Introduction

We’ve all learned the empty can and full can tests to test the integrity of the supraspinatus muscle. Earlier on, these tests had been described as they were thought to create impingement of the shoulder, which would stress the supraspinatus tendon. These claims were later refuted, as it was found that the testing position did not isolate the supraspinatus tendon from other muscles. Yet, physical examination of the shoulder girdle muscles when a supraspinatus lesion is suspected, regularly includes the assessment of these tests. This study tried to find a position for isolated supraspinatus strength testing, where the most supraspinatus muscle activity and the lowest activity from contributing muscle groups were found.

Methods

Healthy subjects within the age group of 18-40 years were included in this study. Surface electrodes were applied over the anterior, middle, and posterior deltoid muscle, the upper trapezius, the pectoralis major muscle, the supraspinatus, and the infraspinatus muscles.

The dominant arm was tested for the maximum voluntary isometric contraction (MVIC) in varying positions of shoulder abduction, horizontal flexion, and rotation with applied manual resistance. Then every position was again tested, while holding a 1-kilogram dumbbell, to obtain EMG activity of each muscle. These muscle activities were standardized by dividing the weighted EMG activity by the MVIC to obtain a comparable value.

To identify the shoulder position where the most supraspinatus activity could be isolated, the results of the EMG activity of the middle deltoid and the supraspinatus were used to calculate the S:D ratio. The standardized weighted EMG of the supraspinatus was divided by that of the middle deltoid. A higher S:D ratio thus means the most activity comes from the supraspinatus muscle compared to that of the deltoid.

Results

The highest S:D ratio was obtained in the position of 30° of abduction, 30° of horizontal flexion, and external rotation. In this position, the contribution of the supraspinatus muscle relative to the action of the deltoid is highest. The next highest S:D ratio was found when slightly changing the position to 60° of horizontal flexion.

Questions and thoughts

The question of whether isolated supraspinatus strength testing adds much to the rehabilitation of subacromial shoulder pain can be posed. Lately, much debate has been held on the importance of a diagnosis. Our profession is characterized by clinical uncertainty, but nonetheless, we can confidently say that physiotherapy has value in shoulder pain rehabilitation. Rather than finding a diagnostic label for someone’s pain, Klintberg et al. in 2015 already postulated that “physiotherapy treatment decisions should be based on physical assessment findings and not structural pathology”.

Minimizing the abduction range isolates the activity of the supraspinatus muscle more. This is a logical finding, as the supraspinatus is a humeral head depressor, rather than an abductor.

Talk nerdy to me

EMG and especially surface EMG are not hundred percent reliable, as the signals may be subject to artifacts of overlying tissue and movement. The findings were, however, comparable to other EMG studies. Malanga et al. in 1996 already found that it was impossible to isolate the activity of the supraspinatus from that of the deltoid. The authors of this study have cleverly solved this issue as they used the S:D ratio.

Take home messages

If you want to test the supraspinatus tendon and avoid much contribution from the deltoid muscle, the best test position is 30° abduction and 30° horizontal flexion with the arm in external rotation. Thus, it is not necessary to include a 90° abduction range as this position will activate the deltoid muscle more.

Reference

Kijkunasathian C, Niyomkha S, Woratanarat P, Vijittrakarnrung C. The preferable shoulder position can isolate supraspinatus activity superior to the classic empty can test: an electromyographic study. BMC Musculoskelet Disord. 2023 Apr 3;24(1):255. doi: 10.1186/s12891-023-06372-3. PMID: 37013546. 

Additional reference

Klintberg IH, Cools AM, Holmgren TM, Holzhausen AC, Johansson K, Maenhout AG, Moser JS, Spunton V, Ginn K. Consensus for physiotherapy for shoulder pain. Int Orthop. 2015 Apr;39(4):715-20. doi: 10.1007/s00264-014-2639-9. Epub 2014 Dec 31. PMID: 25548127.