The law of diminishing returns" applies at some point as you begin to lift heavier weight over time. There will come a point when lifting more weight will not help you kick harder or punch faster. At that future point in time, lifting weight beyond a certain poundage wouldn't help you, and may even slow you down, or make you less flexible. Now how much weight that is will depend largely on your genetics, stretching routine, etc,, but you should tailor your weight training to match your fighting style. Find out what the top fighters in your style are doing with weights and emulate them. Go as far as you can go with weights, then modify your routine and change things up.

No

Contrary to what many will tell you adding muscle won't slow you down if you continue to train. In fact, if you look at top athletes you'll see most have well developed muscle.

So just be sure to stretch, and do your workouts.

no, it wont, if you are that worried about getting slower combine it with olympic lifts which will increase speed and strength. like clean or snatch.

girls squat 90 lbs

It will not know its in the mind.

Will squatting with medium weight (say 90 pounds) affect my speed as a kickboxer?

I want to build muscle in my legs because they are too skinny, but I won't do it if it is going to make me slower in the ring.

Thoughts ?