How to Perform the Incline Dumbbell Bench Press
The pectoralis major, also known as the chest, pectorals, or pecs, is made up of two heads - the clavicular head, also known as the upper chest or upper pectorals, and the sternal head, also referred to as the chest or lower pectorals. The chest is primarily built through pressing movements and movements requiring the arm to move across the midline of the upper body, across the chest.
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The dumbbell incline bench press is a compound push exercise targeting the clavicular head of the pectoralis major. It falls somewhere in between a vertical and horizontal plane movement. The anterior or front deltoid and triceps brachii (comprised of the long, lateral, and medial heads) act as supporting muscle groups during this movement. 
Supporting muscle groups assist the target muscle group during the movement. The short head of the biceps brachii acts as a stabilizer during this exercise.  Stabilizer muscles help maintain a posture or fixate a joint by contracting without significantly moving.  When this exercise is performed at the proper angle, there's no significant involvement of the sternal head of the pectoralis major.
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How to Perform the Incline Dumbbell Bench Press
Approach a free-standing adjustable bench, ensure you've got plenty of room on all sides of it (move it if necessary), and adjust the seat and back angle. Adjust the back pad so that it creates a 45 degree angle with the ground.
You don't need to get out a protractor and measure but it should be somewhere in between vertical and horizontal.
As the back pad angle increases and becomes more upright you'll move emphasis on to the shoulders. you'll notice more involvement of the sternal head of the pectoralis major as the back pad becomes more horizontal. Adjust the seat pad so that you're comfortable and not sliding off the bench.
Now go to the dumbbell rack and select the appropriate working weight. If you've previously performed barbell incline bench press then be prepared to start with a lower initial working weight. Many find that a 200lb barbell incline bench press doesn't translate to being able to use two 100lb dumbbells. Ensure you select two dumbbells of equal weight.
don't lift 50lbs with your left arm and 75lbs with your right arm. Uneven loading won't improve your gains and will likely lead to an injury. If this is your first time performing the exercise then pick a conservative weight that you can safely lift for 8 to 12 repetitions.
Once you've selected the appropriate working weight grasp the dumbbells, pick them up, sit down on the seat pad, and rest the dumbbells on your thighs. You should still be holding the dumbbells and they should be standing vertically so that only one side of each dumbbell is touching your thigh.
In a smooth, controlled, and fluid motion lift the dumbbells off of your thighs and move your upper torso back so that it's resting on the angled back pad. While lowering your upper torso you will be straightening (but not hyperextending) your arms and elbows so that the dumbbells are moving upwards and in-line with your shoulders.
Once the dumbbells are in position ensure you're taking a traditional pronated grip (thumb wrapped around the fingers and palms facing away from you). Ensure your chest is high, shoulders are down, and you're squeezing together the shoulder blades and upper back. This is your starting position.
After setting your starting position take a deep breath, brace your abdominals for impact and begin lowering the dumbbells towards the sides of your upper chest in a controlled manner. As you lower the dumbbells ensure your chest stays up, shoulders stay retracted, and upper back remains squeezed.
In the starting position the dumbbells are vertically in-line with the shoulders but during the lowering portion begin increasing the distance between the two dumbbells so that the internal ends of each dumbbell are significantly outside of shoulder width. Once the dumbbells hit the required depth, which for most will be in-line with or slightly below chest height, hold for 0 to 5 seconds.
At the bottom of the lift, your forearms should be at or almost perpendicular with the ground. If you begin feeling an intense stretch in the chest and shoulders before reaching that depth, then do not go down any farther as the risk for injury dramatically increases.
Hold in the bottom position for the desired duration and then press the dumbbells upwards and towards each other until the arms and elbows are back in the starting position. The movement pattern for both the lowering and pressing of the bar should be the same. Some lifters choose to exhale while pressing the barbell or in between in each repetition. Choose a breathing pattern that feels the most natural and comfortable for you.
This exercise can be performed using straight sets, pre-exhaust sets, drop sets, rest-pause sets, supersets, trisets, giant sets, paused reps, partial reps, forced reps, or slow negatives. As with any exercise, the two most important components are high-quality form and progression. Progression can take a variety of forms (e.g. more weight, sets, or reps, decreased rest period, improved rep quality, etc...) but strive to improve every training session.
Incline Dumbbell Bench Press Form TipsHold at the Bottom - If you're looking to increase intensity then experiment with holding the dumbbells at the bottom position, in-line with or slightly below chest height, for 5 to 10 seconds.
Don't relax your muscles and simply let the dumbbells rest on your shoulders at the bottom position; this is a recipe for serious injury. This hold will not only stretch your chest but will also increase time under tension.
Increased time under tension is an excellent variable to adjust for progressive overload and enhanced muscle growth.
Stay Tight - Staying tight throughout the entire movement minimizes the likelihood of injury. If you've ever seen someone lose tightness during the lowering or pressing portion of the bench press, then you know just how severe the consequences can be. Staying tight begins with a proper setup.
Keep your chest up, shoulders down, and squeeze your shoulder blades together as hard as possible. Those cues, combined with taking a deep breath and bracing your abdominals for impact prior to beginning the set, will ensure your shoulders and pecs stay healthy for a longer period of time.
Avoid Momentum - The incline dumbbell bench press provides maximum benefits when it's performed in a controlled full-range of motion. Check your ego at the door and don't immediately attempt 100lb dumbbells if your incline barbell press is 200lbs.
In addition to staying tight and keeping your head, glutes, and upper back on the padded support, do not bounce the dumbbells at the bottom of the lift. This momentum dramatically increases the likelihood of injury and minimizes the stimulus of the target muscles.
Avoid the False Grip - Do not use the false grip (thumbs and fingers wrapped around the same side of the bar) unless you have a preexisting injury preventing you from using the traditional grip (thumbs wrapped over the knuckles).
While many trainees find they're able to press slightly more weight using the false grip, the likelihood of the dumbbell rolling out of your palms and on to your chest. If you insist on using this insecure grip ensure you have a spotter or plenty of room to drop the dumbbells if you lose control of them.
Don't Slam the Dumbbells Together - Many novice and intermediate trainees think a rep on this exercise isn't complete unless the dumbbells slam together and bounce off of each other at the top of the lift. Not only does this bouncing generate momentum, which can minimize the engagement of the targeted muscle groups, but it also poses an increased risk of personal injury and injury to those around you.
If you're slamming the dumbbells together at the top of the rep and lose control of them then you risk dropping the bells on yourself or others. It is okay for the dumbbells to lightly touch at the top of the lift but ensure that touch doesn't turn in to a bounce.
Use a Full Range of Motion - While advanced trainees can benefit from partial range-of-motion repetitions, most novice and intermediate trainees will experience maximum benefits from using a full range-of-motion on the incline dumbbell bench press. This means every rep should start with the dumbbells vertically in-line with the shoulders, be lowered to the most comfortable depth, and then pressed back to the starting position.
A full range-of-motion maximizes time under tension and stimulus on the target muscles.
References1) "Dumbbell Incline Bench Press." ExRx (Exercise Prescription) on the Internet. N.p., n.d. Web. 5 Nov. 2015.
2) "Kinesiology Glossary." ExRx (Exercise Prescription) on the Internet. N.p., n.d. Web. 5 Nov. 2015.