The classic linear periodization (volume then intensity) shows its limits as soon as we exceed three weekly sessions on the same muscle group or energy pathway. We have observed for two seasons a shift towards daily regulation models, supported by individual physiological markers, which change the way to structure a training cycle.
Managing load through heart rate variability
Heart rate variability (HRV) is no longer a gadget of quantified self. Several studies published between 2022 and 2024 (Kiviniemi et al., Eur J Appl Physiol, 2022; Flatt and Howle, Int J Sports Physiol Perform, 2023) confirm that daily adjusted planning based on HRV improves endurance performance and reduces signs of overtraining compared to a fixed program.
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Specifically, the morning measurement (ring, chest strap, or optical watch) provides a readiness score. When HRV drops below the individual baseline for two consecutive days, the day’s session shifts to technical work or active mobility instead of the planned intervals. The opposite is also true: a spike in HRV signals that a temporary overload will be better tolerated.
We recommend recording values for at least four weeks before modifying a plan. Without this reliable baseline, the day’s figure means nothing. Consumer tools (watches, connected rings) already exploit these approaches, but few practitioners truly know how to transform raw data into training decisions, a topic documented on the ultra-sport.org site for sports with concrete protocols.
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RED-S and relative energy deficiency: the overlooked performance barrier for amateur athletes
The RED-S syndrome (Relative Energy Deficiency in Sport) is not reserved for elite female marathoners. Since the update of the International Olympic Committee consensus in 2023, the scientific community emphasizes that this syndrome also affects recreational athletes who accumulate high training volume and chronic caloric deficit.
The consequences go beyond simple fatigue: hormonal disorders, bone fragility, decreased ability to adapt to effort, and increased risk of injury. A runner who stagnates despite structured training should question their energy balance before adding volume.
Identifying warning signs
- Stagnation or decline in performance over several weeks despite adequate recovery on paper.
- Disruptions in the menstrual cycle in women or a notable decrease in libido in men, early markers of prolonged energy deficiency.
- Repeated stress injuries (periosteum, metatarsals, sacrum), especially in high-impact disciplines like running or trail.
- Persistent irritability, sleep disturbances, and difficulty concentrating outside of effort.
The answer is not to train less, but to better calibrate caloric intake relative to actual expenditure. Occasional nutritional monitoring over two to three weeks, even without weighing every food, is often enough to identify a gap.
Zone 2 training and polarization: beyond the slogan
Everyone talks about zone 2, but few practitioners truly adhere to it. The polarized model (about four-fifths of the volume at low intensity, the rest at high intensity, almost nothing in between) has been documented since the 2000s among Scandinavian cross-country skiers. Its recent diffusion into the general public poses an execution problem.
Zone 2, defined by ventilatory threshold 1 or the first lactate threshold, is slower than most athletes imagine. Without field testing (lactate threshold test or effort test with gas analyzer), many “think” they are cycling or running in zone 2 while they float in zone 3, too fast to build aerobic base, too slow to stimulate VO2max.
Implementing a polarized block over eight weeks
The first step is to determine individual thresholds. A field test with three to four-minute stages and lactate sampling remains the reference. If not possible, a heart rate drift test on a treadmill or home trainer provides an exploitable estimate.
Then, structure the microcycle around two high-intensity sessions (long intervals like 4×8 min or short ones like 30/30) and the rest in strict fundamental endurance. The temptation to “push a little” on easy outings is the main pitfall. We observe that athletes who stick to the protocol progress more in events lasting over an hour than those who maintain “moderately hard” training continuously.
Active recovery and cold exposure: sorting signal from noise
Cold water immersion after effort has contradictory results. Regarding subjective recovery (feeling of freshness, reduction in fatigue perception), the effect is measurable. On long-term muscle adaptation, some data suggest that post-training cold can attenuate gains in hypertrophy and strength if it is systematic.
For an endurance athlete in a closely spaced competition phase, cold immersion remains a useful tool. However, during a muscle development block or overload cycle, it is better to limit cold baths to days without adaptation stimulus.
The most neglected recovery remains sleep. No technology, no cryotherapy protocol compensates for chronic sleep deficit. HRV, in fact, largely reflects the quality of nighttime rest, which ties back to the load management mentioned above.
Sustainable performance gain relies on three pillars rarely aligned at the same time: a training load modulated on individual markers, an energy intake adapted to this load, and recovery that does not sabotage the desired adaptations. The tools exist; the challenge is to articulate them daily without falling into overanalysis.