If you are working out in a commercial gym you have probably seen a lot of people especially men rarely training their legs. They would train chest, shoulders, back, arms and abs 4-5 times a week and even if they do legs its more with leg extension machines. As a result you will see well developed upper bodies but tooth picks for legs. 
“I run for my legs” “My legs are ready big” etc. etc. are the common excuses. Well skipping leg day is a mistake for those who want to achieve their fitness goals and improve performance. Here are the 5 reasons By Fareed Sayed WHY…!!! you should train legs:
1. More Muscle. You can find studies showing that Squats & Deadlifts increase the release of Growth Hormone & Testosterone more than any other exercise. These are natural muscle building hormones.
2. Bigger Upper-body. Squats & Deadlifts work your whole body, not just your legs. Quick examples of how they stimulate upper-body growth too:
a)Your arms squeeze the bar hard during heavy Squats & Deadlifts.
b)Your abs work hard at stabilizing the weight during Squats.
c)Your chest muscles tense hard during heavy Deadlifts.
d)If you can’t train your upper-body because of an injury, you can prevent muscle loss in your upper-body by doing heavy Squats & Deadlifts.
3. More Strength. Stronger legs and core muscles will increase your strength on upper-body lifts and thus help upper-body muscle development. Quick examples of how Squats & Deadlifts can cause strength gains on your other lifts:
a)Leg drive matters on the Bench Press. Stronger legs means more leg drive and a bigger bench, and thus bigger muscles.
b)Squats & Deadlifts strengthen your legs & lower back. These muscles help you stay strong during the Overhead Press.
4. Symmetry. I know I’m preaching all the time you shouldn’t care about what other people think, but a big upper-body with toothpick legs looks ridiculous.
5. … Mental Strength. Here’s the real reason why people don’t train their legs: it’s physically & mentally hard. That’s why doing Squats and Deadlifts will build your mental endurance & character like no other exercise.
Running Doesn’t Count! Running, cycling or soccer will never work your legs like Squats & Deadlifts do. Include both exercises in your routine and you’ll see how you’ll get better at sports and how your upper-body will improve.
Plus well trained legs is also beneficial for your bone health coz as you lift, the bone density becomes more and thus the chances of suffering from any bone degenerative diseases like osteoporosis and arthritis.
