The Cage

One of the biggest things I see with amateur players especially in the northeast when they are inside in a cage for most if not all of the offseason is the hitter works on a swing that he doesn't use in the game. You have to create a routine for yourself in the cage. Obviously you want to start off the tee or with some swings with front flips at 50% percent. Then you should go to a round or two of bat control where you are working on the path of your swings and letting the ball get deep and going the other way. However, after you are loose you need to practice the swing you will take in the game. The video above is of a young Bryce Harper in 2009. He has made some mechanical adjustments to his swing since then but you can see the intent and focus in this cage session. He is also swinging hard like he would in the game. As a hitter you can trick yourself in the cage because if you hit a line drive on the barrel it looks great in the cage with a 60% swing and it is easier to be accurate. That's not the swing you are going to take in the game.
As a hitter you want fast twitch muscles and you need to train and develop those fast twitch muscles. You can only train those muscles if you are swinging at close to 100%. Find out how hard you can swing and still be accurate with the barrel. This varies between hitters based on coordination and body type. Some like Javier Baez or Josh Donaldson can swing at close to 100% where a guy like Adrian Gonzalez swings near 80%. Find your ideal swing effort and you can only find that if you allow your body to swing at 100% and then work down from there.
Remember that anyone can hit off the tee or front flips or bp and make contact on the barrel. You can separate yourself if you develop that ability to be accurate while also swinging close to as hard as you can. As you take ownership of your swing understand that your swing will evolve into a dynamic swing if you have good focus in the cage on each rep and you are determined to push the limit with the amount of bat speed you can create.
Here are a few points of emphasis as you experiment with this concept:
 1)be slower/earlier with your load so that your body has time to be dynamic during the load prior to landing.
2) Don't worry about the results of an individual rep. Don't be afraid to look bad in the cage as you experiment. You will never get a hit in the cage but you can become a great hitter in the cage. Evaluate your progress and focus, not the results
3) In the off season there is always something new to experiment with and fine tune. Be willing to experiment and work on different parts of your swing. You should never have a day in the off season where you are just taking mindless swings to "get your work in"
4)Swing hard and keep trying to be faster. You can never have too much bat speed.
5) Remember why you are in the cage: your goal should be to prepare to mash baseballs in the game. Kenny Powers has a ridiculous quote in Eastbound and Down when he said, "I play real sports, I'm not trying to be the best at exercising" The point relates to the topic today, "You want to rake in the game, you're not trying to be the best at hitting off the tee or taking flips or bp in the cage."
6) The test will be when you get around your teammates, the most discipline hitters create the same focus on getting better when they are in a small group as they do when they are at practice with the whole team. Your teammates opinion of your swing ability in the cage should be irrelevant to what you are working on. This is way easier to say then do but as I said above continue to remind yourself of your goals and what you are trying to accomplish in the games this spring and that will help you keep perspective and focus in your daily work.

Below is a few gifs of big leaguers working on the swing they want to take into the game during practice.