The 2-Year-Old Myth — Why Distal Limb Closure Does NOT Equal a Finished Horse

Some horse owners, trainers, and even veterinarians continue to insist that a two-year-old horse is “fully grown” and that “all growth plates are closed.”

Fact: That claim is scientifically false — and decades of anatomical, radiographic, and histological research prove it (references below).

More importantly: The only region close to done at two years is the distal limb. And yet, people extrapolate that distal limb closure to mean the entire skeleton — including the spine and pelvis — is mature.

This is not a minor misunderstanding. This is negligence.

We are watching an entire generation of horses break down under preventable strain — this is an epidemic

Young horses are breaking down — not because they are weak or defective, but because they are being asked to perform years before the structures responsible for load-bearing, shock absorption, and rider weight are mature.

Then therapists, veterinarians, and owners spend years — and tens to hundreds of thousands of dollars — trying to rehabilitate damage that should never have been caused in the first place.

I am tired of seeing preventable pain.

I am tired of seeing bodies ruined before they ever had a chance.

And I am tired of outdated science being used to justify it.

If you want to be part of the change, here is the actual evidence.

1. Growth Plate Closure Is Not Uniform Across the Body

Horses do not “finish growing” at one age. Growth plates close in a very specific sequence, from the ground up — with distal limbs closing first, and the pelvis and spine closing last.

This means:

YES — some distal limb physes may be closed around age 2.

NO — this does NOT mean the horse is skeletally mature.

Most misuse of science in this debate comes from people who see a radiograph of a closed fetlock or pastern and then assume the pelvis, sacrum, or spine has also fused.

Those areas are not even close.

Equine Growth Plate Closure Chart

(Downloadable PDF below)

Region	Typical Closure Range	Source
Distal phalanx / pastern	~6–12 months	Strand et al., 2007
Fetlock (MC/MT III distal)	~12–18 months	Malone, n.d.
Distal radius (“knees”)	~27–32 months	Strand et al., 2007
Stifle (tibial tuberosity)	~27–40 months	Strand et al., 2007
Distal tibia	~24 months	Malone, n.d.
Pelvic symphysis / ilium / ischium	5.2–5.8 years	Haussler et al., 1997
Lumbosacral vertebral physes	4.9–6.7 years	Haussler et al., 1997
Iliac crest apophysis	~7.2 years	Haussler et al., 1997
Residual vertebral cartilage	up to 22 years	Collar et al., 2019

GET YOUR FREE DOWNLOAD HERE (with clickable links to the research):

Growth Plate Closure Chart_FREE DOWLOAD.pdf

Download PDF • 987KB

This is the point most professionals are missing:

The distal limbs closing ≠ the pelvis and spine closing. They mature on completely different timelines.

2. The Axial Skeleton Matures YEARS After the Limbs

The pelvis and vertebral column are responsible for:

• Shock absorption

• Load transfer

• Carrying rider weight

• Engagement and collection

• Propulsion

  
  

And these are the last structures to fuse.

Key findings from:

Haussler et al. (1997):

• Pelvic growth plates open until ~5.8 years

• Lumbosacral growth plates open until ~6.7 years

• Iliac crest fuses around 7.2 years

Collar et al. (2019):

• Active cartilage found in horses aged 5–8 years

• One horse had residual cartilage at 22 years

Bennett (2001):

• Axial skeleton widely continues maturing into 6–8 years

So a horse at 2 years has:

• a nearly fused fetlock

• a partially fused radius

• a 5–7-year journey before the spine and pelvis are done

How anyone conflates those two realities is beyond comprehension.

Many of us have likely encountered these diagrams that reference the above research. They originate from this research conducted over two decades ago, so this isn't new information. (You can also find these diagrams in your free download above.)

Real-World Examples: Why “Looks Mature” Does Not Mean “Is Mature”

Below are two examples of two-year-old horses—different breeds, different sizes, different genders. They look nothing alike, but they share one critical fact: neither of them is skeletally mature, and the growth plates in their spine and pelvis are nowhere near closed

At 28 months and 15.3 h, this mare looks mature—but imaging would show open spinal and pelvic growth plates still years from completion and not developed enough to carry load without risk of fracture.

This pony is 24 months and proportionate for their type, which can mislead people into assuming they are mature. In reality, their axial skeleton is still developing, and key growth plates remain open.

This is why we cannot assess maturity by appearance alone.

A large or muscular two-year-old may look finished, but internally, they are still an adolescent with open physes throughout the axial skeleton.

When you ride a two-year-old, you are riding on an unstable skeleton. You are, quite literally, placing load on cartilage.

That “wiggly” or “unstable” feeling people describe in young horses is not a personality trait—it is the body struggling to stabilize a load it is not yet capable of managing, relying entirely on soft tissues and immature joint structures.

This is how early damage begins.

3. What the Distal Limb Data REALLY Means

The most generous interpretation of distal limb closure is: “The lowest limb segments are nearing closure by age 2.”

That’s it.

Nothing in distal limb anatomy tells us a single thing about:

• Spine closure

• Pelvic maturity

• Lumbosacral stability

• Sacroiliac development

• Vertebral body fusion

• Ability to bear a rider

Yet, somehow, people take an X-ray of a fetlock and claim the entire horse is “done growing.”

This is the scientific equivalent of a pediatrician saying a child is fully mature because their toes are. It is absurd.

4. Long-Bone Research Confirms Growth Continues at 2 Years

Even within the limbs:

• Fretz et al. (1984) found ongoing long-bone growth at 24 months

• Strand et al. (2007) showed stifle and distal radius still closing after 2 years

• Łuszczyński et al. (2011) confirmed significant breed/sex variation

• Myers (1963) found no evidence that any limb joint universally closes by 2

A 2-year-old is not “finished.”At best, they are mid-process.

5. Why People Still Believe the Myth

Even with this abundant research, people still believe that a two year old horse is fully fused and proceed to start the horse into competitive work.

Here's a few speculations why:

A) Distal limb timelines get misapplied to the whole horse

This is the biggest misunderstanding in the industry — and the most damaging.

B) Outdated teaching

Early vet school curriculum often summarized: “Most growth plates close around 2 years.”

But this referred to distal limb regions only, and even that is not uniform across breeds.

C) Industry pressure

Racing, futurities, and sales markets rely on horses starting early. Financial pressure often overrides biological reality.

D) Misinterpreting the Rogers & Dittmer (2021) paper

This study is often weaponized to argue for riding at 2 years, but:

• It uses anthropometric measures (height, proportions)

• It does not evaluate growth plate closure

• It does not image vertebral or pelvic physes

• It does not contradict the closure research showing 5–7 year timelines

It measures the outside and makes no claims about the inside.

E) Confusing conditioning with maturation

Bone density improves with exercise — this is true. But bone density ≠ growth plate closure. A 12-year-old child can increase bone density too. That does not make them skeletally mature.

6. The Welfare Cost of Getting This Wrong

Loading an immature pelvis and spine leads to:

• Pelvic and lumbar instability

• Lumbosacral malalignment

• Early degenerative changes

• Chronic compensatory patterns

• Persistent back pain

• SI dysfunction

• Shortened athletic lifespan

These are the “mystery cases” therapists see over and over — horses who never stay sound, never stabilize, never move correctly.

And they are non-reversible because:

• You cannot regrow a growth plate

• You cannot remodel malformed adult bone into the shape it should have been

These horses end up in lifelong rehab, or lifelong pain.

7. Clinical Roadmap For Introducing Training

Below is an overview of training that respects the horse’s anatomical and physiological development. Every horse should be evaluated by a qualified professional before beginning any structured program, but this serves as an evidence-based guide to what is clinically appropriate at each stage of growth.

Age 2

• Light groundwork only

• Balance, proprioception, straightness

• No riding

Age 3–4

• Occasional get on and get off for sensitizing horse to mounting

• Continued neuromuscular groundwork

• Avoid heavy or repetitive load

Age 5–6

• Pelvis and lumbosacral region more prepared

• Very light riding walk/trot and limited circling

• Continue to avoid heavy or repetitive load

Age 6–7+

• Horse is finally approaching skeletal maturity

• Introduce collection, hills, light lateral work if horse is prepared

• Suitable for athletic work if horse is prepared

The Bottom Line

A 2-year-old horse is not a finished athlete.The distal limbs may be approaching closure —but the spine and pelvis are years behind.

To claim “fully grown” at 2 years is not only scientifically inaccurate —it is ethically indefensible.

Yet, how often are horses sold off the track as "retired" from racing by the age of 4 or 5?

These horses are often injured before they have finished growing. They suffer from issues like growth plate fractures, which are major contributors to common conditions such as kissing spine, OCD, ringbone, shivers, and early onset arthritis in key joints such as the SI. Why do you think this happens? Trust me, it is not coincidence, it is science.

If we want sound, healthy horses with long athletic careers,we must stop pretending otherwise.

Below is a list of references from veterinarians who have dedicated their careers to advancing equine science. I have also included the often-misunderstood Rogers and Dittmer (2021) paper, specifically because its arguments are frequently cherry-picked online. When read in full, their conclusions about “skeletal maturity” do not contradict the anatomical and radiographic evidence presented in this blog—they simply use different metrics (anthropometric proxies rather than imaging of growth plates).

If you have heard conflicting information—online, from peers, or even from professionals—please return to the research itself. These veterinarians hold medical degrees, conduct rigorous studies, and contribute evidence that is essential to equine welfare. Their work is time-consuming, often underfunded, and sometimes self-funded. The best way to support them—and the horses in our care—is to read their research and share it.

References

Bennett, D. (2001). Timing and rate of skeletal maturation in horses.

Cymbaluk, N. F., & Pharr, J. W. (1984). Quantitative analysis of long-bone growth in the horse. American Journal of Veterinary Research, 45(8), 1602–1609.

Haussler, K. K., Stover, S. M., & Willits, N. H. (1997). Developmental variation in lumbosacropelvic anatomy of Thoroughbred racehorses. American Journal of Veterinary Research, 58(10), 1083–1091.

Łuszczyński, J. et al. (2011). Effect of breed and sex on growth rate and radiographic closure time of distal radial metaphyseal growth plate. Livestock Science, 142, 67–73.

Malone, E. (n.d.). Large Animal Surgery – Physeal Disorders (Angular Limb Deformities). University of Minnesota.

Myers, V. S. (1963). The age and manner of closure of various epiphyses and other centers of ossification in the front limb of the domestic horse. M.S. Thesis, Iowa State University.

Nielsen, B. D. (2023). A review of three decades of research dedicated to making equine bones stronger: Implications for horses and humans. Animals, 13(5), 789.

Rogers, C. W., & Dittmer, K. E. (2021). Growth and bone development in the horse: When is a horse skeletally mature? Animals, 11(12), 3402.

Strand, E. et al. (2007). Radiographic closure time of appendicular growth plates in the Icelandic horse. Acta Veterinaria Scandinavica, 49(19).

Vulcano, L. C. et al. (1997). Radiographic study of distal radial physeal closure in Thoroughbred horses. Veterinary Radiology & Ultrasound, 38(5), 352–354.

Collar, K. et al. (2019). Lower back anatomy, growth plate closure in Quarter Horses. The Horse.