In a study of cyclists, a believed placebo of caffeine reduced performance output by 1.4%. In a separate caffeine placebo study, an unbeknownst placebo provided 23.8% increased leg-extension load (Hurst et al., 5-6).
In studies with mechanical aid, the use of placebo TENS units increased finger extension force capacity by 13.8%, whereas the non-placebo TENS counterpart increased finger extension force by 14.4% (Hurst et al., 7).
Nutritional and mechanical aid placebos were found to provide small benefits and placebo steroids and TENS units had the greatest effect on performance (Hurst et al., 8-9). In conclusion, the effect of a placebo depends upon the type of placebo received (Hurst et al., 9).
Hurst, Philip, et al. “The Placebo and Nocebo Effect on Sports Performance: A Systematic Review.” European Journal of Sport Science, vol. 20, no. 3, 2019, pp. 279–292., doi:10.1080/17461391.2019.1655098.