heart rate is at sixty-eight percent of his age-predicted maximum. His pack weighs twenty-two kilograms. His posterior chain is under sustained axial load. He is not running. He is not in the anaerobic zone. He is not generating the sustained cortisol response that prolonged high-intensity endurance work produces. His HPG axis is not receiving the chronic metabolic stress signal that Hooper’s runners were generating.
His testosterone, if you drew blood at this moment, would be unlikely to be suppressed by the training stimulus he is providing—or so the mechanistic reasoning suggests. That was stated in the prologue as a narrative claim. This chapter has provided the mechanistic basis for that inference, while acknowledging that the inference has not been directly tested. The loaded walk is doing what the resistance training literature says loaded movement does: stimulating the posterior chain, recruiting high-threshold motor units under sustained mechanical demand, generating the metabolic and hormonal signal that the HPG axis interprets as a reason to maintain androgenic output rather than reduce it.
Many men have arrived at this conclusion not through an understanding of exercise endocrinology but through direct physical experience. They have run high mileage and noticed the drift. They have switched to rucking, or added weighted carries, or reduced their running volume, and noticed the return. The energy that was missing. The recovery that normalised. The mood that steadied. These men did not have the vocabulary to describe what was happening. They described it, if they described it at all, as feeling better. What they were feeling, in physiological terms, was a testosterone-to-cortisol ratio moving back toward the anabolic side. What they were feeling was the result of removing the exercise stimulus that was suppressing their HPG axis and replacing it with a stimulus that the HPG axis responded to with increased androgenic output.
They were not imagining it. The signal was real. The mechanism was documented. The evidence was published, replicated, and—for most of the endurance running culture—steadfastly ignored.
The Hormonal Case, Stated Plainly
High-volume endurance running chronically suppresses testosterone in men performing volumes above approximately 81 kilometres per week, without compensatory LH elevation, through a mechanism involving central HPG axis downregulation and possible direct Leydig cell insufficiency. The condition was named and characterised in the 1980s. It has been replicated and extended across multiple independent research populations. It is not a controversial finding in the sports endocrinology literature. It is simply not discussed in the spaces where runners get their information.
Loaded carries—farmer’s walks and strongman-type loaded ambulatory exercise that recruits the posterior chain under sustained axial load—produce acute testosterone elevation in elite male athletes, with p values below 0.01, and are associated with T/C ratio improvement rather than deterioration; rucking at recreational loads operates on the same mechanistic pathway, though direct confirmatory evidence at those intensities is not yet published. Military men performing regular load carriage maintain testosterone profiles comparable to recreational weightlifters who are fifteen years their junior. Periodised resistance training elevates testosterone and reduces resting cortisol in men well into their sixth decade of life.
The hormonal case for rucking over high-volume running is an inferential argument—strong in its mechanistic logic, consistent across independent research streams, but untested in its specific application to recreational rucking. Running at volumes above approximately eighty kilometres per week suppresses the male HPG axis. The evidence from loaded carries at near-maximal loads and from military populations with diverse training suggests that load carriage does not produce the same suppression—and may support the androgenic environment. But the key word is suggests. No study has measured what happens hormonally when a forty-five-year-old man carries sixteen kilograms in a backpack for an hour, three times a week, over twelve weeks. The distinction is not trivial. For men over forty, for whom the baseline testosterone trajectory is already declining and the hormonal buffer against exercise-induced suppression is narrowing year by year, it may be the most consequential exercise decision they make.
The man on the trail understands none of this in the language of endocrinology. He understands it in the language of how he feels when he gets home, when he lifts the pack off his back, when he showers and eats and sits down to work. He feels, in a word, capable. Not exhausted. Not depleted. Not running on the cellular fumes of an overtrained system. Capable. Alert. Himself.
That is what an intact testosterone-to-cortisol ratio feels like from the inside.
The mechanistic evidence offers a plausible explanation for why he feels it. Whether that explanation will survive direct experimental testing remains to be seen. But the man who feels capable after carrying a load has, at minimum, the convergent weight of the resistance training, loaded carry, and military literature behind his experience—even if the definitive study remains unwritten.