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Train at least 3 times a week. Work out hard. Practice explosive reps. Get plenty of rest between workouts. Prioritize weights over cardio.
Many people think that to build muscle you have to train every day, but that's not the case. Muscles get bigger when they get small "breaks" in training, and get stronger when they build up again during REST days. So don't exercise the same muscle groups two consecutive days. Rotate muscle groups so you're training your arms, back, chest, and other areas on the days you aren't working out your legs. Making sure to work out your other muscle groups is important when you're working on your legs. Don't neglect the rest of your body! Make your leg workouts count. They shouldn't last more than 30 minutes, but those 30 minutes shouldn't feel comfortable. You want to go all in and maximize the workout, putting a lot of pressure on your muscles so that they break down and build back up stronger.  For each exercise, you should use an amount of weight that you can lift for about 10 reps before you have to stop. If you can lift an amount of weight 15 times without stopping, you aren't lifting enough. If you can't lift it more than 5 times without needing a break, you're doing too much. Some trainers recommend "training to failure," which is the practice of repeating an exercise until you can't do even one more rep. This is said to build up the muscles more quickly, but it can cause injury if done incorrectly. Work with a trainer to figure out which technique is right for you. Many bodybuilders train with "explosive" movement, but this can cause injury if you push yourself too hard or use the wrong technique. If you're interested in these quick, forceful motions, take the time to learn them correctly:  Start with a lighter than usual weight. Always use a slow, controlled release on the eccentric (the lowering or lengthening part of the exercise). Pause and contract the muscles at the low point of the exercise. Explode into the lift or push. Start with a short range of motion and increase the distance gradually as you train. Keep your joints slightly bent at peak range of motion to prevent damage to connective tissues. Muscles get stronger during the time between workouts, when the fibers heal and strengthen. That's why it's mandatory to get plenty of sleep every night when you're training heavily. On the days when you don't work out, let yourself rest. Don't go on a 10-mile hike or bike all day long - it's okay to put your feet up and relax. Cardio promotes long, lean muscles instead of bulky ones. However, it takes many hours of running to undermine a heavy weight training routine, and 150 minutes of moderate cardio every week is still important for good health. If you are serious about bulking up your legs, get your cardio in by swimming or by using a rowing machine. You can limit cardio to 30 minute sessions after your resistance training, but do not skip cardio completely.