Muscle US

Muscle Injury Detection and Therapies

Tracy A. Turner, DVM, MS, Dipl.Am Col Vet Surgery, Dipl.ACVS, Dipl.ACVSMR
Turner Equine Sports Medicine and Surgery



Muscle injuries occur in horses to various degrees. The injuries can cause anything from poor performance to lameness. However, diagnosis may be difficult as the pain may only manifest itself during performance and may not be palpable. Infrared thermal imaging can be useful to determine the region of injury. Rehabilitation is based on healing, improving flexibility and muscle condition, strengthening the injured muscle, and then slowly returning to full activity.


Muscle pain and injury as a cause of lameness and poor performance in the horse is poorly recognized. In human athletes, muscle fatigue, muscle stiffness, muscle soreness and muscle tears are well recognized and considered among the most common athletic injuries. In horses, muscle injuries are uncommonly documented as a cause of lameness. Fibrotic myopathy, stringhalt, and ruptured peroneus tertius are among the only muscle injuries reported in the horse.

Muscle pain and injury as a cause of lameness and poor performance in the horse is poorly recognized. In human athletes, muscle fatigue, muscle stiffness, muscle soreness and muscle tears are well recognized and considered among the most common athletic injuries. In horses, muscle injuries are uncommonly documented as a cause of lameness. Fibrotic myopathy, stringhalt, and ruptured peroneus tertius are among the only muscle injuries reported in the horse.1 These lamenesses are usually characterized by the resultant gait abnormalities. Other muscle problems such as stress tetany, synchronous diaphragmatic flutter, exhaustion, post exercise fatigue, tying-up (exertional rhabdomyolysis), and azoturia are regarded as specific physiologic disturbances.2 Muscle injuries frequently cause lameness in human athletes and racing greyhounds. Similar injuries therefore would be expected in the horse.

Factors which predispose to muscle strains include cold temperatures or impaired circulation to the muscle, local or generalized muscle fatigue, poor or insufficient training, and insufficient warm-up.1 Cold has been shown to increase muscle tension and cause circulatory disturbances. This phenomenon causes earlier muscle fatigue which can lead to uncoordinated muscle movement and strain. Fatigue predisposes to injury in two ways. First, muscle fatigue is a manifestation of general fatigue which affects those groups that are maximally loaded. As muscles fatigue they decrease in performance and elasticity thus enhancing the likelihood of strain. Further general fatigue results in central nervous system incoordination of movement and predisposition to strain. Therefore, training must be designed to progressively increase the work load to develop the muscle groups, and to decrease early fatigue and permit rapid restoration of muscle function after exertion. Insufficient warm-up of muscles prior to exercise results in decreased circulation and lowered capacity to eliminate muscle waste products. Both these factors decrease the muscle’s ability to sustain maximal performance.

The equine athlete is exposed to these predisposing factors on a routine basis. Hypothetically, if the horse suffers muscle strains, these injuries would most likely be manifested as lameness. The difficulty for the veterinarian is the positive diagnosis of these injuries. In human medicine, the athlete’s description of the pain location is often the single most important factor in diagnosis. This diagnostic aid is obviously lacking in veterinary medicine. Many of these cases probably go undiagnosed in equine medicine because they cannot be confirmed by commonly used diagnostic methods such as radiographs and nerve blocks. As such, these lamenesses are most likely treated empirically with various combinations of rest, analgesics, and anti-inflammatory agents.

Some veterinarians have recognized muscle injury as a cause of poor performance. Sites in the forelimb include the biceps brachii, brachiocephalicus, the pectorals and the musculotendinous junction of the superficial digital flexor. In the hindlimb, the semimembranosus and semitendinosus, adductor, gracillus, gluteal, and gastrocnemius muscle. Muscle tension, spasms and pain have been recognized in the thoracolumbar region. In fact, localized muscle soreness is readily induced by a poor fitting saddle or poorly balanced rider.


As previously stated, the diagnosis of muscle injury in horses can be difficult. It is important to determine if there was a history of a fall or other trauma, the duration of clinical signs, the presence of swelling and whether lameness or poor performance has been documented.

The detection of muscle swelling or muscle loss as a result of fibrosis, chronic injury, or atrophy can be problematic. The horse should stand square, bearing weight evenly on all 4 limbs and with the head straight. The horse can then be assessed visually and the muscles palpated looking for fibrosis, tension or spasms, defects or pain. Acute muscle tears may not be palpable because the defects become filled with hemorrhage, inflammatory debris, exudate, and edema. However, careful palpation can detect most superficial muscle injuries but deeper muscle injury is more difficult to identify.

Serum muscle enzyme concentration assessment while very useful in the diagnosis of systemic muscle disease such as rhabdomyolysis is of limited use in the diagnosis of muscle soreness or muscle tears.

In humans, part of the assessment includes evaluation of joint range of motion associated with the involved muscle as well as strength assessment. There is no data available for horses but this author believes this would be valuable information, especially for the assessment of rehabilitation.

Ultrasonography can be very valuable in the assessment of the injury site. The most difficult aspect in horses may be the detection of the injury. The author has found infrared thermal imaging to be valuable in locating the area of injury.

Thermography is the pictorial representation of the surface temperature of an object.3 It is a noninvasive technique that measures emitted heat. A medical thermogram represents the surface temperatures of skin making thermography useful for the detection of inflammation. Although thermographic images measure only skin temperature, they also reflect alterations in circulation of deeper tissues. This ability to noninvasively assess inflammatory change makes thermography an ideal imaging tool to aid in the diagnosis of certain lameness conditions in the horse.

Thermographic muscle lesions have been described in horses.1,3 These lesions were defined as those with a 1° centigrade disparity in temperature over 25% of symmetrical body areas. These disparities could consist of an increase or a decrease in temperature. Increases in temperature were qAindicative of either chronic scarring and reduced circulation or local edema, swelling, and vascular stasis due to severe inflammation. On the basis of the thermographic and clinical findings, the horse’s injury could be further categorized as one of three types of muscle injury: cranial thigh, caudal thigh, and croup region. The cranial thigh muscle injuries included injuries where the thermographic abnormality was over the quadriceps musculature (Figure 1), the caudal thigh myopathy which included those cases in which the primary thermographic abnormalities were located over the caudal thigh from the sacrum to the gaskin (Figure 2,3), and the croup myopathy (Figure 4)  which included cases shown thermographically to have inflammation involving the caudal loin, sacroiliac region, and hip. Since that report, muscle injuries have been diagnosed thermographically in the foreleg as well but these have been limited to muscles of the shoulder region.3

Figure 1: Thermographs of the dorsum of the right and left stifles. Note the asymmetry with the area above the left patella (arrow) markedly hotter than the right side.

Figure 2: Thermogram of a horse from behind with the tail held up. Note the asymmetry between the left and right sides with the marked increase heat over the right side, this is in the area of the semimembranosus and semitendinosus muscles.











Figure 3: View of the left hip region of a horse. Note the broad heat (arrow). This is directly over the biceps femoris m.

Figure 4: A dorsal view of a horse’s croup. The left side is warmer, the white encircled area represents the gluteal m, note the most heat is at the origin of the gluteal and at its insertion on the greater trochanter (arrow).











According to some manuscripts, once the area is located, the ultrasonography was used to characterize the nature of the injury.4 In each case, ultrasonography of the region of the “hot spot” revealed disruption of normal muscle fibers and varying sizes of hypoechogenicity typical of hemorrhage. A second lesion noted sonographically was a hyperechogenicity thought to be early fibrosis and a disruption of normal muscle/tendon patterns with focal hypoechogenic areas suggesting tearing of the musculotendinous junction.1.4

Another diagnostic technique that may be of use is muscle biopsy. One case was reported where biopsy of the thermographic “hot spot” revealed fasciitis of the fascia surrounding the muscle.4


In humans, the goal of rehabilitating muscle injuries is based on 3 goals: (1) Improving flexibility and muscle condition, (2) Strengthening, and (3) Return to full activity.5 Improving flexibility and muscle condition can be performed using a number of different techniques. Stretching exercises may be done from day 1 as long as they can be done without pain. If pain is felt then stop and wait. Stretching should be done regularly, at least three times a day in the early stages of rehabilitation. Horses stretch best by walking in a long and low frame. Stretching can be performed before exercise, in order to help the muscles warm up, loosen up, and relax; movements should not be forced and methods limited to what is tolerated. Carrot stretches are an excellent method. Work is limited to what the horse can do without pain. Likewise, intensity is limited to the gaits the horse can perform without pain. But if the horse is capable, ground poles or cavalettis can stretch the horse as can bending exercises. After exercise, one can take advantage of warmed up muscle and force the stretch more.

Sports massage techniques are exceptionally useful after the initial acute stage (usually 48 hours).5 This will relax the muscle, loosen and help prevent scar tissue formation and encourage blood flow and healing of the muscle. Massage has many techniques, generally, massage movements should go with the lay of the hair (following the direction of the muscle fibers), sometimes transversely but never counter. Counter movements may induce muscle spasms.

Horses because of their size may be difficult to stretch or massage, that is where therapeutic devices have been helpful. Therapeutic sound and electrical stimulation are among the most common devices used. Therapeutic sound devices can be divided into therapeutic ultrasound and shockwave. Therapeutic ultrasound is a method of stimulating tissue beneath the skin’s surface using very high-frequency sound waves, between 800,000 Hz and 2,000,000 Hz, which cannot be heard by humans.6 Ultrasound is applied using a transducer or applicator that is in direct contact with the patient’s skin. Gel is used on all surfaces of the head to reduce friction and assist transmission of the ultrasonic waves. There are three primary benefits to ultrasound. The first is the speeding up of the healing process from the increase in blood flow in the treated area. The second is the decrease in pain from the reduction of swelling and edema. The third is the gentle massage of muscles, tendons or ligaments in the treated area because no strain is added and any scar tissue is softened. These three benefits are achieved by two main effects of therapeutic ultrasound. The two types of effects are thermal and non-thermal effects. Thermal effects are due to the absorption of the sound waves. Non-thermal effects are from cavitation, microstreaming, and acoustic streaming.6 The thermal effects have been documented in horses, the non-thermal effects have not.7

Extracorporeal Shockwave Therapy (ESWT) are abrupt, high amplitude pulses of mechanical energy, similar to soundwaves, generated by an electromagnetic coil or a spark in water.8 “Extracorporeal” means that the shockwaves are generated externally to the body and transmitted from a pad through the skin. With ESWT, reduced pain and faster healing are reported. The exact physiological mechanisms at this stage are poorly understood, but it appears that the cells undergo microtrauma which promotes the inflammatory and catabolic processes that are associated with removing damaged matrix constituents and stimulates wound healing mechanisms.8

Electrical stimulation or electrical muscle stimulation (EMS), also known as neuromuscular electrical stimulation (NMES), is the elicitation of muscle contraction using electric impulses. EMS has received increasing attention in the last few years because of its potential to serve as a strength training tool for healthy subjects and athletes, a rehabilitation and preventive tool for partially or totally immobilized patients, a testing tool for evaluating the neural and/or muscular function in vivo, and a post-exercise recovery tool for athletes.9 The impulses are generated by a device and delivered through electrodes on the skin in direct proximity to the muscles to be stimulated. The impulses mimic the action potential coming from the central nervous system, causing the muscles to contract. The electrodes are generally pads that adhere to the skin. Electrical stimulation is reported to have many benefits: (1) Pain relief caused by decreased spasticity of muscle, (2) Improved range of motion caused by reduced muscle tension, (3) Reduction in swellings caused by injury, (4) Reduction of scar tissue during healing, (5). Re-education of muscle function to prevent further injury, (6) Strengthening of muscles and tendons, (7) Reversal of muscle wasting, and (8) Decreased rehabilitation time after injury and surgery.9

There are different types of electrical stimulation, among the most common are transcutaneous electrical nerve stimulation (TENS) is the use of electric current devices to assist with short-term or long-term pain relief.9 TENS units are designed to produce analgesia of pain and reduce responses of dorsal horn neurons to painful stimuli. The TENS systems activate the descending inhibitory pathway from the brain stem to the spinal cord. However, the means of reducing pain varies between the specific types of systems and includes activating spinal cord gating mechanisms, endogenous opiates, serotonin receptors, noradrenaline receptors, and muscarinic receptors. Regardless, the electrical signals cause muscles under the electrodes to contract.

Another type of electrical stimulation is functional electrical stimulation (FES).9 FES is the application of an electrical current through surface electrodes to produce a controlled muscular contraction.  A microprocessor generates a train of impulses, which imitate the neural signals that pass between the spinal cord and the peripheral nerves in healthy muscle, producing a muscle contraction. FES is utilized to disturb spastic hyperexcitability, returning the muscle to its balanced contraction and relaxation phases, therefore reducing pain. FES devices are designed to stimulate motor nerves but peripheral nerves are also stimulated when motor nerves are activated, so there is a combined effect. FES provides a means to mobilize muscle, tendon, and the associated ligaments through the generation of controlled muscular contractions. FES can be used for stimulation of deeper tissues, and therefore, the attainment of strong muscular contractions is possible. Stronger contractions have been shown to be more effective in reducing pain, and the benefits have proven to be longer lasting than other forms of electrical stimulation.

Pulsed electromagnetic field therapy (PEMF), is a reparative technique most commonly used in the field of orthopedics for the treatment of non-union fractures, failed fusions, congenital pseudarthrosis and depression.10 More recently, PEMF has been used as a more for healing other types of connective tissue including muscle. Regardless, of the modality, in order to achieve the goal of improving flexibility and muscle condition, it is the author’s opinion, the horse must remain in at least low-intensity exercise.

Strengthening the muscles is important to avoid re-injuring the muscles. It is especially important to strengthen the muscles in the same direction/way that they were injured. Light strengthening exercises can begin after the acute stage or as soon as pain allows. If they are painful then stop and wait. Horses gain strength by flexion (ground poles and cavellettis), through transitions of gait, stress (deep footing, hill work, etc) and lateral work (may be most difficult, especially for inner thigh injuries). For humans, there are protocols for strengthening specific muscle groups, these protocols do not exist for horses at this time. There is work ongoing using, therapeutic banding, treadmills and underwater treadmills but the results are not complete. Most build strength through controlled exercise.

Strengthening exercises may be done on a daily basis in the early stages of rehabilitation and as intensity increases and full activity is regained they may be reduced to 3 times a week. Stretching exercises should be continued throughout the strengthening process both before and after a strengthening session.

The return to full activity should be a gradual process. Do not go straight back into regular work but build up gradually from slow trotting. When the horse can slow trot for 20 minutes without problems then gradually build up speed. Extended trot should lead to canter and canter to gallop gradually increasing to what is needed for competition.

Only when the horse can comfortably manage specific training should they be returned to full training or competition. As training is commenced massage therapy, acupuncture, and in some cases chiropractic adjustments can help the horse recover. But for long-trm the author has found that altering the exercise program can be most beneficial and conditioning is of utmost importance. The work schedule should be a progressive schedule that will take 60 to 90 days, ideal work schedule is 6 days per week. Initially, most exercise should be warm up (stretching). This will consist of walking in a long and low frame until the rider can feel the hips moving equally and a normal overstride behind. Once achieved the horse should be worked over ground poles for 15 to 20 minutes. When ground poles become easy, change to cavellettis. Exercise for at least 1 hour.As work becomes easier then slow trot can be added. Work in a long and low frame (extended frame). This will help stretch the horse’s top line and any tight muscles there. Once the horse can slow trot consistently for 15 minutes, then the intensity (speed) can increase. When 15 minutes of extended trot is achieved, the canter can be added. Work the horse for at least an hour each day. Lateral work should not be added until the horse can consistently canter for 15 minutes without problems.

In addition, vigorous massage of the tight muscles in the horse’s back, croup and thigh before and after exercising the horse may be helpful. Post-exercise body wash with liniments, intense massage, and therapeutic ultrasound treatments may be helpful in alleviating soreness. Attention to the horse’s trimming and shoeing should concentrate on  hoof balance (especially rearlimb sole angle) and traction. During this retraining period, infrared thermal imaging can be used to assess muscle stress to insure that too much stress is not be applied. Stress would be seen as abnormal heat in the skin overlying the muscle.


The most common cause of muscle inflammation is muscle strain.1 A classification of first, second or third degree strain injuries, described in human athletes, has been applied to horses. In one study areas of hindlimb muscle injuries in horses were identified. One, the croup myopathy, involved inflammation over the areas of the longissimus lumborum m., gluteus medius m., gluteus profundus m., the sacroiliac joint, and the gluteal insertions on the greater trochanter and associated fascia. The analogous regions in man would constitute the lower back and hip. The second, the caudal thigh myopathy, involved the areas over the biceps femoris m., the semitendinosus m., the semimembranosus m., and their origins and their upper limb insertions and musculotendinous attachments. This would be analgous to hamstring injury. The third, the cranial thigh injury, constitutes damage to the quadriceps and tensor fascia lata.

A gluteal tendon lameness has been described in the horse; the major clinical sign was pain around the greater trochanter.11 There have been numerous reports of the effect of lumbar and sacroiliac pain leading to lameness. For the purposes of this paper, the author has chose to place all these problems in one of three categories because, although physical examination may reveal soft tissue pain and imaging of the areas by thermographic examination confirmed the location of lesion(s), these methods could not specifically identify the structures involved. Thermography only reflects problems of deeper tissues, ie, it indicates the area of disease but does not reveal any information as to the nature of the organic damage. The grouping of these problems together because of their close anatomical location and similar effects on the horse’s gait.

Muscle injuries in the horse have been described as ranging from loss of performance to pain created by a particular movement to overt lameness.11 The wide range of degree of lameness reported in the horse supports this observation. In one report, the caudal thigh myopathy was more likely to cause severe lameness than the cranial thigh or croup muscle injuries.1 The caudal thigh muscles may be more likely to tear, and tears in this group of muscles have been documented. Fibrotic myopathy, a condition of the horse that involves the semitendinosus muscle and occasionally the semimembranosus and biceps femoris muscles, is thought to originate from trauma to the musculotendinous junction. Another possible reason for the greater pain associated with these injuries is the complex actions of these muscles. This group of muscles extends the hip, flexes the stifle, and extends the hock. Because of the action of the horse’s reciprocal apparatus all three functions cannot occur simultaneously unless muscle contraction is coordinated. Hypothetically, if an injury occurred, the horse should have pain each time the leg was extended because stifle extension would exert a direct opposing force on the caudal thigh muscles as they contract for hip and hock extension. Forelimb muscle injuries occur but they have been poorly reported.3

Pain on palpation is probably the single most important physical evidence of injury. Pain elicited by palpation should be repeatable, but care should be taken not to overdo palpation which may result in the horse “guarding” the injury and thus not responding. The author has found found that firm pressure was more reliable than squeezing muscle masses when trying to differentiate pain from simple annoyance. Stress points have been described that help point to lesions of these muscles. Stress points are the points where the greatest stress produced by movement occurs. However, palpable pain is not reliable with regards to muscle strains and tears.

Thermography can be used in these cases as a diagnostic tool.3 In each case, information from thermography is important in determining the region of injury. Thermography has been shown to be a practical aid in the clinical evaluation of lameness. This modality specifically increases the accuracy of diagnosis by confirming inflammation in palpably sore areas and by showing the area to concentrate further diagnostic testing such as, sonography or muscle biopsy. Clinically, thermography also improves therapy. Once the area of inflammation is determined physical therapy can be applied directly to that area. In this fashion therapeutic ultrasound, massage, or other treatment is applied more specifically to the inflamed area. Further, thermography can be used to monitor the resolution of the inflammatory process and possible recurrence during the rehabilitation.


Muscle injuries occur in horses to various degrees. The injuries can cause anything from poor performance to lameness. However, diagnosis may be difficult as the pain may only manifest itself during

Muscle injuries occur in horses to various degrees. The injuries can cause anything from poor performance to lameness. However, diagnosis may be difficult as the pain may only manifest itself during performance and may not be palpable. Infrared thermal imaging can be useful to determine the region of injury. Rehabilitation is based on healing, improving flexibility and muscle condition, strengthening the injured muscle, and then slowly returning to full activity.



  1. Turner TA. Hindlimb muscle strain as a cause of lameness in horses. Proceedings Am Assn Equine Practioners 1989;34:281-286.
  2. Hodgson DR. Myopathies in the athletic horse. Compend Contin Educ Pract Vet 1985;7:s551-s555.
  3. Turner TA. Diagnostic thermography. Vet Clin North Am Equine Pract 2001;17:95-114.
  4. Tomlinson JE, Sage AM, Turner TA: Ultrasonographic abnormalities detected in the sacroiliac area in twenty cases of upper hindlimb lameness. Equine Vet J. 2003;35(1):48-54.
  5. Herring SA: Rehabilitation of Muscle injuries. Med Sci Sports Exerc 1990, 22(4):453-456.
  6. Miller D, Smith N, Bailey M, et al: Overview of therapeutic ultrasound applications and safety considerations. J Ultrasound Med. 2012;31(4):623-634
  7. Turner TA, Wolfsdorf K, Jourdenais J: Effects of heat, cold, biomagnets and ultrasound on skin circulation in the horse. Proc Am Assoc of Equine Practnr, 1991;37: 249-257.
  8. McClure S: Extracorporeal shockwave therapy: theory and equipment. Clin Tech in Equine Pract. 2003;2(4):348-357.
  9. Schils SJ: Review of electrotherapy devices for use in veterinary medicine. Proc Am Assoc Equine Practnr, 55:68-73.
  10. Auer JA, Burch GE, Hall P: Review of pulsing electromagnetic field therapy and its possible application in horses. Equine Vet J,1983,15(4):354-360.
  11. 11. Aleman M: Review of equine muscle disorders. Neuromuscular Disorders, 2008,18:277-287


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