This condition does not damage muscles but indicates tissue microdamage has occurredand thats not necessarily a bad thingIn training DOMS often happens during early conditioning phases, following a layup, or after introducing a new workload. | Adobe stockMost equestrians have experienced, at some point, the unmistakable aching and soreness that seeps into their muscles the day after getting back in the saddle for the first time in a long time, or following a particularly long, tough ride. Delayed-onset muscle soreness (DOMS) is a well-characterized phenomenon in human sports medicine yet remains relatively underexplored in the context of equine physiology.Recognizing DOMS in horses matters because of its often-subtle presentation, and it could be misinterpreted as behavioral resistance, lameness, or other more serious conditions, such as rhabdomyolysis (tying up). So, what exactly causes DOMS, what does its occurrence tell us about our horses fitness levels, and how can we best manage it? Weve turned to experts in the field and emerging research for answers.Defining DOMS in the HorseResearchers describe DOMS in horses as muscular discomfort and stiff ness that generally emerges 24 to 72 hours after undergoing unusual exercise to which the body isnt accustomed1. DOMS typically occurs in muscles used more intensely or for a longer duration than normal, and this increased activity results in some muscle (micro) damage, explains Michael Lindinger, PhD, editor-in-chief of the European Journal of Applied Physiology and president of the Nutraceutical Alliance, in Guelph, Ontario, Canada. The amount and duration of soreness are generally proportional to the amount of muscle damage.In humans, scientists have established this soreness results from microscopic damage to muscle fibers, particularly following eccentric muscle contractionsmovements in which muscles lengthen while load bearing2. In horses, this type of movement happens in every gallop stride, which produces a mixture of eccentric and concentric muscle contractions, says Catherine McGowan, BVSc, MANZCVSC, Dipl. ECEIM, CertVBM, PhD, FHEA, FRCVS, specialist and emeritus professor of equine internal medicine in the Department of Equine Clinical Science at the University of Liverpool, in England.The current research suggests the muscle microtrauma seen in DOMS initiates a cascade of inflammatory responses that stimulate tissue repair and adaptation. More specifically, the mechanical stress of eccentric contractions results in microdamage to the sarcomeresthe contractile units of muscle fibers2,3. The subsequent inflammatory response results in soreness, stiffness, and a temporary reduction in strength or flexibility.In equine athletes specifically, clinical signs of DOMS can include a shortened stride or reduced impulsion, resistance to forward movement, back or hindquarter soreness on palpation, and pain-associated behavioral changes. Lindinger covers the process in detail below. Timeline and Sequence of Physiological Events in a Horse Experiencing DOMSMichael Lindinger, PhD, editor-in-chief of the European Journal of Applied Physiology and president of the Nutraceutical Alliance, in Guelph, Ontario, Canada, describes how signs of DOMS appear in a horse:Excessive contraction intensity (force, speed) or duration of exerciseespecially with downhill activitiescan trigger muscle damage.An increase in reactive oxygen species (ROS) within muscles results in oxidative damage of lipids in muscle membranes, causing them to leak. There might also be contractile apparatus damage through disruptions in interactions between the muscle filament proteins actin and myosin.During the 24-48 hours after the unaccustomed activity, muscles continue to break down and leak, and an increase in pain-inducing metabolites (chemical compounds) occurs at pain-sensitive nerve endings. This increase in molecules in damaged muscles is what causes the pain.Dissipation of metabolites over a few days eventually results in the resolution of pain sensation.Muscle repair starts occurring within 48 hours of damage and could take up to three weeks to fully repair and recover. Avoid excessive physical activities during this period.DOMS as a Fitness BenchmarkBeyond the aches and pains, what can post-exercise muscle soreness tell us about the health and fitness of our equine athletes? DOMS itself does not cause tissue damage but, rather, is an indicator that tissue (micro) damage has already occurred, Lindinger says. Training programs for horses that have been sedentary need to keep in mind that increased activity may result in tissue damage and, therefore, DOMS.In equine training DOMS often occurs during early conditioning phases, following periods of layup, or after introducing a new workload, he adds. It is most frequently observed in horses returning to work after a period of rest, progressing to more intensive work, or in young horses as they develop strength and stamina. Scientists have suggested DOMS might also occur in horses starting a new exercise or activity or exercising while compensating for lameness, he notes, citing McGowans research1.Researchers suggest DOMS-related soreness in equine athletes reflects muscles responding to new or increased demands by initiating repair and growth4. Small amounts of muscle damage are normal with training and may contribute to beneficial adaptations in the form of hypertrophy (size increase) of myofibers (muscle fibers) alongside the biochemical adaptations, Lindinger explains. However, recurring or excessive soreness may indicate flaws in a horses conditioning program, such as insufficient warmup, inadequate recovery time, or poorly structured workload progression. In these cases DOMS becomes less a sign of adaptation and more a reflection of excessive cumulative strain in training.Does DOMS Diminish Equine Performance?Horses affected by DOMS can experience a reduction in powerby up to 50%, McGowan says, adding in her paper: DOMS does not just cause soreness but is well-established in human athletes to result in a sustained loss of muscle force production capacity, which happens immediately after exercise, before the onset of muscle pain.Fortunately, she says, DOMS and its associated power reduction and stiffness likely do not lead to long-term, detrimental muscle breakdown. Some horses may be stiff and unwilling to exercise, she explains. The only other risk is that the painful muscles could cause altered gait patterns and a risk of injury elsewhere, so during exercise the horse should be monitored carefully.Getting a DiagnosisIn any case veterinarians must distinguish DOMS from more serious muscular disorders. Notably, they need to rule out exertional rhabdomyolysis (aka tying-up), a condition in horses characterized by the breakdown of muscle fibers during or after exercise, typically caused by a combination of intense physical activity, metabolic or genetic factors, and inadequate conditioning or electrolyte imbalance.Exertional rhabdomyolysis may initially resemble DOMS but often involves a history of recurrent, possibly more severe episodes of stiffness, notes McGowan. There may be an identifiable trigger factor, but in general tying-up occurs during usual or low-level exerciseoften just 20-30 minutes after commencing exercise, rather than being delayed, as in DOMS. Equine researchers have suggested exertional rhabdomyolysis can involve more severe muscle enzyme elevation5, pain (evidenced by sweating and tachycardia, an abnormally fast heart rate)6, and dark urine due to the excretion of muscle breakdown products (i.e., myoglobin), which, left untreated, could carry potential long-term implications. Additionally, DOMS is an important differential diagnosis for muscle strain injuries (muscle pulls or tears); however, McGowan reports that muscle injuries resulting in tears tend to be more localized on palpation and likely to be asymmetrical1.A thorough evaluationincluding palpation, gait analysis, and bloodwork (that measures the muscle enzymes creatine kinase and aspartate aminotransferase )can help vets distinguish between DOMS and other muscular disorders 7.Exertional rhabdomyolysis and acute muscle injuries will result in muscle damage and abnormally elevated CK and AST levels in the blood plasma, Lindinger explains. The increase in concentrations of these enzymatic injury markers is proportional to the amount of muscle injury.Furthermore, DOMS is unlikely to present as severely as recurrent exertional rhabdomyolysis (RER) or polysaccharide storage myopathy, adds McGowan. Polysaccharide storage myopathy is a muscle disorder characterized by abnormal accumulation of glycogen and related sugars in muscle tissue, leading to muscle stiffness, pain, and episodes of exertional rhabdomyolysis.Elevations of muscle enzymes are usually mild to moderate with DOMS, and myoglobinuria (dark urine) is unlikely, she says. The enzyme increase is also often delayed in relation to the pain and peaks as the horse recovers, rather than early on. When taking the patients history, she says careful questioning about unaccustomed exercise might reveal a clue. For a more comprehensive differential diagnosis, see the chart, which includes McGowans input, to help distinguish between DOMS and more serious conditions.Management Strategies for DOMS When bringing a horse back into work after being sedentary and/or recovering from an injury, DOMS should be expected, Lindinger explains. Gradual buildup is essential; muscle adaptations occur over the course of weeks and months as the training program progresses.To mitigate the effects of DOMS, our sources recommend gradual loading, proper warmup and cooldown, cross-training, and rest periods that include whats called active recovery, such as hand walking, turning out, and exercise using a hot walker or treadmill.DOMS typically peaks at 48 to 72 hours and resolves within three to five days, says McGowan, though Lindinger notes that at least in mild cases, this can be shorter, and many largely resolve within 48 hours. The inflammation generally resolves on its own, but some supportive therapies might enhance recovery and help keep the horse comfortable during this period.When DOMS is suspected, gentle exercise is indicated (walking and trotting), along with massage, Lindinger says.In McGowans 2024 Traumatic Muscle Injuries paper, she lists cryotherapy, stretching, compression, andagainmassage, as treatment strategies for exercise-induced muscle microdamage1 in equine athletes.Using non-steroidal anti-inflammatory drugs (NSAIDs) in DOMS cases has yielded conflicting results in human studies11. Many horse owners turn to phenylbutazone (Bute) or flunixin meglumine (Banamine) for musculoskeletal pain, but DOMS calls for a different approach. Muscle adaptation requires some degree of inflammation, and researchers say NSAID drug use might interfere with recovery. As long as pain is well managed, anti-inflammatories should not be used to treat DOMS, as this may prevent beneficial training adaptations from occurring in muscles, says Lindinger.Lets take a brief look at nutritions role in supporting muscle health in a way that could reduce the occurrence or impact of DOMS. Amino acids, the building blocks of protein, contribute to muscle protein synthesis. Additionally, antioxidant supplementation, particularly vitamin E12 and selenium13, supports muscle health and might reduce oxidative stress during recovery. As for omega-3 fatty acids, they possess anti-inflammatory properties that might aid in post-exercise recovery, including in cases of DOMS, report authors on a 2024 human study14.Take-Home MessageDelayed-onset muscle soreness is an exercise-induced muscle injury that occurs in response to muscular exertion that reflects both the stress of exercise and the bodys adaptation to it, McGowan says. Though generally benign and able to resolve on its own, DOMS can still hinder performance, disrupt training progression, and mask more serious musculoskeletal conditions if horse owners dont recognize and manage it properly. Develop an exercise program that takes into consideration your horses fitness level, optimizes muscle development without overtaxing his body, and minimizes soreness while maximizing gains.References:McGowan C. Traumatic muscle injuries. Vet Clin North Am Equine Pract. 2024;41(1):181192.Proske U, Morgan, DL. Muscle damage from eccentric exercise: mechanism, mechanical signs, adaptation and clinical applications. J Physiol. 2001;537(2):333345.Peake JM, Neubauer O, Della Gatta PA, Nosaka K. Muscle damage and inflammation during recovery from exercise. J Appl Physiol. 2017;122(3):559570.Damas F, Libardi CA, Ugrinowitsch C. The development of skeletal muscle hypertrophy through resistance training: the role of muscle damage and muscle protein synthesis. Eur J Appl Physiol. 2018;118(3):485500.Valberg SJ, Clancey NP, Salinger A, Waldridge B, Tan JY, Barton MH. Pseudohyperkalemia in horses with rhabdomyolysis reported by an enzymatic chemistry analyzer. J Am Vet Med Assoc. 2024;262(1):15.Cervellin G, Comelli I, Lippi G. Rhabdomyolysis: historical background, clinical, diagnostic and therapeutic features. Clin Chem Lab Med. 2010;48(6):749756.McKenzie EC. Clinical examination of the muscle system. Vet Clin North Am Equine Pract. 2025;41(1):115.Valberg SJ. Sporadic and recurrent exertional rhabdomyolysis. Vet Clin North Am Equine Pract. 2025;41(1):111124.Firshman AM, Valberg SJ. Polysaccharide storage myopathy. Vet Clin North Am Equine Pract. 2025;41(1):125137.Ememe MU. Exertional rhabdomyolysis in horses: a review. J Agr Vet Sci. 2025;18(4):1219.Schoenfeld BJ. The use of nonsteroidal anti-inflammatory drugs for exercise-induced muscle damage: implications for skeletal muscle development. Sports Med. 2012;42(12):10171028.Duberstein KJ, Johnson SE, McDowell LR, Ott EA. Effects of vitamin E supplementation and training on oxidative stress parameters measured in exercising horses. Comp Exer Physiol. 2009;6(1):1725.White S, Warren L. Effects of dietary selenium and training on oxidative stress in young horses. Equine Vet J. 2014;46:33.Fernndez-Lzaro D, Arribalzaga S, Gutirrez-Abejn E, Azarbayjani MA, Mielgo-Ayuso J, Roche E. Omega-3 fatty acid supplementation on post-exercise inflammation, muscle damage, oxidative response, and sports performance in physically healthy adults-a systematic review of randomized controlled trials. Nutrients. 2024;16(13):2044.This article is from the Winter 2025 issue ofThe Horse: Your Guide to Equine Health Care. We at The Horse work to provide you with the latest and most reliable news and information on equine health, care, management, and welfare through our magazine and TheHorse.com. 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