Hello. Gaining weight has become as difficult as loosing weight. One must try to slowly increase their food intake and appetite in order to increase their body weight. It is also important to note that the increase in body weight should not be through fat gain, but muscle gain. To increase muscle mass, one must consume healthy food rich in complete proteins, coming from low fat milk and milk products, lean cuts of chicken, blend of cereals and pulses. With proteins, complex Carbohydrates is also essential that is sourced from whole grain cereals and pulses. Lastly fats, that should be in limited amounts, coming majorly from invisible sources like nuts and oil seeds that are rich in MUFA and PUFA.
Next Steps
As far as exercise is concerned, yoga will help an individual to be fit, if you specifically want to strengthen your muscles, try to engage your muscles doing power yoga, muscle training, core strengthening, etc. This would help you achieve weight gain with muscle strength.