strengthen
     v 1: make strong or stronger; "This exercise will strengthen your
          upper body"; "strenghten the relations between the two
          countries" [syn: {beef up}, {fortify}] [ant: {weaken}]
     2: gain strength; "His body strengthened" [ant: {weaken}]
     3: give a healthy elasticity to; "Let's tone our muscles" [syn:
         {tone}, {tone up}]
Source: WordNet® 2.0