Scapular dyskinesis is strongly associated with an increased risk of ‘overhead’ injuries in athletes. Andrew Hamilton looks at recent evidence on the evaluation of scapular function in overhead athletes and the implications for rehab.
In overhead sports like tennis or volleyball, the shoulder is at increased risk of injury due to the high loading and impact forces experienced in the joint region during serving and smashing. Many shoulder injuries occur over time, with chronic overload leading to injury. As part of this process, pain may also arise from sport-specific adaptations, strength and flexibility alterations, and shoulder joint posture changes. These alterations change biomechanics and movement strategies during serving and striking, aggravating the risk of an overload injury.
Evidence suggests that while the structure and function of the glenohumeral joint are pivotal in determining the risk of shoulder injury, a whole kinetic chain exists in which deficits anywhere along the chain can increase the risk of shoulder injury(1-5). Glenohumeral internal rotation deficit (GIRD) and rotator cuff strength imbalance play an essential role in the kinetic chain in the shoulder. Still, other deficits such as scapular dyskinesis, thoracic spine stiffness and hyperkyphosis, lumbar core instability, and hip range of motion can all contribute to a ‘kinetic chain failure,’ increasing injury risk in both young and old overhead athletes(6,7). However, this article will concentrate on scapular dyskinesis (SD).
Physiologically, the scapula is the stable base of origin for muscles that contribute to the dynamic glenohumeral stability and produce arm motion. Scapular stability is needed for force production from muscles arising from the scapula. Scapular Dyskinesis is the alteration of normal scapular kinematics, resulting in the loss of normal control of scapular motion. SD by itself is not an injury or a musculoskeletal diagnosis. Instead, it has been hypothesized to relate to changes in glenohumeral angulation, acromioclavicular joint strain, subacromial space dimension, shoulder muscle activation, and humeral position and motion.
Classification of Scapular Dyskinesis
Kibler et al. reported the reliability of a visually based classification system for scapular dysfunction that defined three types of motion abnormalities: type 1 = inferior angle prominence, type II = medial border prominence, and type III = excessive superior border elevation. The normal, symmetric scapular motion was considered type IV.
- Type 1: Inferior Angle Prominence-At Rest: the inferior medial scapular border may be prominent dorsally -Abnormal Motion: The inferior angle tilts dorsally, and the acromion tilts ventrally over the top of the thorax. The axis of the rotation is in the horizontal plane -Common Associated Findings: Tightness/shortening of pectoralis minor (Borstad & Ludewig, 2005)
- Type 2: Medial Border Prominence -At Rest: The medial border is prominent dorsally. Abnormal Motion: The medial border of the scapula tilts dorsally of the thorax. The rotation axis is in the frontal plane. Common Associated Findings: weakness/reduced serratus anterior and lower traps activation.
- Type 3: Superior Border Prominence -At Rest: The superior border is generally elevated, and the scapula is often anteriorly displaced. Abnormal Motion: Shoulder-shrug motion is evident. The axis of motion is in the sagittal plane. Common Associated Findings: Tightness/overactive upper trapezius, weakness/ reduced activation lower trapezius.
- Type 4: Normal and symmetric scapular motion
Various shoulder soft tissue pathologies, including impingement (internal and external) anterior capsular laxity, labral injury, and rotator cuff weakness, have been found in association with SD in overhead athletes complaining of shoulder pain(8-13). To confuse matters, however, scapular asymmetries have been noted in overhead athletes who are asymptomatic and those who are injured. This makes it difficult to determine whether SD is simply an effect of shoulder injury in overhead athletes or is implicated as a prime cause.
Some studies have found no causative relationship between SD and shoulder pain. In contrast, others have identified scapular dyskinesis as a possible risk factor for chronic shoulder pain in overhead athletes(14-16). In particular, obvious SD, as defined by Kibler et al., has increased the risk for shoulder pain (see box 1).
As discussed, glenohumeral range of motion, rotator cuff strength or imbalance, and scapular position and movement are essential for assessing healthy and previously injured overhead athletes. By performing a clinical assessment, clinicians can define risk/causal factors and then help guide the athlete in a suitable rehab program with the goal of a total return to play after injury.
However, unlike the measurement of glenohumeral range of motion and rotator cuff strength, where it’s relatively straightforward for the clinician to measure defined ranges of motion or levels of strength and strength imbalances, assessing SD is more challenging. Part of the reason is that evidence supporting ‘recommended values’ for preventing injury or returning to play after injury concerning scapular function is scarce.
Studies have investigated the validity of simple visual observation (performed using the yes/no method for SD being present/absent) as a means of assessment. In general, they suggest that simple visual observation is a relatively reliable and valid method on the proviso that the clinician is trained in an appropriate and standardized manner(17). Categorizing SD into different types based on the specific position of the scapula (i.e., Kibbler’s method) is also a reliable means of assessment when used by a clinician during repeated assessments. However, the evidence also suggests that inter-clinician reliability is not nearly as high(18).
Although it is difficult to separate cause and effect, the evidence does suggest that an obvious degree of SD is a risk factor for shoulder pain in overhead athletes. In one study, for example, researchers rated SD in handball players by dividing them into one of three categories—normal scapular control, slight SD, or obvious SD—and found that obvious dyskinesis was a major risk factor for shoulder pain(16).
Having said that, the studies carried out to date do not support the notion that scapular behavior should be symmetrical in overhead athletes. For example, studies on volleyball and handball players have looked at scapular asymmetry in resting scapular posture and found that a degree of asymmetry was common and not predictive of shoulder pain(19,20). Other researchers have also reported that the prevalence of SD is almost identical in subjects with and without shoulder pain, questioning the clinical value of scapular asymmetry. Although this may sound confusing, it boils down to the fact that clinicians treating overhead athletes should be aware that some degree of scapular asymmetry is common and should not be considered automatically as a pathological sign – but rather an adaptation to sports practice and extensive use of upper limbs.
When assessing scapula function in overhead athletes, measuring several parameters can be helpful. These include scapular upward rotation and inter and intra-muscular strength. Scapular inclination – several studies have measured scapular inclination following upward rotation in healthy overhead athletes(21,22). The data from these studies can be used as a reference base, providing cut-off values for correct scapular positioning at several elevation angles. It’s common to observe a significant variation in scapular upward inclination through the midrange of motion – most likely due to substantial natural anatomical variations between individuals. However, in total elevation, most studies suggest that upward inclination should be at least 45-55 degrees(22,23). Figure 1 shows the use of a digital inclinometer for measuring inclination. The use of a digital inclinometer has been shown to exhibit high inter – and intra-rater reliability. However, this is on the proviso that there is adequate palpation of the reference points in the different humeral elevation angles and that the clinician can control any additional tilting of the inclinometer in planes other than the scapular plane(22).
Assessing strength ratios and absolute strength of the scapular muscles is useful. In a healthy but non-athletic population, the isokinetic ratio protraction/retraction is around 1:1, whereas overhead athletes have slight changes. In the case of throwing athletes, a subtle shift in strength towards the protractors is normal(8,23,24). In one-handed overhead sports such as tennis, an increase of 10% in scapular muscle strength is advised on the dominant side. In particular, the strength levels of the lower trapezius and serratus anterior should receive special attention since these muscles are shown to be susceptible to weakness in injured athletes(24,25). And while in bilateral sports such as swimming, rowing, etc., there shouldn’t be significant side-to-side differences in scapular muscle strength, clinicians should expect to observe some strength bias towards the handed side.
Several protocols have been described in the literature for measuring scapular muscle strength(23, 26). Clinicians should remember that different testing procedures will produce different outcomes depending on the equipment used, the positioning of the dynamometer, and patient positioning. An example of a well-validated set of strength tests for scapular function (lower, middle, and upper trapezius and serratus anterior is shown below in figures 2-5).
The following four tests for scapular muscle strength tests have been demonstrated to provide excellent intra-rater test-retest reliability. Muscle testing is performed by first prepositioning the scapula in the midrange position of scapular motion for the specific muscle test. The midrange position is located by having the subject go through the available scapular range of motion, and then the midrange is estimated as the midpoint of the motion. This midrange position is selected to optimize the length-tension relationship of the tested muscle and, therefore, generate a maximum isometric contraction. A ‘make test’ is then performed – subjects are instructed to maintain the midrange position during each muscle test. At the same time, resistance is gradually applied via the handheld dynamometer (HHD) until the clinician matches the subject’s effort.
The resistance force from the HHD is applied to the scapula’s spine midway between the acromial process and the root of the spine. The force on the scapula is applied in the superior and lateral direction parallel to the long axis of the humerus, which is maintained at 140 degrees of elevation. The scapular motion used is scapular adduction and depression.
The HHD resistance force is applied to the scapula’s spine midway between the acromial process and the root of the spine. The force is applied in the lateral direction parallel to the long axis of the humerus, which is placed in 90 degrees of abduction. The scapular motion used is scapular retraction.
The HHD is placed over the superior scapula. Force is applied directly downward (inferior) through the HHD toward scapular depression. The scapular motion used is scapular elevation.
The elbow is placed in 90 degrees of flexion, and resistance is applied to the ulna at the olecranon process along the long axis of the humerus. The triceps muscle should be monitored visually and by palpation to ensure it does not contribute to force production during the SA muscle test. The scapular motion used in this test is scapular protraction.
Once deficits and imbalances in scapular behavior have been identified, an intervention program to restore flexibility and muscle performance is the logical step. In a recent paper, Cagnie et al. outlined a ‘scapular intervention algorithm’ (see figure 6) to help guide the clinician through the different steps and progressions required(22).
In this paper, Cagnie et al. describe a detailed 4-stage approach for scapular strength rehab consisting of the following:
First stage: conscious muscle control performed in the early stage of scapular training. This stage aims to improve proprioception and normalize the scapular resting position.
Second stage: muscle control and strength necessary for daily activities. The goal here is to focus more on muscle control and co-contraction (advanced control during basic activities) or muscle strength (in case, for instance, manual muscle testing or isokinetic testing shows an isolated strength deficit in one or more scapular muscles).
Third stage – advanced control during sports movements using general scapular exercises to increase muscle strength. This stage aims to exercise advanced scapular muscle control and strength during sport-specific movements. Special attention is given to integrating the kinetic chain into the exercise program and implementing sport-specific demands by performing plyometric and eccentric exercises. In particular, overhead athletes should perform exercises where the external rotators are eccentrically loaded, such as weight balls and elastic resistance tubing.
In addition, the authors provide a protocol for treating flexibility problems in the scapular muscles. They point out that these are often characterized by loss of flexibility in the pectoralis minor and the levator scapulae muscles and stiffness and tightness of the posterior shoulder structures (the capsule and the glenohumeral external rotator muscles). These flexibility deficits may lead to scapular malpositioning, particularly anterior tilting and downward rotation.
The elbow is placed in 90 degrees of flexion, and resistance is applied to the ulna at the olecranon process along the long axis of the humerus. The triceps muscle should be monitored visually and by palpation to ensure it does not contribute to force production during the SA muscle test. The scapular motion used in this test is scapular protraction.
While SD may not be the root cause of shoulder injury in overhead athletes, investigation and rehab (if necessary) of scapular function are important parts of overcoming shoulder injury. In particular, identifying scapular asymmetries, strength imbalances, and deficits in scapular muscle flexibility should form an important part of any treatment protocol.
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