What is the biggest fitness trend for 2019?
Respectfully, many years ago, I disconnected myself from the "trends." I reached that decision after trying and miserably failed "new trends" - usually pushed by some marketing plan selling new supplements or offering new ways of physical exertion/dieting, new products to buy, etc.
Sadly, people who are interested in the topic of trends and inclinations (it does not include you) are sitting in the sidelines and watching others, looking for the next trend.
I no longer care about new trends. Many years ago, I reverted to an old school method of rehabilitation, consumption. It works.