Captain America became the ultimate in human physical perfection because he was injected with a "Super-Soldier" serum and then was zapped with "via-rays." The Department of Defense is actually working on a project that sounds quite similar -- minus the via rays. In the early part of the decade, researchers were investigating how to build a "Metabolically Dominant Soldier." Certain drugs could allow a person to could go for days without food, for example.
One drug company, Rinat (later acquired by Pfizer) was working on a "pain vaccine" that would basically make soldiers impervious to pain for weeks on end. (That research eventually led to trials of a new arthritis drug, Tanezumab). Genetic engineering and gene therapy may also lead to soldiers that sleep less or have better control of their body temperature.