Many people, especially beginners, make one huge mistake that will seriously hinder their aim for building muscle and gaining weight. Yes they are willing to push out endless sets on the bench press and bicep curls but they do not train legs. This is one major mistake in the bodybuilding world. There are many reasons why people neglect training the lower body most likely because it is one of the hardest muscle group to train, we have more control on tensing the upper body and we can see the "pump" in the chest, triceps, biceps, shoulders etc but not so much on the legs. Now an excuse I often hear thrown around the gym is that when a young gentleman is asked "how often do you train your legs?" He then replies "I don't need to because I play football twice per week" Bull!! No matter what sport you are doing if you want to gain muscle mass and put on weight then training legs is a must! If anything training legs would help improve the power, strength and endurance of his legs when playing football, hence improving his game but basically it is an excuse.
So why train legs to make big muscle gains? Firstly, your lower body is around 60% of your full body so the chances of you growing massive upper body with little puny legs are slim to none! Hitting legs hard will promote muscle growth in all areas of your body. Studies have shown that squats and deadlifts provide your body with a larger burst of testosterone and growth hormone than any other exercise and it is this that allows all of your muscles to benefit from the extra testosterone. So if you're not squatting then start NOW! That extra burst of testosterone is the fastest way to build muscle mass.