From growth to performance - the musculoskeletal and neural development during the transition from yearling to 2yo

WORDS: ÅSA BECKMAN

► Nothing happens until it is experienced

We can plan, measure, and structure training down to the smallest detail. However, for the horse, reality begins the moment it passes through the horse's consciousness. Every movement, contact, and environmental change is evaluated through the question 'Am I safe?'.

For horses, as prey animals, safety is not abstract; it is a biological necessity that determines whether the body can relax, learn, and adapt, or redirect energy toward flight and defense. Unlike humans, horses are unaware of what will happen next in their lives. Every new situation, routine, or training exercise is experienced in real time. This places the responsibility on humans to create conditions in which horses can face the unknown without losing their inner balance.

Training begins here, long before the first jog on the gallops, in the mutual experience of safety, and in our capacity to see the horse's vulnerability as an opportunity for trust and learning.

► The Horse as a complex system

A horse is not a machine; it is a living, self-organizing system. Billions of cells, tissues, and nerve signals interact in a continuous flow of information. There is no central conductor controlling every movement, the order emerges from the interaction.

In a complex system, the whole is always greater than the sum of its parts. A change in one part of the body, whether physical or emotional, affects the whole body. When the horse feels safe and present, not only does its behavior change, but also its muscle tension, fascia, breathing rhythm, and heart rate will change.

Therefore, training is not just about muscles, endurance, or technique. Every sensory input such as sound, smell, or the presence of a human nearby, contributes to the system's adaptation and learning. Horses organize themselves in relation to everything they experience.

► The Trainer in the system

When a young horse enters training, it encounters not only a new environment but also an entire system of humans, routines, and expectations. At the centre is the trainer, who weaves together the visible and invisible, the measurable and intuitive.

The horse selection process often combines explicit knowledge and tacit experience. Conformation, pedigree, and movement can be described in words, but often something intangible a gut feeling or subtle observation - determines suitability. This is the same type of knowledge that allows an experienced trainer to sense a problem before any measurement confirms it.

The stable itself is a complex system, where the rhythm of routines creates security. Feeding, turnout, cleaning, and training schedules all influence the horse's physiology and mental state. A disruption in daily flow can immediately be noticed in subtle behavioral cues, such as worried looks, changed eating patterns, or an unusual gait.

The trainer's role extends beyond planning workouts and races. It is about coordinating a network of specialists veterinarians, farriers, equine therapists, dentists, nutritionists and owners - all viewing the horse from different angles. Every voice must harmonize in a living dialogue, with the horse's response as the final authority.

► Planning as communication

When a young horse starts training, every day is an opportunity to shape both body and trust. However, training is not simply filling the schedule with time on the gallops; it is about 'communicating with the horse's nervous system and tissues through movement, rest, and experience'.

The trainer's plan is, in effect, a language. Each exercise, from warm-up to gallop - signals to the horse: 'This is safe, this is a learning opportunity.'. The pace, surface, rhythm, and pauses all convey messages to the body.

Daily planning must also allow for such variations. The body and fascia of a young horse respond to nuances, such as the slope of the track, rhythm of the gallop, and the impact of the hand. Combining fixed routines with thoughtful variations ensures both security and adaptive development.

Planning thus becomes more than logistics - it becomes 'a way to speak to the horse's body'.

► Body adaptation

Tacit knowledge or intuition? As an example.

A trainer and work rider have just been talking about tying-up and how fortunate they'd been not to have horses affected. It was time for a colt's first serious workout. Afterwards, the rider was reasonably satisfied, but when the horse was hosed down back at the stable, he was markedly lame in his right hind leg.

Had he tied up after all? Was their discussion beforehand an intuitive knowing of what was going to happen?. The veterinarian was called and found the hamstrings extremely tight.

The horse was treated, but bloodwork ruled out tying-up. Instead, he was suffering from edema, as his body had failed to circulate extracellular fluid effectively back through the lymphatic system. This was causing swelling and resulted in lameness. Another experience was added to the trainer's overall knowledge.

During half a year the same colt's wither and croup heights were measured. He remained slightly overbuilt growing noticeably taller as the season went on. Measuring methods only provided an indication of growth, but initial assumptions - that the scapula and humerus, as well as the ilium and femur, would lengthen during this phase - proved incorrect.

Interestingly, the scapula and ilium had roughly the same dimensions, contributing to a harmonious conformation. During this time, the horse grew approximately 3 cm at the withers and 5 cm at the croup, although identifying the precise growth zones involved remains challenging.

Fascia does not simply cover the body, it envelops and integrates bones, muscles, tendons, and the nervous system forming a continuous sensory and mechanical network that both supports and communicates throughout the organism. During the first year of training, the young horse undergoes profound whole-body adaptation.

Far more than muscles are changing: connective tissues, cartilage, bones, and neural pathways are all being shaped by experience. Developing a future athlete (both in body and mindset) offers little margin for poorly timed mistakes.

The fascial system, the body's connective-tissue continuum, serves simultaneously as structural support and as a sensory communication pathway. Richly innervated, it perceives tension, pressure, vibration, and subtle shifts in load. It reorganizes itself according to the movement patterns the horse uses most.

Training that offers variation, rhythm, balance, and emotional safety allows the fascial-neural network to 'learn' efficient patterns, improving coordination while reducing the risk of overload.

The nervous system adapts just as dynamically as the tissues it governs. In young horses, neural circuits for balance, proprioception, arousal regulation, and motor control are still developing. Each novel experience surfaces, environments, speed, handling - shapes synaptic pathways.

When early training is predictable, calm, and well-timed, the sympathetic and parasympathetic branches learn to shift smoothly between alertness and relaxation. This supports better movement quality, quicker recovery, and a 'trainable' mindset.

Conversely, inconsistent or overly stressful experiences can reinforce defensive patterns that later appear as tension, poor coordination, or behavioral resistance.

Cartilage develops within a critical window early in life. Extensive turnout with other foals offers the varied play, loading patterns, and social engagement that cartilage needs to mature properly. This window, closing around the foal's first year, has long-term consequences for cartilage quality and joint health. By the time a yearling enters training, that phase is already behind them.

Tendons and ligaments reach near-complete development around two years of age. After this point, their capacity for adaptation is slower. Too much load too early can create micro- injuries that become chronic; too little movement results in weakness and reduced elasticity. Well-designed early training - short gallops, varied surfaces, progressive strength work encourages healthy remodeling and resilience.

Bones respond actively to mechanical stress. Early, moderate-intensity movement walking, trotting, playful acceleration, short gallops strengthens bones without overloading growth plates or joints. Throughout life, bones, muscles, fascia, and neural pathways remodel continuously in response to use. Inactivity drives loss; appropriate load drives growth, coordination, and resilience.

As the body adapts to the physical demands of early training, it does so not through isolated muscle growth, but through the reorganization of the entire support system. This principle becomes clearer when viewed through the lens of biotensegrity, which refers to the body's dynamic architecture.

► Biotensegrity - the body's dynamic architecture

The young horse's body can be understood as a living structure organized according to the principle of biotensegrity, that is, biological tension and structural integrity. In this model, the body is not held up by rigid levers and hinges, but by a 'field of continuous tension' in which every element influences the whole. Muscles, tendons, ligaments, and fascia interact in a constantly adjusting balance between tension and compression.

As the horse begins training, the tension patterns within this network shifts. Fascial fibres align along lines of force, and bones and joints adapt not in isolation but in relation to how the entire system learns to distribute load.

A small imbalance, a single-sided movement habit, or a moment of stiffness doesn't just affect one tendon or joint, it subtly reshapes the whole pattern of support that holds the body together.

Biotensegrity also offers a way to understand how the body organises through movement and learning. Each time a young horse plays, moves freely, or works under saddle, the tissues recalibrate their tension levels.

The nervous system continuously reads and responds to these changes, refining coordination and timing. Training is not only about muscle strength; it is about a 'system-wide conversation between tissue, movement, and perception'.

This process is particularly vivid in young Thoroughbreds. The body is still growing, the connective tissues are maturing, and every training session represents a small renegotiation of internal balance. A trainer who views the horse through the lens of biotensegrity notices subtle shifts, a change in posture, a new ease in a turn, or a fleeting asymmetry as signs of the living network reorganizing itself toward greater coherence.

This constant reorganization of the body's internal tension field is mirrored by equally fine-tuned adjustments within the nervous system. Just as the tissues learn to balance mechanical forces, the brain and autonomic system learn to balance arousal and calm - shaping how the young horse meets the world.

► Arousal - the nervous system balance

For a horse to learn and adapt optimally, it must be in a state of 'arousal' a level of nervous system activation that is alert but not overwhelming.

Arousal is the balance between the 'sympathetic (go)' and 'parasympathetic (rest)' systems.

Too much arousal → tension, poor coordination, impaired learning, flight mode

Too little arousal → low alertness, impaired learning, shut down

Moderate arousal → optimal attention, readiness, adaptive learning

Trainers can influence arousal through environment, intensity, variation, and recovery of the training. Observing subtle cues, such as ears, posture, and sweat helps assess whether the horse is within the functional window. Each exercise, gallop, or rest period is a signal to fascia, muscles, and the nervous system about what is safe and learnable.

► Summary - building the foundation for a career

The first year in training is about preparing for maximum performance - how to build a horse that wants to win, with soundness that lasts.

The trainer is a coordinator, communicator, and observer within complex systems. The conductor who coordinates balance between activity and recovery. Challenges and safety are crucial. Experience, nervous system responses, fascia, and tendon adaptation together form a horse that can perform and remain healthy throughout its career.

By respecting this balance and guiding the horse through safe, structured, and varied experiences, the trainer lays the foundation for a successful, sustainable, and rewarding racing career.

This way training becomes a fulfilling partnership between human and horse.

References:

• Clayton, H. M. (1991). 'Conditioning sport horses'. Sport Horse Publications. Langevin, H. M. (2006). 'Connective tissue: A body-wide signaling network?' Medical Hypotheses, 66(6), 1074-1077. https://doi.org/10.1016/j. mehy.2005.12.032.

• Levin, S. M. (2002). 'The tensegrity-truss as a model for spine mechanics': Biotensegrity. Journal of Mechanics in Medicine and Biology, 2(3/4), 375-388. Luomala, T., & Pihlman, M. (2016). 'A practical guide to fascial manipulation: An evidence- and clinical-based approach'. Elsevier Health Sciences.

• Peters, S. (n.d.). Horse Brain Science [Web resource / podcast]. Benias, P. C., et al. (2018). Structure and distribution of an unrecognized interstitium in human tissues'. Scientific Reports, 8, 4947. https://doi. org/10.1038/s41598-018-23062-6.

• Theise, N. D. (2023). 'Notes on Complexity: A Scientific Theory of Connection, Consciousness, and Being'. Spiegel & Grau.

Lower limb anatomy and how it can be conditioned for racing

Article by Adam Jackson MRCVS 

Better understanding the appropriate levels of exercise and training while the horse’s body grows and develops has been a topic of research for many years. Although it has been shown that young, growing horses are well-suited to adapt to conditioning, it is vital that continued research is performed in order to develop thoughtful and strategic training methods to promote healthy, fit and sound horses with long careers and lives.  

Horses’ limbs consist of dozens of muscles, bones, tendons, ligaments, and joints that allow the horse to move as well as support its body weight. The limbs function to provide thrust and movement while absorbing impact and bearing weight.  Most of the horse’s weight is supported by the fore limbs, while the propulsion of the horse is provided by the hind limbs. In addition, the horse has two apparatuses referred to as the stay apparatus and suspensory apparatus. The stay apparatus allows major joints in the limbs to lock so that the horse may rest and relax while standing. The suspensory apparatus is designed to absorb shock, carry the horse’s weight, and prevent the overextension of joints. Finally, the hooves are important structures that maintain support and traction as well as provide additional shock absorption.  

Since the cardiovascular system provides blood supply throughout the body, by responding to various stimuli, it can control the velocity and amount of blood carried through the vessels, thus, delivering oxygen, nutrients, hormones, and other important substances to cells and organs in the body.  It plays a very important role in meeting the body’s demands during exercise, stress, and activity.  

Exercise is used to increase the body’s ability to withstand repeated bouts of similar exercise with less impact.  With a strong and healthy cardiovascular system, there is an improved ability of the musculoskeletal system receiving oxygen, thus, allowing muscles to better their capacity to use oxygen and energy.  However, the adaptation period for each of these physiological systems do differ as the cardiovascular system adapts faster compared to the musculoskeletal system. This is often an overlooked consideration when developing training programmes for horses. 

It is important to understand the various functions, structures, and adaptive processes of the horse’s musculoskeletal system such as bone, articular cartilage, tendons, and ligaments in order to develop appropriate training regimens. 

Bone has many important roles that involve locomotion, the storage of minerals (especially calcium and phosphate), soft tissue and vital organ protection, and the support and containment of bone marrow. Bone is a specialised connective tissue, and together with cartilage forms the strong and rigid endoskeleton.  The bone is continuously altering through two processes called bone modelling and bone remodelling, involving four cells referred to as osteoclasts, osteoblasts, osteocytes and bone lining cells.  

Osteoblasts secrete bone matrix in the form of non-mineralised osteoid, which is then mineralised over a few weeks to form a bone matrix.  Osteoclasts are involved in resorption of bone as this process occurs faster than the formation of bone. When the bone surfaces are not in the development or resorption phase, the bone surface is completely lined by a layer of flattened and elongated cells termed bone-lining cells.  Osteocytes are derived from osteoblasts and are highly specialised to maintain the bone matrix.  They are designed to survive hypoxic conditions and maintain biomineralisation of the bone matrix.  Osteocytes also control osteoblastic and osteoclastic activities allowing bone remodelling.

The function of bone modelling is to alter and maintain shape during growth. As the horse grows and develops, bone modelling occurs with the acquisition and removal of bone.  While the young horse grows and develops, bone modelling allows the bone to endure strains from everyday work and exercise. The adult skeleton undergoes a minimal amount of bone modelling. Due to the presence of the high frequency of bone modelling in young horses, their skeletal strength is highly influenced by strains to their bones during exercise and daily use. With this knowledge, it has been concluded and confirmed that short-term dynamic exercise of an adolescent can lead to beneficial changes to its bone morphology.  

Bone remodelling is a different process, in which old and damaged bone is renewed, which enables the bone to respond and adapt to changing functional situations. Bone remodelling is usually a coordinated relationship between bone resorption and bone formation. This process occurs throughout the horse’s life with the renewal of primary, damaged or old bone. Osteoclasts absorb old and damaged bone, and the osteoblasts form new bone and lay down new bone matrix until the earlier absorbed bone is replaced. In those animals with musculoskeletal disease or damage, there is an imbalance of osteoblast and osteoclast activity. With the knowledge that osteoblast activity to make new bone takes months whilst osteoclast activity of removing old and damaged bone only takes a few days to two weeks, bone that is being repaired is at a high risk of further injury as bone removed has not been completely replaced.   Multiple studies have shown that exercise while growing can provide lifelong benefits; however, it must be done with care and knowledge.

In addition, many studies have shown that exercise of a dynamic nature in moderate distances, such as that achieved in the pasture or prescribed short-distance high-speed work is beneficial to musculoskeletal development and may prevent injuries when entering race training. It has also been observed that long slow work does not increase bone strength. Below is a summary of the young horse response of the various types of exercise.

Articular cartilage is a highly specialised connective tissue found in joints with the role of providing a smooth, lubricated surface of articulation and to help transmit loads with a low amount of friction. The articular cartilage is a hyaline cartilage (flexible and strong tissue providing a smooth, slippery surface) with a dense “ExtraCellular Matrix” (ECM) consisting of specialised cells called chondrocytes, collagen and proteoglycans. These components help to retain water in the ECM that is required for the joints mechanical properties. As age increases, hydration of the matrix does decrease, resulting in stiffness. Chondrocytes are residential cells in articular cartilage that play a role in the development, maintenance, and repair of the ECM. They do respond to a variety of stimuli, including mechanical loads, growth factors, hydrostatic pressures, piezoelectric forces (formation of electric charge with force). Because of the lack of blood vessels, lymphatics, and nerves as well as being a harsh biomechanical environment, there is a limited capacity to heal and repair. In addition, chondrocytes have limited potential for replication, thus, have limited healing capacity; and chondrocytes survival depends on an optimal chemical and mechanical environment.  

Maintaining joint health is vital, which requires the preservation of healthy cartilage tissue. Inactivity of joints is detrimental to articular cartilage; thus, regular movement of joints and dynamic loads is needed to provide a normal articular cartilage structure and function. Biochemical responses of the cartilage to exercise are not nearly as well known compared to bone. While the confinement of young horses stunts joint development, excessive straining of cartilage can also reduce joint development. It has been observed that pasture access was optimal for the development of joints and the confinement or excessive sprint exercise (12–32 sprints of 40 metres for 6 days a week for 5 months) causes detrimental effects on the joint and may be deemed as unnatural exercise.  It is also thought that exercise is needed well before two years of age to allow cartilage thickening as well as the avoidance of confinement. It can be concluded that further studies are required with respect to level of exercise and type of exercise in order to achieve healthy cartilage tissue as there is clearly a fine line between frequency and intensity of exercise.  

Tendons and ligaments are distinct but closely related tissues that have unique and important roles in musculoskeletal function and musculoskeletal disease. Tendons and ligaments are dense, fibrous connective tissues that connect muscle to bone or bone to bone, respectively.  These tissues transmit mechanical forces to stabilise the skeleton and allow body movement.  Tendons and ligaments consist mainly of collagen type I as well as small amounts of collagen III, IV, V, and VI. There are also various proteoglycans in tendons and ligaments that both organise and lubricate collagen fibre bundles. The elasticity of tendons and ligaments is due to the large amount of type I collagen. During locomotion, the tendon decreases energy cost to the horse by acting as a spring to store and release energy while stretching and recoiling in the stance and swing phases of each stride. Tendons and ligaments have blood vessels and nerves that allow the homeostasis and response to injury.  

Tenocytes are tightly regulated by a series of growth factors and transcription factors that allow the synthesis, maintenance, and the degradation of the tendon extracellular matrix. Tendons are elastic, but tearing may occur if there is excessive loading on the tendon and the repair of collagen is a slow process. In addition, tendons have crimp morphology where the tendons buckle in a state of relaxation and act as shock absorbers.  Unbuckling of the tendon occurs during loading.  This crimp morphology may be disturbed if an injury occurs and also is reduced in older horses.  

Due to the variation of activity of tenocytes in foals and young horses, it has been observed that both a lack of exercise and excess of exercise can impair tendon make-up and subsequent functionality. With the current data and research that has been gathered, it can be concluded that if horses take advantage of spontaneous exercise when in the paddocks (which they often do), the developing tendons may benefit and be at a lower risk of injury when racing training starts. 

Conclusion

It is clear that further research is needed in order to ascertain the optimal amount and type of exercise that is needed in order to provide a strong musculoskeletal system and functional performance. However, it has been shown that prescribed exercise during the growth of the horse can increase the longevity of the horse’s health and performance. It has been observed that confinement and the lack of loading can result in weaker tissues and the loss of function of none, tendons, ligaments and articular cartilage.  However, it must also be recognised that medical attempts to alleviate pain so that a horse can continue to train through an injury can greatly increase tissue damage which is detrimental to the horse’s health and career. It is far more beneficial to provide an adequate amount of time for the injury to heal, thus, putting the horse’s health and wellbeing as a top priority.  

Nutritional Perspective

Bone development in yearlings from the sales ring to racing

Article by Des Cronin B.Ag.Sc, MBA

Maintaining the equine skeleton is vital to ensure optimal development of the young growing horse, minimise risk of injury in the performance horse, and promote longevity and soundness.

The skeletal development and health of a young horse begins in utero and ensuring the broodmare receives the correct intake of key nutrients will be critical to the growth of the unborn foal. Producing high-quality milk places a significant drain on the mineral reserves of the mare. Maintaining mineral intakes during peak lactation is vital to ensure the foal receives the best nutrition to support the rapid skeletal development in the early weeks and months of growth. During this time, bone formation, body size, and muscle mass greatly increase. Risk of defective bone and related tissue formation increases with one of more of the following:

• Poor diet with the incorrect balance of energy and nutrients in the daily ration

• Inadequate amounts of calcium (Ca) and phosphorus (P)

• A reversed Ca:P ratio

• Low zinc (Zn) or copper (Cu) in the diet

• Low Vitamin D

Feeding a young horse for a maximum growth rate is undesirable because bone hardening lags greatly behind bone lengthening. At 12 months old, the young horse could reach about 90 to 95 per cent of its mature height but only about 75 per cent of its mature bone mineral content.

Ideally, young horses should gain weight at a rate that their developing bones can easily support. Growing bones and connective tissues don’t have the strength to support rapid weight gain from overfeeding, especially energy. Rapid weight gain can also make other skeletal anomalies worse. In these cases the risk of developmental orthopaedic disorders (DOD) and unsoundness increases.

DOD and unsoundness can also occur during uneven growth. For example, switching an underfed, slow-growing horse to a good diet that allows quick growth (compensatory growth), increases the risk of DOD. Foals between the ages of 3 and 9 months of age are at greatest risk of DOD.

Fresh forages, for example grazed grass, usually provide enough major minerals such as calcium (Ca) and phosphorus (P) for the growing horse. However, there can be significant variation in calcium and phosphorus levels in all forages but particularly preserved forages (hay and haylage). Forage analysis should always be undertaken to determine mineral composition. 

For young fast-growing horses, the diet must supply the quantities of calcium and phosphorus needed for normal bone formation. In terms of Ca:P ratio, the ratio must be positive in favour of calcium. Horses are much more tolerant of high-dietary calcium than other species. For practical purposes, a good guideline would be to keep the ratio Ca:P between 1.5 to 1 and 2.5 to 1.  Grains (e.g., oats) contain 10 per cent of the calcium level found in typical forages. Grains are poor sources of calcium, both in terms of the amount of calcium supplied and their effect on Ca:P ratio in the diet. Where grains are fed, supplementation will be necessary to balance the diet.  

While some forages may contain adequate calcium and phosphorus, they will typically supply less than 20 per cent of the daily requirements for trace elements. Supplementation of trace elements will generally be necessary to support normal bone development.

Where concentrates are fed (especially low levels), supplementation may still be necessary to balance the overall mineral and trace element intake. Nutritional advice should be sought to ensure the horse's diet is correctly balanced.

To meet the carefully balanced requirements of key minerals, it is advisable to supplement the daily rations of growing horses and young horses entering training with an appropriate nutritional product. 

Make sure that the supplement used contains the correct ratio of calcium and phosphorus, as well as other key nutrients such as vitamin D and chelated trace elements (copper, manganese, and zinc) to support normal bone development.

Supplementing branch chain amino acids in the diet ensures that growth is maintained. Lysine plays a key role when protein concentrations in the body are low. Vitamin A supports collagen formation, which is a key component of the supportive structures of joints (tendons and ligaments). Vitamin D3 is added to enhance calcium absorption.

Although growth rates slow after the age of two, they are still juvenile in their skeletal development with some growth plates, such as the shoulder and stifles, yet to completely close. Although they may look like fully grown adults, it is still important to meet nutritional requirements especially if starting training and work. With the addition of exercise and training, a young horse's nutritional needs change.  The added forces from groundwork on the long bones and increased requirements of other nutrients like electrolytes need to be considered. 

Finally, horses all grow and develop at different rates because of factors such as genetics. Some youngsters will need  more support for longer periods of time than others, so it is important to manage accordingly.