Muscle Function and Pain Management

Although muscle makes up much of the body, the muscular system is often neglected during assessment of a patient for pain and discomfort. The complex muscular system, which is responsible for complicated movements, cardiac and respiratory function, and even some gastrointestinal function, can be compared to a spider web. When the muscular “web” is intact (i.e., the body has no muscle pain or injury), signals (i.e., the “spider”) can travel from the muscle to the central nervous system (CNS) without difficulty. When the “web” is healthy and completely intact, it can function at optimal capacity and serve the “spider’s” needs for catching food and protecting itself. Similarly, when muscles are healthy and pain free, they can properly communicate signals to the CNS and surrounding musculature to complete necessary functions (e.g., maintaining posture, providing stability, sensing pain, creating movement). Much like a spider web, if the muscle is damaged, it cannot send appropriate signals to the CNS, which affects sensory input and motor output. If the musculature “web” is disrupted, the patient displays abnormal movement, which can lead to pain and discomfort. This article describes how to identify painful patients, the physiology of muscle pain, and therapeutic modalities for treating muscle pain.

Identifying Muscle Pain

When patients arrive at the clinic, thorough physical and visual examination is needed to properly assess them for muscle pain. Observing a patient’s walk in the exam room can provide valuable preliminary information. Before the physical examination is completed, visual assessment can help indicate if the patient is experiencing underlying muscle pain (BOX 1).^1-4

During the physical examination, veterinary nurses can assess heart and respiratory rates, which are elevated in animals experiencing pain.⁵ Placing hands on a patient and gliding them over body structures may help detect areas that feel warmer or may be swollen, possibly indicating local inflammation. A complete orthopedic examination includes evaluating the gait; checking joint range of motion during flexion and extension; adducting and abducting the shoulder and hip joints; listening for sounds of crepitus (clicking or popping) during joint manipulation; assessing resistance when manipulating a joint past its normal resting position; and looking for signs of joint thickening, buttressing (scar tissue buildup at the joint capsule), effusion, and/or instability (e.g., cranial cruciate ligament disease, patellar luxation, hip dysplasia, shoulder).

If indicated based on the presenting complaint, special tests should be completed (e.g., checking for cranial drawer, tibial thrust, Ortolani and other joint stress tests).^1-3,6

The muscular system should be palpated for myofascial trigger points (areas in the muscle and surrounding tissue where disrupted muscle fibers have led to taut bands that are associated with pain and discomfort).⁷ Patients with myofascial trigger points often exhibit muscle fasciculations or will try to move away from palpation of those tender areas (VIDEO 1).⁷ Postural evaluation of weight distribution can be checked by lifting individual limbs while the patient is standing still. If the patient is compensating by weight shifting to another area of the body, that limb will be easier to lift, indicating that the patient is not willing or able to bear the appropriate weight on the lifted leg.¹

A neurologic examination should include checking for conscious proprioception deficits, hopping reflexes, and spinal and myotatic (stretch) reflexes (cranial tibial, patellar, biceps, and triceps) as well as testing for superficial and deep pain¹; however, describing a complete neurologic examination is beyond the scope of this article. Because orthopedic and neurologic conditions can look similar, full orthopedic and neurologic examinations should be performed to aid in more detailed localization of which muscles are affected and how the patient is displaying signs of pain.¹

Acute and chronic pain can inhibit even the most basic movements, which is why pain assessment should never be left out of a physical examination. Pain can be referred (travels from its origin to other areas nearby) or compensated for during everyday activities (e.g., shifting weight to other areas of the body to take mechanical load off an affected limb). Pain can cause behavioral changes (e.g., guarding the affected area, aggression toward family members), decreased appetite, increased sleeping or disrupted sleep, and decreased activity levels (e.g., wanting to lie down more; less willing to perform certain activities such as negotiating stairs, jumping up and down from things, going on walks).^8,9

Physiology of Muscle

Muscle Types

The body contains 3 types of muscle: skeletal, cardiac, and smooth.¹⁰ Discussion of smooth and cardiac muscle is beyond the scope of this article, which therefore covers only skeletal muscle. Skeletal muscle is divided into postural muscles (which support the body’s structure) and phasic muscles (which aid in movement and activity). Skeletal muscles are defined as agonists (muscles that produce desired movement) or antagonists (muscles that oppose the desired movement).^10-12

Muscle Contraction

Two of the primary types of mechanoreceptors found in skeletal muscle that aid in muscular contractions are muscle spindle cells and Golgi tendon organs.^13,14 These receptors are located predominantly in the muscle belly and relay information to the CNS regarding the muscle’s length (ability to stretch) to help facilitate movement.¹³ There are 2 primary types of muscle contraction: isometric and isotonic. Isometric contraction occurs when there is a change in muscle tension without a change in muscle length. Isotonic contraction occurs during muscle activation when there is tension despite a change in muscle length. During concentric isotonic contractions, muscle fibers shorten while maintaining constant tension; whereas, during eccentric contractions, muscle fibers lengthen in opposition to a stronger force.¹⁴

Individual muscles do not act alone. For a muscle to contract, afferent signals need to be sent from sensory motor neurons to the dorsal horn of the spinal cord and processed by the brain to release efferent signals from the ventral portion of the spinal cord, relaying the desired movement to the muscle spindle cell. When any part of that process is disrupted, the appropriate muscle contraction cannot occur.¹⁵

Muscle Pain and Injury

Muscle pain (myalgia), aching, or tenderness in the surrounding tissue can result from a wide range of diseases, injuries, and health conditions.¹⁶ To address pain and discomfort that originate in the musculature, recognizing which diseases and conditions contribute to the pain is crucial. Information learned from the signals that patients exhibit can be used to help advocate for patients and their treatment plans.

Common Causes of Myalgia

Arthritis can affect the surrounding muscle(s). Chronic joint changes may lead to changes in weight distribution. When an animal moves weight onto other areas of the body to take weight away from the affected joint, the compensation can cause clinical signs in the surrounding muscles. In addition, if pain is not controlled, joint changes can lead to guarding of affected areas. Most patients with arthritis will exhibit atrophy from decreased use of the muscles surrounding the abnormal joint.¹⁷

Electrolyte imbalances play a role in the membrane potential of nerve and muscle fibers, directly affecting muscle contraction and relaxation. Electrolytes involved with muscle contraction are sodium, phosphorus, potassium, magnesium, and calcium.¹²

Nerve damage to peripheral limbs or nerve entrapment can cause pain or atrophy of surrounding muscle(s). Nerve entrapment can result from poor blood supply to the nerve, increased muscle tone, weakness on the opposite side causing increased weight shifting to the unaffected side, abnormal posture, lipomas, edema or hematomas, and muscle spasms. Common locations for nerve damage include the brachial plexus and lumbosacral plexus.¹⁶ Nerve damage to regions responsible for weight bearing (e.g., radial and femoral nerves) can result in the inability to bear weight on the affected limb(s). Neurogenic atrophy (FIGURE 1) results from denervation and develops quickly (over days to weeks, usually after trauma); disuse atrophy results from improper use of a limb.¹⁶

Figure 1. Canine patient with severe neurogenic atrophy and decubital ulcer from spinal cord injury.

Neurologic conditions affect the muscles and are exhibited as neuropathy, junctionopathy, or myopathy (BOX 2). They affect sensory or motor neurons and result in weakness, muscle atrophy, or loss of reflexes.¹⁶

Trauma can affect the most superficial structures all the way to deeper muscular structures, depending on the injury. Familiarity with the types of trauma (e.g., puncture, laceration, abrasion, tearing) helps in choosing the optimal treatment. Common types of trauma experienced by veterinary patients are vehicle accidents, animal bites, wounds, and intramuscular injections,^5,18 which may require additional interventions.

Myofascial trigger points are areas in which disrupted muscle fibers have led to taut bands within the muscle, resulting in pain and discomfort.^7,19 Myofascial trigger points usually form around the neuromuscular junction after a muscle has been overloaded but continues to perform in a deficient state. At the motor endplate of the muscle, more acetylcholine than normal is released and muscle fibers contract. The contracted muscle knot can then bulge and compress capillaries, resulting in focal hypoxia of the surrounding tissue, leading to local tenderness and sensitivity to nearby nerve endings from the release of bradykinin and other agents.⁷

Muscle tension can result from stress and anxiety. It can be speculated that animals experiencing stress and anxiety may also experience muscle tension as a result of holding muscles in abnormal postures. As an example, animals that sit and pant or stand, pace, or lie in uncomfortable positions while hospitalized could have muscle tension.⁵

Surgical sites and incisions are an overlooked cause of muscle pain and discomfort. Even routine surgeries such as spays, neuters, or dental prophylaxis can result in muscle pain that creates behavioral changes (e.g., not wanting to eat, play, or perform daily activities).⁵ During surgical procedures, muscular tissue is usually retracted out of the way and manipulated to provide access to the surgical site (FIGURE 2), which can result in microinjuries from surgical instruments, increased pressure placed on the muscles during retraction, ligation of nerves, and muscles pulled in abnormal directions.

Figure 2. Muscles being retracted during hemilaminectomy surgery.

Treating Muscle Pain and Discomfort

A variety of factors affect muscle’s ability to heal, such as oxygen and blood flow, inflammation, metabolic changes, nutrition, immobilization of peripheral limbs (e.g., casts, braces, orthotics, external fixators), reduced movement (e.g., crate rest, not bearing weight on limb[s]), and age. Fortunately, many methods are available to veterinary team members to minimize patients’ muscle pain and discomfort, such as supplements, anti-inflammatories, pain medications, and physical modalities.

Physical Modalities

Manual Therapy

Manual therapy is a hands-on approach to help block pain signals from entering the dorsal horn, thereby reducing activation of the muscle spindle cells and lessening reduced muscle tension and contraction.^20-22 It targets muscle receptors such as those in Golgi tendon organs and mesenchymal stem cells, changing the pliability of connective tissue, reducing muscle spasms, and targeting Aδ fibers (which alert the body to pain) and C fibers (which initiate perception of pain).

Therapeutic Exercises

Therapeutic exercises are a more hands-off approach focused on maintaining muscle mass, strengthening muscles, and improving proprioception by getting patients more involved in their own recovery. Exercises include weight shifting, proprioceptive or balance work (FIGURE 3), and retraining of posture and gait. During manual exercises, selected areas of the body and individual muscles are targeted to increase weight loaded on the affected limbs and to promote muscle strength, endurance, stamina, and/or growth. Because there are many therapeutic exercise types, choosing the most effective exercise for the condition being treated can be difficult without additional continuing education training. Consulting with a rehabilitation specialist who can provide an individualized exercise plan for patients while also promoting patient safety and preventing the chance of reinjury is recommended.^19,23

Figure 3. Patient practicing balance exercise after traumatic brain injury.

Kinesiology Taping

Kinesiology tape (often called “kinesio” tape) is specialized therapeutic tape used to help improve blood supply or lymphatic drainage, encourage a muscle to maintain a certain movement, provide mild stability to an area, and decrease pain.²⁴ Kinesiology taping techniques for humans can be applied to animal patients, with modifications based on animal anatomy and presence of fur (FIGURE 4).

Figure 4. Kinesio tape placed on patient to aid with pain and swelling.

Kinesiology taping is thought to work via the gate control theory of pain modulation (the spinal cord contains a neurologic “gate” that either blocks pain signals or allows them to continue on to the brain). More specifically, the dorsal horn of the spinal cord regulates stimulus input from different fibers in response to stimuli. Pain stimuli from smaller fibers travel faster than pain stimuli from larger-diameter fibers¹⁰; thus, stimuli from skin receptors first arrive at the dorsal horn of the spinal cord, which can prevent transmission of stimuli from larger nerve fibers. The tape helps reduce pressure at the nerve endings of the hair follicles by lifting the space between the epidermis and underlying tissues, which modulates pain transmission and sensory input.^24,25

Kinesiology taping can help facilitate muscle contraction or relaxation and can improve circulation for patients with edema or swelling. Tension placed on the muscle origin will lead to contraction; tension placed toward the muscle insertion will lead to relaxation.²⁴ Kinesiology taping to decrease swelling and edema often involves using a webbing technique over the muscle belly to improve circulation.

Photobiomodulation

Photobiomodulation, also known as low-level laser therapy, is projection of light into the body at a desired wavelength (monochromatic), which collimates on the area of interest (VIDEO 2).^26-28 Light photons penetrate the cells to enhance cellular energy and metabolism. The energy delivered to the target areas at a specific dose (measured in J/cm²) causes rapid metabolization of inflammatory mediators and increases production of adenosine triphosphate (ATP) in mitochondria, thus increasing available energy for muscle contraction.^26,27,29-31 Laser therapy also stimulates vasodilation, which increases circulation, increases lymphatic drainage, and decreases inflammation by removing destructive cellular enzymes and proteins.

Therapeutic Ultrasonography

Therapeutic ultrasonography delivers sound waves to tissues to create minute vibrations (> 20 000 Hz) that warm the affected tissues (FIGURE 5). Therapeutic ultrasound waves penetrate tissue more deeply (up to 5 cm) than superficial warming devices (approximately 1 to 2 cm).^29,32 The sound waves help stimulate orientation and increase flexibility of collagen fibers,^31,32 thereby increasing the pain threshold to enable increased muscle range of motion while decreasing muscle spasms during movement or manual therapy.²⁹ Tissue extensibility is also increased as the sound waves increase blood flow, cell membrane permeability, and enhance release of intracellular mediators (e.g., growth factors, cytokines), resulting in improved healing.³ Sound waves are best absorbed in tissues that are more organized and contain more protein (e.g., tendons, ligaments); heating is 3 times faster in tendons than in muscle.^29,31

Figure 5. Patient being treated with therapeutic ultrasonography. Courtesy Rebecca McElhoe, Purdue University

Acupuncture/Dry Needling

Acupuncture is the process of inserting needles into specific points, usually a muscle motor point or along a nerve tract, to stimulate therapeutic effects in the surrounding tissues and organs (FIGURE 6). Acupuncture treatments can target trigger points within the muscle and myofascial layers. The therapeutic effects include decreased pain, stimulated healing process, and increased relaxation.^8,33,34 The mechanism of action of acupuncture pain relief is still not exactly known, but it is believed that needle insertion into the skin triggers the body to release endogenous opioids (neurotransmitters that are a part of the CNS that help process pain).^34,35 Acupuncture may be performed only by a licensed veterinarian certified in acupuncture.

Figure 6. Patient receiving acupuncture for back pain.

Electrical Stimulation

Two types of electrical stimulation are commonly used to address pain or dysfunction: transcutaneous electrical nerve stimulation (TENS) and neuromuscular electrical stimulation (NMES). TENS targets sensory nerves (FIGURE 7), whereas NMES targets motor nerves.

Figure 7. Dog undergoing transcutaneous electrical nerve stimulation therapy after a hemilaminectomy.

TENS is used for modifying pain via gate control and can stimulate endogenous endorphin release. Pain signals are sent to the CNS via smaller-diameter Aδ and C fibers. The electrical current generated by the TENS machine stimulates the larger-diameter Aβ fibers to block peripheral pain signals from entering the dorsal horn of the spinal cord and dampen the pain response.^29,36 Use of TENS does not generate therapeutic muscle contraction; however, NMES does, thereby maintaining and strengthening muscle while preventing disuse atrophy. The goal with NMES use is to depolarize the motor nerve by activating muscle fibers. When used appropriately, NMES can be used to increase joint mobility, which in turn encourages lymphatic drainage and circulation in surrounding tissue, resulting in decreased edema. NMES can help lower the chance of joint contracture, increase sensory awareness, reduce muscle spasms, and decrease pain caused by muscle stiffness.^29,36,37

Extracorporeal Shockwave Therapy

Extracorporeal shockwave (ECSW) therapy can be used to treat muscle pain; nonunion fractures; wounds; osteoarthritis; neurodegenerative disease; spinal pain; and injury to tendons, ligaments, and other soft tissues. ECSW therapy uses single-pulsed acoustic sound waves (1000× the pressure of therapeutic ultrasound waves) and can deliver focally controlled energy to a desired treatment area without having the thermal effect of therapeutic ultrasonography.^29,38 Stimulating cells via sound waves helps increase local blood circulation and decrease inflammation. ECSWs increase levels of hydroxyproline, an amino acid involved in collagen synthesis, as well as mesenchymal stem cell release and tenocyte proliferation to support tendon and ligament healing. ECSW therapy can also help with formation of new blood vessels, stimulation of bone healing, and tissue regeneration. The exact mechanism of action of ECSW therapy for pain control is still not completely known, but it is believed that ECSWs stimulate cells to release ATP as well as serotonin at the dorsal horn of the spinal cord, helping block pain signals.²⁹

Aquatic Therapy

Aquatic exercise, whether swimming or underwater treadmill walking, is useful for improving muscle stamina and maintaining muscle mass (VIDEO 3). Using water to lighten the total weight on joints helps decrease the chance of injury, reduce muscle spasms, prevent muscle atrophy, and increase muscle mass and strength.

While parts of the body are submerged in water, the surrounding muscle can work more efficiently with less impact on adjacent joints. Depending on the water depth, joint range of motion can be improved more by exercising in the water than on land. Hydrostatic pressure on peripheral body segments helps improve circulation, and the water turbulence challenges postural muscles to balance.^31,39,40 Several properties of aquatic therapy help with muscle care (BOX 3).^39,40

Summary

Although musculature makes up a large part of the body, its contribution to pain and discomfort as well as to joint and bone healing is often overlooked. If we compare patients’ musculature to a spider web, it can help us look for areas of the body that are injured and not communicating or functioning properly with the rest of the body. Such injuries could affect functions such as maintaining posture, providing stability, feeling pain, and creating movement. For veterinary nurses, being able to recognize signs of pain and communicate what we observe to veterinarians can help patients receive the best care possible. Knowing how muscle functions and how it can be compromised when injured is valuable for effective management and treatment. Appropriate use of manual and therapeutic exercises and physical rehabilitation modalities to help maintain, strengthen, and prolong muscle function will ultimately improve recovery from musculoskeletal disorders.