For avid readers of scouting content, getting closer to the evaluation process allows greater context over phrases that can otherwise run together. How often do we as prospect-watchers hear terms like “plus”, “below-average”, “fringy”, among plenty of others? At some point, one player’s “above-average” tool is difficult to individualize from another one’s “plus” when all we have as readers are basic word associations corresponding to certain skills (fastball velocity, raw power, foot speed, etc).

Without insight into the individual components of a pitcher’s motion or a hitter’s swing, it’s logical to be in the dark about why one player is performing better and/or more consistently than another—despite evaluators’ consensus being that both players’ tools or pitches grade out similarly. For instance: a primary cause for widely different statistical outputs between one pitcher and another—both with similar 20-80 grades on their raw pitch-types—can be the overall consistency and command-execution of their pitches. By and large, a pitcher’s ability to have mastery over those attributes is a byproduct of clean, repeatable mechanics.

Given the intrinsic subjectivity of scouting, the priority that individual evaluators place on pitching mechanics varies. That said, the “operation” of a pitcher through his delivery is so inextricably linked to a pitcher’s performance and projection, it’s safe to say that every pitcher’s windup and arm-action, at bare minimum, are noted on nearly every scouting report. The goal of this piece is to break down each of those individual mechanical components in more detail.


Readers will frequently see me and other members of the Prospect Team detail what type of delivery a pitcher uses in the "Mechanics" section of an Eyewitness Report. Sometimes it is even described in a handful of acronyms: FWU, SWU, and NWU being the most common. The concept of various forms of windups is pretty straightforward relative to other more detailed aspects of a pitcher’s mechanics; the acronyms themselves, however, likely need to be quickly described.

Full Windup (FWU)
Pitchers throwing from full windups raise and break their hands over their heads before moving into the leg-lift in their deliveries. For a point of reference, pitchers like Adam Wainwright, Cole Hamels, and Nate Eovaldi are notable examples of pitchers that throw from a full windup. Some full-windup pitchers use the over-head hand-break as a timing mechanism: By starting their hands slightly further back as they load up through their leg kick, the pitcher prevents either their upper or lower half from accelerating disproportionately faster than the other through the release of the ball. Additionally, it can be a way to visualize keeping the body going entirely towards the target in a straight line. Others use it as a way to create deception by starting their motion with a slow over-head break before delivering the baseball more quickly. Generally speaking, starting pitchers are more likely to throw from full windups than relievers.

Semi-Windup (SWU)
A semi-windup is the most common type of delivery used by pitchers. It’s a first move in a delivery with a small step sideways or backwards towards second-base. Unlike the full windup, the hands do not generally move up and down vertically nearly as much before the hand-break and leg-lift in the delivery, not reaching above and/or behind the pitcher’s head. Noah Syndergaard and Stephen Strasburg are two notable starters who throw from a traditional semi windup; Wade Davis’ motion is an example of a reliever who uses a semi windup without runners on base.

No Windup (NWU)
A no windup delivery is a delivery style that’s far more common in relief pitchers. Basically, it’s just throwing from the stretch with a leg-lift to load up before the drive to the plate. Relievers generally throw from no-windup deliveries for two basic reasons: the game situations they enter in, and their general inferior strike-throwing abilities (in the aggregate) relative to starting pitchers. The consensus opinion is that it’s easier to throw consistent strikes from a delivery with fewer moving parts, and indeed, there is a great deal less movement before the leg lift in a no-windup delivery. While nothing in scouting works in absolutes, the average relief pitcher throws with more overall velocity than a starter—velocity generated from more effort-laden mechanics that are less conducive to control and command. Some relief pitchers compensate for the natural effort their mechanics feature by cutting out the excess movement of a full or semi windup. A secondary rationale for relief pitchers throwing from no windup deliveries—especially for situational relievers who, unlike closers, often appear in games during the middle of an inning—is that they so frequently enter with runners already on base, it’s best to be as comfortable as possible only throwing out of the stretch. Kelvin Herrera exemplifies a no windup reliever who throws from a no windup in order to harness huge stuff from an effort-filled delivery; Oliver Perez is more of a no windup type of reliever from the standpoint of his general utilization being situational with runners often on base. While it is far less common to see a starting pitcher work from a no windup, it isn’t unheard of; I’ve seen Yusmeiro Petit work in longer stints out of a no windup.

Arm Actions

The phrase “arm action” is both very common and quite vital in the evaluation of a pitcher. It describes the entirety of the movement the throwing arm takes throughout a pitcher’s delivery—from the moment the throwing arm separates from the glove in the hand break of the motion, all the way though the appearance of the throwing arm after it releases the ball. Again, while nothing in scouting works in absolutes, arm actions are sometimes more central to scouts’ positive or negative feelings about a pitcher than his current windup or other mechanics: The consensus opinion is that a pitcher’s arm action cannot be altered nearly as easily as the windup or landing parts of the delivery.

As such, evaluators are more prone to be skeptical of some aspects of projecting improvements and alterations for pitchers with less-than-ideal arm actions. Inversely, the scouting community tends to be more likely to see through present weaknesses in a pitching prospect if the arm action grades out well enough that future positive adjustments can be envisioned through easier-to-address mechanical attributes than arm action.

How the throwing arm “works” through its release point—both behind the body, and out in front of the pitcher as the ball is leaving the hand—correlate with many important components of pitching success. Additionally, arm action (sometimes referred to also as “arm stroke” or “arm path”) can be an indicator of health or future injury concerns depending on how naturally and loosely the throwing arm travels from proverbial point A to point B. Aside from health, things like overall control and command, consistency of action on secondary pitches, and ability to generate quality spin on curveballs all have roots based in a pitcher’s arm action.

Ideal Arm Action
The best arm actions correlate with command, ability to spin the ball while generating natural two-plane depth on breaking stuff, and perhaps most importantly (especially the younger a pitcher is in their physical development): The capacity to make continual mechanical adjustments while experiencing increases in velocity. This is an area where quick-twitch actions and athleticism in general are evident in pitchers. Ideally, the throwing arm out of the glove takes the appearance of a naturally-produced smaller circle behind the body, but not overly-long such that the arm is easily seen and exposed behind the pitcher’s back hip. From there, the throwing arm accelerates from behind the pitcher’s body to its full extension point in a way that could be described as quick and effortless. As the throwing arm releases the ball, the arm-path, by and large, avoids any inhibited or stiff visuals to its forward-accelerating direction.

Examples of pitchers with great arm actions are generally those also associated with things such as overall athletic deliveries and quality control within the strike zone—not just of the fastball, but numerous secondary pitches as well. Zack Greinke and the legendary Greg Maddux come to mind; both feature(d) tremendous body control and loose extension through the end of their deliveries, and as such, are looked upon as some of the best in recent memory at executing numerous, well-located pitches to different quadrants of the strike zone.

Thank you for reading

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Great article!!! Thank you
Good read Adam... Keep these articles coming.