Out of all the tops 5 major muscle groups of the body (chest, back, legs, shoulders, arms), the leg sessions are typically the hardest for most people in that the leg muscles make up more muscle than anything else and it is the biggest muscle group in your entire body. This is likely the cause for the pain leg workout cause.
Working out causes micro-tears in our body (resulting in a bit of pain your feel during workouts) that go on to repair themselves bigger and stronger in the healing process. This recovery process is responsible for the soreness felt after completing difficult training sessions. Since the leg muscles are the biggest, the micro-tears hurt the most.
With this being said, do not worry about going extremely heavy when training legs since they are also the strongest muscle group (you can squat a lot more weight than you can bench press, for example)
Leg training (aside from calf raises and abs like crunches or planks) can be simplified into quads and hamstrings. When performing squatting movements like the basic barbell squat or a simple bodyweight squat, you are training both of them simultaneously, but the body typically favors the quad. Therefore, it is good to finish leg sessions with hamstring-dominant movements like lunges or romanian deadlifts.
Check out this PSA on the significance of LEG DAY Training! [VIDEO]
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