Resistance training, especially to improve vertical jumping ability, is important for basketball players, not only in the off-season, but also during the competitive season. Strength gained in the off-season can be gradually lost if resistance training is halted.
Maintaining muscle mass requires continued resistance training along with proper diet, especially during recovery period from a hard training session. Athletes should get a source of carbohydrate and 10-20 grams of quality protein within 30 minutes of finishing a training session. This can be done with a simple meal, or a drink that provides carbohydrate and protein. A peanut butter and jelly sandwich, a glass of milk and a piece of fruit would fill the bill, as would a recovery shake of frozen berries, juice, and a scoop of whey protein powder.
The proper ratio of carbohydrate to protein in a recovery meal is 3-4:1, so don't just eat the protein and forget the carbs! Carbohydrate depleted muscles are more prone to injury, just like if you don't get proper protein to repair and rebuild the muscle.