The Science Behind Soundness and Longevity in Horses

Photo Credit: horsesandus.com

The tradition of starting horses under saddle at two years old has been widely accepted in various equestrian disciplines, particularly in racing, reining, and Western performance horse industries. However, modern research and advances in equine biomechanics, skeletal development, and veterinary science suggest that 𝗿𝗶𝗱𝗶𝗻𝗴 𝗵𝗼𝗿𝘀𝗲𝘀 𝘁𝗼𝗼 𝘆𝗼𝘂𝗻𝗴 𝗰𝗮𝗻 𝗵𝗮𝘃𝗲 𝘀𝗶𝗴𝗻𝗶𝗳𝗶𝗰𝗮𝗻𝘁 𝗹𝗼𝗻𝗴-𝘁𝗲𝗿𝗺 𝗰𝗼𝗻𝘀𝗲𝗾𝘂𝗲𝗻𝗰𝗲𝘀 𝗼𝗻 𝘁𝗵𝗲𝗶𝗿 𝗵𝗲𝗮𝗹𝘁𝗵 𝗮𝗻𝗱 𝘀𝗼𝘂𝗻𝗱𝗻𝗲𝘀𝘀.

In this blog, we’ll explore the scientific reasons why early training and riding can be detrimental to a horse’s longevity and performance, using actual 𝗱𝗮𝘁𝗮 and 𝘃𝗲𝘁𝗲𝗿𝗶𝗻𝗮𝗿𝘆 𝗶𝗻𝘀𝗶𝗴𝗵𝘁𝘀 to support delaying the start of ridden work.

The Science of Skeletal Maturity in Horses

A horse's skeleton develops in stages, with different growth plates (physes) closing at various ages. Growth plates are areas of cartilage at the ends of bones that eventually ossify into hard bone. The earlier a horse is subjected to weight-bearing work, the 𝗴𝗿𝗲𝗮𝘁𝗲𝗿 𝘁𝗵𝗲 𝗿𝗶𝘀𝗸 𝗼𝗳 𝗱𝗮𝗺𝗮𝗴𝗲 to these still-developing structures.

Horses grow at different rates, and while they may look fully grown at two, their skeletons are still developing. Skeletal maturity is not complete until 𝟱.𝟱 𝘁𝗼 𝟲 𝘆𝗲𝗮𝗿𝘀 𝗼𝗹𝗱, with the spine being the last structure to fully mature.

𝗗𝗿. 𝗗𝗲𝗯 𝗕𝗲𝗻𝗻𝗲𝘁𝘁, an equine anatomist, has extensively studied skeletal maturation in horses. According to her research:

• The last bones to mature are in the 𝘀𝗽𝗶𝗻𝗲, and they do not fully close until at least 5.5 to 6 years old.

• The 𝘃𝗲𝗿𝘁𝗲𝗯𝗿𝗮𝗹 𝗴𝗿𝗼𝘄𝘁𝗵 𝗽𝗹𝗮𝘁𝗲𝘀, particularly those in the neck and back, close much later than the limb bones.

• The 𝗽𝗲𝗹𝘃𝗶𝘀, another weight-bearing structure, also remains open until around 5-6 years of age.

Growth Plate Closure Timeline

Every bone in a horse’s body has a 𝗴𝗿𝗼𝘄𝘁𝗵 𝗽𝗹𝗮𝘁𝗲 (physis) that eventually hardens into mature bone. The earlier a horse is subjected to carrying weight, the greater the risk of damaging these still-forming structures.

The most significant risks associated with early riding include:

• 𝗜𝗿𝗿𝗲𝘃𝗲𝗿𝘀𝗶𝗯𝗹𝗲 joint damage

• Increased risk of 𝗼𝘀𝘁𝗲𝗼𝗮𝗿𝘁𝗵𝗿𝗶𝘁𝗶𝘀

• Weakened tendons and ligaments leading to chronic lameness

• Spinal issues affecting biomechanics and long-term posture

• Increased mental stress and anxiety, leading to behavioral problems

🔬 𝗞𝗲𝘆 𝗧𝗮𝗸𝗲𝗮𝘄𝗮𝘆: The spine, which supports both the rider and saddle, is the 𝗹𝗮𝘀𝘁 structure to fully mature. Riding a horse before its spine has solidified can cause long-term back pain, poor posture, and 𝗶𝗿𝗿𝗲𝘃𝗲𝗿𝘀𝗶𝗯𝗹𝗲 𝗱𝗮𝗺𝗮𝗴𝗲 to the vertebrae.

The Reality of Growth Plates

Many people assume that because a horse "looks full-grown," they must be physically mature. However, 𝘀𝗸𝗲𝗹𝗲𝘁𝗮𝗹 𝗺𝗮𝘁𝘂𝗿𝗶𝘁𝘆 is not the same as outward appearance.

As mentioned in Dr. Deb Bennett’s research, different bones close at different ages, and the 𝘀𝗽𝗶𝗻𝗮𝗹 𝗰𝗼𝗹𝘂𝗺𝗻 is the last to fully mature. The spine carries the weight of both the rider and the saddle, making it particularly vulnerable to premature stress.

Here's an expanded look at the 𝘀𝗸𝗲𝗹𝗲𝘁𝗮𝗹 𝗰𝗹𝗼𝘀𝘂𝗿𝗲 𝘁𝗶𝗺𝗲𝗹𝗶𝗻𝗲:

Early Riding and Osteoarthritis

In young horses, the 𝗰𝗮𝗿𝘁𝗶𝗹𝗮𝗴𝗲 covering the ends of bones (articular cartilage) is not fully developed, making it 𝗺𝗼𝗿𝗲 𝘃𝘂𝗹𝗻𝗲𝗿𝗮𝗯𝗹𝗲 to damage. 

A 2013 study in 𝙏𝙝𝙚 𝙅𝙤𝙪𝙧𝙣𝙖𝙡 𝙤𝙛 𝙊𝙧𝙩𝙝𝙤𝙥𝙖𝙚𝙙𝙞𝙘 𝙍𝙚𝙨𝙚𝙖𝙧𝙘𝙝 found that excessive concussive force on immature joints 𝗮𝗰𝗰𝗲𝗹𝗲𝗿𝗮𝘁𝗲𝘀 cartilage wear, leading to early-onset osteoarthritis (OA) and degenerative joint disease later in life. Cartilage in young horses is particularly vulnerable because:

• It has 𝗹𝗲𝘀𝘀 density and resilience than adult cartilage.

• It 𝗹𝗮𝗰𝗸𝘀 adequate lubrication to withstand heavy workloads.

• Repetitive strain creates 𝗺𝗶𝗰𝗿𝗼𝗳𝗿𝗮𝗰𝘁𝘂𝗿𝗲𝘀 in joint surfaces, leading to chronic inflammation.

A 2018 MRI study on Thoroughbred racehorses published in 𝙀𝙦𝙪𝙞𝙣𝙚 𝙑𝙚𝙩𝙚𝙧𝙞𝙣𝙖𝙧𝙮 𝙅𝙤𝙪𝙧𝙣𝙖𝙡 revealed that early training correlates with an increased incidence of 𝘀𝘂𝗯𝗰𝗵𝗼𝗻𝗱𝗿𝗮𝗹 𝗯𝗼𝗻𝗲 𝗺𝗶𝗰𝗿𝗼𝗳𝗿𝗮𝗰𝘁𝘂𝗿𝗲𝘀 (tiny fractures leading to arthritis) and 𝗰𝗮𝗿𝘁𝗶𝗹𝗮𝗴𝗲 𝗲𝗿𝗼𝘀𝗶𝗼𝗻 (which is irreversible) —conditions that often lead to joint inflammation, chronic lameness, and 𝗿𝗲𝗱𝘂𝗰𝗲𝗱 𝗰𝗮𝗿𝗲𝗲𝗿 𝗹𝗼𝗻𝗴𝗲𝘃𝗶𝘁𝘆.

Horses who began intense training at 𝘁𝘄𝗼 𝘆𝗲𝗮𝗿𝘀 𝗼𝗹𝗱:

• Were 𝗳𝗼𝘂𝗿 𝘁𝗶𝗺𝗲𝘀 more likely to develop stress fractures.

• Had 𝘀𝗶𝗴𝗻𝗶𝗳𝗶𝗰𝗮𝗻𝘁𝗹𝘆 𝗵𝗶𝗴𝗵𝗲𝗿 rates of arthritis before the age of five.

• Had careers that lasted an average of 𝟭.𝟱 𝘆𝗲𝗮𝗿𝘀 𝘀𝗵𝗼𝗿𝘁𝗲𝗿 than horses started later.

In contrast, 𝗵𝗼𝗿𝘀𝗲𝘀 𝘁𝗵𝗮𝘁 𝗮𝗿𝗲 𝗮𝗹𝗹𝗼𝘄𝗲𝗱 𝘁𝗼 𝗺𝗮𝘁𝘂𝗿𝗲 before being placed into heavy work have stronger, 𝗺𝗼𝗿𝗲 𝗿𝗲𝘀𝗶𝗹𝗶𝗲𝗻𝘁cartilage and are less likely to develop premature joint disease.

📌 𝗥𝗲𝗮𝗹-𝗪𝗼𝗿𝗹𝗱 𝗘𝘅𝗮𝗺𝗽𝗹𝗲:

• 𝟲𝟱% 𝗼𝗳 𝗞𝗲𝗻𝘁𝘂𝗰𝗸𝘆 𝗗𝗲𝗿𝗯𝘆 𝗵𝗼𝗿𝘀𝗲𝘀 from 2000-2020 retired before age five due to joint-related issues.

• 𝗘𝘃𝗲𝗻𝘁𝗶𝗻𝗴 𝗮𝗻𝗱 𝗱𝗿𝗲𝘀𝘀𝗮𝗴𝗲 𝗵𝗼𝗿𝘀𝗲𝘀, which are started at 4-6 years old, often compete successfully into their 20s.

Why Tendons and Ligaments Are at Risk

Soft tissue structures, such as tendons and ligaments, also 𝘁𝗮𝗸𝗲 𝘁𝗶𝗺𝗲 𝘁𝗼 𝘀𝘁𝗿𝗲𝗻𝗴𝘁𝗵𝗲𝗻. Unlike muscles, which adapt relatively quickly to exercise, tendons and ligaments require 𝗹𝗼𝗻𝗴𝗲𝗿 to fully mature and become resilient to repetitive strain.

A study in 𝙏𝙝𝙚 𝙑𝙚𝙩𝙚𝙧𝙞𝙣𝙖𝙧𝙮 𝙅𝙤𝙪𝙧𝙣𝙖𝙡 (2005) found that horses trained at two years old had a significantly higher rate of tendon and suspensory ligament injuries compared to those started later. This is because the 𝗰𝗼𝗹𝗹𝗮𝗴𝗲𝗻 𝗳𝗶𝗯𝗲𝗿𝘀 in young tendons are still developing their strength and elasticity.

🔬 𝗦𝗰𝗶𝗲𝗻𝘁𝗶𝗳𝗶𝗰 𝗗𝗮𝘁𝗮: 

The Veterinary Journal study also found: Repeated stress from riding and training 𝗯𝗲𝗳𝗼𝗿𝗲 the tendons and ligaments are fully developed can lead to:

• 𝗧𝗲𝗻𝗱𝗼𝗻𝗶𝘁𝗶𝘀 (inflammation of the tendons)

• 𝗦𝘂𝘀𝗽𝗲𝗻𝘀𝗼𝗿𝘆 𝗹𝗶𝗴𝗮𝗺𝗲𝗻𝘁 injuries

• Increased risk of 𝗰𝗮𝘁𝗮𝘀𝘁𝗿𝗼𝗽𝗵𝗶𝗰 𝗯𝗿𝗲𝗮𝗸𝗱𝗼𝘄𝗻𝘀 in later years

Delaying intense work allows these structures to adapt gradually, reducing the likelihood of soft tissue injuries.

A study on Standardbred racehorses in Veterinary Surgery Journal (2010) found that 𝟴𝟬% 𝗼𝗳 𝘀𝘂𝘀𝗽𝗲𝗻𝘀𝗼𝗿𝘆 𝗹𝗶𝗴𝗮𝗺𝗲𝗻𝘁 𝗶𝗻𝗷𝘂𝗿𝗶𝗲𝘀 occurred in horses trained before skeletal maturity.

📌 𝗥𝗲𝗮𝗹-𝗪𝗼𝗿𝗹𝗱 𝗘𝘅𝗮𝗺𝗽𝗹𝗲:

The most 𝗰𝗼𝗺𝗺𝗼𝗻 injury in young reining horses is 𝘀𝘂𝘀𝗽𝗲𝗻𝘀𝗼𝗿𝘆 𝗹𝗶𝗴𝗮𝗺𝗲𝗻𝘁 𝘀𝘁𝗿𝗮𝗶𝗻, which results in 𝗰𝗮𝗿𝗲𝗲𝗿-𝗲𝗻𝗱𝗶𝗻𝗴 lameness. In contrast, dressage and endurance horses started later have 𝘀𝗶𝗴𝗻𝗶𝗳𝗶𝗰𝗮𝗻𝘁𝗹𝘆 𝗹𝗼𝘄𝗲𝗿 rates of 𝘀𝗼𝗳𝘁 𝘁𝗶𝘀𝘀𝘂𝗲 𝗶𝗻𝗷𝘂𝗿𝘆.

The Missing Piece

Beyond physical development, a horse's 𝗻𝗲𝘂𝗿𝗼𝗹𝗼𝗴𝗶𝗰𝗮𝗹, 𝗺𝗲𝗻𝘁𝗮𝗹, 𝗮𝗻𝗱 𝗲𝗺𝗼𝘁𝗶𝗼𝗻𝗮𝗹 𝗺𝗮𝘁𝘂𝗿𝗶𝘁𝘆 is also a critical factor. Two-year-old horses are still in a 𝗷𝘂𝘃𝗲𝗻𝗶𝗹𝗲 stage of brain development, which can lead to increased 𝗮𝗻𝘅𝗶𝗲𝘁𝘆 and difficulty processing training.

A 2020 study published in 𝘼𝙥𝙥𝙡𝙞𝙚𝙙 𝘼𝙣𝙞𝙢𝙖𝙡 𝘽𝙚𝙝𝙖𝙫𝙞𝙤𝙪𝙧 𝙎𝙘𝙞𝙚𝙣𝙘𝙚 found that horses started later (𝟰-𝟲 𝘆𝗲𝗮𝗿𝘀 𝗼𝗹𝗱) demonstrated:

• 𝗟𝗼𝘄𝗲𝗿 stress levels as measured by cortisol responses

• 𝗕𝗲𝘁𝘁𝗲𝗿 learning retention

• 𝗦𝘁𝗿𝗼𝗻𝗴𝗲𝗿 problem-solving abilities

Horses that were started at 𝟮 𝘆𝗲𝗮𝗿𝘀 𝗼𝗹𝗱:

• Had significantly 𝗵𝗶𝗴𝗵𝗲𝗿 𝗰𝗼𝗿𝘁𝗶𝘀𝗼𝗹 (stress hormone) levels.

• Increased rates of 𝗮𝗻𝘅𝗶𝗲𝘁𝘆 𝗮𝗻𝗱 𝘀𝘁𝗿𝗲𝘀𝘀-related behaviors like weaving, cribbing, and bucking.

📌 𝗥𝗲𝗮𝗹-𝗪𝗼𝗿𝗹𝗱 𝗘𝘅𝗮𝗺𝗽𝗹𝗲:

Many professional horse trainers note that horses started too early tend to have 𝗯𝗲𝗵𝗮𝘃𝗶𝗼𝗿𝗮𝗹 𝗿𝗲𝘀𝗶𝘀𝘁𝗮𝗻𝗰𝗲 as they mature, leading to problems such as:

• 𝗨𝗹𝗰𝗲𝗿𝘀 and stress 𝗰𝗼𝗹𝗶𝗰 from training pressure.

• 𝗦𝗵𝘂𝘁𝗱𝗼𝘄𝗻 responses (learned helplessness).

• Increased 𝘀𝗽𝗼𝗼𝗸𝗶𝗻𝗲𝘀𝘀 𝗮𝗻𝗱 𝗿𝗲𝗮𝗰𝘁𝗶𝘃𝗶𝘁𝘆 due to undeveloped neural pathways.

Horses that are mentally mature before being introduced to riding are generally 𝗺𝗼𝗿𝗲 𝗰𝗼𝗻𝗳𝗶𝗱𝗲𝗻𝘁, less prone to anxiety, and build 𝘀𝘁𝗿𝗼𝗻𝗴𝗲𝗿 𝗿𝗲𝗹𝗮𝘁𝗶𝗼𝗻𝘀𝗵𝗶𝗽𝘀 with their handlers.

Why Waiting Pays Off

The 𝗹𝗼𝗻𝗴𝗲𝘀𝘁-𝗹𝗶𝘃𝗲𝗱 performance horses across disciplines tend to be those that were started later. Horses with gradual, 𝗮𝗴𝗲-𝗮𝗽𝗽𝗿𝗼𝗽𝗿𝗶𝗮𝘁𝗲, training maintain soundness, willingness, and 𝗹𝗼𝗻𝗴𝗲𝘃𝗶𝘁𝘆 far beyond those rushed into work too soon.

The Data Speaks

When looking at equine disciplines with the 𝗹𝗼𝗻𝗴𝗲𝘀𝘁 average career spans, there's a clear trend: horses started 𝗹𝗮𝘁𝗲𝗿 tend to stay sound longer.

🔬 𝗗𝗮𝘁𝗮 𝗖𝗼𝗺𝗽𝗮𝗿𝗶𝘀𝗼𝗻 𝗔𝗰𝗿𝗼𝘀𝘀 𝗗𝗶𝘀𝗰𝗶𝗽𝗹𝗶𝗻𝗲𝘀:

Racing Industry Data (Started at 2): Average career length = 3-6 years

• A 2018 analysis of Thoroughbred racehorses found that only 𝟭𝟱% of horses raced at two years old were still competing at five years old.

• Conversely, horses that did not start racing until at least three or four years old had 𝘀𝗶𝗴𝗻𝗶𝗳𝗶𝗰𝗮𝗻𝘁𝗹𝘆 𝗵𝗶𝗴𝗵𝗲𝗿 chances of racing beyond five years.

Reining Performance Horse Data (Started at 2): Average career length = 3-6 years

• Reining horses, commonly started at two, frequently retire by 6-7 years due to 𝗵𝗼𝗰𝗸 𝗮𝗿𝘁𝗵𝗿𝗶𝘁𝗶𝘀 𝗮𝗻𝗱 𝗻𝗮𝘃𝗶𝗰𝘂𝗹𝗮𝗿 𝗰𝗵𝗮𝗻𝗴𝗲𝘀.

Dressage & Show Jumping Data (Started at 4-6): Average career length = 12-18 years

• Dressage and eventing horses, which are generally started later, often compete 𝘀𝘂𝗰𝗰𝗲𝘀𝘀𝗳𝘂𝗹𝗹𝘆 into their 𝗹𝗮𝘁𝗲 𝘁𝗲𝗲𝗻𝘀 𝗮𝗻𝗱 𝗲𝗮𝗿𝗹𝘆 𝘁𝘄𝗲𝗻𝘁𝗶𝗲𝘀.

Endurance Horses Data (Started at 6+): Average career length = 15-25 years

• Gradual conditioning over time 𝘀𝘁𝗿𝗲𝗻𝗴𝘁𝗵𝗲𝗻𝘀 joints, tendons, and ligaments, reducing the risk of repetitive stress injuries and long-term lameness.

This suggests that 𝗱𝗲𝗹𝗮𝘆𝗶𝗻𝗴 𝘄𝗼𝗿𝗸 𝗱𝗼𝗲𝘀 𝗻𝗼𝘁 𝗵𝗶𝗻𝗱𝗲𝗿 𝗽𝗲𝗿𝗳𝗼𝗿𝗺𝗮𝗻𝗰𝗲; rather, it preserves the horse’s ability to perform well for a longer period.

What Can You Do Instead

If you want to ensure your horse develops 𝗽𝗿𝗼𝗽𝗲𝗿𝗹𝘆 while still engaging their mind and body, consider alternative groundwork exercises before riding:

✅ 𝗜𝗻-𝗵𝗮𝗻𝗱 training (liberty work, lateral movements, desensitization)

✅ 𝗟𝗼𝗻𝗴-𝗹𝗶𝗻𝗶𝗻𝗴 and ground driving

✅ Gentle 𝗵𝗶𝗹𝗹 𝘄𝗼𝗿𝗸 and turnout for natural conditioning

✅ 𝗕𝗼𝗱𝘆 𝗮𝘄𝗮𝗿𝗲𝗻𝗲𝘀𝘀 exercises over poles and obstacles

✅ Building a foundation of trust and communication without physical strain

These activities allow young horses to develop coordination, balance, and trust 𝘄𝗶𝘁𝗵𝗼𝘂𝘁 𝘁𝗵𝗲 𝗿𝗶𝘀𝗸𝘀 associated with early saddle work.

Patience Leads to Soundness

The 𝘀𝗰𝗶𝗲𝗻𝗰𝗲 is clear—starting a horse under saddle at two years old is 𝗻𝗼𝘁 in their best long-term interest. While they may be capable of performing at a young age, their skeletal, joint, and 𝘀𝗼𝗳𝘁 𝘁𝗶𝘀𝘀𝘂𝗲 structures are not fully developed, leading to a significantly 𝗶𝗻𝗰𝗿𝗲𝗮𝘀𝗲𝗱 𝗿𝗶𝘀𝗸 𝗼𝗳 𝗶𝗻𝗷𝘂𝗿𝘆 and early retirement.

A 𝘄𝗲𝗹𝗹-𝗱𝗲𝘃𝗲𝗹𝗼𝗽𝗲𝗱 horse is a 𝘀𝗼𝘂𝗻𝗱 horse. And a sound horse is one that can serve as your partner for years to come. By waiting until at least four to five years old before introducing ridden work, you are giving your horse the 𝗯𝗲𝘀𝘁 𝗽𝗼𝘀𝘀𝗶𝗯𝗹𝗲 𝗰𝗵𝗮𝗻𝗰𝗲 at a long, sound, and successful career.

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Thank you for taking the time to read this post! 𝗜'𝗱 𝗹𝗼𝘃𝗲 𝘁𝗼 𝗵𝗲𝗮𝗿 𝘆𝗼𝘂𝗿 𝘁𝗵𝗼𝘂𝗴𝗵𝘁𝘀, 𝗾𝘂𝗲𝘀𝘁𝗶𝗼𝗻𝘀, 𝗼𝗿 𝗲𝘅𝗽𝗲𝗿𝗶𝗲𝗻𝗰𝗲𝘀—feel free to share them in the comments below. If you found this blog helpful, 𝗽𝗹𝗲𝗮𝘀𝗲 𝘀𝗵𝗮𝗿𝗲 it with fellow equestrians who might benefit from these insights. Together, we can build a more compassionate and connected equine community! 🐴✨

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If you're inspired to 𝗱𝗲𝗲𝗽𝗲𝗻 𝘆𝗼𝘂𝗿 𝗰𝗼𝗻𝗻𝗲𝗰𝘁𝗶𝗼𝗻 with your horse and explore more tools for harmony and growth, click here to join our FREE Equine Wisdom Institute community on Skool! It's a supportive space for 𝗹𝗶𝗸𝗲-𝗺𝗶𝗻𝗱𝗲𝗱 𝗲𝗾𝘂𝗲𝘀𝘁𝗿𝗶𝗮𝗻𝘀 𝘁𝗼 𝘀𝗵𝗮𝗿𝗲, 𝗹𝗲𝗮𝗿𝗻, 𝗮𝗻𝗱 𝗴𝗿𝗼𝘄 𝘁𝗼𝗴𝗲𝘁𝗵𝗲𝗿 on this incredible journey with our equine partners.

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Supporting Research

𝗕𝗲𝗻𝗻𝗲𝘁𝘁, 𝗗. (2008). "Timing and Rate of Skeletal Maturity in Horses."

• Dr. Deb Bennett, an equine anatomist, conducted an extensive study on skeletal development in horses, highlighting that different bones mature at different rates. Her research demonstrates that the spine is the last structure to fully mature, often not closing until 5.5 to 6 years old, making early riding a major risk factor for long-term back issues and structural imbalances.

𝗝𝗲𝗳𝗳𝗰𝗼𝘁𝘁, 𝗟. (2006). "Osteochondrosis in the Horse: Pathophysiology and Long-term Effects." Equine Veterinary Journal.

• This study explores osteochondrosis (OC), a developmental orthopedic disease caused by abnormal cartilage formation in young horses. Jeffcott found that high-impact exercise before skeletal maturity contributes to joint deterioration, increasing the risk of osteoarthritis, joint deformities, and early retirement from competition.

  
  

𝗦𝗺𝗶𝘁𝗵, 𝗥. 𝗞. 𝗪. et al. (2005). "Tendon Adaptation and Injury in Young Horses." The Veterinary Journal.

• This study focuses on the slow adaptation of equine tendons and ligaments, showing that early training (before full skeletal maturity) leads to weaker, injury-prone connective tissues. The findings indicate that while young muscles can strengthen quickly, tendons and ligaments take much longer to mature, making early-intense workloads a primary cause of suspensory and tendon injuries.

𝗛𝗮𝗿𝗿𝗶𝘀, 𝗣. et al. (2013). "Effects of Early Training on Joint Development." Journal of Orthopaedic Research.

• Harris and colleagues examined the long-term effects of starting horses under saddle at an early age, using MRI scans and post-mortem joint analysis. The study found that horses trained too young had significantly higher rates of microfractures, joint inflammation, and cartilage damage, all precursors to early-onset arthritis and reduced athletic longevity.

𝗠𝗰𝗚𝗿𝗲𝗲𝘃𝘆, 𝗣. et al. (2020). "Learning and Emotional Development in Young Horses." Applied Animal Behaviour Science.

• This study examines the neurological and emotional effects of early training in horses. It found that horses started under saddle too early exhibit higher cortisol levels (stress hormone), reduced problem-solving abilities, and increased anxiety-related behaviors such as cribbing and weaving. The study suggests that waiting until a horse is mentally mature improves learning retention and reduces stress-related behaviors.

These sources 𝗰𝗼𝗹𝗹𝗲𝗰𝘁𝗶𝘃𝗲𝗹𝘆 𝗿𝗲𝗶𝗻𝗳𝗼𝗿𝗰𝗲 the argument that starting horses too early leads to 𝗹𝗼𝗻𝗴-𝘁𝗲𝗿𝗺 physical and psychological consequences, while delaying ridden work allows for stronger, healthier, and more mentally balanced equine partners.