<span>VO2 max is considered to be the most valid measure of cardiorespiratory fitness. It measures the capacity of the heart, lungs, and blood to transport oxygen to the working muscles, and measures the utilization of oxygen by the muscles during exercise. One way to test is using the Rockport Fitness test.</span>
Benzodiazepines are a central nervous system depressant, and have muscle-relaxing properties (such as valium). Opioids may also be considered CNS depressants, but I am not sure that they cause muscle relaxation.
