Background: Underweight persons have problems in increasing muscle size, body weight and also fitness levels. Resistance training [RT] has remarkable effect in increasing muscle size [hypertrophy].
Aim of the work: To determine the effect of resistance training on muscle strength, physical performance and psychological state in underweight females.
Patients and Methods: Sixty underweight females were recruited in this study. They were selected from the clinical nutrition department, Kafr Al Zayat general hospital. They aged from 20 to 30 years old with a BMI between 16 to 18.4 kg/m². They randomly distributed into two equal groups; Group A [Study group] [Resistance training group]: 30 female subjects received strengthening training exercises in the form of upper body exercises [push-up, dips, bridges and planks] and lower body exercises [squats and lungs], in addition to high caloric diet, and group B [Diet group] received only high caloric diet. BMI, muscle mass, muscle strength measurements, physical performance and Rosenberg self-esteem scale were assessed pre and post the study.
Results: There was a significant increase in all variables that represent BMI [2.92%], skeletal muscle mass [10,46 %], muscle strength measures for upper limb [shoulder abduction, elbow flexion, elbow extension  [15.21% , 55.16% , 26.50 %  respectively] and for lower limb [planter flexion, ankle dorsiflexion, knee extension [13.51%, 15.53%, 43.34% respectively], six minute walk test [6.90%], one minute sit-up test [43.99%] and Rosenberg self-esteem scale [37.05%] in resistance training  group [A] than diet group [B] but there was no significant difference between all these variables in between groups pre-treatment [p>0.05].
Conclusion: Resisted training has significant optimistic effects on muscle bulk, muscle strength and improving physical performance and psychological state for underweight females. Therefore, it is recommended as a basic program for underweight females.