Going back a bit to talk muscle activity, Escamilla and Andrews (2009) reviewed many previously published research studies that measured shoulder muscle activity during different sports movements. Most of the data came from studies using electromyography (EMG), which records when muscles turn on and how hard they work. The review focused heavily on overhead throwing motions, especially baseball pitching, football throwing, and windmill softball pitching, but also included volleyball, tennis, baseball hitting, and golf. The authors combined EMG findings with biomechanical data such as joint angles, movement speed, forces, and torques to better understand how and why specific shoulder muscles are active during different phases of motion. Rather than collecting new data, the goal was to compare results across studies and explain injury risk and performance demands.
They found:
- Shoulder muscle activity is highest during fast throwing phases, especially arm cocking, acceleration, and deceleration.
- Rotator cuff muscles are highly active to stabilize the shoulder and resist distraction forces that can reach 80–120% of bodyweight in overhead throwing.
- Scapular muscles (serratus anterior, trapezius, rhomboids) play a major role in positioning and stabilizing the shoulder socket.
- During the windmill softball pitch:
- Supraspinatus and infraspinatus peak during the upward arm swing
- Posterior deltoid and teres minor peak near the top of the arm circle
- Subscapularis and pectoralis major peak just before ball release
- In softball pitching, the greatest shoulder forces occur before ball release, unlike baseball where deceleration forces are highest after release.
- Athletes with shoulder instability often show higher muscle activation, suggesting compensations rather than efficiency.
Implications: The authors explain that overhead sports place very high demands on the shoulder, requiring many muscles to work together to both produce movement and protect the joint. The rotator cuff and scapular muscles are especially important because they keep the shoulder stable while the arm moves at extremely high speeds. In windmill softball pitching, the shoulder experiences large forces during the acceleration phase, which helps explain why pitchers can develop shoulder pain even though the motion is underhand. The review also highlights that efficient athletes tend to use their whole body better, which reduces the need for excessive shoulder muscle activity. When mechanics are poor or athletes are fatigued, the shoulder muscles must work harder, increasing injury risk. The authors emphasize that injury prevention and training programs should focus not only on the shoulder but also on the entire kinetic chain, including the legs, trunk, and scapular stabilizers.
Also…
I love to study throwers of all types, and I have also experienced the benefit of doing rehabilitation as a method of avoiding injury. Here is a post by Resilient Performance that shows some great exercises that could help some athletes with poor moving shoulders.