In a recent study, published in Nutrition Research, scientists from the University of Oklahoma tested the effects of a supposed synergistic cocktail of commonly prescribed preworkout supplements (C sinensis, arginine AKG, Kre-Alkalyn, citrulline AKG, Eleutherococcus senticosus, taurine, leucine, R rosea, sodium chloride, valine, isoleucine, caffeine, whey protein concentrate) on anaerobic performance and aerobic power. After familiarization and baseline testing, subjects completed two test days where they ran to exhaustion on a treadmill. On test days only, subjects drank either a preworkout drink or an isocaloric placebo 30 minutes before testing. The scientists reported that the preworkout energy supplement substantially increased anaerobic running capacity and time to exhaustion, with no difference in aerobic power. Although the authors did not test each ingredient separately in the study, they speculated that caffeine was likely the key player in increasing performance.
Fukuda DH, Smith AE, Kendall KL, Stout JR. The possible combinatory effects of acute consumption of caffeine, creatine, and amino acids on the improvement of anaerobic running performance in humans. Nutr Res. 2010 Sep;30(9):607-14.