Stress and Strain

Stress or load is given in units of force/area, where the units may be pounds per square inch or Newtons per square centimeter.⁹⁶ Therefore stress is directly related to the magnitude of force and inversely related to the unit area, but is independent of the amount of a material.⁹⁷ The complementary measure related to stress is strain or deformation. Strain is usually dimensionless, since the units of measure cancel each other out, but units are often provided (e.g., inches/inch) to give a perspective of scale applied to the loads.⁹⁶

The relation of stress and strain is dependent on several factors. These include the material properties used, the magnitude of the stresses, and the rate of stress application. Subsequently, the graphical representation of stress and strain is different for each type of material discussed. This representation is known as the stress-strain or load-deformation curve.⁹² Figure 3-2 demonstrates the various regions within the stress-strain curve, with each region demonstrating a biomechanical property of a tissue.

The toe region represents how a slightly pulled tissue (i.e., a ligament) produces only a small amount of tension within the tissue; it indicates that the collagen fibers within the tissue must first be pulled tight before a stretch can be induced. This minimal amount of tension simply results in the slack of the tissue being taken up with no stretch being encountered. The elastic region represents a linear change in strain, which occurs if the tissue continues to be pulled at higher stress levels. A good example of this linear relation is similar to the model of a stretched rubber band. The greater the load on the rubber band, the more the rubber band stretches. So, as the tension on the rubber band is removed, the band shortens, nearing its original length. Given this behavior, it is referred to as the elastic zone. The amount of stretch (strain) applied to the tissue in this zone is generally experienced during many body movements. Note that the slope (the angle of the curve) in this zone is indicative of the relative stiffness of the tissue. Stiffness is defined as the change in force per unit change in length.⁴¹ As such, the stiffer the tissue, the steeper the slope. So tissues such as ligaments will only change length at higher levels of stress compared with tissues of less stiffness, such as loose connective tissue.

The upper level of the elastic region marks a transition from the linear slope to one where the slope begins to flatten. This is called the plastic region and is a point where an increasing level of stress on the tissue results in proportionately increased changes in tissue length, probably due to microscopic failure of the tissue. This is termed the plastic zone because of the tissue damage resulting in permanent deformation (plastic deformation). Unlike the elastic region, the plastic energy is not recoverable in its entirety when the load is removed; additionally, there is a change in its resting length. Continued stretch in the plastic region would result in additional deformation of the tissue and would occur until it reached its initial point of failure. The toe region can include strains up to 3%; the elastic region, from 6% to 10%; and the plastic region, from 10% to 15%.41,112

Viscoelasticity

All connective tissue exhibits the property of viscoelasticity, which is simply a combination of the behavior of the properties of elasticity and viscosity.73,76 Elasticity refers to a material’s ability to return to its original state following strain or deformation (i.e., change in length or shape) after a removal of the stress or load. This mutable change is termed elastic deformation and is similar to the changes that occur in a rubber band under high rates of strain. The rubber band rapidly conforms to a new length and is able to return to its original resting length when the stress is removed. However, the rubber band breaks if the degree of stress exceeds the strain capabilities (Fig. 3-3). Viscosity refers to a material’s ability to resist a change in form or to dampen shearing forces. Tissues exhibiting viscosity have time-dependent and rate-dependent properties when forces are applied to them.

The rate at which tissues are stretched has a profound effect on the degree or percent of strain. As mentioned, tensile (distractive) or compressive forces will produce deformation on viscoelastic materials, but will allow return to their original state after removal of the force. Given normal conditions, however, viscoelastic materials do not immediately return to their original state. Viscoelastic materials, unlike pure elastic materials, have time-dependent properties. Slower rates of stress produce greater amounts of strain or elongation, whereas faster rates of stretch produce much smaller amounts of elongation.29,35,119 As such, they are sensitive to the applied force duration. Tissues gradually lengthen when they are subjected to constant or repeated stress of long duration.

So a viscoelastic material subjected to either a constant compressive or tensile load will initially respond by rapidly deforming; it continues to deform over a finite length of time even if the load remains constant. This deformation of the tissue will continue until a state of equilibrium is reached when the load is balanced. This is called the creep phenomenon (Fig. 3-4),⁷⁶ which simply means the gradual increase in tissue length that occurs when maintaining a constant stress (or force). For example, if one uses a constant force against the muscle with slow and passive stretching, the muscle will eventually elongate. However, the tissue does not return to its original length immediately when unloaded because of the collagen’s viscous property.⁷³ Essentially, the longer the duration of the applied force, the greater the deformation or stretching of the tissue.

Another term frequently used is stress-relaxation, or force-relaxation. Unlike creep, this occurs when a viscoelastic material experiences a constant strain (no deformation occurs). In this case, a high initial stress placed upon a tissue decreases over time until equilibrium is reached and the stress equals zero, resulting in relaxation of the tissue. Consequently, no change in length is produced. For example, if a muscle is held at a certain length over time, a reduction in stress would occur, but there would be no change in length (no stretch occurs).

The loading and unloading of tissue is influenced by its viscoelastic properties. This loading and unloading is quantified by measuring the hysteresis area (Fig. 3-5). The loading (solid line) and unloading (dashed line) phases represent a measurement of the stress and strain that is applied to a tissue and then released. The hysteresis area lies between the two lines and represents the amount of energy dissipated in the loading and unloading process.^41,73

The ability of tissues to recover after stress is extremely important in relation to flexibility. Woo and colleagues¹³⁶ have shown that increasing the levels of stress produces an increase in collagen within ligaments and tendons, whereas reducing the levels of stress causes weakening in connective tissues. Recovery is the ability of tissues to return to their previous resting state. However, it does not imply that permanent elongation or microscopic damage has not occurred. Plastic deformation is force dependent under slow rates of stress and is used to describe permanent change in a tissue. For example, when a low degree of stress is applied to a plastic spoon, the spoon slowly deforms to a new shape. The spoon breaks if the stress is applied too fast (Fig. 3-6). Lehman and colleagues⁷⁴ and Warren and co-workers¹²⁶ have demonstrated that recovery of the tissue’s resting length had occurred after microscopic failure had begun.

Along with stress and the rate of stress applied to tissues, temperature also affects connective tissue extensibility as well as the rate of creep. High temperatures in the range of 37° C (98.6° F) to 40° C (104° F) affect the viscoelastic properties of connective tissue and increase the rate of creep.29,73 The higher the temperature (approximately 45° C [113° F] is the therapeutic upper limit), the greater the degree of elongation with stress before tissue failure.¹⁰⁸ Because connective tissue’s viscoelastic and plastic changes occur at higher temperatures, there is less microscopic damage under stress at these temperatures. Studies by Warren and colleagues¹²⁷ have demonstrated that a temperature of 45° C is needed to reduce tissue damage during strains of 2.6% or less. So to stretch out a connective tissue structure, one should heat it and use a large load over a long period of time to produce creep.⁹⁶ Low temperatures decrease the rate of creep.

Muscle or contractile tissue responds to stretch by elastic and plastic deformation properties in ways similar to connective tissue. Obviously the contractile properties of muscle allow for the greatest degree of freedom of movement around a joint. Although connective tissue is considered a passive resistant to joint motion, muscle tissue is considered an active restraint to joint motion by virtue of its elastic and contractile elements. Active exercise (muscular contractions) affects intramuscular temperature. Increases to approximately 39° C are observed in exercised muscle.⁶ Commonly used passive thermal agents that increase tissue temperature are moist heat and ultrasound. The judicious use of active exercise and passive thermal agents before and during stretching programs enhances the effectiveness of the prescribed program.

Measuring Flexibility

It is important to assess overall flexibility by measuring ROM at specific joints, because no one composite test provides an index of an individual’s flexibility characteristics.53,54 Subsequently, some measurements of overall flexibility are somewhat difficult, since they involve a complexity of movements over several joints (e.g., sit and reach test).

Measuring joint ROM is accomplished by using standard goniometric instruments. The measuring device most commonly used to measure ROM is the universal goniometer (Fig. 3-7), which is a protractor with degree measurements. The goniometer arms are placed along the proximal and distal components of a joint with the axis centered over the joint, which are then moved in accordance with the body part being measured. The goniometer can be used to measure joint position and ROM at almost all joints of the body.⁹⁵ Other types of goniometers used less frequently in the clinical setting are the inclinometer, the pendulum goniometer, and the fluid goniometer.

Consistency in measurement is crucial; therefore it is important to use the same type of goniometer for a specific patient each time that patient is measured. Because different goniometers may provide slightly different results, they should not be interchangeable in their use in the clinical setting. For example, when using an inclinometer as the measuring device, it should be used for all future measurements for that patient to ensure consistency.⁹⁶ For documentation purposes, most authorities recommend using goniometry rather than visual estimates because of its increased accuracy and reliability of measurement.^128,137

Joint stability differs from joint ROM in that the ligaments and surface geometry of joint articulations dictate static joint integrity (stability). A patient may demonstrate limited ROM in knee flexion and extension (by goniometry); however, anterior and posterior joint mobility may be excessive and unstable (Fig. 3-8). On the other hand, a patient may demonstrate normal joint ROM, yet when tested statically the joint may be very stable, tight, and unyielding to pressure.

The sit and reach test (Fig. 3-9, A-B), standing toe touch for back and hamstring flexibility (Fig. 3-9, C), seated hip external rotation test (Fig. 3-9, D), and standing knee recurvatum test generally are less specific flexibility tests. These tests and others are used to provide very general assessment of multijoint flexibility. Such tests also can be used as stretching techniques to improve limitations in movement. However, objective clinical documentation of joint ROM is made by joint goniometry.

Factors Affecting Flexibility

In addition to muscles, tendons, and their surrounding fascia, other structures, such as bone, fat, connective tissue lesions, skin, and postural problems, may all lead to flexibility and joint ROM limitations.⁵ Other factors that may affect flexibility are age and gender.^14,25

Stretching Principles

Stretching is effective primarily because of its mechanical and neurophysiologic effects.69,119 As mentioned, the muscle-tendon unit responds viscoelastically during stretching. However, some stretching exercises are based on the neural inhibition of the muscle undergoing stretch as well. In this case, a decreased reflex activity would result in reduced resistance to stretch, resulting in further gains of joint ROM. Most likely, both mechanisms are responsible.

It is important to note that stiffness relates to a tissue’s ability to resist stretch and indicates the amount of deformation proportional to the load applied.⁸¹ So, the stiffer the tissue, the less compliance it has and the less likely it is to stretch; conversely, less stiffness means greater compliance. It stands to reason then that flexibility training is designed to decrease the stiffness of the muscle-tendon unit.^69,119 Furthermore, a rapidly applied stretch results in a greater resistance to that stretch.

Both mechanical and neurophysiologic components are inherent within muscular flexibility.43,50,105,118,119 However, some researchers believe that the ability of stretching to produce greater flexibility is primarily a result of the viscoelastic nature of muscle and connective tissue.^84,119 As such, the most resistance to stretching probably is due to the connective tissue framework and sheathing from within and around the muscle and not the muscle itself (since the muscle itself can be stretched to 150% of its resting length); this includes the epimysium, perimysium, and endomysium and may even include the sarcolemma.^{107,109,116,117}

On the other hand, many researchers suggest that the immediate result of stretching is attributed to neurophysiologic phenomena.⁶⁹ They categorize these phenomena as either a stretch tolerance or active contractile responses and claim that the limiting factor during stretching is the muscular resistance secondary to reflex activity. A stretch tolerance simply means an accommodation to the discomfort of stretching over time. With this philosophy, the aim of stretching is to inhibit the reflex activity, subsequently decreasing the resistance and improving ROM. Several other researchers agree and recommend certain types of stretching for its reduction in reflex activity.^3,43,50 Muscle spindles and Golgi tendon organs are the mechanoreceptors responsible for the contractile responses (Fig 3-10).

Stretching Techniques

In discussion of flexibility and associated stretching programs, the stretching of nonpathologic muscle must be separated from stretching noncontractile connective tissue. Improving muscle extensibility in nonpathologic conditions and in adaptive muscle shortening after injury or immobilization requires a complement of active exercise techniques and thermal agents. Types of stretching exercises are static stretching (active and passive), ballistic stretching, dynamic stretching, and proprioceptive neuromuscular facilitation (PNF) techniques (contract-relax, contract-relax with agonist contraction, and hold-relax). Research has shown that all four techniques have been shown to increase flexibility.31,56,61,80,82,105 Essential to a successful stretching program for improving flexibility is the proper execution of the exercises.

Static stretching

The most common form of stretching exercise that is used to safely increase the ROM in a joint is static stretching. This form of stretching allows one to sustain a controlled stretch by placing a muscle in a fully elongated position and holding that position for a period of time (Fig. 3-11). A passive static stretch implies that the force is applied externally (i.e., with a partner or gravity-assisted) (Fig. 3-12). If an opposing muscle action is used to aid the stretch, then the stretch is called an active static stretch. However, to obtain optimal passive stretching, all voluntary and reflex muscular resistance must be eliminated.

Research supports that slow passive stretching through the ROM will not produce muscle activity in normal relaxed subjects.13,107 As such, the muscle should be slowly and passively stretched to a new length, so that the stretch reflex is not elicited. Additionally, the stretch should not elicit pain.^3,105 As is described later, the stretch is maintained for an extended period of time before being returned to its starting position. Subsequently, a lengthening of the muscle is accomplished.

Athletes use static stretching before sports activities as part of a warm-up before a workout and as part of a cooldown after a workout. Because intramuscular temperature rises to approximately 39° C during exercise,⁶ active general body movements can improve muscle temperature before stretching is done. Most warm-up programs use primarily active static stretches, using gravity to assist the stretch.

Studies on rat tail tendons⁷⁴ demonstrated that ruptures occurred with 31% of normal loads when temperatures were at 25° C, whereas increasing temperatures to 45° C delayed tendon rupture until 102% of normal load. This demonstrates that stretching at normal body temperatures may damage tissue,¹²⁷ but elevating tissue temperature before and during prolonged stretch is less damaging.^126,127

Static stretching has distinct advantages, such as reduced chance of exceeding strain limits of tissues, reduced energy requirements compared with other forms of stretching, and reduced potential for muscle soreness.29,108,136 The ease and practicality of teaching patients to perform static stretching is another advantage. For example, hamstring stretches can be taught with the patient in various positions (Fig. 3-13). The general goals of static stretching are to prevent or minimize the risk of soft-tissue injury from participation in sports or physical activities, improve movement and increase flexibility, and prevent contracture.^8,64

Ballistic stretching

Another technique used quite often by athletes is ballistic stretching (Fig. 3-14). Many believe that this is the least desirable technique, because it potentially places the tissue at risk secondary to the use of jerking or bouncing movements at the end of the ROM to stretch the muscles.⁷¹ Athletes use dynamic, high-velocity, and even violent motions during sporting events and require extraordinary flexibility to prevent or reduce the risk of potential musculoskeletal injury. Relatively high-velocity or quick bouncing may not be appropriate for many patients. The potential for tissue damage exists in all forms of exercise, but ballistic stretching may increase the risk of connective tissue and contractile tissue trauma, although a narrow segment of patients may benefit.

The reason ballistic stretches are considered undesirable is that they stimulate the muscle spindles during the stretch. This results in a continuous resistance to further stretch, which then causes a high rate of tension strong enough to potentially injure the musculotendinous unit.65,107,123 Ballistic stretching does not imply aggressive, violent, high-velocity stretches throughout the ROM; instead it involves a slight but progressively greater bounce at the end of the range achieved through static stretching.

Dynamic stretching

A type of stretching technique that has become increasingly popular is dynamic stretching. With dynamic stretching, muscular contraction is used to stretch a muscle; the effect is to increase or decrease the joint angle where the muscle crosses, thereby elongating the musculotendinous unit as the end ROM is obtained.¹⁰ What sets dynamic stretching apart is that it uses activity-specific movements, thereby preparing the muscles by taking them through the movements used in a particular sport. An example of this would be a sprinter who would walk using exaggerated long strides, thus emphasizing hip flexion and extension (Fig. 3-15). This subsequently actively contracts and stretches the muscles used by the sprinter, namely the hip flexors and extensors.⁶⁰ Dynamic stretching does not incorporate end range ballistic movements that are bouncy or jerky in nature. Rather, all movements are under control.⁹⁶

Proprioceptive neuromuscular facilitation

Another technique used to facilitate ROM increases via stretching is called proprioceptive neuromuscular facilitation (PNF). PNF is a system of therapy that uses different techniques designed to promote neuromuscular responses via stimulation of the proprioceptive system.¹²² These techniques use volitional contractions to increase ROM by decreasing the resistance caused by the spinal reflex pathways.²⁴ PNF stretching uses movements in diagonal patterns along with an isometric contraction before the stretch; this allows greater gains in ROM than stretching alone. However, depending on the clinical setting, many clinicians have modified the various PNF patterns, deviating from the proposed diagonal patterns originally described by Knott and Voss^64a to straight plane patterns. As a result, it is difficult to determine if the effects of PNF treatment are consistent among practitioners and in the research literature.

PNF stretching techniques, which are based on the stretch reflex, appear to increase ROM through the stimulation of the proprioceptors² and have been found to produce greater increases in ROM than static stretching.^91,101 Two neurophysiologic sensory receptors involved with the stretch reflex are the Golgi tendon organs (GTOs) and the muscle spindle. The GTOs are inhibitory sensory receptors located within the musculotendinous junction that signal tension in a muscle (see Fig. 3-10, B). Stimulation of the GTO results in self-inhibition of that muscle, termed autogenic inhibition. They also signal minute changes in muscle tension, thereby providing information about muscle contraction.⁶³

Muscle spindles are excitatory specialized fibers within the muscle (see Fig. 3-10, A) that are sensitive to changes in muscle length as well as maintenance of that length. When a muscle is stretched, the spindles send messages to the spinal cord, which in turn signals the muscle to contract. The classic clinical demonstration of the stretch reflex is produced by tapping the relaxed patellar tendon, which causes the reflexive contraction of the quadriceps. The muscle spindles within the quadriceps are activated by the quick stretch of the patellar tendon, causing the quadriceps to contract reflexively (Fig. 3-16).⁶³

Despite the advantages, PNF stretching has some disadvantages as well. For example, it is more time consuming than other methods, requires skillful application by trained professionals to be effective, and may lead to mild complaints of patient discomfort.¹⁰⁸ Some of the more commonly used techniques are contract-relax, contract-relax with agonist contraction, and hold-relax.

The contract-relax technique involves instructing the patient to relax the affected muscle while the therapist passively moves the limb to the limit of motion (until the person feels a stretch). The patient is instructed to actively contract the restricted muscle (antagonist) against the manually applied resistance of the therapist for 5 to 8 seconds.¹ The patient is then instructed to relax while the therapist passively moves the limb to the new limits of motion, at which point the clinician holds the stretch for 10 seconds (Fig. 3-17). This relaxation of the antagonist muscle during contraction is an example of autogenic inhibition. The process is repeated by continuing from the new limit of pain-free motion until the required number of repetitions is performed or until no more range is gained.¹

In the contract-relax with agonist contraction technique, there is a contraction of the opposite (agonist) muscles instead of the shortened or restricted muscles.¹ In this case, the limb is taken to the point of stretch while the clinician applies manual resistance against the muscle being stretched for 5 to 8 seconds (as in the contract-relax method). The muscle is then relaxed while the agonist muscle concentrically contracts; this facilitates the stretch using the principle of reciprocal inhibition (Fig. 3-18). In reciprocal inhibition, voluntary isometric contraction of the agonist muscle group results in a subsequent reflex inhibition on the muscle groups being stretched (reciprocal inhibition). The clinician again takes up any slack and holds the new position for 10 seconds before continuing.

The hold-relax technique is similar to contract-relax. However, with this technique, the patient actively moves the limb to the end of pain-free motion. The patient isometrically contracts against the force applied by the therapist at the end of the ROM for 5 to 8 seconds as resistance is slowly increased. After holding the contraction for the requisite amount of time, the patient is instructed to relax. The patient then actively stretches the limb to the new limits of motion (Fig. 3-19).¹

Duration, Frequency, and Intensity of Stretching

The specific duration, frequency, intensity, and number of stretching repetitions varies in the literature. Stretch duration depends upon the joint targeted, the flexibility goal, and the type of stretching technique used. Generally, static stretches may be held in a fully elongated position for anywhere from 10 to 60 seconds, with most research recommending that stretches be held between 15 and 30 seconds.8–10,81,103,122 An attempt is then made to extend the stretched position farther within tolerable limits (Fig. 3-20). However, because of the structural diversity of tissue, predicting the duration of stretch for each tissue becomes very difficult.⁷³ Current American College of Sports Medicine (ACSM) guidelines recommend 3 to 5 repetitions for each stretching exercise.⁹⁸

The limits of motion achieved during a stretching program depend on the patient’s tolerance, age, pathologic condition (if any), motivation, and commitment. The muscle’s ability to adapt is a prolonged process. Patients must be cautioned not to exceed their pain limits and must receive counseling about the fact that many sessions of stretching are needed to produce change and lasting improvement. Approximately 6 weeks of stretching are necessary to demonstrate significant increases in muscular flexibility.³⁷ An individual must stretch at least three times per week to improve flexibility. An individual must stretch at least 1 day per week to maintain the flexibility gained during the program.¹⁰⁸

Intensity

Increases in ROM can be significantly affected by the intensity of the stretch.107,125 Structural damage subsequent to stretching could result with increased levels of force and may result in structural weakening of the muscle-tendon unit, thereby increasing the risk of injury.^94,120 The appropriate intensity for static stretching is to stretch slowly and hold the position at low force levels. This is most often communicated to the individual as pain-free or mild discomfort.

How long does it last?

There is little reported on the long-lasting effects of stretching. Some researchers have reported that stretching techniques using cyclic and sustained stretching for 15 minutes on 5 consecutive days increased hamstring muscle length with a significant percentage of the increased length being retained 1 week posttreatment.¹¹⁵ Others reported that the knee ROM was able to be maintained for 3 minutes, but had returned to prestretched levels in 6 minutes³⁶; whereas other researchers have reported that increased ROM from stretching remained for up to 90 minutes.^45,89 These differences have been explained by variations in warm-up, stretching position, stretching force, and stretching duration.¹³⁸ Some research has shown that stretching at least once a week after a 30-day training program will maintain the gained flexibility.¹²³

Comparisons of Different Stretching Techniques

All stretching techniques previously discussed have been shown to increase flexibility and ROM.47,49,79 However, it has not been adequately defined in the literature as to the appropriate stretching frequency and duration for long-lasting changes in flexibility.¹⁰⁰ It is also not completely evident which stretching technique increases flexibility most effectively. Ballistic stretching and static stretching appear to be similar in their effects on flexibility,^56,61,105 yet existing research indicates that dynamic stretching may produce about the same flexibility gains as static and ballistic stretching.¹⁰

Although many studies have revealed that PNF may be the preferred method for increasing flexibility over static and ballistic stretching,27,42,52,101,118 direct comparison between methods is very difficult to determine because of the differences in methodology, experimental design, procedures, and measurement instruments and the use of inadequate control groups.⁹⁶ Additionally, some studies found no significant differences between techniques.^56,79,86,91 Some authors have reviewed stretching studies and reported conflicting evidence as to the efficacy of these methods^44,52; they reported that most of the differences between studies were probably due to variations in training methods, measuring instruments, and confounding variables.

Research has shown that a long-term, routine stretching program can result in increased long-term flexibility, whereas an acute stretching program does not.10,66 Wiktorsson-Moller and colleagues¹³⁰ reported that stretching significantly increased hip ROM for flexion/extension, hip abduction, knee flexion, and ankle dorsiflexion. The effect was significantly greater than that obtained by massage and warming up separately or combined. Similarly, Williford and associates¹³² showed that increases in flexibility could occur as a result of a static stretching training program.

When to Stretch

Very little literature is available to determine when stretching should take place in an exercise program. Some believe that stretching should be part of the warm-up to enhance musculotendinous extensibility, but should not constitute the entire warm-up.⁷⁰ Wiktorsson-Moller and associates¹³⁰ showed that warm-up before stretching resulted in significant changes in joint ROM. Some think that stretching should not be performed at the beginning of a warm-up routine.^70,107 They believe that the tissue temperatures may be too low for optimal muscle-tendon function, subsequently leading to a less compliant tissue, which may not adequately prepare the tissue for activity. As such, many authors recommend at least 5 minutes of light progressive exercise before stretching.¹⁰⁷ Conversely, some discourage exercise before stretching, stating that warmed up muscle tissue does not necessarily stretch better nor is it less likely to be injured.⁹³

Some researchers found that stretching should be performed after an exercise session because of the improved joint ROM.⁸⁵ However, Cornelius and colleagues²⁶ revealed that performing static stretching before, after, and both before and after each workout in a 6-week program did not make a difference in increasing ROM. All produced significant increases in ROM.

Stretching of Soft-Tissue Contractures

The stretching of soft-tissue contractures involves muscle, capsule, tendon, ligament, bursa, and skin. Many options exist for the therapist when prescribing stretching exercises for patients after immobilization or injury, which differ from static, ballistic, or PNF stretching. Long-duration, low-load static stretching has been an effective technique that produces long-lasting connective tissue changes.29,74,126,127 The PTA must recognize adaptive changes that occur in various soft tissues after injury or immobility.

Initially, scar tissue is formed, which may result in the development of a scar contracture if the tissue is not adequately mobilized. A contracture, in this case, is defined as a permanent or transient limitation of movement or shortening of muscle or other soft tissues; in other words, a contracture is the result of healed tissue that forms a fixed, rigid scar and causes cosmetic deformity or functional deficits.⁵⁸ Scar tissue formation may be the result of adhesions and other tissue damage (Fig. 3-21, A). An adhesion involves a limitation of function resulting from scar tissue that forms between structures. For example, when scar tissue forms after knee surgery, it can bind down and cross-link collagen tissue, resulting in adhesions around the patella, suprapatellar pouch, and quadriceps tendon (Fig. 3-21, B).

Generally, immature scar is defined as adaptable for up to 8 weeks and becomes progressively less changeable for up to 14 weeks. Scar becomes quite inextensible at 14 weeks and is termed inadaptable, or mature scar.⁴ Adaptable scar is highly vascular, with many cells (including myofibrocytes) that give the scar the ability to contract. Immature scar tissue also has a high rate of remodeling,⁴ which is the process of tissue restructuring in response to stress or immobilization.²⁹

As new scar tissue is formed, the collagen fibers become highly unorganized and arranged randomly, creating an immobile structure.⁷⁸ Adaptable or immature scar tissue becomes increasingly organized and oriented, with specific directional lines of stress. The formation of collagen fibers are in response to stress imposed from mechanical loads.⁵⁸ Where stretching is concerned, the PTA must be attentive to the following critical components:

Clinically, the following areas are involved in a low-load, prolonged stretch technique:

1. Preheat the involved structures with moist heat or ultrasound.²⁹

2. Place the involved structures in a position of comfort, not maximum stretch. This is an extremely important point. To elicit relaxation, the involved structures must be placed in a supported and comfortable gravity-assisted position.

3. Maintain moist heat application during the entire course of treatment (20 to 60 min).

4. Apply stress or load gradually and minimally. With new immature scar, gravity alone may be enough to clinically effect change. With mature scar, only slightly greater loads should be used. This is a critical point. Lentell and colleagues⁷⁵ found that the magnitude of force used in their study (0.5% of body weight) to create a significant long-lasting change in motion fostered such relaxation that many subjects were not even aware of stretching taking place during the procedure. In an effort to gain knee extension after surgery, for example, it would behoove one to use this technique to avoid reflexive splinting or muscle guarding (Fig. 3-22).

5. Allow the patient to rest or recover for a few minutes during the course of treatment if the sensation of stretch becomes too uncomfortable.

6. Maintain heat application for 5 to 10 minutes after removal of the loads. Some researchers¹⁰⁷ have advocated the use of ice packs after stretch in this protocol. Lentell and colleagues⁷⁵ did not find cooling to be effective in their study. However, cooling the involved structures after stress may be effective in selected cases where pain and an inflammatory response are present.

7. Initiate isometric contractions after the application of heat and passive stretching to enhance strength gains at the new end of ROM.

Lentell and colleagues⁷⁵ demonstrated the effectiveness of applying heat before and during low-load, prolonged stretching and external rotation of nonpathologically involved shoulders. Heat application before and during such stretching was clinically superior to bouts of stretching alone, stretching plus ice, and a heat-stretch-ice protocol. Clinically, few contraindications exist when attempting to gain motion after specific surgical procedures.

Adhesions are desirable and, in fact, are a surgical goal in selected cases. Desirable permanent shortening of connective tissue is needed to prevent a functional loss of movement in some knee surgeries and surgical correction of some shoulder instabilities. If an attempt is made to fully regain external shoulder rotation after surgery to correct recurrent dislocation, the intent to scar down and protect the joint from further dislocation may be derailed. In this case, it is wise to gain functional motion very slowly to allow enough time for a mature scar to form (14 weeks).

The clinical application of low-load, prolonged stretch can be modified to varying degrees depending on the surgical procedure, time constraints of healing, and goals of the rehabilitation program. For example, supine wall slides are a modified technique that uses some of the points of low-load, prolonged stretch (Fig. 3-23). When attempting to gain knee flexion range, it is wise to preheat the quadriceps muscles and suprapatellar pouch before stretching. Next, the patient is placed in a supine position and the foot of the involved limb put on a towel against a wall. To reduce friction against the wall, the contact surface of the towel is lightly coated with baby powder so it will slide more easily against the wall. As the patient relaxes, gravity assists in knee flexion and the foot slides down the wall.

This concept can be modified further. In keeping with the example of gaining knee flexion range, the use of isotonic exercise equipment can be helpful. With the patient in a seated position on a knee extension machine, moist heat or ultrasound can be used before and during the stretch (Fig. 3-24). Many knee extension machines are manufactured with an adjustable range-limiting device that allows the patient to adjust the starting and stopping angle of the exercise. Before the stretch is begun, the patient’s hips are secured with straps to keep them from rising during the treatment. An angle is selected that is comfortable to the patient. As the tissues are continually heated, a very gradual increase in the flexion angle is initiated. The angle does not have to be excessive to be effective. Thus the protocol remains essentially the same, but the equipment and the position of the patient are changed.

The knee serves as an excellent example to further describe and clarify methods to improve ROM by prolonged static stretching. To gain knee extension range, the patient can be supine with moist heat applied behind the knee (popliteal fossa) and on the hamstring and quadriceps. The heel of the involved limb is placed on a small folded towel (Fig. 3-25, A). If the knee is contracted to −20°, for example, towels are added under the hot packs under the knee to ensure a very comfortable starting position. During the course of treatment, small layers of towel can be removed gradually to allow for improved range of knee extension. As a progression to this technique, a small vertical force can be applied on the knee. Care should be taken to ensure that this force is sufficiently small (1 to 2 lb or lighter) and that it is applied superior to the patella to avoid compressive forces between the patella and femur (Fig. 3-25, B-C).

The patient is brought to a sitting position to enhance this stretch further. A towel is used to dorsiflex the involved foot, and the patient is instructed to slowly lean forward to stretch the hamstrings (Fig. 3-26, A). Simultaneous isometric quadriceps sets also are used to improve strength at the new limits of knee extension.

Gaining knee extension can be achieved in a prone position as well (Fig. 3-26, B). However, care must be taken to elevate the patella off the table and thereby prevent excessive patellofemoral compression. This is done by placing a small folded towel superior to the patella. This position works well when only slight degrees of motion are needed (5° to 10° of knee extension). This procedure also can be done on an isotonic exercise apparatus following the same process as described with gaining knee flexion on an isotonic exercise machine. Knee extension range also can be improved in a sitting position, with or without the aid of isotonic exercise equipment (Fig. 3-26, C).

There are many commercially available tools that use the concept of low-load, prolonged stretch. Dynasplint (Dynasplint Systems, Inc.) and Pro-glide (LMB) are two examples of dynamic splints used to progressively load selected joints to gain motion (Fig. 3-27). An arrangement of pivot points and incrementally adjustable degrees of tension provides the levels of stress needed to effect change in joint motion. The selection of patients for use of one of these splints must be made carefully. Skin integrity is an issue that must be addressed in the elderly population. Metal hinges and spring-loaded tension flanges may not be appropriate for this population because of the weight of the devices and the patient’s potential for skin breakdown.

Simple tools for dynamic stretching can be used at home. A wand, cane, or shortened broomstick can be used for general shoulder flexibility (Fig. 3-28, A). Increased mobility can be gained by using the unaffected arm to assist the affected extremity (Fig. 3-28, B).

Codman’s pendulum exercises are effective for gaining relaxation and small degrees of motion in the shoulder. Relaxation is paramount to the effectiveness of this exercise. In one exercise technique, the patient is placed prone on a treatment table and a very light weight is held in the hand of the affected extremity. This light distraction force is used in conjunction with gradual, light oscillations in various directions (Fig. 3-29). Relaxation is enhanced by applying moist heat followed by ultrasound to the affected joint before pendulum exercise. When one is teaching the oscillation component of this exercise, it must be made clear that muscular contractions must not be used to initiate and maintain the prescribed motions. The oscillation movements can be initiated by gently swinging the upper body or torso.