A gym workout routine based around compound exercises, which engage multiple muscles simultaneously, is going to help you build muscles. These exercises are ideal for fast weight gain in men, but women generally gain muscle weight in smaller amounts, depending on body type. If you're a mesomorph, you'll gain muscle weight more quickly than women with endomorph and ectomorph body types. Muscle is denser than fat, so as you gain muscle, your weight increases.
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Barbell Bench Press
The barbell bench press will help you gain weight in your chest by working the pectorals. Lie on a bench press machine and grip the bar at wider than shoulder-width. Take the weight of the bar, and slowly lower it to about 3 inches above the chest. Feel the tension in your chest muscles, and then quickly power the bar back up to full extension, according to the Shape Fit website.
The barbell squat is one of the best muscle-gaining exercises. Take the weight of the bar on the back of your shoulders. Use padding if necessary for comfort. Squat in a slow and controlled motion. Upon reaching the point where the buttocks are parallel with the knees, come back up to start position.
Adopt a stance in front of a weighted barbell with your feet shoulder-width apart. Grip the bar with an overhand grip, using powder for grip purposes if necessary. Bend at the knees, keeping you back straight and head up at all times during the motion. Lift the bar quickly, puffing out your chest and rolling back your shoulders at the top of the motion where the bar reaches groin height. Slowly lower the bar to the floor for one rep.
Standing Military Press
The standing military press is an exercise that will add weight and mass to the upper back and shoulders. Grip a barbell wider than shoulder-width as it sits on a squat wrack at chest height. Position the bar at the front of the chest. Power the bar straight up to full extension, being careful to move your chin out of the way on ascent. As you lower the bar, do it in a slow, controlled motion.