Intensity Distribution

Colin Ward
Apr 16
3 min read

Updated: Jun 3

What is Intensity Distribution in Endurance Training?

Understanding intensity distribution is crucial for effective training. It often refers to how often high, medium, or low-intensity sessions happen within a mesocycle, typically spanning a four-week period.

The Structure of a Mesocycle

Within a mesocycle, you’ll find microcycles, usually lasting a week. In general, you should only engage in 2 or 3 intense sessions each week. The total training volume, measured in hours, also plays a significant role.

Many athletes overlook the importance of intensity distribution during individual training sessions. This is especially true during easier endurance rides. Neglecting to consider this can lead to suboptimal training. The consequence? Easy days can become too intense, while hard days might not challenge you enough.

The Importance of Consistent Effort

Consider a 2 or 3-hour endurance ride focused on maintaining a steady intensity around Zone 2 (Z2). Ideally, there should be minimal fluctuations in power and heart rate. If your effort varies too much, you risk spiking your power and increasing fatigue. This is not sustainable for a 3-hour ride. Alternatively, failing to maintain a consistent effort means you're not effectively maximizing your time during training. It’s all about finding the right balance.

Real-World Examples: Two Different Athletes

Below are two examples from different athletes, both of whom had the same training prescription: to go out and ride steadily for 2.5 to 3 hours while maintaining consistent effort.

Analyzing the Ride

When evaluating their rides, I look for:

Crank Time: How much time was spent actively pedaling? Was there significant coasting?
Variability Index (VI): What does the index show? Is it a steady ride, or is it spiking?
Heart Rate and Power Profile: This includes the amount of time spent in the designated endurance power zone.

Athlete 1’s Performance

HR trace from the ride, mostly within the prescribed range

Key Observations:

Minimal Coasting Time: The athlete pedaled throughout most of the ride, actively engaging their legs and aerobic system.
Low VI Index (1.06): This indicates a stable performance.
No HR/PwR Drift: The heart rate trace appears optimal.
Power Distribution: Most effort was at the endurance intensity, with little time spent above tempo.

Overall, this execution is commendable. The athlete will likely recover quickly while accumulating valuable endurance work over 3 hours.

Athlete 2’s Performance

HR trace shows a lot more variation and is often way above or below the range

Key Observations:

Significant Coasting Time: Over 20 minutes (14%) were spent not pedaling. This is time that could have been used effectively, even if at a lower intensity.
High VI (1.18): This ride was spiky, which is typical in crit races where power output fluctuates.
Heart Rate Analysis: The HR profile shows considerable variability.

Despite the average heart rate landing in Zone 2, the workload on the legs was likely higher than necessary. If too many rides are conducted this way, it can lead to excessive fatigue or inadequate stress response. It’s important to note that both rides occurred over similar terrain, eliminating differences in difficulty.

Understanding Training Stress Score (TSS)

A noteworthy observation is that both rides yielded similar TSS scores. This raises questions about the usefulness of TSS as a metric. From the perspective of energy utilization—both fat and muscle glycogen—the ride with more variability was more demanding. Consequently, it would require a longer recovery period.

More glycogen depletion means your body needs time for replenishment. Also, more muscle damage may occur, necessitating repair time. Managing your effort during rides is an invaluable skill, especially when racing or participating in time trials.