Over the past few months I’ve been reading The Nature of Training by Manuel Sola Arjona, and it’s helped crystallise a lot of ideas I’ve quietly held for years. The book helped clarify what I’ve long sensed instinctively - that athletes are complex, living systems with patterns we can sometimes predict and moments that simply can’t be explained.

Around the same time, I read a Substack article by the same author about the pitfalls of single-day fitness testing. It echoed the exact patterns I’ve witnessed again and again - athletes producing wildly different numbers on different days despite being in the same training block, under the same conditions, with supposedly the same level of ‘fitness’.

I’ve seen these variable patterns of behaviour unfold for years and have learned that athletes aren’t predictable. They’re adaptable, they’re variable and they’re complex. And because of that, single-day tests often fail to capture what’s truly going on.

The Problem with One-Off Fitness Tests

For decades, endurance sport has viewed the body like a machine, performing the same way every day, under the same fuel, same routine, same environment. But anyone who’s coached real people knows that isn’t how it works. You’re not a machine, you’re an organism. Constantly adjusting, adapting, responding, compensating and shifting. And because of that, a single fitness test is simply a snapshot, not a reliable picture of who you are as an athlete.

Here’s why…

1. Your Body Changes from Day to Day

Even if your training is consistent, your daily physiology isn’t. Performance can swing noticeably depending on:

sleep
mood
stress levels
accumulated fatigue
hydration
fuelling
muscle soreness
hormones
environment
cognitive load
motivation

In other words, even a ‘controlled’ test has uncontrolled variables. A single maximal test might catch you on:

a great day (inflating your perceived fitness),
an average day, or
a day where your system just isn’t firing.

It doesn’t tell you your true capacity, it tells you who you were in that particular moment.

2. Humans Are Complex, Adaptive Systems. Not Linear Engines

Training adaptations don’t follow a straight line. They’re influenced by dozens of interconnected systems:

aerobic metabolism
neuromuscular recruitment
autonomic nervous system state
heat load
emotional regulation
endocrine function
psychological arousal
nutrition and energy availability

These systems don’t always peak together. Some adapt quickly, others lag, some fluctuate based on stress or life factors. This is why two tests a week apart can produce completely different outcomes, even if your fitness hasn’t changed at all. Complex systems fluctuate and that’s not a problem – that’s normal.

3. A Single Test Can’t Capture the Bigger Picture

A one-off test:

can mislead you if it’s a ‘bad’ day
can inflate your confidence if it’s an unusually ‘good’ day
ignores the direction of your adaptation
ignores the reality of normal biological ups and downs

You wouldn’t judge your health based on one blood pressure reading. You wouldn’t judge your mood based on one hour. Yet in sport, we often judge fitness from one test.

It’s no surprise athletes are often confused: ‘How can I feel fitter but test worse?’ or ‘How can I test better but race poorly? Because the test isn’t measuring the whole system, just that moment.

A Better Way: Look at Trends, Not Snapshots

Instead of hanging everything on a single maximal effort, a more honest way to assess fitness is serial, lightweight, repeatable assessments

short bike tests
submaximal pace/HR checks
recurring swim indicators
occasional maximal anchors for high-intensity zones

None of these on their own tell the full story, but together they reveal a pattern, and it’s the pattern that shows whether you’re adapting. It’s the pattern that helps predict fitness adaptation and race performance. And it’s the pattern that respects the reality of how human physiology works.

Why Trends Work Better

1. They smooth out the ‘good-day/bad-day’ noise

Averages do what biology does naturally, they balance out variability.

2. They show the direction of your adaptation

Are you trending up?
Flat?
Down?
Recovering?
Stagnant?

You can’t see that from a one-off test.

3. They respect real life

You’re juggling work, training, family, stress and environment - not living in a lab. Trend-based monitoring embraces that reality.

What This Means for You as an Athlete

Stop treating a single test as a verdict.
Stop assuming a bad day means regression.
Stop assuming a great day means you’ve suddenly levelled up.
Start looking at multiple data points.
Start using submaximal indicators regularly.
Start paying attention to context and how your body is responding over time.

Most importantly, trust the pattern, not the snapshot.

Final Thought

Athletes aren’t predictable they’re adaptable, they’re variable and they’re complex.

If we want to understand our fitness honestly and usefully, we have to think in terms of trends, patterns and the overall story, not isolated high points or low points.

What counts is the work you repeat, not the occasional standout session. Embracing complexity means moving past reductionism - the outdated idea that one metric or one test can define an athlete’s fitness.

Bevan McKinnon / December 2025

Why One-Off Fitness Tests Don’t Tell You the Truth About Your Fitness