The preferable shoulder position can isolate supraspinatus activity superior to the empty can

Key points

1. 01The classic empty can (EC) test position produced nearly the smallest supraspinatus-to-deltoid (S:D) ratio (median 0.8) of all 24 positions tested, suggesting it is poorly selective for supraspinatus isolation.
2. 02The highest S:D ratio (median 3.4, range 0.5-9.1) occurred at 30 degrees abduction, 30 degrees horizontal flexion, and external humeral rotation - about 4.25 times better isolation than the EC position.
3. 03S:D ratio increased significantly with lower degrees of shoulder abduction, lower degrees of horizontal flexion, and external over internal humeral rotation (all p < 0.05).
4. 04The study tested 24 comprehensive shoulder positions by combining three abduction angles (30/60/90 degrees), four horizontal flexion angles (0/30/60/scapular plane), and two rotations (internal/external).
5. 05Infraspinatus and pectoralis major showed relatively flat EMG activity across positions, suggesting they are not major confounders in this test.

How it was conducted

Design

Controlled laboratory EMG study

Participants

21 healthy male volunteers, mean age 29.0 +/- 0.9 years, mean BMI 24.6 +/- 2.9 kg/m2, all with dominant right arm, no history of shoulder disorder

Muscles monitored

Seven periscapular muscles via surface EMG: middle deltoid, anterior deltoid, supraspinatus, upper trapezius, posterior deltoid, infraspinatus, and pectoralis major

Positions tested

24 shoulder positions: factorial of 30/60/90 degrees abduction x 0/30/60/scapular plane horizontal flexion x internal/external humeral rotation

Primary outcome

Supraspinatus-to-middle-deltoid (S:D) ratio calculated from standardized weighted EMG normalized to maximum voluntary isometric contraction (MVIC) for each position

Analysis

Kruskal-Wallis test for non-normally distributed data, multilevel mixed-effects linear regression, multiple regression with parsimonious model

What they found

• The greatest S:D ratio occurred at 30 degrees abduction, 30 degrees horizontal flexion, external rotation: median 3.4 (range 0.5-9.1).
• The second highest S:D ratio occurred at 30 degrees abduction, 60 degrees horizontal flexion, external rotation: median 3.0 (range 0.6-21.9).
• The classic EC test position (90 degrees abduction, 30 degrees horizontal flexion, internal rotation) showed near the lowest S:D ratio: median 0.8 (range 0.2-1.2).
• The proposed position offered approximately 4.25-fold better SSP isolation than the EC position based on S:D ratio comparison.
• S:D ratio significantly increased with lower degrees of abduction (coefficient for 60 vs 30 degrees: -0.95, 95% CI -1.37 to -0.53, p < 0.001; 90 vs 30 degrees: -1.23, 95% CI -1.65 to -0.81, p < 0.001).
• Middle deltoid activity significantly increased with higher abduction, lower horizontal flexion, and internal over external rotation (all p < 0.05).
• SSP activity significantly increased with higher abduction, lower horizontal flexion, and external over internal rotation (all p < 0.05).
• Kruskal-Wallis p-value for S:D ratio comparison across 24 shoulder positions: 0.0001.
• Multiple regression with parsimonious model showed shoulder position, body weight, and scapular plane were inversely associated with S:D ratio; overall F-test p < 0.0001, adjusted R-squared 0.0545.

Limitations

• Study used only healthy young males aged 18-40; findings may not translate directly to patients with chronic shoulder pain or confirmed supraspinatus tears.
• Surface EMG electrodes were used rather than fine-needle intramuscular electrodes, which may introduce some signal cross-contamination.
• Subscapularis was not included in the analysis, and it may influence abduction torque in some positions.
• Diagnostic accuracy (sensitivity, specificity) of the proposed position in clinical populations was not assessed; this study only characterizes EMG muscle activation ratios.

Why it matters

For patients

Patients suspected of having a supraspinatus tear may benefit from being examined in this lower-arm position, which more specifically targets the supraspinatus muscle and could reduce false-negative strength tests caused by compensatory deltoid activation.

For clinicians

Clinicians performing supraspinatus strength testing should consider using 30 degrees abduction with 30 degrees horizontal flexion and external rotation rather than the classic empty can position, as this significantly reduces deltoid co-activation and may improve the specificity of the physical examination.

For readers

This EMG study provides a biomechanical rationale for reconsidering the standard empty can test position, though prospective diagnostic accuracy trials in shoulder pain patients are needed before the proposed position can be recommended for routine clinical practice.