Begins when your front or lead foot makes contact with the ground. As your foot hits the grounds, your body twists to face home plate, or the target you are throwing to and your glove hand flexes at the elbow and tucks into your shoulder.
Scapula of your throwing shoulder maintains upward rotation using the middle/lower fibers of the trapezius muscle and serratus anterior muscle. The scapula on your glove hand shoulder downward rotates using your rhomboids and pectoralis minor muscles.
Throwing Hand-Slightly starts to diagonally adduct the shoulder joint by concentrically contracting your pectoralis major, anterior deltoid, and coracobrachialis muscles.
Glove Hand- Adduct shoulder joint by eccentrically contracting your latissimus dorsi, teres major, and lower pectoralis major muscles.
Flexion to 30
Your lead foot causes the hip to externally rotate by eccentrically contracting your piriformis, gemellus superior and inferior, obturator internus and externus, and quadratus femoris muscles. This causes a left transverse rotation. The opposite side of your hip internally rotates by concentrically contracting your gracilis, semitendinosus, and semimembranosus muscles causing a right transverse rotation pelvic tilt.
Lead knee slightly flexes by concetrically contracting your biceps femoris, popliteus, semimembranosus and semitendinosus. The opposite knee internally rotates by isometrically contracting your popliteus, semimembranosus and semitendinosus muscles.