1. Given added strength or support
   "strengthened concrete contains steel bars or metal netting";
   - reinforced

1. Make strong or stronger
   "strengthen the relations between the two countries"; "This exercise will strengthen your upper body";
   - beef up [informal], fortify
    
2. Gain strength
   "His body strengthened";
   - toughen
    
3. Improve muscle strength and firmness through exercise
   "Let's strengthen our muscles";
   - tone, tone up

See also: strong

Type of: alter, change, exercise, modify, work out

Antonym: weaken