Challenge Level (CL) – Knowledgebase

Definition

Challenge Level (CL) is a Magic Buckets concept that describes how deep you’re going on each interval—specifically, how much fatigue you’ve accumulated at the current interval target power. It’s scored from 0 to 10 and is closely related to HIE depletion & proximity to MPA.

In simple terms: higher CL = closer to a Breakthrough at that interval power.

CL 3 ≈ ~30% of the way to a Breakthrough at the target power
CL 6 ≈ ~60% of the way to a Breakthrough at the target power
CL 10 ≈ Breakthrough-level effort (maximal)

What It Means

Challenge Level is best thought of as “how close am I to my limit during the work interval?”

Important: CL does not change the interval’s target power. Instead, it describes the fatigue state you’re working in while holding that target.

A helpful analogy is a 20-minute FTP test:

Intervals done at CL 1–2 feels like the first few minutes: the power might be hard, but you haven’t accumulated much fatigue yet.
Intervals done at CL 5–6 feels like the midpoint: you’re still pushing similar power, but fatigue is starting to build and the effort feels heavier.
Intervals done at CL 9–10 feels like the final 1–2 minutes: power is still roughly the same, but you’re very close to your limits—highly efficient training, but very challenging.

How It Works in Practice

In Magic Buckets, interval targets and recoveries are designed to help you hit your daily Low/High/Peak XSS targets efficiently.

Riding at a higher CL usually makes training more time-efficient, because strain accumulates faster when you’re closer to your limits.
Riding at a lower CL is more conservative and often takes longer, but can be more sustainable day-to-day.

Interval duration stays the same. If your work intervals are 20 seconds, they remain 20 seconds whether you’re at CL 2 or CL 8.

CL rises as you accumulate fatigue (typically when riding above Threshold Power) and falls as you recover (when riding below Threshold Power).

Recovery changes with CL: At higher CLs, recovery intervals often become shorter to help you maintain that fatigue level. If you recover too long/easily, your CL will drop—so Magic Buckets typically keeps recoveries tighter when you’re training deep.

Why it Matters

Challenge Level helps you balance:

time efficiency (higher CL = often less time), and
completion likelihood (lower CL = training is generally easier)

It’s especially helpful when:

you’re time-crunched and need a shorter session, or
you want a more controlled session on a day you’re not feeling 100%

Where You’ll See It

In the Magic Buckets widget on the Today Page — click through the various CL's to see the workout chart update along with the estimated workout duration & difficulty rating
The CL gauge is also visible on the real-time data fields, Garmin & Xert EBC.

Common Misunderstandings

CL is only relevant when you’re doing High-Intensity training

For pure endurance sessions, Challenge Level isn’t used because you’re only applying strain on your low intensity system & there are no High/Peak XSS targets.

CL is not directly a “difficulty rating.”

It’s a measure of how close you get to your limit at the interval power. That being said, doing your training at higher CL's will result in a higher difficulty scores.

Interval duration doesn’t change with CL.

The interval length stays the same—CL reflects how close you are to your ceiling when doing your intervals.

Higher CL isn’t always better.

It’s a tool for time-efficiency and motivation, but it can be more demanding and may not be appropriate every day.

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