A capsaicin capsule before lifting left leg press repetitions and jump height unchanged, and expectancy did not move them either

What the expectancy effect is

An expectancy effect is the change in performance that comes from believing a supplement will work, rather than from anything the supplement does. It is why a placebo arm exists at all. Adding a third arm where the participant takes nothing lets a trial separate the drug from the belief, because the placebo and the no supplement condition differ only in what the participant thinks they have taken.

Drawn from standard trial methodology, not from this paper.

Why they ran it

The authors note that experimental evidence suggests capsaicin supplementation improves calcium metabolism in skeletal muscle, which points to ergogenic potential, but that previous studies of the question carry numerous methodological biases. They set out to test the effect on countermovement jump height and strength endurance in trained individuals while addressing those limitations.

Drawn from the paper's introduction.

Sixteen advanced resistance trained men, averaging 24 years and 73.9 kg, completed one repetition maximum testing and then, after familiarisation sessions, performed four sets of leg press at 70 percent of one repetition maximum and countermovement jump tests under three conditions at least 72 hours apart: capsaicin at 12 mg, a matching placebo, and a control condition with no supplement at all. Capsules were taken 40 minutes before testing in a double blind, counterbalanced crossover.

Nothing separated the conditions. Total repetitions came out at 55 plus or minus 15 on capsaicin, 52 plus or minus 16 on placebo and 54 plus or minus 16 on control, at p 0.854. Jump height was 36.0 plus or minus 3.3 cm, 36.3 plus or minus 3.6 cm and 36.2 plus or minus 3.7 cm respectively, at p 0.976.

The expectancy probe came back empty as well. Neither positive nor negative expectancy effects from capsaicin ingestion were detected on the performance outcomes, both at p greater than 0.05.

The numbers

Total repetitions, four sets leg press at 70% 1RM	capsaicin 55±15 vs placebo 52±16 vs control 54±16, p=0.854
Countermovement jump height	capsaicin 36.0±3.3 cm vs placebo 36.3±3.6 cm vs control 36.2±3.7 cm, p=0.976
Expectancy effect on performance	neither positive nor negative detected, both p>0.05
Dose and timing	12 mg capsaicin, 40 minutes before testing
Washout between sessions	at least 72 hours

Why this might happen

Not available The full discussion is not openly available and the abstract offers no mechanism, so we do not know what the authors think caused this.

Drawn from no open full text was reachable, and no mechanistic measure was taken.

What this does not show

This is a failure to detect a difference, not a demonstration that the difference is zero. The repetition totals actually favoured capsaicin by about three repetitions out of roughly 52, and the analysis could not separate that from noise. No equivalence margin was declared, so the trial rules out a large benefit far better than it rules out a small one.
It does not show that capsaicin does nothing for anyone in any setting. One acute amount, taken 40 minutes before one session, was tested on two lower body tasks in men who were already advanced lifters. Other amounts, longer loading periods, other tasks and less trained people were not examined.
The absent expectancy effect does not mean belief never shapes supplement trials. Expectancy was probed inside this particular protocol, with these particular tasks and this particular group. A null expectancy signal here says nothing about protocols where the task is longer, more fatiguing or more open to pacing decisions.
Nothing here speaks to what capsaicin does inside muscle. The trial measured repetitions and centimetres. Calcium handling, oxidative stress markers, substrate use and any other physiology that motivated the question in the first place were not measured, so the performance null cannot confirm or rule out an underlying biological effect.

Where this leaves us

The ergogenic case for acute capsaicin, which rested on calcium handling work in muscle, now has a cleanly designed performance trial against it, and the belief effect that could have rescued a positive result was looked for and not found.

Advanced resistance trained men in their twenties, one acute dose, two lower body tasks.

A trial with a declared equivalence margin and enough participants to hold it would turn this from a failure to detect into a statement about how small the effect can be.

Caveats worth holding

Sixteen participants, all male and all advanced lifters.
No equivalence margin and no power calculation are reported, so the trial constrains a large effect far better than a small one.
Acute single dose only; no loading period was tested.
No open full text was reachable, so this rests on the abstract.