Nick Pelios
Freediver, Creator
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Nick Pelios
Freediver, Creator
Freedivers are cast as figures of calm transcendence, slipping into the deep as if human physiology is suspended along with breath. But this vision is only half the truth. Beneath the silence is a storm, and it happens inside the muscles. Every kick, every undulation, every stroke sets off a metabolic chain reaction. Oxygen demand rises, but delivery is restricted. Energy production shifts, lactate builds, and fatigue follows. It is here, in this quiet battle with physiology, that the real story of freediving unfolds.
Lactate is not simply waste. It is the chemical currency of compromise. When muscles run short on oxygen, they convert pyruvate into lactate to keep contracting, to keep moving even under duress. The accumulation of lactate signals a reliance on anaerobic metabolism, and its clearance reflects how quickly a body returns to readiness. In freediving, the dive reflex alters the playing field entirely. Blood is redirected toward the brain and heart, while limbs, the very engines of propulsion, are left in partial deficit. What results is a range of fatigue profiles that depend on how a diver moves through the water. The choice of finning style, monofin, bifins, or no-fins, is not just about hydrodynamics or preference. It is about how muscles are stressed, how lactate accumulates, and how recovery is managed.
Different ways of moving create different patterns of fatigue. The monofin is often hailed as the ultimate freediving tool, capable of translating human effort into smooth, dolphin-like propulsion. Its undulating motion recruits large muscle groups, the core, glutes, hamstrings, and spinal erectors, spreading the effort across the body. Efficiency is high, drag is reduced, and distance per stroke is unmatched. Yet this same engagement of broad muscle groups means lactate accumulation can be significant. When the body is deprived of oxygen during apnea, large active muscles become strong producers of lactate. The diver experiences both the grace of glide and the heaviness of burning legs when surfacing.
Bifins offer a different story. The movement is simpler, the equipment more familiar. Two fins, flutter kicking, legs doing most of the work. It may appear more accessible, but research shows bifin diving can generate the fastest muscle desaturation rates and some of the highest lactate readings. The reason lies in frequency and localization. With each kick, the quadriceps and calves work relentlessly, often without the relief of glide phases. Blood flow is restricted by the dive reflex, oxygen debt builds, and anaerobic metabolism takes over. Divers report the distinct sensation of leg heaviness earlier and more acutely than with monofin. Efficiency is sacrificed for familiarity, and the biochemical cost is steep.
No-fins is the purist’s choice, the stripped-down discipline that demands coordination of arms and legs without mechanical aid. It is, by design, inefficient. Every stroke and kick moves the diver forward, but at a greater energy cost per meter. Studies confirm that constant weight no-fins and dynamic no-fins produce some of the highest post-dive lactate levels recorded, often exceeding 7 mmol/L. For context, static apnea barely lifts lactate above 1 mmol/L. In no-fins, the diver feels the effort not just in the legs but across the whole body. Arms, shoulders, and trunk all contribute, each drawing from limited oxygen stores. It is perhaps the purest expression of the freediver’s paradox: the body is working hard, yet the environment demands calm.
What these styles reveal is not just differences in efficiency but differences in physiology. Monofin spreads the load and rewards those with whole-body coordination. Bifins concentrate stress locally and punish inefficiency quickly. No-fins tests endurance across the body, demanding resilience where hydrodynamics offer little help. Each style produces a unique lactate signature, a biochemical fingerprint of effort and adaptation.
Sports science has increasingly turned its gaze toward freediving, and the findings are striking. In a landmark study on competitive freedivers and synchronized swimmers, constant weight no-fins recorded the highest net lactate accumulation, averaging 6.3 mmol/L, while constant weight with monofin reached 5.9 mmol/L. Synchronized swimming, with its combination of artistic movement and apnea, came close at 5.0 mmol/L. Static apnea, by contrast, was almost flat, averaging just 0.7 mmol/L. These numbers highlight how movement under apnea changes the metabolic landscape entirely.
Another study comparing monofin swimming under normal breathing and under breath-hold painted a similar picture. While lactate under normal breathing remained modest—around 2.2 mmol/L in elite divers—the same distance performed under apnea saw lactate surge to over 6 mmol/L in intermediates and 4 mmol/L in elites. The difference between groups is telling. Elite freedivers, through years of adaptation, generate less lactate under the same conditions. Whether through improved efficiency, better buffering of acidosis, or enhanced clearance, training changes the profile of fatigue.
Near-infrared spectroscopy has added another layer of insight. By measuring oxygen saturation and hemoglobin levels in muscles during dives, researchers have tracked in real time how quickly muscles desaturate and how lactate correlates with that drop. Bifin diving showed the fastest rate of desaturation at almost one percent per second, while monofin and no-fins followed closely. The resulting lactate measurements mirrored this pattern, with bifin and no-fins pushing above 7 mmol/L, monofin around 6 mmol/L, and static remaining near baseline. These findings confirm what divers feel: the burn is not imagined, it is measurable.
Such data does more than validate experience. It informs training, strategy, and safety. By knowing which styles are most demanding metabolically, divers can plan recovery intervals, adjust session structures, and build conditioning targeted to their chosen discipline. It also underscores why freediving should not be portrayed as effortless. The numbers tell a story of muscle stress, of biochemical compromise, of resilience in the face of oxygen debt. That is as much a part of the sport as the silence of the deep.
Understanding lactate is one thing; measuring and managing it is another. In the laboratory, blood samples from the fingertip or earlobe provide precise readings, taken before dives, immediately after, and at intervals during recovery. Portable analyzers now make this feasible even poolside, allowing divers and coaches to track lactate clearance over time. A typical protocol might chart lactate every minute for five minutes post-dive, then every two minutes up to fifteen minutes. The shape of the clearance curve reveals how quickly a diver is ready for the next attempt.
Technologies like near-infrared spectroscopy add another dimension. By monitoring muscle oxygenation continuously, NIRS allows divers to see when muscles hit their lowest saturation point, when blood volume peaks, and how quickly recovery begins. These measures correlate strongly with lactate accumulation and clearance, providing a non-invasive tool to complement blood sampling. For freedivers, this is not just academic. It is a way to train smarter, to avoid stacking fatigue, and to personalize recovery strategies.
Management extends beyond measurement. Active recovery has long been shown to accelerate lactate clearance compared to passive rest. Light swimming, gentle finning, or even walking in shallow water can help circulation and reduce recovery times. Nutrition plays a role too. Carbohydrates support the Cori cycle, where lactate is recycled back into glucose by the liver. Hydration ensures blood flow remains efficient. Heat aids perfusion, while cold may prolong clearance by sustaining vasoconstriction. Freedivers who treat recovery as seriously as descent find themselves better prepared, safer, and more consistent.
Training adaptation is the ultimate tool. Over time, repeated exposure to apnea with effort builds buffering capacity against acidosis, enhances mitochondrial density, and improves lactate clearance rates. Elite divers demonstrate this clearly, showing lower lactate accumulation for equal work. What feels like supernatural ability is in fact years of physiological conditioning, honed by data and discipline.
Mainstream depictions of freediving rarely capture this side of the sport. Advertisements highlight grace. Documentaries focus on transcendence. Feature films simplify the challenge into willpower or romance. What is missing is the invisible battle: the chemical storm in the muscles, the sensation of legs turning heavy with lactate, the precise timing of recovery before another attempt. To understand freediving only as serenity is to misunderstand it.
This is not about stripping the magic away. On the contrary, it deepens it. The ability to turn biochemical struggle into fluid movement is extraordinary. To glide while the body burns, to remain composed while muscles drown in lactate, is a feat of adaptation and resilience. It is the union of physiology and psychology, of data and art. Freediving deserves to be portrayed in its full truth. The silence is real, but so is the fatigue. The calm is genuine, but it is earned against the background noise of muscle chemistry.
When a diver surfaces after a long dynamic or a deep constant weight dive, the relief on their face is not just about making it back. It is the body’s celebration of clearance beginning, of lactate being carried away, of balance restored. That moment is as much part of freediving’s story as the descent itself. It is time we acknowledged it, in training, in science, and in culture.
References
Rodríguez-Zamora L, Engan HK, Lodin-Sundström A, Schagatay F. Blood lactate accumulation during competitive freediving and synchronized swimming. Undersea Hyperb Med. 2018
Drviš I, Vrdoljak D, Dujić G, Foretić N, Dujić Ž. Aerobic and Anaerobic Metabolism During Monofin Swimming in Trained Breath-Hold Divers. J Funct Morphol Kinesiol. 2025
Vrdoljak D, Dujić Ž, Foretić N. Muscular Oxygen Saturation and Hemoglobin Concentration during Freediving: A Case Study. Oxygen. 2024
Huang T, et al. Novel insights into athlete physical recovery concerning lactate metabolism, lactate clearance and fatigue monitoring. PMC. 2025
IJREP. Effects of Various Recovery Modalities on Lactate Clearance and Subsequent Exercise Performance. Int J of Reha and E-P, 2021