Why 40sec Z5+ Reps on The Bike Matter

Short 40-second Z5+ reps on short rest are one of the best tools you can use on the bike to build anaerobic capacity, repeatability, and high-end power under fatigue. These are not just “hard efforts” — they target a very specific part of the energy system that most endurance athletes undertrain.

Let’s break down what they actually do physiologically and why they’re so effective. These intervals are designed to build anaerobic capacity, repeatability, and high-end power under fatigue, which are all critical for racing and for maintaining strength as an endurance athlete.

First, these efforts target the anaerobic glycolytic system, which is the energy system you use when the effort is too hard for oxygen delivery to keep up. Around 40 seconds is long enough that you burn through the quick energy stores and are forced to rely on anaerobic metabolism. This improves your ability to produce power above threshold, tolerate lactate, and keep working even when the legs feel flooded. This is exactly what you need for hills, surges, passing, racing into the wind, and finishing strong.

Second, the short recovery between reps is intentional. Because the rest is incomplete, your body has to learn how to produce high power while already fatigued. This builds repeatability, which is much more useful for racing than a single hard effort. In competition you rarely get full recovery — you surge, settle, surge again, and keep going. These workouts train your ability to clear lactate while still riding and to maintain coordination and form under stress.

These intervals also push your heart rate and oxygen demand very high, even though the focus is anaerobic. Because of this, they also help raise your VO₂ ceiling and high-end aerobic power, which supports both bike performance and overall endurance.

Another important benefit is that this type of work forces recruitment of fast-twitch muscle fibers, which endurance athletes often underuse. Training these fibers helps maintain power, strength, and muscle mass, which becomes especially important as athletes get older. High-force, high-intensity work like this helps preserve the ability to accelerate, climb, and respond to changes in pace.

There is also a mental component. These intervals create a very specific kind of discomfort — burning legs, heavy breathing, and incomplete recovery. Learning to stay relaxed and keep good form in that state is a skill, and it carries directly into racing situations where things get hard late in the event.

The reason we use about 40 seconds is because it sits in the ideal range. Shorter efforts rely too much on quick energy stores, and longer efforts become more aerobic. Around 30–45 seconds gives the best combination of high power, high lactate, and high oxygen demand, which makes it one of the most effective durations for building anaerobic capacity.

These sessions are most useful when we want to improve race strength, climbing ability, surge tolerance, and late-race durability. They are a very effective tool when used at the right time in the program, but they also need to be balanced with aerobic work and recovery so you can actually adapt to them.

These 40-second reps are there to make you stronger at the top end, more durable under fatigue, and better able to handle the unpredictable demands of racing.

Targets the Anaerobic Glycolytic System (your high-power engine)

40 seconds is long enough that you burn through phosphocreatine quickly and are forced to rely on anaerobic glycolysis.

That means you are training the ability to:

• Produce power when oxygen supply can’t keep up

• Tolerate high lactate levels

• Keep working when the legs are flooded

Adaptations:

• ↑ glycolytic enzyme activity

• ↑ buffering capacity

• ↑ lactate tolerance

• ↑ ability to produce power above FTP / CP

This is exactly what you need for:

• Attacks

• Hills

• Surges in racing

• Technical courses

• Short climbs

• Passing / responding to moves

Short rest = trains repeatability, not just peak power

If recovery is short (ex: 20–40 sec, 1:1, or incomplete recovery), you never fully clear lactate.

That forces the body to learn how to:

• Reproduce high power while fatigued

• Clear lactate while still working

• Maintain coordination under stress

This is huge for racing because races are never: one sprint → full recovery → one sprint

They are: surge → settle → surge → climb → surge → sprint

Adaptations:

• ↑ lactate clearance while riding

• ↑ fatigue resistance at high intensity

• ↑ ability to hold form under stress

• ↑ neuromuscular durability

Raises VO₂ ceiling indirectly

Even though these are anaerobic-focused, short hard reps with limited rest push VO₂ very high.

Because:

• Oxygen demand spikes fast

• Recovery is incomplete

• Heart rate stays elevated

• You accumulate time near VO₂max

This can improve:

• VO₂max utilization

• Oxygen delivery

• Stroke volume response

• High-end aerobic power

This is why these sessions often feel like: harder than VO₂ workouts but different

They sit in the overlap of:

• VO₂

• anaerobic

• neuromuscular

Improves recruitment of fast-twitch fibers

Most endurance athletes live in slow-twitch world.

40 sec Z5+ forces recruitment of:

• Type IIa fibers

• Type IIx fibers (if power is high enough)

Adaptations:

• Better motor unit recruitment

• Higher peak power

• More force per pedal stroke

• Improved acceleration

This matters for older athletes especially:

• Fast twitch declines with age

• High force work preserves it

• Helps maintain power and muscle mass

Builds mental tolerance to discomfort

These intervals hurt in a very specific way:

• Burning legs

• Breathing hard

• High tension

• No full recovery

That teaches:

• Staying relaxed under stress

• Holding form when it hurts

• Not backing off when lactate rises

That skill transfers directly to racing.

Why 40 sec is a sweet spot

Too short (<20s):

• Mostly phosphocreatine

• Not enough glycolytic stress

Too long (>1 min):

• Becomes more VO₂ / threshold

• Less peak power

~30–45 sec:

• Max glycolytic stress

• High lactate production

• High power output

• High VO₂

• Big neuromuscular load

That’s why coaches use:

• 30s

• 40s

• 45s

• 1 min (upper end)

40 sec sits right in the middle.

When these are most useful in a program

Best used when you want to build:

• Race punch

• Short climb strength

• Sprint durability

• Late-race power

• VO₂ support work

• Off-season / early build / pre-race sharpening

Less useful when:

• Athlete is exhausted

• Aerobic base is low

• Injury risk is high

• Too close to key race (unless very small dose)

8. Example set

• 2 × 8 × 40 sec Z5+

• 30 sec easy between reps

• 4–5 min between sets

or

• 10 × 40 sec hard

• 20 sec easy

• continuous block

or

• 3 × 6 × 40 sec

• 40 sec rest

• 5 min between sets

All of these build anaerobic capacity + repeatability

40 sec Z5+ on short rest trains the ability to:

• Produce high power

• Repeat it without full recovery

• Tolerate lactate

• Recruit fast fibers

• Hold form under fatigue

• Handle race surges

For endurance athletes, this is one of the best ways to build the missing top end without turning training into sprint-only work.