Admit it: When you see a workout program based on saving time, your gut reaction is, "You can't build muscle in x minutes!" To be clear, I agree with you. Building muscle takes time and consistent high-volume training, and under normal circumstances, you should devote adequate time to your lifts.
But what happens when you don't have normal circumstances or adequate time?
We all feel that annoying itch when we can't make it to the gym. The reason doesn't matter. Maybe you overslept, left work later than you should have, or made plans that prevented you from getting to the gym. Whatever the reason, your day doesn't feel complete if you don't get in some sort of workout.
By this logic, a 20-minute workout serves two purposes. First, it eliminates the "not enough time" excuse. If you can spare 20 minutes, you can get in a good workout. Second, it scratches that itch. You still made it to the gym. You still got it done. A 20-minute shoulder workout won't be a staple in your routine, but it can be a big confidence booster to maintain progress and help move you closer to reaching your goals.
Side Lateral to Front Raise
Stand holding a pair of dumbbells at your sides. Lift the dumbbells out to the sides. Once your arms are roughly parallel with the floor, move the dumbbells toward one another until your arms are pointing straight ahead. Lower the weights, the raise them back up. Now move the dumbbells out to your sides and then lower them. That's one rep. You'll need to go light here, but do this move correctly, and all three heads of the deltoids will be on fire.
Machine Shoulder Press
Add weight as the reps decrease. Keep in mind that you likely won't be able to use as much weight as you normally would because you pre-exhausted the shoulders with the side lateral to front raise. Remember, the focus is on training the muscle, not showcasing power, so add weight cautiously.
Partial Lateral Raise
Use a pair of heavier dumbbells and perform half reps. It's a small range of motion, but by the time you get to the end of the set, your delts will burn and the pump will be very real. Lift, don't swing. If you need to use body English to lift them up, lighten your load. Once again, add weight on each set as the reps decrease.
Grip the band so there's tension throughout the entire rep. Even though you're short on time, on each rep feel your rear delts contract before you release the tension. Go up in resistance on each set. If you have only one band, hold the weight for 3 seconds on the second set and 5 seconds on the last. If you don't have any bands, perform rear lateral raises with light dumbbells instead.
Once you finish all three exercises, rest 30 seconds before starting over. Complete a total of 3 trisets.