From R. C. Schafer, DC, PhD, FICC's best-selling book:
“Applied Physiotherapy in Chiropractic”
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The topics of this chapter have been adapted from Volume 1, Chiropractic Rehabilitation, by K. D. Christensen, DC, © 1990, and used here with permission.
Strengthening exercises for the muscular system play an essential role in the chiropractic management of various neuromusculoskeletal disorders. Knowledge of various training methods and exercise techniques are thus among the most important requirements for effective treatment.  Properly conducted individual exercise programs help prevent many injuries and serve to shorten the recovery period necessary to restore the patient back to health.  Exercise programs can be designed to increase strength, aid weight loss, increase cardiorespiratory efficiency, or simply improve overall musculoskeletal performance.
All exercise programs should have specific goals in mind. The cornerstone of exercise is Davis’ Law, or the (SAID) principle that states that the body makes specific adaptation to imposed demands.  The more specific the exercise, the more specific the adaptation. Exercise, therefore, should be as specific as possible to the individual’s goals and needs.
The patient who participates in a well-devised, scientifically based, properly instructed exercise program should benefit in at least four areas: [4
1. Enhanced musculoskeletal performance
2. Decreased risk of injury
3. Decreased severity should an injury occur
4. Accelerated rehabilitation and return to activity after injury.
Reid and Schiffbauer indicated that hypertrophy of muscle through exercise protects against bodily injury.  To avoid injury, Gallagher states patients should supplement recovery activities with exercises to increase the size and the strength of the muscles, which will then protect joints from injury.  Thorndike reports that exercise to strengthen joints can reduce the incidence of injuries.  Adams reports that habitual exercise can cause a significant increase in the strength of ligaments surrounding a joint and therefore prevent injuries.  Kraus reveals that while exercise is an important factor in the prevention of injury, it is also important in the prevention of reinjury 
TYPES OF EXERCISE
Different type exercise programs are available that apply Davis’ Law to produce better integrity of the joint. There are three basic types outlined: isometric, isotonic, and isokinetic. Isotonics can be divided into a concentric (positive) movement, an eccentric (negative) movement, and variable resistance. The isokinetic type can also be divided into a concentric or eccentric movement or both.
These types of exercises can be defined and contrasted in terms of the speed of movement and the resistance applied. In isometrics, we know that the speed of movement is fixed and the resistance is fixed. In isotonics, on the other hand, the speed is variable, generally fluctuating around 60° as demonstrated in weight lifting where there often is a slowing of movement at the weak points in the range of motion (ROM). The resistance in isotonics is fixed (the amount of weight lifted). In isokinetics, the speed is fixed but the resistance is variable. Thus we can see that isokinetics is the opposite of isotonics.
These types of exercises can be designed in musculoskeletal rehabilitation to optimize joint integrity by strength. Strength has been defined as the “maximum voluntary force exerted in a single muscular effort.” 
Mechanical muscle-strength testing has played a key role over the years in the prescription of exercise programs, the identification of the sequelae of orthopedic disabilities, and the prevention of musculoskeletal injuries.  The evaluation of strength is crucial in determining overall musculoskeletal fitness. Indeed, the lack or impairment of strength can seriously affect an individual’s success in any musculoskeletal endeavor.
“Active exercise may be either static or kinetic. Static (isometric) exercise is performed without producing joint motion. The muscle exercised maintains a fixed length. Kinetic (isotonic) exercise is performed to produce joint movement. Contracting muscles shorten, causing movement of the joint at which they are attached. Isokinetic exercises produce joint motion at a controlled rate of speed. Concentric contraction occurs when a muscle is contracted from an extended to a shortened position. Eccentric contraction occurs when a tense-shortened muscle lengthens. Power is the ability to release muscular work as a function of time. Endurance is the ability of muscles to perform work by holding a maximum contraction for a given length of time or by continuing to move a submaximal load.” 
Progressive Resistive Exercise
DeLorme published his first papers on the benefits of “progressive resistive exercises (PRE)” in regard to increasing strength in 1945.  Since then, studies have substantiated his results. It is now accepted that the degree of tension developed during contraction is responsible for increasing strength.
There are various tensions that can be developed depending on the type of exercise applied. For example, an eccentric isotonic exercise, which would be releasing a load or another negative portion of an exercise, produces the highest tension developed in muscle tissue. Next would be isometrics, followed by concentric exercise.
While performing isometric exercise, no work is done in the physical sense in this type of muscle work. When applying the physical formula:
Work = Force × Distance
isometric exercise lacks the component of distance. However, a high degree of tension is achieved (static work) that can be strenuous. This would include sitting upright without support and maintaining good posture in general for the long back extensors (erector spinae). The rapid tiring from static work is caused by the compression of capillaries during contraction of the muscle, which prevents a sufficient supply of oxygen and removal of metabolic wastes. Thus, the work must be accomplished anaerobically. 
It is generally known that when a muscle is immobilized after a fracture, or even during partial immobilization in an orthopedic support after a strain, muscular atrophy begins to occur within 6 hours.  The lack of exercise caused by immobilization results in the loss of muscle tone; it atrophies and can no longer fulfill its normal function. This fact is of great importance to justify isometric exercise after musculoskeletal injuries and for the maintenance of muscle strength during immobilization.
The interruption of normal function from musculoskeletal injury and immobilization leads to a loss of strength not only in the muscles of a damaged extremity but also in those of uninjured extremities. This loss can be largely avoided with the proper use of isometric exercise. In most cases, uninjured body parts can be exercised even on the first day of immobilization of the injured part. The injured and immobilized extremity can be isometrically tensed and thus exercised while still in a cast or in a splint after the immediate pain has subsided. In this way, much of the potential loss of muscle strength can be avoided. The muscle atrophy already setting-in can also be countered. A primary advantage of isometric exercise in musculoskeletal rehabilitation lies in the opportunity for localized muscle exercise without moving involved joints.
Strength increases more rapidly in isometric than in dynamic exercises.  On the other hand, strength is also lost more rapidly after cessation of exercise. A primary disadvantage is that the muscular coordination necessary for many types of musculoskeletal activities is not integrated in this exercise, which is why isometrics must, in time, be combined with dynamic exercises in the treatment of various musculoskeletal conditions. Isometric exercise places great compression stress on the joint; thus, people with arthritic conditions should not participate in intense isometric contractions. Also, people in danger of heart attacks should not engage in intense isometric training because of the danger of compression narrowing blood vessels.
There are thus unique advantages to isometrics due to the fact that they do not move a joint and therefore can be used early in a rehabilitation program. Static strength increases, and atrophy retards. Most other advantages to isometrics focus around the lack of special facilities and equipment needed to perform them.
Disadvantages must also be carefully understood. A major disadvantage is a limited overflow of strength development. Approximately 20° of overflow from the angle of application occur: strength gains will be noticed for 10° on each side of the application. This is a small amount, and therefore, realistically, isometric exercise at one point in a plane will not increase strength at another point in the plane. Other problems also exist with isometrics such as difficulty with patient motivation, minimal gains of endurance, and lack of eccentric workloads.
When we apply an isometric contraction, the general rule to follow is known as the “Rule of Tens,” wherein we build tension in 2 seconds, hold the desired tension for 6 seconds, and then gradually relax tension in 2 seconds. Applying this to isometrics, we usually recommend a 10-second contraction, 10 seconds of rest between each contraction, ten repetitions, and ten sets at ten different angles in the ROM.
One should exercise at various angles in the ROM because of the limited physiologic overflow. If we apply isometrics only at one point, there is approximately a 10° overflow on each side.  How then would we apply isometrics when there is a painful point in an arc of motion? We would exercise at every 20° using the Rule of Tens throughout the range of motion, beginning at 10° on each side of the painful position. Thus we would get a physiologic overflow into the painful deformation and achieve our goal to increase strength at that point and decrease related pain.
In contrast to isometric exercise, isotonic exercise involves work in a physical sense. This is called dynamic muscular work. For example, when the biceps contracts and shortens and the lower arm is bent or a weight is lifted, movement is accomplished. The physical formula of work = force × distance is fulfilled.
Dynamic muscular work does not involve lengthy contractions but instead is distinguished by the alternation between contraction and relaxation. In the concentric contraction phase, individual muscle fibers shorten, and their origin and insertion approximate. In the eccentric relaxing phase, individual muscle fibers go through a lengthening process with their origin and insertion moving apart. In each motion, the agonist and antagonist muscle groups are involved. In isotonic exercises, the prime mover (agonist) produces a concentric muscular contraction (eg, quadriceps producing knee extension). This is followed by an eccentric contraction of the same muscle group (ie, quadriceps slowly lowering the leg toward flexion). When multiple repetitions are performed by the same muscle group concentrically and then eccentrically, a transient muscle ischemia is produced that compromises blood flow.
From two to three times more force can be generated with eccentric contractions.  The clinical implications of this are evident when dealing with a patient who cannot initiate a concentric contraction. An example may be the use of eccentric straight-leg raises in a knee rehabilitation program. Immediately postsurgery, after the patient can perform quadriceps isometric sets, immediate progression to eccentric straight-leg raises is often effective using the iliopsoas. The clinician passively assists with hip flexion, or a sling mechanism can be developed to allow the patient to work independently after which active eccentric straight-leg lowering is performed by the patient. (Note: The quadriceps can isometrically contract while the iliopsoas muscles eccentrically contracts.)
Although eccentric contractions are useful in early rehabilitation programs or in gaining muscle mass and strength, there is a distinct disadvantage involving residual muscle soreness that may cause decreased performance due to pain and biochemical changes in the involved muscle. Eccentric isotonic exercises are thus used at the two extremes of a rehabilitation program. They generate more tension early in rehabilitation and can perhaps help prevent a reflex disassociation by maintaining a neurophysiologic pathway for muscle contraction. They are then used near the terminal stage of rehabilitation to maximize the eccentric joint strength needed during daily activities.
Regarding circulation and isotonic exercise at the moment of contraction, the intramuscular pressure increases. This forces blood into the veins and can be accomplished with only one-fifth of maximal contraction. During relaxation, the increase in the capillary bed is then so extensive that the circulation is 15—20 times greater than when the muscle is at rest.  In this way, the circulation can supply the tissues with oxygen and remove metabolic wastes. Dynamic muscular work thus promotes circulation and metabolism and eases the pumping work of the heart. Thus, isotonic exercise is accomplished aerobically.
It is emphasized that strengthening exercises increase muscular performance. Without them, an improvement in performance in any functional activity is impossible. A proper system of exercise is necessary for the preservation and restoration of muscles after injury. However, strength-building exercises alone would leave much to be desired in developing endurance.
CONSTANT- AND VARIABLE-RESISTANCE ISOTONICS
Isotonic exercises are generally grouped into constant or variable types, being both concentric and eccentric. Examples of constant-resistance isotonics include free weights. Variable-resistance isotonics include such systems as Nautilus, Universal, Eagle, and others. Constant-resistance isotonic exercise is easily shown by use of a weighted barbell. As the weight is lifted, the resistance remains the same throughout the entire ROM. Variable-resistance isotonic exercise is provided by several commercially available machines such as Nautilus, Universal, Eagle, etc. Due to the shape of the cam or changes in the length of the lever arm, the resistance varies through the range of motion. Although the resistance (specific amount of weight) is the same, due to the engineering of the equipment (cam), it permits varying amounts of resistance at specific points in the range of motion. Constant resistance exercise reflects “real life” requirements and simulation, whereas variable resistance exercise does not simulate “real life” resistance requirements.
As a muscle contracts and the extremity travels through its ROM, strength increases to a peak and then decreases as the range ends. By correlating the Nautilus cam system with the muscular torque, one can understand how the Nautilus cam works as an isotonic exercise and why we call it variable resistance. The point where the cam is efficient corresponds to points where the muscle is inefficient. When the muscle is efficient, the cam is not. Thus, the cam alters resistance to better suit muscular capabilities.
ADVANTAGES OF ISOTONICS
The advantages of isotonics are numerous, affordable, and accessible to most people. They can be highly motivational as a person increases the weights. Isotonics also provide exercise for the entire range of motion in both concentric and eccentric phases. There are improvements to the circulatory and neurologic systems. Isotonics can be varied easily to meet the demands desired, and the results are experienced quickly. Regarding application to musculoskeletal rehabilitation, muscle loading is accomplished at the weakest points in the range of motion.
In the late 1960s, James Perrine developed the concept of isokinetic exercise, which proved to be a revolution in exercise training and musculoskeletal rehabilitation.  Instead of applying traditional exercises involving a constant weight or resistance performed at variable speeds, Perrine developed the concept of isokinetics, which involves a dynamic preset fixed speed with variable resistance.
Testing, clinical use, and the rehabilitative potential of isokinetics are still in their infancy. Isokinetics is a relatively new form of rehabilitation made popular by the Cybex and Orthotron machines, and has gained increasing acceptance. 
As isokinetics have a fixed speed with a variable resistance throughout the ROM, the velocity is constant at a preselected dynamic rate where resistance varies to exactly match the force applied at every point in the ROM. This accommodation allows maximal dynamic loading throughout the entire ROM. By controlling the velocity of exercise, maximum resistance throughout the full ROM is developed by exercising at that velocity. Keep in mind that isokinetic and isotonic exercises are essentially opposites.
COMPARISON OF STRENGTH TRAINING PROCEDURES
Isometric, isotonic, isokinetic, and variable-resistance procedures all produce substantial gains in muscle strength. Most studies demonstrate that isometric exercise provides smaller gains.  “Among isotonic, isokinetic, and variable-resistance procedures, there appears to be no consistent difference in the magnitudes of strength gains when appropriate controls are used. Further, no single system or product has been found to be superior to any other when the studies have been well-controlled.  Evaluation of the muscle activity of maximal isometric, isotonic, and isokinetic contractions as measured by integrated electromyography reveals no single method of muscular contraction elicits the greatest microvolt-second level for every subject. 
In studies assessing the relative effectiveness of isokinetic and isotonic exercise for quadriceps strengthening, results strongly suggest that the Cybex offers no particular advantage over ordinary weights.  Additionally, variable-resistance versus constant-resistance strength training result in increases for all training procedures with neither demonstrating superiority. No significant differences between groups are demonstrated. This allows the conclusion that constant-resistance exercise and variable-resistance exercise are equally effective in developing muscle strength and endurance. 
Soft-Tissue Healing Phases
The physiologic process of soft-tissue healing can be divided into three predominant phases: 
1. Acute Inflammatory Phase. This phase occurs after the initial trauma to soft tissue. The classic signs/symptoms of the acute inflammatory response include:
The humoral responses during the acute phase of an inflammatory soft-tissue injury include:
a. Blood coagulation to minimize blood loss
b. Fibrinolytic responses to modify coagulation and prevent widespread clotting
c. Kinins to vasodilate blood vessels, increase capillary permeability, and increase edema
d. Complement phagocytosis to remove cellular debris and decrease inflammatory cells (chemotaxis).
The cellular responses involve:
a. Mast cell degranulation, resulting in release of histamine and serotonin
b. Granulocytes releasing prostaglandins causing vasodilation and chemotaxis.
2. Repair and Regeneration Phase. This phase begins after the initial inflammatory phase and predominantly concerns the synthesis and deposition of collagen. During and following the inflammatory period, the body produces collagen in the form of scar tissue that continues production for 3 weeks. During this time, the collagen and soft-tissue structures are significantly weak.
The essence of trauma is that oxygen deprivation produces cell death resulting from disruption of blood vessels. This appears to play an important part in the repair process with the development of a new blood supply by a process of “vascular budding.” 
The repair of tissue is only possible when the wound has become clean.  A soft-tissue sprain/strain leads to disruption of muscle fibers. Fibrous tissue (collagen) and soft-tissue elements result in platelets releasing thrombin, which converts fibrinogen to fibrin. This ultimately organizes into collagenous scar tissue.  The characteristics of scar tissue include:
a. Decreased circulation
b. Decreased tensile strength
c. Decreased flexibility
Healing of sprains and similar soft-tissue injuries occur by fibrous repair (scar tissue) and not by regeneration of damaged tissue. Fibroblasts are the major cells involved in the repair process. In minor injuries, however, skeletal muscle fibers may regenerate. 
The regeneration process of skeletal muscle fibers becomes intensive between the 5th and 21st day, decreasing markedly during the next 3 weeks.  Unless new muscle fibers are accompanied by a nerve supply, they fail to mature and remain at the myotube stage, eventually atrophying.  Fibroblasts and resultant collagen deposition impede muscle regeneration, which ultimately interfere with the in-growth of axons and result in uninnervated and useless muscle tissue.  The effect of collagen synthesis during the healing stage is an increase in innervation in the collagen tissue resulting in a paradoxical condition called denervation supersensitivity.
3. Remodeling and Rehabilitation Phase. Davis’ Law states that soft tissue will model according to imposed demands. Stearns states that the “fibroblastic activity and the healing of connective tissue were reflected to the effects of movement responsible for the development of an orderly arrangement of the fibrils.”  Collagen has the inherent capacity to contract approximately 3—14 weeks after injury. 
When motion is introduced appropriately and timely following a neuromusculoskeletal injury, the fibroblasts align and orient themselves in a controlled and efficient matrix, the arthritic degenerative condition is reversed, and the result is a small flexible scar that does not impede functional performance. 
When to Begin and End Rehabilitation
Each of the three phases of soft-tissue healing has a minimum length of time required to complete the phase as well as a maximum amount of time.  Listed in Table 13.1 are the minimum and maximum amounts of time required to complete recovery. This indicates that rehabilitative exercise could begin as soon as 5 days following a soft-tissue injury but no later than 6 weeks after the initial injury. Additionally, rehabilitation may be completed in 3 weeks or require over 12 months for complete soft-tissue remodeling and strengthening to be achieved. Strength and endurance levels should be rehabilitated to within 10% of the normal uninjured comparison.
Table 13.1. Appraised Minimum and Maximum Recovery RatesPhase Minimum Maximum I. Acute inflammatory 48 hours 72 hours II. Repair and regeneration 48 hours 6 weeks III. Remodeling and Rehab 3 weeks 12 months +
Pain-Free Range of Exercise
All exercise should be strictly limited to the pain-free range of motion. Resistive exercise progression is based on the points shown in Table 13.2. Progression to the pain-free full-range of motion is accomplished by a neurologic process called physiologic overflow. When exercise is performed in a limited pain-free range, there is an occurring physiologic overflow to each side of the range exercised. 
Table 13.2 Basis for Resistive Exercise ProgressionExercise Stage Range of Motion Velocity of Exercise 1 <———— SHORT ————>Slow Progressing to 2 <———— FULL ————>Fast
Speed of Exercise
Due to findings of neurologic and functional improvements with varied speeds of exercise, it is now recognized that during the rehabilitative regimen the speed of exercise should be varied. Initially, the exercise session should be slow and carefully conducted. As coordination and confidence are gained, the speed can be increased. The following chart represents a classic month-to-month progression of exercise speed of the concentric and eccentric phases of movement.
Month Concentric Eccentric 1 3 seconds 6 seconds 2 2 seconds 4 seconds 3 1 seconds 2 secondsConcentric exercise is the initial shortening component of a muscle contraction, and eccentric exercise is the lengthening component.
Frequency of Exercise
The number of rehabilitative strengthening training sessions per week required to achieve maximum strength gains is from three to five for 7 weeks.  Five sessions produce greater results than four sessions that will produce greater strength gains than three.  However, three sessions per week is the minimum number to achieve significant strength gains. The number of training sessions per week required to achieve maximum strength gains with 12 weeks of isotonic exercise would be from 3 to 5 exercise sessions for 7 weeks followed by 2 sessions per week for the remaining 5 weeks.
Maintenance of Acquired Strength Gains
Acquired strength gains decay at 1/3 the rate gained.  However, one exercise session per week of “maximal contractions prevents such strength loss.”  This indicates that one exercise session per week maintains strength but generally will not increase tensile strength. Two exercise sessions per week provide moderate gains in strength but are not maximal during the initial 2 months.
Strength Versus Endurance Exercise
The optimum sets and repetitions prescribed for development of strength are three sets of six repetitions.  Exercising beyond six repetitions begins to develop endurance. Thus, the classic three sets of 10 repetitions develop strength and endurance. However, strength and endurance can be obtained despite the isotonic method by exercising to fatigue.  Weekly increases in strength can be directly associated with training at maximal isotonic weight-training loads. 
Concentric Versus Eccentric Exercise
A common side effect of maximal eccentric training is muscle soreness occurring 48 hours later.  It should be noted that this only occurs with maximal eccentric exercise. Since isotonic exercise loads a muscle at its weakest concentric angle, typical constant or variable-resistance exercise will not usually cause this soreness.
Maximal and submaximal eccentric exercises produce earlier strength gains.  However, there is no difference in the long-term differences in strength gains when comparing eccentric versus concentric exercise.
Strength and Endurance Evaluation/Re-evaluation
Safety protocols require during the first session of rehabilitative exercise to determine levels of resistance that may be unsafe to approach and possibly cause pain. During the first exercise session, it is therefore standard to determine the one-repetition maximum that is the greatest amount of weight used to achieve one repetition through the pain-free range of motion. This is called the “1 RM,” which is synonymous with either pain or discomfort, or maximal capability. Re-evaluation of the 1 RM should occur at least every 30 days during an active rehabilitation strengthening program. During the program, however, the patient typically exercises using a set number of repetitions in which the patient performs at either a six-repetition maximum (6 RM) or a ten-repetition maximum (10 RM). It is important that all such exercise be performed pain-free.
Endurance abilities are assessed after determining the 1-RM level by taking 75% and 50% of the level and performing repetitive contractions to fatigue or an inability to complete a repetition. Bilateral comparisons, called the endurance ratio, compare the total amount of work (endurance) between relative anatomy. Re-evaluation of the 75% and 50% endurance ratio should also occur at least every 30 days during an active rehabilitation program.
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