It depends on how much time you have. Ideally, you’d go into the gym and walk on a treadmill for five minutes or jump rope until you break a sweat. Then you’d go through a series of mobility exercises—leg swings, arm circles, and similar moves that take you through the range of motion you’re going to use on your exercises. Then you’d do a few stretches for tight areas like the hips and pecs, pick up the weights and do a few light sets to find your groove, and get to work.
But the truth is, few people have time for all that, and we suspect you’re among them. To keep it simple, follow this rule: Never start lifting cold. Find a way to start sweating, whether it’s with calisthenics or a quick jog. The foam ¬roller—a long cylinder of dense foam your gym probably has parked in a corner—can be great for warm-ups. Lie on it and roll your muscles over it, pausing on spots where you feel tenderness. The foam roller acts as a sort of massage to work knots out of your muscles and enhance blood flow, and it improves flexibility right away.
From there, Bruno recommends stretching your pecs (chest), lats (back), hip flexors (the front of your hip), hip rotators (sides of your hips), and calves. Do two or three stretches per side for 30 seconds each. If you’re really short on time, you can build flexibility training into your workout. For instance, between sets of deadlifts, you can bend over and touch your toes, then squat down (called a toe-touch squat). Repeat for up to 10 reps, continue your rest period, and then do another set of deadlifts. You’ll get more mobile and warmer each set until you’re ready to do your hardest set.
One caveat here: Don’t do static stretches—where you hold a stretched position for time—between sets of an exercise that uses the same muscles. For instance, you wouldn’t want to stretch your pecs between sets of bench presses, as this may temporarily weaken these muscles, hurting the quality of later sets. However, it’s a good idea to stretch out the lower body while working the upper body, and vice versa.