The Serratus Anterior and Trunk Stability: Why They Work Together

When we think about the serratus anterior, we usually think about the shoulder blade. The serratus plays a huge role in controlling the scapula during reaching, pushing, and overhead movement, but it does not work in isolation. Because it attaches from the ribs to the shoulder blade, the connection between the serratus anterior and trunk stability matters more than most people realize. It needs a stable rib cage and trunk to work from. This is why someone can have a relatively strong shoulder but still struggle with scapular control during a plank, push-up, overhead lift, or throwing motion. Sometimes the issue is not just the shoulder itself, but how well the shoulder blade, rib cage, and trunk are working together.

If you have not read it yet, we recently covered how the serratus anterior works and how to strengthen it. This post picks up where that one left off and looks at what the muscle is working from.

Why Does Trunk Stability Matter for the Shoulder?

The shoulder is designed to be extremely mobile, allowing us to reach, throw, push, pull, and move our arms in almost every direction. In order to control all of that mobility, however, we also need stability. The scapula sits on the rib cage, and the rib cage is part of the trunk. If the trunk cannot maintain a controlled position while the arm moves, the muscles around the shoulder are working from a constantly changing base.

Think about trying to produce force from an unstable surface compared to solid ground. The more control you have at the base, the easier it is to transfer force farther down the chain. The same concept applies to the shoulder. This becomes especially important during higher-level activities like throwing, swimming, climbing, lifting, or playing an overhead sport.

Where Does the Serratus Anterior Fit In?

The serratus anterior originates along the ribs and attaches along the inside border of the scapula. Because of this position, it creates an important muscular connection between the rib cage and shoulder blade. Its main functions include scapular protraction, upward rotation, posterior tilt, and keeping the scapula controlled against the rib cage.

When the serratus contracts, it is not simply moving the shoulder blade. It is controlling the relationship between the scapula and the rib cage underneath it. If the rib cage is moving excessively or the trunk cannot maintain an appropriate position, the serratus now has a much more difficult job.

What Happens When Trunk Stability Is Poor?

A plank is a good example of how the shoulder and trunk have to work together. When someone performs a plank, we might initially look at the shoulder blades and see whether they are winging or whether the person is adequately pushing away from the floor. But we also need to look at the rest of the body. Are the ribs flaring? Is the lower back excessively arching? Are the hips dropping? Is the trunk rotating or shifting from side to side?

All of these compensations change the position of the rib cage underneath the scapula. This is why simply telling someone to “push the floor away” does not always fix poor shoulder mechanics. If they cannot control the trunk underneath the shoulder blade, they may just find another compensation to create the movement we are asking for.

Rib Cage Position Matters

The scapula does not move independently of the rib cage. It moves on top of the rib cage, which means that when the position of the rib cage changes, the starting position of the scapula changes as well. This is especially noticeable with overhead movement.

For example, someone may appear to have great shoulder mobility when reaching overhead, but if you watch closely, they may be getting a significant amount of that movement by extending through their lower back and flaring their ribs. Instead of getting the necessary movement through the shoulder and scapula, they are using the trunk to compensate.

This does not mean the spine should never move during overhead activity. Our bodies are supposed to move together. The question is whether the person can control that movement and whether they have enough motion available at the shoulder that they do not have to compensate somewhere else.

The Serratus Anterior, Core, and Force Transfer

The relationship between the serratus anterior and trunk stability becomes even more important when we look at athletic movements. Think about throwing a baseball or hitting a volleyball. The movement does not start at the shoulder. Force is generated from the ground, travels through the legs and hips, continues through the trunk, and eventually reaches the shoulder, arm, and hand. If the trunk cannot effectively transfer that force, the shoulder may have to work harder to create the same result.

This concept applies to throwing, swimming, tennis, volleyball, boxing, rock climbing, overhead lifting, and many other activities. The serratus anterior helps connect the arm to the trunk by controlling the scapula on the rib cage. Instead of thinking about the serratus as just a shoulder muscle, it can be helpful to think about it as part of the system connecting the upper extremity to the rest of the body.

Exercises for Serratus Anterior and Trunk Stability

One of my favorite ways to work on serratus anterior and trunk stability together is through closed-chain exercises. When the hand is planted on the floor or another surface, the serratus has to stabilize the scapula while the trunk is controlled over the arm. Instead of isolating one muscle, we are teaching multiple areas of the body to work together.

Quadruped Serratus Press

Start on your hands and knees with your hands underneath your shoulders. Keep your elbows straight and allow your chest to sink slightly toward the floor before pushing the floor away from you. The shoulder blades should move around the rib cage while the rest of the trunk stays relatively controlled. Try to avoid arching the lower back or shifting the hips to create the movement.

Bear Plank

Start on your hands and knees and lift your knees a few inches off the ground. Push the floor away while keeping your ribs and trunk controlled. This increases the demand on the serratus while also requiring the abdominals and hips to maintain your position.

Bear Plank or High Plank Shoulder Taps

From a bear plank or high plank, slowly lift one hand and tap the opposite shoulder. When one hand leaves the floor, the body naturally wants to rotate. The serratus on the weight-bearing side has to stabilize the scapula while the trunk works to resist excessive rotation. The goal is not to eliminate every bit of movement, but to be able to control it.

Plank With Reach

From a high plank, slowly reach one arm forward while maintaining control through the opposite shoulder and trunk. Because the arm moves farther away from your center of mass, this creates an even greater challenge for both scapular stability and anti-rotation control.

Bear Crawls

Bear crawls are a great higher-level exercise because you are constantly transferring weight from one arm to the other while controlling the shoulder blades, rib cage, pelvis, and trunk. Slowing the exercise down often makes it significantly more difficult, because you cannot use momentum to compensate for poor stability.

Wall Slide With Trunk Control

Place your forearms against a wall and gently press into it while sliding your arms upward. Allow the shoulder blades to naturally upwardly rotate as you reach overhead, but pay attention to what happens at your trunk. If the ribs immediately flare and the lower back arches, decrease the range and focus on maintaining better control as the arms move overhead.

Pallof Press With Reach

Stand perpendicular to a resistance band or cable and perform a Pallof press. From the extended position, add a forward or slightly upward reach. The trunk now has to resist rotation while the serratus controls the scapula during the reaching movement. This is a great way to start bridging traditional core strengthening with functional upper-extremity movement.

Stability Does Not Mean Rigidity

One important thing to remember is that trunk stability does not mean that we need to brace as hard as possible or prevent the spine from ever moving. The trunk is supposed to move. Throwing, reaching, lifting, swimming, and almost every athletic movement require coordinated movement throughout the entire body.

The goal is not rigidity. The goal is control. Can you keep your trunk stable when you need stability? Can you rotate when you need rotation? Can you reach overhead without creating all of that movement from your lower back? Can you transfer force from your legs and trunk into your arm? Those questions are much more important than simply asking how long someone can hold a plank.

Common Questions

Can a weak core cause shoulder pain?

Not directly, but poor trunk control changes the base the shoulder blade works from, which can make scapular control harder during planks, overhead lifting, and throwing. In the clinic we usually see it as one contributing factor rather than the whole explanation.

Why do my ribs flare when I reach overhead?

Often it is a trade-off. If there is not enough available motion at the shoulder or the scapula is not upwardly rotating well, the body borrows the movement from the lower back and rib cage instead. Restoring shoulder motion and trunk control together tends to work better than cueing the ribs alone.

Are planks enough for shoulder stability?

Holding a plank longer is not the goal. Adding controlled movement, such as shoulder taps, reaches, and crawling, challenges the serratus and the trunk in the way they actually have to work during real activity.

The Bottom Line

The serratus anterior is usually discussed as a shoulder muscle, but its function is closely connected to the rib cage and trunk. It helps control the shoulder blade against the rib cage while the muscles of the trunk provide the base that allows the shoulder to move and produce force efficiently.

This is why serratus strengthening does not always need to look like an isolated shoulder exercise. Exercises like bear planks, shoulder taps, crawling, plank reaches, wall slides, and resisted reaching allow us to strengthen the serratus while also challenging the trunk to control rotation, extension, and changes in body position.

As someone progresses through physical therapy, the goal should be to move beyond strengthening individual muscles and start teaching those muscles to work together. Ultimately, we do not just need a strong serratus anterior or a strong core. We need them to be able to work together during the movements that person actually needs to perform.

Working on Shoulder and Trunk Control in Chicago

If your shoulder bothers you during planks, push-ups, overhead lifting, or throwing, it is worth having someone look at more than just the shoulder. Every session at Dynamic Physical Therapy Chicago is one-on-one with the same doctor of physical therapy, so we can watch how your shoulder blade, rib cage, and trunk are working together and build your program from there. Illinois direct access means you do not need a physician referral to start, and we are in network with Blue Cross Blue Shield.

Call 312.643.1555 or request an appointment to get started.