Wind is key for strong stems. It causes the plants to wiggle a bit and helps them to build strong stems.
It's May 8th and looking outside I can see some snow.
It's important to pre-grow tomatoes indoors otherwise you will
have mostly green tomatoes at the end of the summer. However plants pre-grown indoors tend to shoot up high with
rather slim stems that will easily break. This is because there is no wind indoors. Some wind is key for
ensuring strong stems.
In some years you can start pre-growing indoors and by May it's already warm enough to bring the plants outside.
This is the best way to get strong healthy plants. Outside during the day and inside at night. However
there could be a week in-between where that is not possible. I found that a computer fan placed on the side of
the plants can help a lot. I attach it to a timer and let it run every 30min with a pause of 30min. This way
you get strong plants even indoors.