MLB Reliever Fatigue in the Modern Era

Everyone knows a fatigued bullpen is trouble. It's intuitive, it matches what we know about every other athlete, and it gets reinforced every time a gassed reliever leaves one hanging over the plate on his third straight day of work. Projection systems dock tired arms. Broadcasters flag heavy workloads. Fans groan when the same closer trots out for the third time in four days. It's one of the few things in baseball everyone agrees on.

So we tested it.

We rebuilt the fatigue state of every relief appearance from two seasons six years apart, roughly 33,000 outings in all: pitch counts over the previous two and four days, days of rest, consecutive-day streaks, and each pitcher's fastball velocity measured against his own baseline for the season. Essentially, recreating a snapshot of the pitcher's "fatigue factor" going in to every game.

And what we found might surprise you.

The radar gun cooperated with the conventional wisdom. Tired relievers throw measurably softer, in every quality tier, in both seasons. The box scores did not cooperate. The results of those tired outings were nearly indistinguishable from fresh ones, and in some of the highest-pressure spots in baseball, slightly better.

Both things are true, and the gap between them is the story. The physical cost of fatigue shows up on every radar reading, yet the damage almost never reaches the scoreboard. Resolving that contradiction means asking a question most fatigue research skips: not "what does fatigue do to a pitcher," but "which pitchers are being deployed at less than 100% in the first place?" The answer turns out to say less about arms than about the people deciding when to use them.

What We Looked At

We reconstructed the fatigue state of every relief appearance in the 2019 and 2025 regular seasons, roughly 17,000 outings in 2019 and 15,900 in 2025.

For each appearance we knew the pitcher's cumulative pitch count over the previous two and four days, his days of rest, his consecutive-day streak, and his fastball velocity relative to his own established baseline. After requiring enough track record to set a trustworthy baseline, about 14,900 and 14,100 outings qualified for analysis.

Every outing was assigned a quality tier from the pitcher's season-to-date ERA at the moment he entered the game:

Why tier by current-season ERA rather than reputation? Because that running ERA is what the manager sees when he picks up the bullpen phone. A name-brand closer who has been getting hit all year is, for deployment purposes, a different pitcher than one who has been untouchable.

Why did we pick these two particular years? We compared 2019 and 2025 because they bracket a genuine divide in eras of bullpen baseball: 2019 was the last full season before the three-batter minimum was implemented, before the 13-pitcher roster cap, and before the pitch clock. All of which reshaped how relievers get used. Six years apart, two different rulebooks, same physics.

Finding 1: Fatigue Costs Velocity in Every Tier, Both Eras

Tired relievers throw softer. This is the one result that holds no matter who is getting tired or how managers choose to deploy them.

Eight tier-season cells, eight negative numbers, all clustered around half a mile per hour. Whatever else changed in baseball between 2019 and 2025, the physical price of pitching tired did not. One methodological note for the skeptics: fresh relievers actually run slightly above their own season baseline, because the baseline includes their tired outings and because velocity builds through the early months of a season. The fresh-to-tired swing is the honest measure, and it points the same direction everywhere.

Finding 2: Bullpens Have Gotten Dramatically Deeper

Modern roster construction carries more quality arms and far fewer liabilities. The tier distribution makes the case by itself.

Half of all 2025 relief outings came from above-average or elite arms, up from 37 percent in 2019. Below-average arms went from handling more than a third of the league's relief work to less than a quarter. The change shows up in workloads too: the average below-average reliever logged 12.5 qualifying outings in 2025 against 15.2 in 2019, while above-average arms rose from 15.3 to 16.4. There is simply a better arm available on most rosters now, and managers lean on it.

Finding 3: Managers Route Work by Quality, Relentlessly

When the game is on the line, the ball goes to the best available arm, and the gradient is steep. In 2019, a fresh elite reliever's outing was a save situation 19.7 percent of the time; a fresh below-average reliever's, 2.0 percent. In 2025 the same comparison runs 11.7 percent against 2.8. The best arms live in the highest-leverage innings, an order of magnitude more often than the bottom of the bullpen.

And the share of save situations does not fall off as the good arms accumulate fatigue. Moderately fatigued elite relievers in 2019 saw save situations on 32.4 percent of their outings, well above their fresh rate. The pattern is gentler in 2025 (17.0 percent against 11.7 fresh) but points the same way. A tired arm, in any tier, is usually an arm the manager has been actively choosing all week. Fatigue in this data is less a warning label than a receipt: proof of recent trust.

The Payoff: Elite Arms Answer the Call, Fresh or Not

Here is the table that ties it together. Within each tier, does recent workload change how often save opportunities convert cleanly?

Read the elite row first. Roughly two of every three save chances convert cleanly, and the rate simply does not degrade with recent workload. If anything, the moderately fatigued cells run a touch higher across most tiers, which is the signature of selection rather than a benefit of fatigue: when a manager sends a fatigued arm back out, he has already judged that arm capable of the moment, and he is usually right.

That is the resolution of the apparent paradox. Aggregate the data naively and tired relievers look fine, even good. Stratify by quality and the mechanism appears: the tired-pitcher population is loaded with the arms managers trust most, used in the spots they have earned. The velocity cost is real and universal. The outcome cost rarely surfaces, because the men making the decisions have become exceptionally good at knowing which arms can pay it.

What This Means

For fans

The bullpen choreography you watch every night is more sophisticated than it looks. Each call to the pen weighs quality, matchup, and accumulated workload, and the weighting is mostly invisible because it works. Elite closers pitch fatigued because they remain the best option. Lesser arms get the night off before the damage shows. The data catches managers running this calculation with remarkable consistency across two seasons six years apart.

For sabermetricians

Public research on reliever fatigue keeps coming up empty in conclusive outcome data, and this is why. The aggregate tired-versus-fresh comparison is dominated by selection: the tired population is disproportionately composed of trusted, high-quality arms. Controlling pitcher quality is the differentiating factor, as is the in-game situation. The cleanest design is within-pitcher, comparing each arm's fresh outings to its own tired ones, and the velocity channel suggests there is a real physical effect that can be measured there.

For DFS players / Sports Bettors

This should change how you read a reliever's workload column. Fatigue penalties in projection models are fighting a signal that manager behavior has largely neutralized. When a heavily fatigued arm takes the mound, the manager has already vouched for it, and the conversion data backs his judgment. The signal that actually pays runs the other way. When a reliever who has been sitting for days suddenly turns up in a high leverage or save situation, treat it as a warning rather than a gift, because it usually means the arms ahead of him are unavailable and you are looking at the manager's fourth option, not his first.

Based on roughly 17,000 relief appearances in the 2019 MLB regular season and 15,900 in 2025, with per-outing fatigue states reconstructed from pitch counts, rest days, and appearance streaks, and quality tiers assigned from each pitcher's season-to-date ERA entering the game. About 14,900 and 14,100 outings respectively carried enough pitcher track record to qualify for analysis. Velocity comparisons use each pitcher's own cumulative fastball baseline; aggregate figures exclude position players and extreme outliers. Fresh, lightly fatigued, and moderately fatigued refer to escalating recent-workload states; the heaviest workload buckets are excluded from tables where samples fall below publishable size.

Published by InsidethePen Staff
6/30/2026