I can’t afford a gym membership and can’t build a home gym. Can I still gain muscle?” — Peter V., Chicago, IL
Weights are so popular because anyone can use them—but, believe it or not, there was a time when barbells and dumbbells didn’t exist and people (mostly gladiators and Spartans, I imagine) still got ripped and strong.
A bodyweight training regimen will work, provided you have a few objects to use as tools to help you make the best use of your own weight. Pullups and rows on a bar, tree limb, or sturdy pipe will work your back and biceps; pushups and dips on playground equipment will cover your chest, shoulders, and triceps; and squats, lunges, and jumps (preferably onto a box to start) will strengthen your legs.
The main problem with body-weight training is that your own weight is either too much or not enough, depending on the exercise. When it feels too heavy, adjust the angle of your body so you’re lifting less of your weight (e.g., do inverted rows instead of dead-hang pullups). When it feels light, slow down your reps to work on endurance and stability.
Ultimately, your body-weight training options are as broad as your imagination, so get creative.