Not every high-intensity effort can be sustained for a long time. When exercise intensity exceeds the body’s ability to produce and use enough energy to maintain that pace, signs of fatigue begin to accumulate, making it increasingly difficult to continue at the same level.
This can happen during training, on a climb, during a cycling attack, or even at the beginning of a race, when an athlete starts at an intensity higher than they can sustain.
The Body Needs to Change How It Produces Energy
As intensity increases, energy demands rise rapidly. During higher-intensity efforts, the contribution of anaerobic energy pathways increases to meet energy demands, while the body also has to deal with increased production of exercise-related metabolites.
At the same time, ventilation, heart rate, and perceived exertion increase. When this intensity is maintained for long enough, the athlete begins to struggle to sustain the same speed or power.
That moment when it feels like the body is "no longer responding" does not happen randomly. It is the result of a demand that is becoming greater than the body’s ability to sustain that effort at that moment.
How Does This Appear During Training?
A classic example occurs during interval training.
Imagine a runner who needs to complete 5 × 1 km at a high intensity. They start the first repetition well above the planned pace because they feel good and believe they can maintain that performance.
The first kilometer is fast. During the second, breathing becomes much more intense. By the third, perceived exertion increases and pace starts to drop. During the final repetitions, they need to slow down even further to complete the workout.
In this case, the athlete has not necessarily lost fitness. They started above an intensity they could metabolically sustain for that period.
This Can Also Happen During a Race
In competition, this mechanism becomes even more evident. Starting above a sustainable intensity may seem advantageous during the first few kilometers, but the physiological cost appears later.
In a long race, for example, increasing intensity too much on a climb can rapidly increase energy demands and accelerate fatigue. The athlete may gain a few seconds on that section, but later have to significantly reduce their pace.
That is why pacing strategies are so important in endurance sports: it is not enough to produce a lot of energy. The athlete needs to be able to sustain the intensity for the required amount of time.
Is Training Above Your Limit Always Bad?
No. Efforts above a sustainable intensity are part of many training sessions and can be important for developing different adaptations. The problem is failing to distinguish a planned training stimulus from excessive and poorly distributed effort.
If every workout is performed at very high intensities, recovery can be compromised and the quality of subsequent sessions may decline.
Training needs to consider the relationship between intensity, duration, and recovery.
Metabolic Capacity Can Be Trained
With appropriate training, the body improves its ability to produce and use energy, transport oxygen, and sustain higher intensities. Over time, this allows a pace that was once very difficult to become more sustainable.
Therefore, progress does not only mean being able to reach a higher speed or power output. It also means being able to sustain a high intensity for longer before fatigue limits performance.
In sport, knowing how to accelerate is important. Knowing when and how much to accelerate can be even more important.
