What is scaption in external rotation?

Scaption in External Rotation

Scaption in external rotation (also called "full can" position) is a shoulder exercise performed by elevating the arm in the scapular plane (approximately 30-40° anterior to the frontal plane) with the thumb pointing upward, combining scapular plane abduction with humeral external rotation to optimize rotator cuff activation while minimizing impingement risk.

Biomechanical Definition and Positioning

• Scaption refers to arm elevation in the scapular plane, which lies approximately 30-40° anterior to the pure frontal (coronal) plane 1
• External rotation (ER) in this context means positioning the humerus with the thumb pointing upward ("full can" position), as opposed to internal rotation with thumb down ("empty can") 1
• This combined movement optimally aligns the humeral head within the glenoid fossa while allowing the scapula to upwardly rotate 45-55°, posteriorly tilt 20-40°, and externally rotate 15-35° during maximum elevation 1

Muscle Activation Patterns

Rotator Cuff Recruitment

• The infraspinatus and subscapularis generate forces two to three times greater than supraspinatus during scaption, though supraspinatus remains more effective at abduction due to its superior moment arm 1
• During scaption with ER ("full can"), infraspinatus and subscapularis activity is higher compared to the "empty can" position, while posterior deltoid activity is lower 1
• Supraspinatus activity remains similar between "full can" and "empty can" exercises, but the external rotation position reduces impingement risk 1

Force Distribution

• Estimated force contributions during abduction include: middle deltoid 434 N, anterior deltoid 323 N, subscapularis 283 N, infraspinatus 205 N, and supraspinatus 117 N 1
• The rotator cuff generates these forces not only for abduction but also to neutralize superior translation forces from the deltoids at lower elevation angles 1

Scapular Muscle Engagement

• The serratus anterior is critical during scaption with ER, contributing to scapular upward rotation, posterior tilt, and external rotation while preventing scapular winging 1, 2
• Middle and lower serratus anterior activation is significantly higher when combining arm external rotation with scapular protraction exercises compared to neutral position 3
• The middle trapezius provides medial scapular stabilization, while lower trapezius assists in both medial stabilization and upward rotation 2

Clinical Advantages Over Alternative Positions

Reduced Impingement Risk

• Scapular external rotation and posterior tilt are greater during "full can" compared to "empty can", which decreases scapular internal rotation and anterior tilt—both of which narrow subacromial space and increase impingement risk 1
• Scapular retraction during humeral elevation increases subacromial space width and enhances supraspinatus force production compared to protraction 1

Optimal for Rehabilitation

• In patients with rotator cuff tears, supraspinatus is more active during abduction than scaption and flexion, and its activity plateaus after 40° of elevation 4
• If minimizing supraspinatus stress is desired (such as in acute rotator cuff pathology), active arm elevation should be directed toward flexion and scaption rather than pure abduction 4
• Infraspinatus demonstrates similar activity patterns across all three planes with an increasing trend according to elevation angle 4

Pathophysiology Context in Adolescent Athletes

• In skeletally immature athletes, the combination of excessive external rotation moment at the distal humerus and increased internal rotation torque at the proximal humerus can lead to proximal humeral physis injury with overuse 5
• Maximum shoulder internal rotation velocity can approach 7000° per second in professional athletes, creating shear forces that—combined with rapid rotation—lead to physeal or labral injury 5
• The epiphyseal plates remain open until late teens (proximal humerus: 17-18 years) and are weaker than surrounding joint capsules and ligaments, making them vulnerable to repetitive overhead stress 6

Common Pitfalls to Avoid

• Avoid combining scaption with internal rotation ("empty can"), as this increases scapular internal rotation and anterior tilt, narrowing subacromial space 1
• Do not position the shoulder in abduction with external rotation after anterior shoulder dislocation, as this recreates the mechanism of injury and stresses damaged anterior capsulolabral structures 7
• Avoid overhead pulleys during rehabilitation, as these encourage uncontrolled abduction that can worsen instability 7
• In patients with shoulder impingement, decreased activation of middle/lower serratus anterior and rotator cuff, along with increased upper trapezius and middle deltoid activation, represents pathologic compensation patterns that should be addressed 2

Load Considerations

• External loads applied to the hand during scaption affect scapular three-dimensional position, particularly scapular protraction during scaption and scapular latero-rotation during abduction 8
• The magnitude of external load influences the scapulo-humeral rhythm, with effects most visible on scapular protraction during scaption 8