Placebo-induced changes in force production for types of muscle contractions

Purpose: To test the impact of a placebo drink on acute force production during isometric and isokinetic leg extensions in male and female college students. Methods: Nine male and five female subjects apparently healthy and free of leg injury completed familiarization testing and two counterbalanced trials. In one trial, participants were told they were consuming a performance-enhancing drink, although the drink contained only flavoring. In the other trial, participants were not given any drink (control). Both trials then included concentric and eccentric strength tests performed at 60 degrees per second, and isometric strength tests with the knee at a 70 degree angle. A repeated-measures analysis of variance (ANOVA) was performed with treatment (placebo, control) and contraction (concentric, eccentric, isometric) as the within- subjects effects. Post-hoc testing was performed using polynomial contrasts. Results: There was no significant treatment effect for the drink for concentric (132.9 33.8 vs. 130.5 35 Nm), isometric (139.7 36.5 vs. 136.6 28.9 Nm), or eccentric (190.9 50.3 vs. 195.3 55 Nm) quadriceps contractions compared to control, and no treatment x contraction interaction. Eccentric contractions exhibited significantly higher peak torque compared to concentric or isometric contractions (p \u3c 0.05). Conclusion: Findings suggest the placebo effect may not play a significant role in isokinetic or isometric contractions. Findings add to recent, but limited evidence that the placebo effect may not be as universal as currently thought. Future studies should investigate the difference between placebo-induced improvements of isotonic and isokinetic contractions