A baseball pitch takes about two seconds from first movement to release. The part that actually damages arms lasts a fraction of that.
Biomechanics labs have measured what happens in those milliseconds, and the numbers are hard to believe. The shoulder rotates internally at speeds exceeding 7,000 degrees per second — among the fastest motions the human body produces. The elbow experiences valgus torque in the region of 64 newton meters during late cocking, which is roughly at or beyond what an isolated ulnar collateral ligament can withstand in cadaver testing. During deceleration, distraction force pulling the arm out of the shoulder socket approaches one to one and a half times body weight.
Your arm survives this because the load is shared — across ligament, muscle, timing, and the energy delivered up from your legs and trunk. Injuries happen when that sharing breaks down.
This article walks through the throw phase by phase. For each one: what’s happening, what’s under stress, and which symptom points back to it.
Phase 1: Windup
What’s happening: From first movement to the point where the lead leg reaches its highest position. The arms separate, and the body begins organizing itself.
Stress on the arm: Essentially none. This phase generates no meaningful load.
What goes wrong here: Nothing directly — but errors here propagate. Poor balance on the back leg, a rushed tempo, or an early loss of posture means every subsequent phase has to compensate. Problems that appear at the arm frequently originate here.
Clinical note: Single-leg balance and hip control are worth assessing in any thrower with arm pain, even though they seem unrelated to the symptom.
Phase 2: Stride and Early Cocking
What’s happening: The lead leg strides toward the target and the front foot plants. The throwing arm begins moving into abduction and external rotation.
Stress on the arm: Building, but still modest.
What goes wrong here: This is where the kinetic chain either delivers or doesn’t. Roughly half of the energy in a throw comes from the legs and trunk. Restricted hip rotation, weak glutes, a stride that’s too short or lands closed, or poor trunk control all reduce that contribution — and the arm has to make up the difference.
The symptom that points here: An athlete whose arm pain worsens as they fatigue, or whose velocity drops while effort rises. The arm is being asked to generate what the body should be delivering.
Phase 3: Late Cocking — Maximum External Rotation
What’s happening: The front foot is planted, the trunk begins rotating forward, and the throwing arm lays back into extreme external rotation — often around 170 to 180 degrees in high-level throwers.
Stress on the arm: This is the peak stress phase, and it’s where most throwing injuries originate.
At the elbow: Enormous valgus force — the forearm being levered away from the upper arm. The UCL is the primary restraint, and it’s operating at or beyond its isolated capacity. The flexor-pronator muscles on the inside of the forearm share that load, which is why forearm strength matters so much.
At the shoulder: The rotator cuff works to hold the humeral head centered while the capsule is under extreme tension. The posterior shoulder is being stretched; the anterior structures are being stressed.
What goes wrong here:
- UCL injury — the sprain or tear that leads to Tommy John surgery
- Medial epicondyle apophysitis in skeletally immature throwers, often called Little League elbow — the growth plate, not the ligament, is the weak link before maturity
- Proximal humeral epiphysiolysis — Little League shoulder — stress at the growth plate of the upper arm
- Internal impingement — the undersurface of the cuff contacting the labrum in extreme external rotation
- Labral injury, including SLAP tears
The symptoms that point here: Pain on the inside of the elbow during the cocking phase or at ball release. Deep pain at the back of the shoulder at maximum lay-back. In a young thrower, any medial elbow pain should be treated as growth plate involvement until proven otherwise.
Phase 4: Acceleration
What’s happening: From maximum external rotation to ball release — roughly 40 to 50 milliseconds. The shoulder rotates internally at over 7,000 degrees per second.
Stress on the arm: Extremely high, though very brief.
What goes wrong here: The elbow extends with tremendous velocity, and the olecranon — the bony point of the elbow — can slam into its fossa repeatedly. Over time this produces valgus extension overload: bone spur formation, posterior elbow pain, and sometimes loose fragments.
The symptom that points here: Pain at the back of the elbow at or just after release, sometimes with a sense of the elbow catching or locking. This is a different presentation from medial pain and points to a different problem.
Phase 5: Deceleration
What’s happening: The most violent phase. After release, the arm has to be stopped — and the muscles doing that work are contracting while lengthening, which is the highest-demand form of muscular work.
Stress on the arm: Distraction forces approaching one to one and a half times body weight, pulling the arm away from the body.
What goes wrong here: The posterior rotator cuff and the muscles controlling the shoulder blade absorb this load. When they can’t, structures fail:
- Posterior cuff tendinopathy — particularly infraspinatus and teres minor
- Labral pathology from repeated traction
- Biceps tendon irritation at its attachment
- Scapular dyskinesis as the muscles controlling the shoulder blade fatigue
The symptom that points here: Pain in the back of the shoulder after release, or a deep ache that shows up in the innings following a heavy outing rather than during it.
Clinical note: Deceleration capacity is the most commonly under-trained quality in throwers. Most throwing athletes have trained the muscles that accelerate the arm far more than the ones that stop it.
Phase 6: Follow-Through
What’s happening: The remaining energy dissipates as the body decelerates and rebalances.
Stress on the arm: Declining, but still meaningful.
What goes wrong here: Abbreviated follow-through — cutting the motion short, often from fatigue or from trying to “aim” the ball — concentrates deceleration forces into a shorter window and a smaller range. The trunk and legs should absorb a large share of this energy. When they don’t, the shoulder does.
The symptom that points here: A thrower whose mechanics visibly shorten late in an outing, often alongside a rise in arm soreness the following day.
The Thread Running Through All Six
Two patterns account for a large share of throwing injuries.
First: the arm pays for what the body doesn’t provide. Restricted hip rotation, poor trunk control, weak legs, and inadequate single-leg stability all show up as arm stress. This is why assessing a thrower’s shoulder and elbow alone misses most of the picture.
Second: fatigue is the amplifier. Every phase above degrades when the athlete is tired. Mechanics shorten, timing slips, the kinetic chain contributes less, and the arm absorbs more. Research on youth pitchers has found that throwing while fatigued is associated with a dramatically elevated risk of eventual surgery — by some estimates more than thirtyfold.
That’s why “how much” and “how tired” matter as much as “how well.”
What About Non-Baseball Throwers?
The phase framework broadly applies to javelin, water polo, team handball, cricket, and quarterbacks, with sport-specific differences in load and volume.
Softball is genuinely different. The windmill motion produces a distinct stress profile — more distraction and biceps demand, less valgus torque at the elbow — and softball injury patterns don’t mirror baseball’s. Guidelines developed for baseball pitchers don’t transfer directly.
Symptoms That Need Assessment Now
Stop throwing and get assessed for: a pop or sudden sharp pain during a throw, medial elbow pain in a skeletally immature athlete, numbness or tingling into the ring and little fingers, pain that persists at rest or at night, loss of velocity accompanied by pain, or inability to fully straighten the elbow.
Arm pain in a young thrower is never something to push through. The growth plate is the weakest link in an immature skeleton, and damage there has consequences that a ligament injury doesn’t.
Find Out What Your Mechanics Are Costing You
If you’re throwing through pain — or you’d rather find the weak link before it finds you — we’d like to take a look.
Motus RX Physical Therapy offers a free discovery visit at no cost and no obligation. You’ll get an assessment of your shoulder and elbow, but also your hips, trunk, and single-leg control, plus a clear picture of where in the throwing motion your symptoms are coming from and what to do about it.
If your presentation warrants imaging or a surgical opinion, we’ll tell you directly.