If you asked most athletes — or most coaches, honestly — which muscle matters most for top speed, you’d hear “hamstrings” or “quads” nine times out of ten. New research on collegiate football players says otherwise, and the answer has real implications for how we train and rehab speed at Kinetix Sport + Spine.
The Study
A team of researchers led by Jack Martin at the University of Wisconsin–Madison, working with David Opar’s lab and Springbok Analytics, scanned 66 Division I football players with MRI to measure the size of 35 different lower-body muscles. They also measured each player’s peak sprint speed on the field using motion sensors. Then they asked a simple question: how much of the difference in speed between players can be explained just by how big specific muscles are?
The answer surprised a lot of people in sports science circles.
The psoas major — a deep hip flexor most people have never consciously trained — explained 50% of the variance in sprint speed all by itself. Glute medius was next (45%), then glute maximus (37%). Everything below the hip — quads, calves, even the hamstrings — explained far less.
How Each Muscle Stacked Up
Approximate variance in sprint speed explained by individual muscle volume, based on reported study findings.
Maybe the most interesting finding: stacking more muscles into the equation barely helped. Adding two more muscles to psoas major only pushed the explained variance up another 7–9%. The hip flexors and glutes are so tightly linked in how they develop that once you account for one, the others don’t add much new information.
Why the Psoas?
The psoas major sits deep in your core, connecting your lumbar spine to your femur. Its job is hip flexion — pulling your leg forward and up. During sprinting, this is exactly what has to happen, over and over, as fast as possible: snap the leg through, minimize ground contact time, repeat.
The researchers offered a few theories for why psoas size tracks so closely with speed:
Direct Mechanical Contribution
A bigger psoas may simply generate more force to swing the leg forward faster — straightforward force production translating directly into limb speed.
A Marker of Overall Hip Development
Psoas size may just reflect how well-developed someone’s entire hip flexor/extensor system is — the engine room of sprinting rather than a single isolated lever.
Proximal Mass Efficiency
Muscle mass closer to the hip (proximal) adds strength without adding much rotational inertia to the leg. Muscle mass at the ankle (distal) does the opposite — it makes the leg harder to swing quickly. Interestingly, in this study, smaller calf muscles were mildly associated with faster sprinting.
What This Doesn’t Mean
This is correlational research — bigger psoas correlates with faster sprinting, but the study can’t prove psoas training causes speed gains. It’s also worth noting: even their best combination of every muscle in the lower body topped out at explaining 59% of the variance in speed. That leaves over 40% explained by other things entirely — tendon stiffness, muscle fascicle length, fiber type, foot and heel anatomy, and sprint technique. Nobody’s becoming Usain Bolt by deadlifting alone.
What This Means in Clinic
This study lines up with what we already see and coach for athletes who come through Kinetix Sport + Spine looking to build speed or return from injury.
Hip Flexor and Glute Strength Deserve Real Priority in Programming
Not as an afterthought tacked onto a leg day, but as a central pillar. Loaded hip flexion work (weighted marches, banded hip flexion, sled work) and posterior hip strength (hip thrusts, RDLs, lateral band work for glute medius) earn their place at the top of a speed-focused program.
This Is Especially Relevant for Linemen and Bigger-Bodied Athletes
The study found that, relative to body size, linemen had smaller psoas and glute volumes than skill players — and they were also the slowest group. That’s not a knock on effort; it’s a training opportunity. If you’re working with an athlete who’s “strong but not fast,” hip flexor and glute development relative to total mass is worth a hard look.
Speed Work Isn’t Just Sprint Mechanics Drilling
You can clean up someone’s technique all day, but if the underlying tissue capacity in the hip isn’t there, technique has a ceiling. We treat strength and size development in the hip complex as foundational to speed work, not separate from it.
It Reinforces Our Approach to Return-to-Play After Hip, Groin, or Hamstring Injuries
Hip flexor strength and capacity isn’t just about reducing reinjury risk — based on this data, it may be one of the single biggest levers for getting an athlete back to their pre-injury top speed.
This is the same hip flexor and psoas complex we address constantly with Full Body ART — the structure most commonly adhered and restricted in golfers, runners, and desk-bound athletes alike. If the psoas is tight, restricted, or full of adhesions, it can’t produce the force or speed of contraction this research shows it’s responsible for — regardless of how much you’ve strengthened it in the gym. Tissue quality and tissue size are two different variables, and both need to be addressed for the psoas to do its job at the level this study suggests it can.
The Takeaway
Sprint speed is complicated, and no single number tells the whole story. But if you’re an athlete, a parent, or a coach trying to figure out where to focus training time, this research gives a clear, evidence-backed answer: build the hips first. The psoas and glutes aren’t just core stabilizers — they may be the single biggest lever for raw speed that most training programs underuse.
If you’re working through a hip, groin, or hamstring issue — or just want a sports-focused chiropractic and performance assessment to find out where your speed ceiling actually is — that’s exactly the kind of evaluation we do at Kinetix Sport + Spine.
Reference: Martin JA, Joachim MR, Blemker SS, Opar DA, Mortensen B, Heiderscheit BC, The HAMIR Study Group. Lower-Body Muscle Volumes Can Explain Half of the Variance in Sprint Speed Between Collegiate American Football Players. Scandinavian Journal of Medicine & Science in Sports. 2026;36(4):e70283.
Find Out Where Your Speed Ceiling Actually Is
Hip, groin, or hamstring issue holding you back? A sports-focused chiropractic and performance assessment identifies exactly what’s limiting your speed — tissue quality, strength, or mobility.
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