Zourdos 2016 — 1RM fluctuates ±3–5% day-to-day in trained lifters

Zourdos and colleagues ran a 36-day case series on three trained powerlifters / weightlifters, testing 1RM strength daily. The point of the study isn’t the long-run trend (though all three athletes did get stronger over the month). The point is the noise — daily 1RM readings swing ± 3–5 % from one day to the next, even with no change in programmed load. That noise is precisely why fixed-percentage prescription mis-loads the bar most days.

How to read this chart

X-axis is days, 1 to 36. Y-axis is percentage change in 1RM relative to day 1. Each line is one athlete, all anchored at 0 % on day 1. The signal-lime line ends highest (+9 %), the teal line in the middle (+5 %), and the dim line lowest (~+3 %) — but the long-term ranking matters less than the shape of each line.

Look at any single line and trace day-to-day. The local jaggedness — the ± 3–5 % swings between adjacent days — is the practical takeaway. When yesterday read 5 % above day 1 and today reads 0 %, the athlete didn’t get weaker; they’re just inside the normal noise band of daily readiness.

When to use this evidence

• Defending auto-regulation. A program that prescribes “85 % 1RM” five days in a row treats day-to-day fluctuation as zero. The chart shows it isn’t. Daily readiness drifts.
• Justifying VBT to a sceptical lifter. If 1RM swings by 3-5 % each day, your “85 % 1RM” target load is off by ± 5 % most sessions. Reading bar speed corrects that mis-load in real time.
• Coaching mental framing. A bad lifting day isn’t a regression. It’s the normal bottom of the daily-readiness wave. The next session is likely to bounce back.

Why daily 1RM moves so much

Strength on any single day reflects the integrated state of the whole system: sleep, hydration, glycogen, central-nervous-system fatigue, mood, hormonal cycles, even ambient temperature. Each of those moves on its own clock. The combined effect averages out over weeks, but on a single morning it can produce ± 5 % variation in true working capacity. Programs that don’t account for it are betting against the noise.

Pitfalls

• Three athletes isn’t a meta-analysis. This is a case series, not a controlled trial. The qualitative pattern (high day-to-day noise) is robust across the broader literature; the specific magnitudes vary.
• The trend line matters. All three athletes climbed over 36 days — the noise is overlaid on actual strength gain. Don’t read the noise as proof that training doesn’t work.
• Daily 1RM is brutal. This study was a research protocol; you wouldn’t program daily max attempts in real training. The principle generalises to working loads that are near daily max — they fluctuate too.

Where to go next

For the practical “how to auto-regulate against daily readiness” guide, see Readiness and autoregulation with VBT. The Load–velocity profile chart and its interactive generator are the primary tools for catching daily fluctuation in working sets.