As long as you're breaking a sweat and getting a good workout during your pick-up games, they can absolutely take the place of designated gym sessions a few days per week.
In fact, New York-based exercise physiologist and physical therapist Scott Weiss says he'd recommend playing a sport over going to the gym almost any day. "You're interacting with friends, you're moving in all different directions, and you're not focused so much on the workout," he says. "When your goal for the hour is simply to put the ball in the hole, it's less of a mental grind than counting reps in the weight room or slogging away on the treadmill."
But even pro athletes don't spend all of their time on the court or the field; they spend most of it practicing technique, running drills, and strengthening their muscles. (We're gonna guess that isn't something your pick-up league does on a regular basis.) So yes, you have to do those things, too.
The good news? You can do less of it.
Say you play basketball or soccer two nights per week, and you get in a good hour of running during each of those games. In this case—as long as your goal is general fitness and wellness and you're not training for an endurance event—you don't need to do any other cardio or interval training throughout the week, says Toronto-based exercise physiologist and personal trainer Craig Ballantyne.
Instead, he says, use your gym time to focus on strength training, and aim for three sessions per week. (Save two days, ideally, for recovery.) Your workout regimen will vary depending on your goals and the time you can devote, but it's smart to focus at least part of your time on sport-specific moves and drills.
"If you're a weekend warrior with a game every weekend, you can't just show up every Saturday and play great," says Weiss. "During the week, the best thing you can do is work those muscles and practice those skills that are important to your sport."
Picking up games more than once or twice week? That's totally fine, but don't ditch the weight room entirely. "Lifting weights makes everyone a better athlete and gives everyone a better body," no matter how much running and passing and catching you're doing, says Ballantyne. "Two short 20-minute sessions per week is enough, doing squats, dumbbell presses, a single-leg exercise like squat splits, and pullups or dumbbell rows."
One thing you should not do, however, is double up on gym time and a hard-played game on the same day. "If your schedule says you're supposed to lift but your buddy calls you to play basketball, don't try to do both—you'll just increase your risk of injury," says Weiss. "Save the gym for the following day. Really, alternating game days and strength-training days is ideal."
And this should go without saying, but we're assuming you're really working hard during these games. If you're in a casual league that does more standing around than actual running—or you're on the bench more than you're in the game—you can still benefit from an actual workout before or after the event.
Finally, make sure you're taking at least one or two days off per week from lifting and strenuous cardio to avoid overtraining and potentially getting injured. "We don't like our patients telling us they lifted six days last week and played basketball for five," Weiss says. Something physically easy, like practicing foul shots or taking a yoga class, is okay on an off day.
Bottom line: The way you choose to balance pick-up sports and gym time should depend on your goals, says Ballantyne. "Do you want to perform at an all-star level on the court? If yes, decrease your volume of gym time," he says. "Do you want to gain 20 pounds of muscle? Then limit your time on the court."