The investigators will clarify rate of electromyography (EMG) rise and rate of force development in overhead athletes on scapular muscles, including upper trapezius, lower trapezius and serratus anterior. The correlation between rate of EMG rise and rate of force development will also be examined.

The contributing factor of scapular dyskinesis can be bony and joint-related issues, neurologic problems, soft tissue problems. Patient with thoracic kyphosis, pectoralis minor stiffness, long thoracic nerve injury, and so on can lead to scapular dyskinesis and further shoulder dysfunction. During shoulder movement, the neuromuscular control of the scapular muscles also play an important role. Previous studies found that participants with pattern 1 and 2 scapular dyskinesis had lesser lower trapezius (5%, P =.025) and serratus anterior activity (10%, P =.004), and higher upper trapezius activity (14%, P =.01) in pattern 2 participants during arm lowering compared to normal participants. Furthermore, the intervention focus on neuromuscular control can change the recruitment pattern of participants with scapular dyskinesis. Significant increases in activation of the middle and lower trapezius (MT: 4.9 ± 2.4% of the maximal voluntary isometric contraction (MVIC); LT: 10.2 ± 6.8% MVIC, p < 0.0 25) were found with conscious control in 3 exercises among the 3 dyskinesis groups, and increased serratus anterior activation (11.2 ± 4.8% MVIC, p < 0.0 25) was found in the concentric phase of side-lying external rotation in the pattern 1 and 1 + 2 groups. The studies show that the muscle recruitment is highly related to the scapular dyskinesis.

However, there are some limitation in the previous studies which presented the outcome by EMG amplitude. First, no matter with or without intervention, previous studies fail to show difference between groups in some condition. Although, there are some difference in lowering phase, the results fail to show difference in elevation phase and some degree of lowering phase. Second, substantial cancellation of the EMG interference signal can occur. The positive and negative signal will be offset. Last, not only neural effect but also contractile effect will be captured. Contraction type, including concentric, eccentric or isometric, will occur in a movement, so the signal will be affected. As the result, another method to represent neuromuscular effect should be considered.

The rate of EMG rise (RER; Formula: ΔEMG/Δtime) has been used to evaluate the rate of muscle activation in order to account for the neural factors that contribute to rate of force development (RFD; Formula: Δforce/Δtime). The onset (<75 ms) of a rapid contraction indicates a role for neural factors. Previous studies with RER outcome have been conducted to see the effect of pain, aging or training. It has been reported that RER reduce with pain and aging while increase after training, and better sensitivity to distinguish difference than peak EMG amplitude (PEMG). The significant difference is found in both upper trapezius and deltoid for RER but only in upper trapezius for PEMG. However, most of the studies about RER are conducted on lower extremity or female worker and no study conducted on athletes, not to mention athletes with scapular dyskinesis.

The overhead sports are characterized with forced and rapid movement. The more sensitive and functional measurement of RER may detect the difference of overhead athletes with different type scapular dyskinesis. Therefore, the purposes of this study are to compare the RER, PEMG, RFD and peak force on scapular muscles (UT, LT, SA) among different types of scapular dyskinesis at 2 arm elevation angles (30, 90 degree). Additionally, to investigate the correlation between RER and RFD. The investigators hypothesize that overhead athletes with scapular dyskinesis will demonstrate significant lower RER and RFD, and there will be significant positive correlation between RER and RFD.

Inclusion Criteria:

• Playing overhead sports for at least 1 year.
• Still active in training or competition.
• The frequency of training or game should be at least 2 times per week, 1 hour per time.

Exclusion Criteria:

• Subjects with shoulder pain onset due to trauma, a history of shoulder fractures or dislocation, cervical radiculopathy, degenerative joint disease of the shoulder, surgical interventions on the shoulder, or inflammatory arthropathy.
• Visual analog scale (VAS) > 5 during movement in the experiment.

Scapular dyskinesis is defined as abnormal scapular position and motion. It can be divided in 4 patterns. It has been reported that overhead athletes have higher prevalence (61%) compare to non-overhead athletes (33%). Additionally, athletes with scapular dyskinesis have 43% greater risk of developing shoulder pain than those without scapular dyskinesis. Due to higher prevalence with greater injury risk, scapular dyskinesis plays the important role of injury process that need to be concerned for the overhead athletes.