Most people do pushups in a way that only works the front of shoulders and can lead to impingement over time.
Your anterior deltoids (front of the shoulders) are not equipped to resist the weight of your body, so to optimize a pushup, it's best to utilize your chest muscles as well.
Today, I'll talk about a physics concept that will completely change how you feel pushups, reduce the risk of shoulder pain and injury, and improve your strength.
If you love to learn about these principles and want to apply them to teaching a fitness class, join the 200-hour Fitness Teacher Training that I'm hosting, starting in April 2021. The last day to sign up is March 1st.
This physics principle is called the ground reaction force.
This force is extremely important in exercise, yet it is rarely mentioned (and definitely isn't taught in most teacher trainings). It can completely change what muscles are targeted and the amount of stress moving through your joints.
The ground reaction force has to do with Newton's Third Law of Motion, which is that every action has an equal and opposite reaction.
How does this apply to exercise?
When you're pressing into the floor with a certain magnitude or direction, the floor will be pushing back up with the same magnitude and opposite direction.
Think about a trampoline. If you jump hard, you will bounce higher. The trampoline is responding with an equal and opposite force, directing you higher into the air the harder you jump.
Additionally, it will push back in the opposite direction of your landing. If you land at an angle, you will be bounced backward at that same angle. I think about doing a front handspring on a trampoline. I was so bad at them because I always landed on my feet, but at an angle, so I would get bounced backward and fall down, instead of landing straight up and down on my feet, bounding straight up, and sticking the landing.
Exercise on the floor is the same idea. When you push into the floor in a different direction, the upward force will be directed in the opposite direction. This is how you can optimize your pushups.
Quick anatomy before we get into how to apply ground reaction force to your pushups.
To work a muscle effectively, the insertion of that muscle has to move toward the origin of the muscle. In other words, the two ends of the muscle have to approximate.
When it comes to optimally working the chest muscles, the humerus (upper arm bone) has to adduct (move closer towards the midline). This is why a dumbbell chest press is generally better at strengthening the pecs than a barbell bench press. In a dumbbell chest press, the arms are free to move closer to the midline as you press and can fully adduct the arm. However, a barbell bench press inhibits the humerus from adducting towards the midline, as you just push straight up and down. You aren't able to fully adduct the arm, which means you will miss the full opportunity to challenge the pecs. Similarly, in a pushup, the floor is blocking you from fully adducting the arm. In fact, a pushup and barbell bench press probably works the anterior deltoid (front of the shoulder) and some tricep more than the pecs.
Of course, working the anterior deltoids is not a bad thing at all. However, the problem arises because the anterior deltoid isn't equipped to resist your body's weight, so many times, you will compensate by tilting the scapula, rolling the shoulder in, dropping the head, etc. This can lead to impingement pain in the shoulder over time and poor results when it comes to strength building.
However, this problem can be improved by utilizing the ground reaction force.
When you are doing a pushup and pressing straight down, the anterior deltoid is doing most of the action. However, changing the direction of force to be more angled inward will allow the pecs a better advantage. You do this by energetically gliding your hands together as you press up from the bottom of a pushup. I use the cue to "act as if you were kinking your mat lengthwise down the middle." This will align the ground reaction force more through the pecs (at an angle), stabilize your shoulder girdle more, and lead to better strength results.
If you want to learn more detail about this and how to apply the ground reaction force to other exercises, join the 200-hour Fitness Teacher Training! You can read more about it here.