According to our systematic search, IR weakness, ER weakness, and limited range of ER at the glenohumeral joint have been reported to be associated with the development of rotator cuff tendinopathy. Supraspinatus weakness and imbalance in IR/ER strength have also been reported as a risk factor in the development of shoulder injuries.
Limited evidence support that weakness and stiffness of the rotator cuff muscles are risk factors to the development of rotator cuff tendinopathy
Our findings showed the significance of pre-injury muscle strength and stiffness on the incidence of tendinopathy. The reported risk factors include weakness of the shoulder external rotators, weakness of the internal rotators, and a small range of motion for external rotation at the glenohumeral joint. It was demonstrated in previous studies that these muscle changes, in the absence of pain, may increase the risk of newly developing rotator cuff tendinopathies in the upcoming season.
Other risk factors such as a low IR/ER ratio and supraspinatus weakness were also mentioned in included studies. Although the significance of these factors were associated with shoulder injuries, rotator cuff tendinopathies make up 70 out of 105 total cases of shoulder injuries included. There is a possibility that the risk factors may also be valid for the development of rotator cuff tendinopathy, but the calculations were unfortunately not commenced in our included studies(22,24,25).
Risk factors were classified into five evidence levels in a previous systematic review on rotator cuff tendinopathy.(27,28) Using the same method, evidence supporting that weakness and stiffness of rotator cuff muscles are risk factors to the development of rotator cuff tendinopathy could be classified as limited evidence. Very limited evidence support that a low IR/ER ratio and supraspinatus weakness are risk factors to the development of rotator cuff tendinopathy.
Hypothesis on how muscle weakness may lead to rotator cuff tendinopathy
Tendon loading, in terms of tension or compression to the tendon, is one of the risk factors leading the development to tendinopathy. this suggestion is supported by previous in vitro studies, demonstrating that tension or compression of tendon cells will lead to apoptosis and increased expression of inflammatory markers.(5,6,29) As mentioned in the introduction, several mechanisms have been proposed by various groups to explain how muscle weakness may lead to increased loading. Strength in the rotator cuff muscles are crucial in counteracting the deltoid muscle, keeping the humeral head adhered to the glenoid.(8) with weakened or fatigued rotator cuff muscles, humeral head elevation may occur, decreasing the size of sub-acromial space, leading to compression of the supraspinatus tendon.(9,10) Another possible mechanism was described in a previous study suggesting that IR and ER strength is crucial for the deceleration of arm swinging movements.(11) It was illustrated that the requirement for ER strength is increased to effectively decelerate an arm from forceful IR. Imbalance in athletes with repetitive arm swings is hypothesized lead to strains and tears to the tendon.(12)
Muscle weakness may not be an isolated problem
Associated factors such as muscle stiffness posture, and movement patterns may provide more insights on the causes of muscle weakness, as well as a direction on how to effectively correct it. Unfortunately, it was demonstrated in our systematic search that the association of these factors to the development of rotator cuff tendinopathy is under-investigated, with only one existing study discussing the significance of shoulder range of motion to the development of rotator cuff tendinopathy.(26) This study demonstrated that limited ER range of the shoulder is associated with the development of rotator cuff tendinopathy. Unfortunately, whether range of motion was active or passively measured was not described. This piece of information suggests a cause of the underlying muscle weakness. Limited range in ER may imply stiffness of internal rotators such as pectoralis major, teres major, and subscapularis, latissimus dorsi, and deltoid muscles. Considering the concept of reciprocal inhibition, tightness in this muscle group would lead to weakness in ER muscles, which is also consistent with our result demonstrating that a weakened ER compared with IR strength is associated with the development of rotator cuff tendinopathy.(8) This finding provide evidence that muscle weakness is not the only muscle change which may lead to the development of rotator cuff tendinopathy.
Could the history of a previous injury be a confounding factor
As shown in Table 1, the history of previous injuries were inconsistently recorded in the included studies. Of the two studies which documented for injury history, 19 out of 24 cases of shoulder injury also had a history of previous shoulder injury. However, current evidence is inconclusive whether the history of a previous shoulder injury may act as a confounding factor between rotator cuff weakness and the development of rotator cuff tendinopathies.
Rotator cuff tendinopathies are described to be persistent and recurring.(30) However, A previous systematic review on the risk factors on the development of rotator cuff tendinopathy have not been able to identify existing studies on whether a previous injury history is a risk factor to the development of future rotator cuff tendinopathies.(27) Also, rotator cuff tendinopathies are well known to present as shoulder weakness(31), but little is known on whether a previous injury would lead to residual weaknesses.
One of our included studies have attempted to address this issue.(21) According to this study, 47% of 108 athletes were documented for previous shoulder injury. However, it was shown in the study that previous injury was not a significant risk factor to development of new injury (p=0.28). There were also no significant difference in isokinetic results between players with and without previous injuries (p>0.05)(21). The significance of previous injuries were not assessed in the other included studies.
Rotator cuff tendinopathy affects a huge population, leading to disability and heavy socioeconomical impacts.(3) The results from this study provided an insight on how we may attempt to decrease such burden via primary prevention. It was demonstrated that not only in absolute strength, but imbalance in external and IR strength of the shoulder may contribute to the development of rotator cuff tendinopathy. Therefore, screening programs can be conducted in high risk groups, such as athletes competing in overhead sports, to identify individuals who are at a further high risk of the development of rotator cuff tendinopathy, and indicated for correction of muscle imbalance. There is a possibility that correction of muscle imbalance in existing cases of tendinopathy may also improve symptom control by modifying mechanical load to the tendon. However, its effectiveness on symptom control is yet to be investigated.
Regarding the method to correct muscle imbalance, it is to be acknowledged that a successful protocol would include, but is not limited to training of the weakened muscle. As mentioned in previous sections, muscle tightness would lead to reciprocal inhibition of the antagonist muscle. Attempts to relief muscle stiffness may augment the effectiveness of training to restore muscle imbalances. This provides a rationale to some existing treatments to tendinopathy, including eccentric exercises to train weak muscles, soft tissue release to relax tightened muscles. Systematic reviews on the effectiveness of these treatments alone have shown promising but inconclusive results on symptom relief of tendinopathy.(14,32,33) However, the effectiveness of combined treatment of muscle relaxation followed by training have not been investigated in rotator cuff tendinopathies, which opens a direction of future clinical trials.
All included cohort studies recruited patients in the athletic population. Therefore, the significance of muscle imbalance on the development of rotator cuff tendinopathy in the general public is currently unclear.
Rotator cuff tendinopathy observed in the included studies also presented in a rather sub-acute manner, developing in one season time. In comparison, tendinopathy in the general public is more known as a chronic degenerative disease with failed healing.(4) One explanation can be that weakness in the rotator cuff leads to an unfavorable mechanical environment, causing subclinical microtraumas chronically. Tendinitis would then develop as an acute on chronic condition from triggers such as an intense competitive season.
This hypothesis is particularly relevant as participants in the included cohort studies were painless at the beginning of follow-up, but no confirmatory imaging tests were done to confirm the absence of tendinopathy. Muscle weakness or stiffness could therefore be a result of adaptation to painless cases of tendinopathy, which its existence was previously described in an MRI study.(34) The diagnostic criteria of rotator cuff tendinopathy was also not clearly documented in several included studies(21,22,24,25), providing a potential source of inconsistency.
Athletes in the included studies also varied in age, gender, and sports participated. These factors may explain the inconsistencies between the results of our included studies.
With a goal to develop screening programs and lower injury risk, future studies could be directed in three steps. To further investigate and provide more details on the profile of muscle weaknesses that may lead to rotator cuff tendinopathy, to investigate on the most effective method to correct muscle weakness, and at last to investigate whether correction of muscle weakness would lead to a decrease in injury risk.
Unfortunately, existing evidence is still far from achieving step one. The significance of muscle weakness or stiffness have only been investigated on rotator cuff muscles, while there have been hypothesis that muscles of the spine, chest, and back may also have an impact on shoulder injuries.(35) Results from this review have also pointed out several limitations on the existing cohort studies, and that a future cohort could further improve on the existing evidence with a longer follow-up time, clearly documented injury history, and a stringent diagnosis to rotator cuff tendinopathy.
Prospective investigations on muscle stiffness, posture, and movement patterns may also be fruitful, as it may provide a cause to the existing muscle imbalance, as well as provide an insight on how this problem could be more effectively corrected.