Phases of Throwing / Throwing Biomechanics

Overhead throwing, the pitching cycle, is divided into six phases: wind-up, stride or early cocking, late cocking, acceleration, deceleration and follow-through. Knowing them explains when and why specific injuries occur. Examiners expect the precise definitions, not just the names: each phase is bounded by a specific event, and its dominant muscle activity explains where the loads arise.

Energy from the ground up. Throwing is a kinetic-chain activity. Energy and momentum are generated in the legs and pelvis, beginning with the stride, transferred through the trunk, and delivered to the shoulder, elbow and hand. Efficient, well-timed transfer from the lower body protects the arm.

Proximal-to-distal sequencing. The segments accelerate and then decelerate in sequence, legs and pelvis, then trunk, then arm, then hand, each passing momentum distally like a whip. This is the summation of speed principle, and well-timed sequencing lets the small, fast distal segments reach high velocity without excess joint load.

When the chain fails. The timing of the transfer, particularly the separation between pelvis and trunk rotation, determines how much load reaches the shoulder and elbow. When the lower body and trunk do not generate or transfer energy efficiently, through poor conditioning, faulty mechanics or mistimed pelvic-trunk separation, the arm has to "catch up" and make up the energy. The shoulder and elbow then see higher rotational and valgus loads, and the risk of injury rises.

Mechanics can be changed. Altering throwing mechanics changes the load on the shoulder and elbow, and with it the injury risk. Stride length is the example to quote: it changes pelvic-trunk separation and the timing of peak trunk rotation.

The peak loads become memorable, and explain UCL failure, once they are quantified. These are established pitching-biomechanics figures.

At the shoulder. Maximal external rotation in late cocking is around 170-180 degrees, an apparent figure that includes the scapulothoracic and trunk contribution. Internal-rotation angular velocity near release reaches roughly 7000 degrees per second, among the fastest of all human joint motions. In deceleration the distraction force is around 1 to 1.5 times body weight, resisted eccentrically by the posterior cuff.

At the elbow, and why the UCL fails. Peak elbow valgus torque across late cocking and acceleration is approximately 64 Nm. That is roughly double the in-vitro failure strength of the ulnar collateral ligament, around 30-35 Nm, and the UCL supplies only about half of the valgus restraint.

Sharing the load. The UCL therefore cannot withstand a single maximal pitch on its own. The dynamic stabilisers (the flexor-pronator mass), the bony articulation and an efficient kinetic chain must share the load, and when that sharing fails the UCL is overloaded and injured. UCL injury and valgus extension overload are developed in their own topics.

Each throwing injury maps to the phase and the load that produce it. At the elbow the injury is to the ulnar collateral ligament, from valgus torque. At the shoulder it is internal (posterosuperior) impingement, SLAP and labral tears, the glenohumeral internal rotation deficit (GIRD) of the "thrower's shoulder", and biceps and posterior cuff pathology.

Naming the phase that overloads a structure is only half the answer; examiners then ask by what mechanism the load produces the lesion. Three chains are worth being able to recite.

Posterior capsule to GIRD. Repetitive eccentric deceleration loads the posteroinferior capsule and cuff, which respond with contracture. The result is GIRD, conventionally a loss of more than 20 degrees of internal rotation compared with the non-throwing side, measured at 90 degrees of abduction with the scapula stabilised.

Adaptation or disease. The deficit alone does not distinguish them, because the thrown shoulder also gains external rotation through bony humeral retroversion. What matters is the total rotational motion, internal plus external. Preserved total motion suggests physiological adaptation; lost total motion suggests pathological posterior tightness.

GIRD to internal impingement. A contracted posteroinferior capsule shifts the glenohumeral contact point posterosuperiorly and allows the scapula and humerus to hyperangulate in late cocking. The articular (undersurface) side of supraspinatus and infraspinatus is then pinched between the greater tuberosity and the posterosuperior glenoid. This internal (posterosuperior) impingement produces undersurface partial cuff tears and posterosuperior labral fraying in the abducted, externally rotated arm.

Peel-back to SLAP. In the same laid-back position the long head of biceps vector rotates posteriorly and vertically, twisting at its base and peeling the posterosuperior labrum off the glenoid rim. This peel-back mechanism is the biomechanical explanation for the type II SLAP lesion of the thrower. It is also the basis of the arthroscopic peel-back test, in which the labrum is seen to displace medially over the glenoid neck when the abducted arm is externally rotated.

Measurement technique, imaging and management for each are developed separately in GIRD, internal impingement, SLAP tears, scapular dyskinesis and UCL injury of the elbow.

Prevention follows from the mechanics and targets the whole chain:

• Lower-body and core conditioning, so that energy is generated and transferred efficiently
• Coaching of mechanics: pelvic-trunk separation, the timing of peak trunk rotation, and parameters such as stride length
• Restoring deficits in the thrower: stretching for GIRD and posterior capsular tightness, scapular dyskinesis and cuff strength
• Managing workload

Workload. Volume and fatigue, not pitch type, are the dominant modifiable risks. Age-based pitch counts with mandatory rest days are the practical expression of this: the USA Baseball / MLB Pitch Smart limits are roughly 95 pitches a day at 13-14 years, with rest days scaling with the day's count, and no more than about 100 innings in a year. The age-specific tables and rest-day rules are set out in full in our Little League Shoulder topic.

Fatigue and joint load. The acute version of the fatigue argument, a within-game rise in measured joint load, was not confirmed when Escamilla and colleagues measured it. The established association is with cumulative volume.