Purpose: This study aims to quantify the magnitude of the placebo effect of an injected placebo ("OxyRBX") purporting to have effects similar to those of recombinant human erythropoietin on endurance running performance in "real-world" field-based head-to-head competition settings.
Methods: Fifteen endurance-trained club-level men (mean ± SD: age, 27.5 ± 6.8 yr; body mass index, 22.9 ± 2.0 kg·m) with 10-km personal-best record times of 39.3 ± 4.4 min completed a randomized cross-over study of 3-km races before and after 7-d "control" and "placebo" phases. During the placebo phase, participants self-administered subcutaneous saline injections daily, believing it to be OxyRBX, with no intervention during the control phase. At the start and end of each 7-d phase, 3-km running performance was assessed. Qualitative assessments of participants' perceptions and experiences were recorded throughout and during semistructured interviews on completion.
Results: Race time improved significantly more in response to placebo intervention (9.73 ± 1.96 s faster, P = 0.0005) than in response to control (1.82 ± 1.94 s faster, P = 0.41; Pinteraction = 0.02). In response to placebo, participants reported reductions in physical effort, increased potential motivation, and improved recovery. Beliefs and congruence between positive expectations of the effects of placebo and perceptions of physical change during training also appeared to impact on competitive performance.
Conclusions: Compared to control, the injected placebo improved 3-km race time by 1.2%. This change is of clear sporting relevance but is smaller than the performance improvement elicited by recombinant human erythropoietin administration. Qualitative data suggest that placebo may have improved performance by both reducing perception of effort and increasing potential motivation, in accord with the psychobiological model for exercise performance, and that cognitive and noncognitive processes appear to have influenced placebo response.