Microstructure of Muscle

The muscle consists of muscle fibers and connective tissue (Fig. 11-1). There are three layers of connective tissue: the epimysium, the perimysium, and the endomysium. The epimysium surrounds the muscle and (along with the perimysium and the endomysium) is continuous with the muscle’s tendons. Within the muscle, groups of muscle fibers (known as a fasciculus) are surrounded by the perimysium. Within each fasciculus, each individual muscle fiber is surrounded by the endomysium.

A muscle fiber (muscle cell) is further divided into myofibrils and myofilaments (see Fig. 11-1). Each muscle fiber is surrounded by a sarcolemma, the muscle fiber’s cellular membrane. Within the muscle fiber, the myofibrils are contained within the sarcoplasm. The myofibrils are bundles of the contractile proteins myosin and actin grouped together as a sarcomere (the basic functional unit of the myofibril). The interaction between the myosin and actin during the muscle’s contraction contributes to functional movement. The structural relationship between the contractile and noncontractile components allows skeletal muscles to perform both concentric (shortening) and eccentric (lengthening) muscle contractions as well as the ability to return to a resting position.¹⁸

Microstructure of Tendon

Tendons serve two functions: to facilitate movement or joint stability by transmitting forces between muscle and bone, and to store energy for later movement. An example of this is the energy created during plyometric exercises. The series elastic component stores elastic energy created during plyometric exercises in response to an eccentric stretch of the muscle. Tendons have the ability to withstand significant physiologic loads; however, they may be at risk of injury by either trauma, degeneration, or overuse. A tendon’s structure is divided into three locations: the muscle–tendon junction, the midsubstance, and the bone–tendon junction.

Tendons are dense connective tissue consisting of fibroblasts (cells) and an extracellular matrix composed of collagen fibers, elastin, and ground substance (Fig. 11-2). The fibroblasts are responsible for synthesizing components of the extracellular matrix.²⁸ The ground substance provides support to collagen fibers.²⁸ The collagen fibers, which are oriented in a parallel arrangement, provide strength to resist tensile forces.

The structural hierarchy of tendons is similar to skeletal muscle. Collagen fibrils are grouped together into the tendon’s basic functional unit, the collagen fiber. Groups of collagen fibers (primary fiber bundle) form a subfascicle.¹⁶ Several subfascicles grouped together form a fascicle (secondary fiber bundle).¹⁶ Several fascicles grouped together form a tertiary bundle.¹⁶ These three groups (bundles) are surrounded by a connective tissue called the endotenon.¹⁶ The epitenon surrounds the entire tendon as a fine, loose connective tissue sheath.^16,28 Blood vessels and nerves are also contained within the epitenon.²⁸ The paratenon is a loose areolar connective tissue attaching to the outer portion of the epitenon.^16,28 The function of the paratenon is to allow gliding over adjacent structures.¹⁶

The transition from tendon into its osseous attachment site includes several connective tissue changes.¹⁶ The first change is from the primary tendon structure into fibrocartilage. Next, the fibrocartilage converts into a mineralized fibrocartilage. The mineralized fibrocartilage secures the attachment of the tendon to the bone. In addition to this arrangement, there are collagen fibers from the tendon that pass through these transition zones and attach to the bone. These fibers are known as Sharpey’s fibers.

Muscle and Tendon Injury Mechanisms

A patient’s rehabilitation program will be developed by the PT based on the patient’s medical history, the patient’s subjective report, and the PT’s physical evaluation of the patient. When guiding a patient through his or her treatment program, the PTA should be familiar with the patient’s diagnosis and mechanism of the injury. Possessing this knowledge will help the PTA advance the rehabilitation program per the PT’s plan and will alert the PTA to cease treatment and consult with the PT when the patient demonstrates a negative response to treatment.

A muscle injury may be caused by disease, a direct (external) mechanism, or an indirect (internal) mechanism. Some diseases may have profound effects on the muscular system and contribute to significant disability. Examples of diseases that weaken muscles include muscular dystrophy, multiple sclerosis (MS), Parkinson disease, and Huntington disease. Patients who have been diagnosed with a neuromuscular disease may benefit from treatment by the physical therapy team. However, for purposes of this chapter, the pathomechanics and only the rehabilitation strategies for direct and indirect muscle injuries will be addressed.

A direct muscle injury will occur when an external force applied to the body results in a trauma. Examples of direct muscle injuries include a contusion caused by a blunt trauma or a deep laceration caused by a sharp implement. In each of these examples, the applied supraphysiologic load was greater than the muscle’s maximum tissue tolerance. An indirect muscle injury occurs independent of an applied external force. Indirect muscle injuries are the result of either a supraphysiologic load (e.g., a violent or excessive muscular stretch or excessive muscular stretch combined with an eccentric muscular contraction) or repeated insults (e.g., overuse injury) to the tissue without proper time for recovery.8,18 Indirect muscle injuries primarily occur at the musculotendinous junction; however, they may also occur in the belly itself.¹⁸

A muscle strain, frequently referred to as a muscle tear, is an indirect injury (Box 11-1). A pulled hamstring (or sometimes referred to as a hammy) is an example of a common muscle strain experienced in sports.^5,6,26 Muscle strains can cause significant pain and greatly reduce function.^5,6,26 Hamstring injuries can plague an athlete with a lengthy rehabilitation period and a slow return to sport.²⁵ Muscles that cross two joints, like the hamstrings, are frequently strained.^8,25 Sports medicine specialists believe that these muscles may become injured in response to an intense eccentric load.⁸ Many of the most frequently strained muscles are composed of type II muscle fibers, the fibers that are least resistant to fatigue.⁸

Muscle strains are frequently described by the degree of damage to the muscle. Table 11-2 presents the characteristics associated with a first degree, second degree, or third degree muscle strain.

Because muscle attaches to bone via tendon, the two structures are interrelated and are frequently injured together (e.g., musculotendinous junction muscle strains). However, there are a number of injuries seen clinically that are specific to the tendon itself (e.g., tendinitis or Achilles tendon ruptures).

Tendon injuries range from acute tendinitis to chronic tendinopathy (Table 11-3). Tendinitis is an acute injury of the tendon associated with an inflammatory response. A patient with a “true” tendonitis may present to physical therapy with pain, functional loss, and signs associated with inflammation (pain, warmth, swelling, redness). A patient with tendinitis often describes performing an unfamiliar, repetitive activity from 1 day to a week before seeking medical attention. If a patient presents with a tendon injury that lacks inflammatory signs, it is likely that the patient has a tendinopathy. It is not uncommon to have a patient describe that he or she has experienced pain for many months to years before seeking medical attention. The Achilles tendon, the quadriceps tendon, and the supraspinatus tendon are common locations that one might develop a tendinopathy. The manner in which each condition is treated differs greatly. A tendon may also rupture, requiring surgical repair.

When a tendon experiences strain levels that overload the tissue’s tensile capabilities, microtrauma will result. A frequent, repeated strain to the tendon will continue the microtrauma (damage) to the microstructure of the tendon. An injury occurs when the rate of damage surpasses the tendon’s ability to repair itself. Over time, if the tendon fails to heal, a tendinosis region will develop.

Some tendons may also be at risk for significant tearing or rupturing. In the event of a tendon rupture, surgical intervention is necessary to restore function. Orthopedic surgeons frequently perform tendon repairs to the tendons of the supraspinatus (in addition to the other rotator cuff muscles), extensor carpi radialis brevis, quadriceps tendon, and Achilles tendon.

Age Effects on Tendons

Over time, the aging tendon has an increased risk of sustaining an injury.³³ PTAs will frequently provide treatment for patients who are 40 years and older who have experienced a tendon injury. In some cases, patients will receive conservative physical therapy treatment, whereas in other cases patients may require invasive procedures such as injections or surgical repair.

Acute Stage: Immediate Response for a Muscle or Tendon Injury

A sports medicine team consisting of the PT, the PTA, or the PTA with a dual credential as a certified athletic trainer (ATC), will provide the first line of treatment once an athlete is injured. Many sports certified PTs who are dual-credentialed as an ATC or who are sports certified specialists (SCS, board certified sports physical therapist by the American Physical Therapy Association) provide game coverage within their community. The immediate treatment provided by the sports medicine team for the athlete who has been sidelined because of a sport-related injury will include protection, rest, ice, compression, and elevation (PRICE).

If an athlete sustains an injury that impairs function or if continued sports participation might worsen the injury, then the athlete should be removed from competition and allowed to rest the injured area. If possible, the injured region should be elevated. Elevation of the joint or extremity should be above the level of the heart (e.g., an athlete who has injured his leg should have the lower extremity elevated and supported while the athlete lies on his back). Elevating the extremity will reduce the swelling that occurs immediately post injury. Ice should be immediately applied to the injured area to reduce further inflammatory damage and control pain. Compression should be applied in conjunction with the ice for up to 20 minutes, every 30 minutes to 1 hour. Finally, the injured region should be protected from further injury. For example, the athlete who has sprained an ankle might benefit from the use of bilateral crutches and decrease weight-bearing stress. Depending upon the severity of the injury, the athlete should be referred to the emergency room to rule out a fracture (if necessary) or to the appropriate sports medicine or orthopedic physician the following day.

Acute Stage: Treatment in the Physical Therapy Clinic

The clinical goals associated with this stage are to facilitate healing to the injured tissue, control or reduce the effects of the inflammation, decrease pain, initiate controlled movement to restore ROM, and reduce the loss of muscular strength

Physical agents are frequently used during the acute stage of healing to decrease pain, reduce inflammation, facilitate healing, and to restore muscular function. In addition, passive range of motion (PROM) and active assisted range of motion (AAROM) exercises are initiated to maintain or restore movement around the joint. Manual therapy techniques are performed to reduce pain and improve soft tissue and joint mobility. Isometric strengthening may also be performed during this phase to retard strength loss. Table 11-5 presents a list of evidence-supported treatments for patients in the acute stage of healing.

Patients should be provided with a home exercise program to perform between sessions. The effectiveness of each technique should be assessed during and between treatment sessions. Treatments that fail to impact the patient’s recovery should be terminated. In addition, treatments that worsen the patient’s state should be immediately stopped. A failure to remove stress to the injured tissue during the acute phase may increasing swelling, pain, and the length of this phase.

Subacute Stage: Treatment in the Physical Therapy Clinic

The clinical goals during the subacute phase are to restore full active and PROM, initiate muscular strengthening, continue to address residual swelling, reduce pain, and initiate functional movement tasks.

During the subacute phase of healing, the body deposits immature collagen fibers in the injured region. Prescribing therapeutic exercises that appropriately stress the injured region will help to promote the normal healing process and the development of a mobile scar. However, overstressing the area may injure the new collagen fibers. Pain during exercise should be avoided. Make it clear to patients that they should stop performing an exercise if it reproduces their pain.

A primary goal of this phase is to progress and restore range of motion. During the acute phase, PROM and AAROM exercises are prescribed. As tolerated, the patient should be progressed to active range of motion (AROM) techniques and static stretching exercises. Typically, multiple sets and repetitions of AROM exercises are prescribed. Manual therapy techniques (joint mobilization, massage) may continue to be necessary to restore joint and soft tissue mobility. Static stretching exercises should initially be performed gently, holding each stretch for 30 seconds.13,24,37

The prescription of resisted exercises will assist the collagen orientation and improve tensile capabilities. Initially, exercises should be performed with high repetitions and low weights. Performing a muscular endurance program will allow the patient to gradually gain strength while reducing the risk of damaging the new collagen fibers. Consider patients performing 15 to 25 reps of 1 to 2 sets per exercise. The initial number of exercises prescribed by the PT will be dependent upon the patient’s presentation. The inclusion of neuromuscular electrical stimulation may further assist the restoration of muscular strength.¹⁹

Chronic Stage: Treatment in the Physical Therapy Clinic

During the chronic stage of healing, collagen aligns to the stresses applied and the tissue is maturing and remodeling. Physical therapy interventions should facilitate this through the continued prescription of therapeutic exercises.

Increasing the endurance capacity of the muscle may be continued as necessary, but now the therapy team should be able to progress strengthening as tolerated. Exercise prescription should be functional in nature and should account for muscle fiber structure within muscle groups. For example, muscles of the core (type I muscle fibers) should continue to be trained using endurance training strategies. Strength training variables may be applied to muscle groups containing higher percentages of type II muscle fibers. Once the patient is pain free, plyometrics or power training may be initiated if functionally necessary (e.g., athletes, industrial workers).⁴

Special Considerations: Tendon Repairs and Tendinopathy

The patient who has been referred to physical therapy with a postoperative tendon repair requires special consideration. Failing to follow the physician’s postoperative protocol may put the repaired tendon at risk for injury or rerupturing.

Patients who have been referred with a tendinopathy diagnosis will benefit from performance of eccentric exercises. There is a growing body of evidence that suggests the inclusion of eccentric exercises will help to reduce pain and restore function in individuals with either a tendinosis at the Achilles tendon or patella tendon.10,14,17,36