"My personal opinion is that there's absolutely no evidence that the CVAC system works. The duration of the stimulus is way too short for your biological systems to become activated," Levine says.
Levine is also concerned that the rapid changes in altitude could harm those using the CVAC pod. When you intermittently get low levels of oxygen, as opposed to adapting to a static high altitude, low oxygen state, it mimics the effects of sleep apnea, where sufferers periodically stop breathing while asleep. This oscillation between oxygen levels can cause high blood pressure, insulin resistance, and other negative side effects that Levine worries might also be a byproduct of using the CVAC chamber.
"Everyone is looking for Djokovic's secret, but his secret is he has a stupendous backhand, great intensity on the court and is just a great tennis player in general," Levine says. "I think every athlete is looking for an edge, but I don't think you'll find it with CVAC."
Others are slightly less skeptical: Dr. Laith Jazrawi, chief of sports medicine division at NYU Langone Medical Center, says that while the CVAC's effects need further study to really support its use, the logic of it fits into research on altitude training.
"While 20 minutes seems like a short time frame, any stress will allow for some changes. More studies need to be done to confirm some of the red blood cell changes, but there also other adaptive enzymatic changes that may not be so easily measured," he says.
The pod also might be having a placebo effect, as is the case with many athletes' secret training tips -- offering athletes with that extra boost of confidence they need to excel during competitions.
"Ultimately, the pod may be more of a psychological edge with mild improvement in stamina. However, at the professional level -- with extremely talented individuals -- any minor edge might make a big difference," says Jazrawi.