Is Shoulder Pain Not Caused by Muscle Atrophy? The Mechanism of 'Pain-Induced Muscle Inhibition' and Clinical Application That Athletic Trainers Should Know
As an athletic trainer caring for athletes with shoulder pain, it is not uncommon to find that the cause is muscle weakness. Weakness in the rotator cuff is a very common finding in shoulder pain. For such cases, we often tend to think, 'The muscles are thin (i.e., muscle atrophy), so let's prescribe strengthening exercises aimed at muscle hypertrophy.'
However, a paper published this year, 'Influence of Pain on Rotator Cuff Muscle Size and Function,' challenges this common assumption. This study states that in cases of chronic shoulder pain without complete structural tears, the primary cause of muscle weakness is not physical muscle atrophy, but rather neurological inhibition due to pain.
This finding could be a key point that fundamentally changes the priority of programs for athletic trainers and physical therapists in the field. This time, I have summarized the methods, results, and practical approaches of this important study that can be applied to clinical practice starting tomorrow.
Overview and Verification Methods of the Study
This study is a strictly matched case-control study published in the IJSPT.
Participants
Shoulder Pain Group (12 individuals): Patients with chronic right shoulder pain (mean symptom duration: 8.6 months). MRI examination confirmed that there were 'no complete tears' in the rotator cuff tendons. However, heterogeneous structural abnormalities commonly seen in clinical practice, such as tendinosis, partial tears, acromioclavicular joint arthrosis, and labral lesions, were included, gathering a model that realistically reflects the actual clinical setting.
Healthy Control Group (12 individuals): Healthy subjects who had no shoulder pain in the past 3 months and were strictly matched with the pain group in terms of age, sex, height, weight, and dominant hand.
Evaluation Items and Measurement Methods
Muscle Evaluation via Ultrasound Echo: Using an ultrasound diagnostic imaging device, the cross-sectional area (CSA) of the supraspinatus and infraspinatus muscles was measured. The thickness of the supraspinatus tendon was also quantified at the same time.
Measurement of Isometric Muscle Strength: Using a handheld dynamometer, 'external rotation strength' and 'abduction strength in the Full Can position' were measured using a standardized protocol.
Measurement of Muscle Endurance: The Posterior Shoulder Endurance Test (PSET) was performed. The subject lies prone, holding a dumbbell equivalent to 2% of their body weight, and repeats horizontal abduction of the shoulder joint (in external rotation, thumb pointing toward the ceiling) in time with a metronome (30 beats/min). The number of repetitions until the movement could not be continued due to the appearance of compensatory movements or fatigue was counted.
Study Results: Size is the Same, But Function is Significantly Reduced
The results brought about by this study overturned our conventional 'anatomical hypothesis'.
1. There is no difference in muscle size (CSA) and tendon thickness
The most notable finding is that despite having chronic pain for an average of 8.6 months, the cross-sectional area of the supraspinatus and infraspinatus muscles in the painful shoulder showed no difference at all compared to the healthy shoulder or the healthy control group (p > 0.19). Furthermore, no statistical difference was seen in the thickness of the supraspinatus tendon between groups or between left and right (p > 0.86). Even with chronic pain, 'structural muscle atrophy' that can be distinguished by the naked eye or ultrasound did not occur.
2. Significant decrease in muscle strength and muscle endurance
Although muscle size was maintained, a serious decline was confirmed in terms of function.
External rotation strength and full-can strength: The painful right shoulder showed significantly lower muscle strength compared to the left shoulder and the healthy control group (p = 0.003).
Posterior Shoulder Endurance Test (PSET): The painful shoulder resulted in an average of 13 fewer repetitions of dumbbell lifts compared to the opposite shoulder, and 17 fewer compared to the healthy control group (p = 0.021).
3. The 'impact of pain' revealed by covariance analysis
When the influence of 'pain felt during the test' was removed through statistical processing, a very interesting phenomenon occurred.
In terms of muscle strength (external rotation/full-can), when the influence of pain was corrected, the difference in muscle strength between groups disappeared. In the statistical model, it was found that 36-56% of the variance in muscle weakness was explained by 'pain itself'.
On the other hand, the decline in muscle endurance (PSET) was not completely resolved even after correcting for the influence of pain. This suggests that factors other than pain are involved in the decline in endurance.
Why does it get weaker even though the size does not change?
The pathological interpretations to be derived from these results can be summarized into the following two points.
① Involvement of arthrogenic muscle inhibition
When a tendon is completely ruptured, mechanical tension is no longer transmitted, so the muscle rapidly undergoes disuse atrophy. However, in cases like this study where the tendon structure is preserved, muscle size was maintained even after 8 months had passed. The reason for the decline in muscle strength despite this is that nociceptive signals (pain) emitted from intra-articular inflammation, tendinopathy, or partial tears block neural activity to the rotator cuff at the spinal and cortical levels. This is a phenomenon called 'arthrogenic muscle inhibition.' In other words, it is not a state where 'the muscle is thin and cannot produce force,' but rather a state where 'the muscle is present, but the switch is not turned on due to pain.'
② Decline in endurance is not a problem of a 'single muscle'
The fact that the decline in Posterior Shoulder Endurance Test (PSET) remained even after correcting for pain suggests that PSET reflects not only the ability of the infraspinatus or supraspinatus muscles alone, but also the stamina of the entire 'scapulothoracic joint stabilizer kinetic chain,' including the periscapular muscles (middle and lower trapezius, serratus anterior, etc.) and the posterior deltoid. It is highly likely that athletes with chronic shoulder pain have disrupted scapular alignment and motor control, which leads to a decline in the cooperative endurance of multiple muscle groups.
Practical applications that ATs can perform in the field
Based on this scientific evidence, how should we update our evaluation and rehabilitation protocols starting tomorrow? I propose three concrete steps.
Step 1: In the early stages of rehabilitation, focus on 'pain relief and recovery of neural drive' rather than 'progressive loading'
Just because an athlete's shoulder is weak, prescribing high-intensity tube exercises or external rotation exercises with dumbbells from the beginning can be counterproductive. Forcible strength training accompanied by pain promotes 'arthrogenic muscle inhibition' and further reduces motor unit recruitment.
Utilizing isometric contraction: Start with exercises that maintain isometric external rotation at a pain-free, comfortable angle (3/10 or less on the pain scale) for several seconds. Isometric contraction is known to have a central analgesic effect and is effective in releasing arthrogenic muscle inhibition and 'switching on' the rotator cuff nerves again.
Combining manual therapy and physical therapy: By performing soft tissue mobilization or appropriate physical therapy immediately before exercise to temporarily reduce local pain, you can dramatically increase muscle recruitment efficiency in subsequent exercise therapy.
Step 2: Sharing insights via ultrasound (improving motivation)
If your team or clinic has an ultrasound device, please make use of measurements. Visual feedback showing that there is no structural atrophy is a powerful tool to remove an athlete's anxiety and increase their psychological commitment to rehabilitation.
Step 3: A comprehensive approach to scapular stabilizers (endurance measures)
As the results of the study show, even if muscle strength returns, a lack of endurance in the posterior shoulder remains deeply rooted.
Closed kinetic chain exercises: Introduce exercises such as scapular protraction and retraction in a plank position (Push-up Plus) or wall slides to stimulate the serratus anterior and lower trapezius.
Low-load, high-repetition pattern exercises: Incorporate PSET movements, perform horizontal abduction with light resistance, or do Y-T-W exercises at a constant rhythm using a metronome, etc., to relearn the synchronized movement endurance of the scapula and humerus rather than strengthening a single muscle.
Summary
This study clearly proved that 'in chronic shoulder joint pain without complete tears, the size of the rotator cuff and the thickness of the tendons are maintained, but functional decline such as muscle strength and endurance occurs,' and that 'this weakness is not due to structural atrophy, but mainly due to nerve inhibition caused by pain.'
The role of an AT is not simply to make muscles bigger. It is more about Neuromuscular Control—how to get the muscles that are there to move 100% efficiently and without pain.
If you have an athlete you are currently working with whose inner shoulder muscle strength is not returning, it might be caused by pain from excessive load. In such cases, why not shift your approach to 'pain relief' and 're-education of neural drive with low load'?
Well then,
Reference URL: https://doi.org/10.26603/001c.160558