Welcome to our series of breaking down why NFL running backs decline at age 28! The series starts here.
NFL running backs are some of the most explosive athletes in the world — but even the best often see performance decline around age 28. One big reason? Changes in muscle fiber composition.
Let’s break down the three main muscle fiber types and why they matter for power-driven athletes like running backs.
1. Type I – The Endurance Masters
- Also Called: Slow-twitch fibers
- Best For: Long-duration, low-intensity efforts
- Fuel Source: Aerobic metabolism (oxygen-based)
- Key Strengths: High endurance, fatigue-resistant
- Example Use in Football: Sustained movement over a long game — keeping your legs fresh through all four quarters
Limitation: Not built for big bursts of speed or explosive cuts
A few other examples of this type of muscle fiber working at it’s best would be for marathon running, long-distance cycling, and hiking. All of these activities are endurance based activities where you do the same motion over and over again. These muscle fibers are made to help you keep pushing through those long efforts!
2. Type IIa – The Hybrid
- Also Called: Fast-twitch oxidative fibers
- Best For: Strong, repeated bursts with moderate endurance
- Fuel Source: Mix of aerobic and anaerobic metabolism (they use both blood and glycolytic stores)
- Key Strengths: Balanced power and stamina — can go hard, recover, and repeat
- Example Use in Football: Consecutive explosive plays, breaking through tackles while still holding speed into the next drive
- Limitation: Less endurance than Type I and less peak force than Type IIb
Great examples of type IIa muscle fibers are middle distance running (about a mile), playing soccer, and doing crossfit circuits. All of these need stamina combined with bursts of power.
3. Type IIb – The Powerhouse Specialists
- Also Called: Fast-twitch glycolytic fibers
- Best For: Maximum speed and force in short bursts
- Fuel Source: Anaerobic metabolism (no oxygen needed) and works with ATP-PC systems
- Key Strengths: Strongest and fastest contraction with the highest force output
- Example Use in Football: Exploding through the line of scrimmage, sprinting 40 yards for a touchdown
- Limitation: Fatigue quickly — can’t sustain output for long
The best example of Type IIb muscle fibers are giving it all on a 100m sprint! You’re looking for pure explosiveness with no need for long term endurance. This type of muscle fiber is also great for olympic weightlifting.
Fiber Types Change With Age
As we age — even in our late 20s — our bodies naturally shift toward more Type I (slow-twitch) fibers and fewer Type IIb (fast-twitch) fibers. This means less raw explosiveness, even if overall endurance stays solid. For running backs, that can translate into slower acceleration, less top-end speed, and reduced agility off the line.
The good news? You can fight this decline with targeted training. Power-focused lifts, sprint work, and plyometrics help preserve fast-twitch fibers and keep explosive performance in play — even as the years stack up.
Watch Dr. Jon Explain The Muscle Fiber Types
Stay tuned! In our next installment of why NFL running backs decline around 28 we’re talking about neuromuscular efficiency! Aka the brains ability to tell your muscles to fire properly. To stay up to date make sure you join our mailing list!
Training For Football?
Use our 10 minute daily mobility training to get your whole body ready season! Whether you use it as a warm-up, cool down, or both, these 10 minute mobility drills will help you move easier on the field. Whether you’re running an 30 yard touchdown or throwing the perfect pass, don’t let your mobility be the thing that stops you from making the perfect play!
