The definition of a multisport athlete and a single sport athlete are about as simple as can be. Sports are seasonal in nature. There are fall, winter, spring, and summer sports. Summer sports vary by state. For example, my home state (Iowa) held baseball and softball during the summer, whereas Ohio currently holds organized baseball and softball during the spring. So, regardless of how many seasons your state association holds, a multisport athlete would be an athlete who participates in two or more sports during the year. Perhaps they play a fall and winter sport. Or a winter and spring sport. In contrast, a single sport athlete focuses on one sport all year round. This is also known as specialization.
The application of a multisport athlete and a single sport athlete is also very simple. When a multisport athlete is in season, their focus and energy is devoted to training, practicing, and competing within that said sport. The other sports are put aside. If an athlete is not competing year-round in sports, their off-season is usually devoted towards developing for one primary sport with the possibility of some time being dedicated to a secondary or tertiary sport. Typically, the sport most trained for in the offseason is the sport that the athlete is the best at, enjoys the most, and is more likely to compete in at the next level (college).
A single sport athlete, in contrast, is devoted year-round towards improving themselves in one single sport. This is why they are called "specialized" athletes. Depending on the sport, the specialized athlete competes during the traditional middle or high school season and then either trains for the next season or competes in additional leagues throughout the year.
So, what happens when a youth athlete is specializing in multiple sports? What differences are we seeing between the multisport athlete of yesterday and the multi-specialization of today's athlete? What are the problems associated with this new approach to youth athletics? What roles do we as coaches, trainers, and parents have in its perpetuation? What role do we have in its prevention?
The following 5 categories often require extra consideration to properly support young athletes:
The role of the parent
The role of the coach
The role of the strength coach/private trainer