WON'T GETTING STRONGER MAKE ME SLOW AND STIFF?
Speed is a function of force output to body weight. In simpler terms- leaner, stronger athletes will be faster.
Becoming stiff is due to a lack of mobility work.
Strength training paired with mobility work, increase joint range of motion.
WON'T LIFTING WEIGHTS DURING THE SEASON MAKE ME TOO SORE TO PERFORM WELL?
I've written a short article further explaining why this way of thinking is severely flawed. I'll be happy to share it with you.
If you strength train correctly, you won't be very sore. Most "gym bros" train to get sore. Good strength coaches minimize soreness without sacrificing progress.
WHEN IS IT APPROPRIATE TO START?
Research does not support rumors of strength training having adverse effects on growth development. The earlier you start, the better.
Injury in young strength-training athletes stems from improper technique, knowledge, and lack of appropriate supervision. The strength training itself is not problematic. As an example,
What does a young athlete do in physical therapy? Strength train!
'The best time to plant a tree is 20 years ago. The second best time to plant a tree is right now.'
WILL LIFTING HEAVY WEIGHTS HURT MY ARM?
Lifting heavy with poor form, poor exercise selection, and without proper mobility work and prehab will likely hurt your arm.
Properly periodized heavy lifting matched with appropriate movement selection will significantly reduce the risk of injury.
Appropriate strength training, proper mobility work, and efficient mechanics increase throwing performance.
WHAT SHOULD I EAT?
I can't think of a more nuanced topic than nutrition. There's conflicting evidence everywhere you look.
Eat plenty of protein (1g per pound bodyweight per day), eat things that come directly from the earth and animals, and eat a consistent amount (calories matter).
Nothing in nutrition is a simple yes/no, all or nothing answer. I'd be happy to answer more specific questions.
HOW DO I GAIN MUSCLE BUT NOT FAT?
To manage expectations- It is impossible to gain muscle without simultaneously adding some fat purely.
The goal is to minimize fat gain while still getting leaner (I can explain this concept further).
The short answer: Remain in a slight calorie surplus while training consistently, eating enough protein, eating well, and most importantly sleeping consistently enough.
HOW DO I LOSE FAT WITHOUT LOSING STRENGTH?
In a growing athlete, you never want to restrict calories much. It has negative growth implications and will wreck their metabolism.
The goal should be to very slowly improve body composition by increasing muscle mass.