Athletes commonly focus their off-season on building muscle. It's a great idea in theory, but a singular concentration on that area can lead to overtraining, which can heighten the risk of a chronic injury. (See Understand Overtraining to Avoid Performance Declines.) To combat this problem, I believe an athlete's off-season focus should be on a progressive sport-specific program.
For an athlete in any sport to build useful muscle requires sport-specific training. But that doesn't mean daily time on the field, court or ice. To build quality off-season muscle, first review your previous season and assess your weaknesses. From that knowledge, address how to correct them—along with any other incapacity that prevented you from playing your best. (See Todd Durkin on Adding Skill Development to a Workout.)
For instance, if a hockey player's personal off-season goal is to increase foot speed, he shouldn't perform heavy Olympic lifts during every training session. Progress doesn't come from hard work alone. It comes from quality work done in the right areas. Developing endurance, strength and power will help an athlete surpass his previous training level. Each individual is different, but in order to achieve a greater level of fitness, each must focus on his or her weaknesses.
That said, even following a progressive sport-specific program can lead to overtraining. It has probably happened to every athlete at least once in the off-season, without their even realizing it. Generally, we associate overtraining with plateauing, but there's more to it. Have you ever reached a point where you begin to lose strength, where the weight you lifted last week is suddenly too heavy? If this sounds familiar, guess what? You were overtraining.
The remedy is to construct an appropriate and realistic plan and stick with it. I promise if you take this approach, you'll be better next season than you were in the last.
Want to create your own plan? Check out these STACK articles to get started: