Thresholds, Load, and Process in Endurance Training
Gabriel Espinosa explains how to interpret training zones, avoid the Zone 3 trap, and dose strength work in triathlon.
Understanding physiological thresholds is essential for dialing in training stimuli in endurance sports. Physiologist Gabriel Espinosa details laboratory and field testing, the hazards of constantly training at moderate intensity, and proper strength training prescription.
In endurance sports, training more does not always mean training better. The relentless pursuit of progress leads many amateur athletes to make classic mistakes in intensity distribution, falling into the trap of going too hard on easy days and lacking the energy for maximum-effort sessions. In a conversation on Stemma Performance with Bruna Mahn, physiologist Gabriel Espinosa, from the Human Performance Laboratory in Rio de Janeiro, discussed how exercise physiology helps individualize training loads, define thresholds, and build a consistent long-term process.
Measuring thresholds: laboratory testing and field monitoring
The foundation of accurate training prescription begins with identifying an individual's metabolic milestones. The gold standard for this assessment is the cardiopulmonary exercise test with ergospirometry (gas analysis), often paired with blood lactate testing. This protocol maps everything from light intensity up to maximal effort (VO2 max), identifying the first threshold (LT1) and the second threshold (LT2), the latter representing the maximal metabolic steady state—corresponding to FTP or critical power in cycling and the anaerobic threshold in running.
While laboratory testing provides diagnostic precision, Gabriel Espinosa emphasizes that the best approach combines lab assessments with continuous field monitoring. Rather than relying solely on isolated test sessions—which trigger anxiety and do not always reflect an athlete's true maximal output—coaches can extract real-world data from training and racing using power meters and GPS, analyzing power and velocity duration curves (such as 3-, 6-, and 12-minute intervals or distances from 800m to 3,000m).
The Zone 3 trap and the importance of low intensity
One of the most common mistakes among runners and triathletes is the so-called 'flattening' of training zones. By underestimating easy-pace work due to ego or biomechanical discomfort at lower cadences, many runners refuse to train at slower paces and spend most of their weekly volume in Zone 3 (Z3).
Zone 3 often creates a satisfying sense of sustainable effort and hormonal rush during workouts, but it extracts a steep metabolic price: it depletes significant glycogen stores and causes chronic cellular damage. The result of this pattern is an inability to recover properly to hit high intensities during interval workouts (Z4 and Z5). The athlete ends up perpetually training in a moderate middle zone, sacrificing the polar spread needed to develop maximal mitochondrial efficiency at the base (Z2) and peak power at the top.
For beginners or athletes returning from injury—whose Z2 often overlaps with a walking pace—breaking up volume with strategic walk breaks (such as alternating running intervals with one-minute walks) or turning to non-impact cross-training (cycling and swimming) are effective ways to build cardiorespiratory and peripheral adaptations without overloading the musculoskeletal system.
High intensity and the mental barrier of Zone 5
Reaching and sustaining maximal effort zones (Z4 and Z5) requires more than physical conditioning: it demands psychological tolerance for extreme discomfort. Often, the urge to cut an interval short stems from a neural self-preservation drive. Gabriel highlights that the ability to withstand this metabolic stress is trainable.
Strategies such as group workouts on a track, direct in-person coaching with immediate feedback, and training alongside peers of similar fitness levels help athletes overcome the fear of all-out efforts. In addition, undergoing prior medical and cardiological screening provides the peace of mind athletes need to confidently push their limits during prescribed hard sessions.
Strength training for endurance: context without training to failure
Integrating strength training into a triathlete's routine must respect their individual context—including age, training history, and available time—while avoiding the logic of bodybuilding or conventional fitness. The goal of lifting for runners and cyclists is to enhance neuromuscular qualities and protect joint structures against overload, not to reach concentric failure or pursue fatiguing hypertrophy.
For most younger athletes, one or two well-dosed sessions per week are sufficient. Peri- and post-menopausal women, on the other hand, may benefit from higher training frequencies to counteract muscle mass loss. When structuring the weekly schedule, pairing strength sessions on the same days as high-intensity workouts is a practical strategy to protect recovery days, ensuring that rest is genuinely restorative.