Comparison of Scapulothoracic Joint Repositioning Ability and Muscle Strength in Individuals With and Without Scapular Dyskinesis

This study aims to compare the difference in scapulothoracic joint position sense (JPS_ST)and scapular muscle strength between individuals with and without SD.

Ten asymptomatic males were recruited (five with SD and another five without SD) for the following evaluations. During the JPS_ST test, participants sat with their shoulders in neutral and arms resting by their sides. A laser pointer was attached to the lateral acromion to project the scapular orientation to a screen in front of participants. JPS_ST were tested as the points at one- and two-thirds of maximal range of scapular protraction and retraction, respectively. The distance between the project point and target point was measured as the JPS_ST error. In addition, the maximal strength of periscapular muscles, including upper, middle, and lower trapezius, pectoralis major, and serratus anterior, was measured. Mann-Whitney U was used to examine the difference between groups, and Spearman’s correlation was used to analyze the relationships between JPS_ST error, muscle strength, and maximum project displacement. A significant level was set at p0.05.

No significant differences in JPS_ST errors were found between groups. However, the non-SD group showed significantly greater pectoralis major strength than the SD group(non-SD: 18.6 ± 2.1%WB, SD: 14.8 ± 2.0%WB, p=0.028). Pectoralis major strength negatively correlated with JPS_ST error of protraction to 2/3 target (r = -.830, p = 0.003), and middle trapezius strength correlated with JPS_ST error of retraction to 1/3 target (r = 0.721, p = 0.019). The maximum protraction distance was significantly associated with JPS_ST error of retraction to 1/3 target (r = 0.648, p = 0.04). 

A large range of protraction was associated with increased JPS_ST error of retraction, possibly due to the limited proprioception provided by the lengthened scapula retractor. The relationship between muscle strength and JPS_ST was the opposite between testing movements, possibly due to the varied coordination between agonists and antagonists.

This study demonstrated the feasibility of using a laser pointer to assess the reposition sense capability of the scapulothoracic joint. Current results indicate the effects of muscle strength and muscle coordination. Future research assessing the altered JPS_ST in patients with shoulder disorders is essential to realize the role of scapulothoracic position sense.