You can learn a lot about someone from a fitness assessment:

• • VO₂ max.
  • Strength.
  • Body composition.
  • Blood pressure.
  • Resting heart rate.
  • Movement quality.

Those numbers help describe capacity (what the body is capable of doing under relatively controlled conditions).

But there is another question that may matter just as much:

What happens to that capacity when you are tired, stressed, traveling, under-slept, several hours into a demanding day, or coming off three difficult days in a row?

That is where performance durability becomes important.

In endurance physiology, durability is an emerging concept describing how resistant an athlete's physiological characteristics are to deterioration during prolonged exercise.  Two people can begin an event with similar VO₂ max values, thresholds, and efficiency but experience very different declines as fatigue accumulates (Maunder et al., 2021).  More recent work has reinforced the idea that measuring an athlete only while fresh may miss an important part of the performance picture (Hunter et al., 2025).

For executives and adults over 40, I think the concept has an even broader application.

Capacity tells us what you can do at your best.  Durability tells us how much of that capacity remains available when life starts pushing back.

And life almost always pushes back.

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Executive Insight

Your Best Day Is Not the Test

Imagine two executives.

Both are 48 years old.

Both can run a strong 5K.

Both strength train three times per week.

Both have respectable body composition, normal blood pressure, and good laboratory results.

On paper, their performance profiles look almost identical.

Then both leave Monday morning for a three-day business trip.

They take an early flight.

Sleep six hours instead of eight.

Spend most of the day sitting in meetings.

Eat restaurant meals.

Have a late dinner with clients.

Wake up early Tuesday.

Work another ten-hour day.

Fly home Wednesday night.

Executive A is tired, but still functional.  Thursday's workout is adjusted slightly, but completed.  Energy normalizes quickly.  By Friday, he feels close to baseline.

Executive B is wiped out.  Training disappears for four days.  Soreness lingers.  Sleep deteriorates further.  Motivation drops.  He feels sluggish through the weekend and needs nearly a week before his normal routine returns.

Their maximum capacity may have been similar.

Their durability was not.

That difference becomes increasingly important when your goal is not merely performing well during a fitness test, but maintaining physical and cognitive capacity across decades of work, travel, family responsibilities, training, and aging.

Capacity vs. Durability

Think about a successful company.

Revenue during ideal economic conditions tells you something about the business.

But what happens when a major client leaves?

Expenses rise?

Several employees are unavailable?

Deadlines compress?

The market changes unexpectedly?

The strongest company is not necessarily the one that produces the greatest result during perfect conditions.

It is the one that can absorb disruption without its entire system falling apart.

Human performance works similarly.

Your VO₂ max represents aerobic capacity.

Your squat or deadlift demonstrates strength.

Your power output demonstrates explosiveness.

Your laboratory markers give information about health and metabolic function.

But those measurements usually happen under relatively controlled circumstances.

Durability asks another question:

How much of those abilities remain available after stress begins accumulating?

This distinction matters because fatigue is not one single physiological event.  Fatigue emerges from interactions between the nervous system, working muscle, metabolism, perception, motivation, and the demands of the task itself.  As Enoka and Duchateau (2016) explain, the relationship between fatigue and actual performance is complex; two people can experience the same external workload while showing very different reductions in performance.

That is exactly why someone's maximum numbers do not tell the entire story.

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Why Performance Declines as Fatigue Accumulates

Every demanding task uses resources.

A workout creates stress.

So does a difficult workday.

So does poor sleep.

So does psychological stress.

So does travel.

The body does not necessarily categorize them into neat boxes labeled "business stress" and "training stress."  They overlap.

A particularly interesting study by Stults-Kolehmainen and Bartholomew (2012) found that people reporting greater psychological stress recovered muscular force more slowly following strenuous resistance exercise...even after accounting for factors including workload, fitness, and training experience.

That matters for anyone who has ever wondered:

Why did a workout that normally feels easy suddenly feel terrible during an extremely stressful week?

The workout may not have changed.

The environment surrounding the workout did.

That is an important part of durability.

Your body is always responding to the total demand placed upon it, not simply the workout written on today's program.

Aerobic Durability

Can You Maintain Output?

One of the clearest examples of durability comes from endurance sport.

Traditional endurance profiling often evaluates VO₂ max, threshold, and economy while an athlete is relatively fresh.  But Maunder et al. (2021) argued that these characteristics can shift during prolonged exercise...and that the timing and magnitude of that deterioration may itself be an important performance characteristic.

Hunter et al. (2025) later described durability as resilience against deterioration in physiological and performance variables during or following prolonged exercise.  Importantly, they also emphasized that exercise intensity, duration, nutrition, and environmental conditions all affect how durability appears.

You do not need to be a marathon runner for this to matter.

Imagine cycling at the same workload for 90 minutes.

During the first 20 minutes, your heart rate is 125 beats per minute and the effort feels easy.

Seventy minutes later, maintaining the same power requires a heart rate of 140 and substantially more perceived effort.

The external workload has not changed.

Your internal cost has.

Improving aerobic durability means becoming better able to maintain useful output without an excessive rise in physiological cost.

For an executive, that might translate into something much less glamorous than winning a bike race.

Walking through an airport without feeling drained.

Playing golf after a long workweek.

Hiking on vacation.

Training after work.

Keeping up with your children.

Or simply reaching 5:00 p.m. with energy still available.

Strength Durability

What Happens After Repeated Effort?

Maximum strength is valuable.

I have written before that strength functions almost like a retirement account; build enough early, maintain it, and you create a larger reserve for later life.

But there is another layer.

Can you repeatedly access that strength?

Carrying one heavy suitcase once may be easy.

Carrying luggage through an airport, loading it into a car, walking through a hotel, sitting through meetings, training the following morning, and doing it again the next day creates a different demand.

Durability is not simply asking:

How strong are you?

It asks:

How quickly does your usable strength deteriorate?

That is why a complete training program should not consist exclusively of testing or training maximum output.  There is also value in moderate loads, repeated efforts, unilateral training, carries, circuits, controlled conditioning, and longer submaximal work.

You are teaching the body to produce force, but also to continue producing useful force after the first effort is over.

Movement Durability

Does Your Technique Survive Fatigue?

Watch someone squat during the first set of a workout.

Then watch their final repetitions after fatigue accumulates.

The movement may look completely different.

The knees shift.

The torso position changes.

Range of motion decreases.

Tempo becomes inconsistent.

Coordination deteriorates.

That tells you something important.

Movement quality is not simply whether you can perform a movement correctly once.

Movement durability is whether you can continue controlling that movement as fatigue increases.

The objective is not to train until technique collapses. Poor repetitions are not a badge of honor.

The objective is to progressively increase the amount of high-quality work you can tolerate before technique begins deteriorating.

That creates usable fitness rather than fitness that exists only while completely fresh.

The Sleep Test

Perhaps nowhere is durability challenged more obviously than through inadequate sleep.

In a systematic review and meta-analysis covering 69 publications, Craven et al. (2022) found that acute sleep loss negatively affected exercise performance across multiple categories, including strength, endurance, high-intensity exercise, power-related performance, and skill.  Across the pooled outcomes, sleep loss was associated with an average performance reduction of approximately 7.6%, although the magnitude varied substantially between studies and conditions.

Sleep loss also becomes more problematic when it accumulates.

In a classic controlled experiment, Van Dongen et al. (2003) restricted healthy adults to four, six, or eight hours of time in bed for 14 consecutive nights.  Cognitive performance deficits progressively accumulated in the sleep-restricted groups, even though people were not always fully aware of how impaired they had become.

That last point is particularly relevant to high-performing professionals.

You can feel like you are functioning normally while objective performance is quietly deteriorating.

Durability does not mean becoming immune to sleep loss.

Nobody wins that battle indefinitely.

It means building enough capacity, maintaining good enough habits, and managing workload intelligently enough that one poor night does not automatically destroy the following three days.

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The Travel Test

Travel provides another useful real-world stress test.

Flying itself can mean prolonged sitting, unusual meals, dehydration risk, disrupted routines, early departures, late arrivals, and shortened sleep.  Crossing time zones adds circadian disruption.

Research on travel and athletic performance remains imperfect, but reviews have documented enough evidence to recognize travel fatigue and jet lag as legitimate challenges to sleep, physiology, and performance (Leatherwood & Dragoo, 2013).

The executive version is easy to recognize.

You land late.

Your body thinks it is midnight.

Your calendar thinks it is dinner.

You sleep poorly.

Breakfast happens at the wrong biological time.

Then you are expected to give a presentation at 8:00 a.m.

This is where physical performance and professional performance begin to overlap.

You are no longer asking whether you can perform when your schedule is optimized.

You are asking whether your system can absorb disruption without completely losing structure.

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Fitness That Only Exists Under Perfect Conditions Is Fragile Fitness

This may be the most important concept in this Brief.

Many people unknowingly develop fitness that depends on everything going right.

They need their normal gym.

Their normal breakfast.

Their normal bedtime.

Their preferred workout time.

Their normal equipment.

A low-stress workday.

Eight hours of sleep.

Nothing is wrong with optimizing those conditions.

You should.

Sleep matters.

Nutrition matters.

Recovery matters.

Routine matters.

But real life will occasionally remove them.

A child wakes up during the night.

A flight gets delayed.

A meeting runs long.

A parent becomes sick.

Work becomes unusually demanding.

Your hotel gym has three dumbbells and a treadmill from 1997.

Durability means your system bends without completely breaking.

It does not mean ignoring recovery.

It means having enough physical capacity and behavioral flexibility that disruption becomes a temporary adjustment instead of the beginning of a two-week collapse.

That creates an important distinction:

Recovery asks: How quickly can I return toward baseline?

Reserve asks: How much excess capacity do I possess?

Durability asks: How much of that capacity remains accessible while stress is accumulating?

Together, those concepts describe something much closer to real human performance than a single fitness score ever could.

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Why High Fitness Does Not Automatically Mean High Durability

A high VO₂ max is excellent.

Being strong is excellent.

Having low cardiovascular risk is excellent.

But none of them guarantees that your performance will hold up equally well under fatigue.

That is partly why durability has attracted attention in exercise physiology.

Two athletes can begin with similar physiological profiles yet experience different rates of deterioration during prolonged work (Maunder et al., 2021).

The same principle likely applies more broadly in life.

One individual may tolerate three challenging days and rebound quickly.

Another may need several days to recover.

Age, training history, health status, sleep, nutrition, psychological stress, recent workload, environmental conditions, and individual biology can all influence the response.

So the objective is not to compare your durability with someone else's.

It is to determine:

What causes your own performance to begin falling apart and can you move that point farther away?

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How to Measure Your Own Durability

You do not need a laboratory.

And there is currently no universally accepted "Executive Durability Score."

Instead, look for performance drift.

Use repeatable conditions whenever possible and observe what changes as time or workload accumulates.  You might monitor the heart rate required to maintain the same walking, running, cycling, or rowing pace; how perceived effort changes during a longer session; whether movement quality deteriorates late in a workout; whether usual strength performance remains available after several demanding days; and how quickly resting heart rate, sleep, soreness, energy, and normal training performance return toward your baseline after travel or unusually stressful weeks.

The goal is not perfect numbers.

The goal is recognizing patterns.

If a two-day trip reliably destroys your training for a week, that tells you something.

If one difficult workout leaves you sore for five days, that tells you something.

If your normal walking pace suddenly produces an unusually high heart rate after several stressful nights, that tells you something.

Data become valuable when they help you adjust the plan.

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Executive Action Plan

Build a Body That Holds Up

Durability is built gradually, not through constantly beating yourself up.  A useful approach is to build a large aerobic and strength foundation, spend plenty of training time below your maximum, progressively increase the amount of quality work you can tolerate, practice maintaining technique when moderately fatigued, preserve sleep and fueling whenever possible, and occasionally train under realistic, but safe, less-than-perfect conditions rather than requiring every workout to occur in a perfectly controlled environment.

Most importantly, do not confuse durability training with deliberately depriving yourself of recovery.

You do not become better at functioning on four hours of sleep by repeatedly sleeping four hours.

You do not become more resilient to excessive stress by remaining chronically overloaded.

Durability comes from building capacity and then giving the body enough recovery to adapt.

That distinction matters.

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High Performance Is What Remains Available

You should absolutely work to improve your maximum capacity.

Get stronger.

Improve your VO₂ max.

Build muscle.

Improve your metabolic health.

Improve your mobility.

Increase your performance reserve.

But eventually another question becomes important:

How much of that fitness is still available when life stops cooperating?

Because high performance does not happen exclusively after eight hours of sleep, perfect nutrition, and a stress-free week.

It happens during travel.

During deadlines.

During long days.

During difficult weeks.

During periods of family responsibility.

During aging.

During seasons when everything cannot be optimized simultaneously.

The goal is not to become invulnerable to stress.

That is impossible.

The goal is to build a system with enough capacity, enough reserve, and enough adaptability that normal disruption does not immediately overwhelm it.

Build your ceiling.

Build your reserve.

But also build the ability to hold onto your performance when conditions become imperfect.

Because ultimately:

Your ceiling determines what you can do at your best. Your durability determines how often you can actually live near your potential.