08 Jun Serratus anterior and overhead athletes: don’t underestimate its importance!
Serratus anterior is an important muscle for the overhead athlete. Dysfunction in this muscle can lead to shoulder injuries such as impingement, rotator cuff breakdown and performance decrements during overhead tasks. Chris Mallac looks at its anatomy and biomechanics, and highlights some clinically relevant
exercises designed to retrain serratus anterior function.
Shoulder pain is a common complaint in overhead athletes involved in sports such as swimming, tennis and the throwing sports. Overhead upper extremity movements place incredibly high demands on the shoulder complex, requiring high muscular activation around both the scapula-thoracic joint and glenohumeral joint. Researchers have reported that abnormal biomechanics of the shoulder girdle and repeated overhead
movements can lead to injuries in overhead throwing athletes (1) .
In particular, muscular imbalances around the shoulder complex in the form of altered activation patterns and inherent myofascial restrictions, may lead to diminished scapular control and dyskinesis resulting in glenohumeral joint injuries, such as instability and impingement (2) . The serratus anterior (SA) is one of the scapula muscles that provides a link between the shoulder girdle and the trunk and has often be implicated as a dysfunctional muscle in shoulder pathologies (3,4) . The SA is a prime mover of the scapula, contributing to the maintenance of normal scapulohumeral rhythm and motion (4) . It has large moment arms to produce upward rotation and posterior tilting due to its insertion on the inferior and medial border of the scapula. Poor activation of the SA muscle may result in reduced scapular rotation and protraction, resulting in relative anterior-superior translation of the humeral head in relation to its glenoid articulation, causing subacromial impingement and rotator cuff tears (5) .
Anatomy and biomechanics
The SA is a flat sheet of muscle originating from the lateral surface of the first nine ribs (see figure 1). It passes posteriorly around the thoracic wall before inserting into the anterior surface of the medial border of the scapula (6) . Overall, the main function of the SA is to protract and rotate the scapula, keeping it closely opposed to the thoracic wall allowing optimal positioning of the glenoid fossa for maximum efficiency for
upper extremity motion (7) . The SA can be broken down into three functional anatomical components (8,9) :
1. The superior component originates from the first and second ribs and inserts into the superior medial angle of the scapula. This component serves as the anchor that allows the scapula to rotate when the arm is lifted overhead. These fibres run parallel to the 1 st and 2 nd rib;
2. The middle component of the SA originates from the second, third and fourth ribs and inserts onto the medial border of the scapula anteriorly (sandwiched between the scapula and ribs). This component is the prime protraction muscle of the scapula;
3. The inferior component originates from the fifth to ninth ribs and inserts on the inferior angle of the scapula. The fibres form a ‘quarter fan’ arrangement, inserting onto the inferior border of the scapula. This third portion serves to protract the scapula and rotate the inferior angle upward and laterally. Inman (1944) proposed that the lower part of the serratus anterior is the stabiliser of the inferior border of the scapula, and works with the lower trapezius to create a force couple to upwardly rotate the scapula during overhead movement(10).
Figure 1: Serratus anterior overview
The primary functional roles of the SA are to(9):
- Upwardly rotate the scapula during shoulder abduction, particularly from 30 degrees of shoulder abduction onwards;
- Stabilise and protract the scapula during shoulder flexion movements;
- Rotate the inferior angle anteriorly (posterior tilt of the scapula);
- Stabilise the scapula against the thorax during forward pushing movements in order to prevent the scapula ‘winging’ (see below);
- Hold the medial border of the scapula firmly against the thorax so that with the hand fixed, it can displace the thorax posteriorly during a push up.
In the athlete, particular specific movements require a high level of function of the SA to achieve either full scapular protraction and/or upward rotation. Examples of athletic endeavours requiring this SA function include:
- Throwing a punch in boxing to achieve maximum reach of the arm. Hence the SA is often referred to as the ‘boxers muscle’.
- In the boxer, the SA is needed to brace the scapula on impact with the punch. This allows maximum transfer of force from the lower limbs into the torso then for it to be imparted into the upper limb and punch. If the scapula was to ‘collapse’ into retraction upon impact of the punch, the boxer would then lose power in the punch.
- In swimming, the swimmer needs full upward rotation to achieve maximum reach in the water upon hand entry in freestyle and butterfly.
- The overhead athlete such as a tennis player needs full upward rotation in the act of serving.
- The sweep style rower needs full protraction on the ‘long’ side to achieve necessary reach during the catch phase of the rowing stroke.
- In baseball, the pitcher needs high levels of protraction during the follow through of the baseball pitch. Similarly in the throwing events in athletics.
The SA is innervated by the long thoracic nerve, originating from the anterior rami of the fifth, sixth, and seventh cervical nerves (see figure 2)(7,8). Branches from the fifth and sixth cervical nerves pass anteriorly through the scalenus medius muscle before joining the seventh cervical nerve branch that courses anteriorly to the scalenus medius. The long thoracic nerve then dives deep to the brachial plexus and the clavicle to pass over the first rib. Here, the nerve enters a fascial sheath and continues to descend along the lateral aspect of the thoracic wall to innervate the SA muscle.
Figure 2: Long thoracic nerve (from Safran et al 2004)(11)
SA dysfunction associated with scapula dyskinesis
Proper positioning of the humerus in the glenoid cavity, known as scapulohumeral rhythm, is critical to the proper function of the glenohumeral joint during overhead motion. A disturbance in normal scapula movement may cause inappropriate positioning of the glenoid relative to the humeral head, resulting in injury such as impingement and instability(2,12,13). Precise timing of muscle activation and adequate levels of muscle recruitment are also needed to position the scapula in the ‘ideal’ position. Small changes of activation in the muscles around the scapula can affect its alignment, as well as the forces involved in upper limb movement(14). One of the primary muscles responsible for maintaining normal rhythm and shoulder motion is the SA(15).
Clinically, it has been shown that if a therapist actively repositions a patient’s scapula into an ‘ideal’ posture by reducing the anterior tilt, then it is noticed that impingement pain is reduced, and strength increases in the shoulder during overhead activities(16). The SA is a muscle that will actively work to position the scapula into posterior tilt during overhead activities.
Lack of strength or endurance in the SA allows the scapula to rest in a downwardly rotated and anterior tilted position, causing the inferior border to become more prominent. Furthermore, gross pathological inhibition of the SA or an imbalance between the SA and the other protracting muscle, the pectoralis minor, may result in a ‘winging scapula’. Scapular winging may precipitate or contribute to persistent symptoms in patients with orthopaedic shoulder abnormalities(17,18).
This scapular winging is best appreciated on watching the scapula posture during a push up exercise. Often if the winging is due to a muscle imbalance and the primary scapula stabiliser is the pectoralis minor, this will usually correct if the patient is asked to ‘plus’ and protract the scapula. If the wing disappears then the cause is most likely muscle imbalance, if it remains then it may be a pathological inhibition of the SA. Examples of this are shown below in figures 3-6.
Figure 3: Scapular winging on push up bilaterally
Figure 4: Winging corrects on execution of a ‘plus’
Figure 5: Scapular winging on push up bilaterally (right greater than left)
Figure 6: Left scapula corrects with ‘plus’ however note the right is still winged
The gross examples of scapular winging can also be due to a pathological lesion to the long thoracic nerve that innervates the SA muscle. For the purposes of this discussion, direct nerve insults to the long thoracic nerve will not be discussed as often these injuries will seriously curtail athletic participation in an athlete. The reader is directed to references 19-23 for a more detailed discussion on these pathological nerve lesions.
The importance of a conditioned serratus anterior muscle has been highlighted in EMG studies of sports such as swimming(24), throwing(25), and tennis(26). A fatigued serratus anterior muscle will reduce scapular rotation and protraction and allow the humeral head to translate anteriorly and superiorly, possibly leading to secondary impingement and rotator cuff tears(27). More direct studies on the role that SA plays in shoulder pathologies has been studied by other researchers. The pertinent points of some of these studies can be summarised below;
- When the trapezius and SA EMG is investigated in people suffering from shoulder pathology is compared with those without pathology, it has been found that upper trapezius can show an increased activity during arm elevation and lowering, and that SA shows decreased activation at some elevation angles (usually 70-100 degrees)(28).
- When the muscle activation patterns of swimmers with shoulder pain is compared to those without, it has been found that middle and lower SA show decreased activity in all phases of swimming motion. This can be a possible cause of the shoulder pain or a consequence of a painful shoulder whereby the swimmer uses compensatory muscle activation patterns(29).
- Similarly, other researchers have found a ‘latency’ or activation delay in the SA in the shoulders of painful swimmers as they raise their arms in the scapular plane(30).
- Ludewig and Cook (2000) hypothesised that patients with decreased SA activation are associated with more shoulder pain and/or instability, and that an increase in lower trapezius activity was an attempt to compensate for decreased serratus anterior activation(2).
- Lin et al (2005) studied subjects with various types of shoulder dysfunction and found decreased serratus anterior activity and increased upper trapezius activity, without a change in lower trapezius activity in injured shoulders when compared to normal subjects(31).
Scapula position has also been associated with the ability of the rotator cuff to function. Excessive anterior tilt of the scapula, internal rotation, or excessive elevation of the acromion are factors that decrease the rotator cuff activation and cause an inadequate distribution of tension along the tendons. Such situations impair the optimum length-to-tension ratio of these muscles, leading to a loss of stabilisation and increasing the chance of muscular disruption or degeneration(32). It has been shown that the rotator cuff function improves in the presence of functioning scapula muscles such as the SA and lower trapezius.
Exercises for SA
A significant amount of research has been conducted on finding the best rehabilitation exercises for the SA. The majority of these studies look at movements such as push ups, push up-plus exercises, cable and dumbbell ‘punch’ type movements. These exercises essentially emulate the function of the SA in its protraction role. Some of the findings of the more noted studies are;
- Decker et al (1999) looked at the EMG activity and applied resistance associated with eight scapulohumeral exercises performed below shoulder height that target the SA muscle and how to design a continuum of SA muscle exercises for progressive rehabilitation or training(33). The best exercises according to these researchers are the push ups, dynamic hug, scaption and SA punch exercises.
- Barreto et al (2012) found high levels of activation of the SA in scaption exercises and adequate levels of activation of SA in MMP (modified military press)(34).
- Kim et al (2014) studied the interesting effect of vibration on SA activation and found that the push-up plus exercise performed using the Redcord system with mechanical vibration at 50 Hz increases SA muscle activity(35).
- Park and Yoo (2011) evaluated the effect of unstable surface on the upper and lower parts of the SA, while performing variations of the push-up exercise (push up and push up plus)(36). The results indicated that the different parts of SA have distinct functions in the stabilisation process and therefore are recruited differently. The authors concluded that the main role of the lower SA is the fixation of the scapula onto the thoracic wall and therefore recommend performing the push-up plus on an unstable surface as a more effective strategy for the selective mobilisation of this component of the SA.
- Seo et al (2013) also found that the performance of a push up and knee push up on an unstable surface (Swiss ball) increased activation of the SA(37).
Below are some examples of clinically used SA activation exercises that anecdotally seem to recruit SA to high levels of function:
The wall slider (figures 7a and 7b)
- Using a foam roller or pool noodle on the wall, place the wrists against the roller so that the forearm commences in a neutral pronation/supination position. The hands are approximately six inches apart and the elbows are about twelve inches apart.
- Protract the scapula so that the space between the shoulder blades is filled in with an increase in thoracic kyphosis.
- Slowly flex the shoulder so that the roller moves up the wall. This creates scapula upward rotation.
- As the shoulder is flexing, slowly supinate the forearms. This then creates obligatory shoulder external rotation.
- Reach as high as possible into shoulder flexion.
- Although the feet do not move, the trunk will gently lean into the wall as the arms are raised in shoulder flexion.
- The finish point is when the little fingers touch and the forearm is in maximum supination.
- This should be felt as a strong serratus anterior contraction on the ribcage wall.
- Slowly descend and return to the starting point with the forearms in neutral pronation/supination.
- Perform three sets of ten repetitions.
Figure 7a: Wall slider (start position)
Figure 7b: Wall slider (finish position
(notice the forearm is supinated and shoulder externally rotated)
2. Swiss ball rotations (figure 8)
a. Hold a Swiss ball (55cm) between the arms with the ball gently resting on the chest and the forearms in neutral pronation/supination.
b. Slowly rotate the trunk to the left. As this is done, the right arm is made longer by actively protracting the scapula and the left arm is made shorter by actively retracting and depressing the scapula. The arms ‘glide’ around the Swiss ball.
c. As the arm is made longer (protraction) slowly rotate the palm upwards to encourage supination of the forearm and external rotation of the shoulder.
d. Return to the start position and commence again.
e. This can be progressed by holding Theratubing in the hands, which wraps around the upper trunk. This provides resistance to the scapula protraction.
Figure 8: Swiss ball rotations
(notice the hand is turning into supination)
3. TrX Serratus rollouts (figures 9a and 9b)
- Place the forearms through the loops of a Trx or other suspension device.
- The straps are placed at wrist level for a high-load exercise or just below the elbows for a shorter lever low-load exercise.
- The commencing hand position is similar to the wall slider above. The hands are approximately six inches apart and the elbows are about twelve inches apart.
- Slowly flex the shoulder so that the arms elevate above the head and at the same time start to slowly externally rotate the shoulder by supinating the forearms.
- The finish point is when maximum flexion/elevation is achieved and the little fingers touch.
- Although the feet do not move, the trunk will start to lean forward as the arms are raised.
- This finish position involves maximum scapula upward rotation.
- Return to the start and perform three sets of fifteen repetitions.
- This is a progression on the wall slider as the suspension device is inherently unstable and requires more muscular effort throughout the kinetic chain to control.
Figure 9a: TrX Serratus rollout (start position)
Figure 9b: TrX Serratus rollout (finish position)
4. TrX push up plus (figures 10a and 10b)
- On a TrX, position the handles two to three feet off the ground. The lower the handles, the more difficult the exercise.
- Holding the handles in a push up position, slowly perform a push up and slowly ‘screw’ the hands into supination and shoulder external rotation.
- At the top of the movement, protract the scapula and fill in the space between the shoulder blades with a slight thoracic kyphosis.
- This is a progression from a push up plus as the Trx is unstable and requires more kinetic chain muscular effort.
- Return to the bottom position and perform three sets of ten repetitions.
Figure 10a: TrX push up plus(start position)
Figure 10b: TrX push up plus (finish position)
The serratus anterior (SA) is a muscle that plays an important role in the dynamic movement and control of the scapula during pushing movements and overhead elevation. Specifically it is a protractor, upward rotator, posterior tilt muscle of the scapula and additionally it fixes the scapula against the rib cage during movement. It is an important muscle for the overhead athlete as dysfunction in this muscle can lead to shoulder injuries such as impingement, rotator cuff breakdown and performance decrements during overhead tasks. This article has highlighted some clinically relevant exercises designed to retrain SA function in the athlete with SA dysfunction.
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