More than 60% of adults in the United States are considered either overweight or obese.16,21 Obesity is a leading risk factor for developing heart disease, diabetes, hypertension, and some cancers. Obesity may also contribute to the development of certain musculoskeletal injuries.¹⁵ An individual with a chronic disease who initiates an aerobic exercise program may experience a decrease in the severity of symptoms associated with their disease. Likewise, participating in a regular aerobic fitness program may help to reduce the risk of developing a chronic disease.

There are many positive physical and physiologic changes that occur when one participates in a cardiovascular fitness program. Individuals who perform a cardiovascular fitness program have a lower risk of developing many chronic diseases including cardiovascular disease, type 2 diabetes, and some cancers.3,5,12,17–19,26,29,32,33 Additional training benefits associated with regular participation in an aerobic exercise program include reduction/control of one’s weight, an increase in muscular strength, a reduction in the risk of falls, and a reduced mortality.^9,13

The most notable physical and physiologic changes associated with regular participation in an aerobic exercise training are 4,23,25:

Increased size and number of mitochondria

Increased myoglobin content

Increased heart weight and size

Increased cardiac output and stroke volume

Improved mobilization and use of fat and carbohydrates

Selective hypertrophy of type I slow twitch oxidative muscle fibers

Decreased resting heart rate and submaximal heart rate

Decrease in adipose tissue

Increased blood volume and hemoglobin

Reduced systolic and diastolic blood pressure

Significantly improved oxygen extraction rates from the blood

MINIMUM AEROBIC EXERCISE GUIDELINES FOR AMERICANS

The Centers for Disease Control and Prevention (CDC) has provided physical activity guidelines for children, adults, healthy pregnant or postpartum women, and older adults.1,9 Table 5-1 presents the minimum aerobic exercise guidelines suggested to improve one’s aerobic fitness.^1,9

Table 5-1

Minimum Weekly Aerobic Exercise Guidelines for Adults

Intensity Level	Adults	Older Adults(65 years of age or older)	Healthy Pregnant or Postpartum Women
Moderate intensity aerobic activity	2 hours and 30 minutes weekly	2 hours and 30 minutes weekly	2 hours and 30 minutes weekly
Vigorous intensity aerobic activity	1 hour and 15 minutes weekly	1 hour and 15 minutes weekly	May continue vigorous intensity activities, such as running, if performing these exercises before her pregnancy.
Combination	Equal amount of moderate and vigorous intensity aerobic activity	Equal amount of moderate and vigorous intensity aerobic activity	N/A

From Centers for Disease Control and Prevention: Physical activity for everyone (website): www.cdc.gov/physicalactivity/everyone/guidelines/index.html. Accessed March 15, 2010.

Exercise Guidelines for Adults

The minimum exercises guidelines for adults, older adults, and healthy pregnant or postpartum women are based on the volume of moderate or vigorous intensity aerobic activity one performs.⁹ The CDC has provided both relative and absolute guidelines to help one appreciate the intensity of his or her exercise session.⁹ The talk test may be used to determine the relative intensity of an exercise session.⁹ If one is able to talk during exercise, that exercise is of moderate-intensity. If one is unable to speak more than a few words before needing to pause for a breath, then they are likely performing a vigorous intensity exercise. The absolute intensity guidelines are based on the amount of energy one typically uses during 1 minute of exercise.⁹ Examples of moderate intensity exercises include walking briskly, water aerobics, doubles tennis, and cycling at a pace less than 10 miles per hour.⁹ Running, swimming laps, singles tennis, cycling faster than 10 miles per hour, and hiking are all examples of vigorous intensity exercises.⁹

The CDC recommends that as adults’ fitness level improves, they should increase their level of either moderate intensity aerobic activity to 5 hours a week, or to 21⁄2 hours of vigorous intensity aerobic activity each week, or to an equal mix of both types.⁹

Exercise Guidelines for Children

The number of children and adolescents who are considered overweight or obese has at least doubled during the past twenty years.²¹ Recent studies have demonstrated that the likelihood that one will be obese as an adult increases if he or she was obese as a child.³¹ To combat pediatric obesity, children and adolescents need to participate in daily physical activity. A lack of physical education in many school districts limits the opportunities during the day for a child to exercise.²⁰

The CDC recommends that children perform at least 60 minutes of moderate intensity exercises each day.9,30 In addition, children should perform vigorous intensity exercises at least 3 times a week.⁹ The physical therapy team may play a crucial role in educating a family as to the importance of daily physical activity for a child as well as developing and implementing a fitness program for the child.⁸

ADDITIONAL METHODS TO MEASURE EXERCISE INTENSITY

In a clinical exercise physiology setting the efficiency of one’s aerobic fitness may be determined by measuring the maximum volume of oxygen consumed during exercise. This measure has been termed the maximal oxygen uptake or the Vo₂ max.2,11,25 Using this number, a clinician can prescribe a particular exercise intensity based on a percentage of one’s Vo₂ max.

Most physical therapy clinics do not possess the necessary equipment required to record a patient’s maximal oxygen uptake. Several methods have been developed that allow clinicians to measure and prescribe a particular exercise intensity without needing high-tech equipment.

The aforementioned talk test (previous section) is one manner to determine a client’s aerobic exercise intensity. Two other methods used to measure aerobic intensity are the target heart rate/estimated maximum heart rate method and the perceived exertion method.⁹

Target Heart Rate and Estimated Maximum Heart Rate

The CDC recommends that an individual who is performing moderate intensity exercise should do so at a target heart rate range of 50% to 70% of one’s maximum heart rate (beats per minute).⁹ When an individual performs vigorous intensity exercise, their target heart rate (THR) should be 70% to 85% of their maximal heart rate (MHR).⁹ How is the MHR measured? To calculate a client’s/patient’s MHR, the individual’s age is subtracted from 220 (e.g., 220 − 30). To establish the THR, the MHR is multiplied by the desired intensities. These two calculations (Table 5-2) will provide the lower and upper limit target heart rates for someone who is performing moderate or vigorous intensity exercises.⁹

Table 5-2

Target Heart Rate Calculations for a 30-Year-Old Individual

	Moderate Intensity Exercise	Vigorous Intensity Exercise
Formula	MHR × (0.50) or (0.70)	MHR × (0.70) or (0.85)
Lower limit	(220 − 30) × 0.50 = 95 beats per minute	(220 − 30) × 0.70 = 113 beats per minute
Upper limit	(220 − 30) × 0.70 = 113 beats per minute	(220 − 30) × 0.85 = 162 beats per minute

Maximum heart rate (MHR) = 220 − age.

The Karvonen method has been suggested as an alternative method of calculating THR.²⁵ The Karvonen method differs from the aforementioned technique in that it accounts for one’s resting heart rate. In the previous example (see Table 5-2), the THR for a 30-year-old individual performing moderate intensity exercise is 95 to 113 beats per minute. If this individual has a resting heart rate of 70 beats per minute, the THR range would be 130 to 154 beats per minute (Table 5-3).

Table 5-3

Target Heart Rate Calculations Using the Karvonen Method

	Moderate Intensity Exercise
Formula	MHR (or HR_max) = 220 − 30 = 190
50% MHR	[(190 − 70) × 0.50] + 70 = 130
70% MHR	[(190 − 70) × 0.70] + 70 = 154

Target heart rate (THR) = [(HR_max – HR_rest) × % Intensity] + HR_rest; MHR, maximum heart rate.

Borg Rating of Perceived Exertion

The Borg Rating of Perceived Exertion scale may be used to assess exercise intensity based on an individual’s perception of exertion. While the client is exercising, ask him or her to rate how hard he or she is exercising based on the Borg scale. The Borg scale ranges from 6 to 20 points, with a 6 corresponding to “no exertion at all” and a 20 corresponding to “maximal exertion.”⁶ The client/patient should be asked to view the scale each time when measuring perceived exertion. The Borg scale has also been found to correlate with one’s heart rate.^6,7 This is a helpful feature allowing the clinician to monitor exercise intensity level based on an estimate of heart rate. To determine heart rate from the Borg scale, multiply the perceived rating (e.g., 13) by a factor of 10 (e.g., 130 beats per minute).

METHODS OF AEROBIC TRAINING

Aerobic conditioning programs are either continuous or discontinuous. Continuous aerobic activities provide no rest interval during the entire bout of exercise. Examples of continuous activities are jogging, walking, running, cycling, and stair climbing.

Discontinuous aerobic activities are also known as interval training activities. A discontinuous aerobic exercise routine may include similar exercises used during continuous aerobic programs; however, during interval training, repeated exercise bouts are interspersed with rest intervals. Discontinuous training routines may be beneficial for patients who have limited exercise tolerance.

AEROBIC EXERCISE FOR PATIENTS WITH AN ORTHOPEDIC INJURY

Patients who have been diagnosed with an orthopedic injury will benefit from the inclusion of aerobic exercise as part of their rehabilitation program. The physical therapy team must consider tissue healing parameters when implementing an exercise. An acute injury or surgery may require a period of rest (or activity avoidance) in order to avoid additional injury or protect the surgical repair.

Evidence suggests that aerobic fitness activities should be included in a rehabilitation program for a patient recovering from a back injury.²⁷ Riding a stationary bicycle or walking on a treadmill (Fig. 5-1, A–B) may help to facilitate initial aerobic training. However, sitting on the saddle seat of a stationary ergometer may be uncomfortable or provoke symptoms for many patients with back problems. A recumbent cycle, with its large bucket seat (Fig. 5-1, C) to provide lumbar support, may be preferred by many clients. Patients who are unable to tolerate land based aerobic exercises may benefit from walking on an underwater treadmill. Immersion in the water can provide enough buoyancy during walking, unloading the spine, allowing the patient to exercise without exacerbating symptoms (Fig. 5-1, D).

Fig. 5-1 A, Seated stationary bicycle ergometer. B, Standard treadmill. C, Recumbent bicycle ergometer. Large bucket seat used in a recumbent position may allow some patients to tolerate seated aerobic activities. D, Underwater treadmill. The buoyancy of the water may allow early vertical loading and the initiation of normalized gait mechanics.

Patients who have sustained a lower extremity injury or are recovering after a lower extremity surgery can maintain or improve cardiorespiratory fitness using an upper body ergometer (UBE) (Fig. 5-2, A). The UBE is ideal for individuals who are contraindicated from either bearing weight or performing range of motion activities with their involved lower extremity. The single-leg stationary bicycle ergometer exercise (Fig. 5-2, B) may also be safely initiated by a patient before beginning double-leg cycling.

Fig. 5-2 A, For patients with lower extremity injuries, an upper body ergometer allows continued aerobic activities during periods of immobilization. B, In some cases, a single-leg stationary cycle ergometer can be used for cardiovascular fitness during periods of immobilization.

Older patients with hip, knee, or ankle osteoarthritis (degenerative joint disease) may also benefit from a UBE training program. The vertical compressive loads experienced during treadmill walking, stair climbing, or even stationary cycling may cause or increase one’s symptoms.

Patients with upper extremity conditions can use a stationary cycle or treadmill for endurance training. Patients also can be instructed to use one-arm cycling on a UBE (Fig. 5-3) to maintain upper body aerobic fitness.

Fig. 5-3 A one-arm, upper body cycling activity with an upper body ergometer is also an effective aerobic exercise activity.

Modifications can be made on stationary cycles to allow for continued aerobic conditioning after an ankle injury or surgery. Typically the seat height should allow for slight knee flexion (~10°) at the end of the pedal stroke. With the seat in normal position, the foot generally plantar flexes toward the end of the pedal stroke, causing stress to the anterior talofibular ligament. Therefore the seat height is lowered for a patient with a severe ankle sprain to allow for a complete pedal stroke and keep the ankle joint in neutral (Fig. 5-4).

Fig. 5-4 A, Normal seat elevation for the performance of a seated bicycle ergometer allows for greater plantar flexion motion. Plantar flexion may be contraindicated with acute and subacute sprains of the lateral ligament complex of the ankle. B, With the saddle seat lowered, the affected ankle can be maintained in a more appropriate neutral position during periods of aerobic activity on the cycle ergometer.

Stair climbing, seated rowing, and cross-country ski machines are popular aerobic tools but must be used judiciously. Stair climbers require the patient to be correctly positioned vertically and maintain balance (holding the hand rails) to perform the exercise correctly. Therefore stair climbers are inappropriate or unsafe for many patients during the acute stage or immediate postorthopedic surgery period. Rowing machines require both a pulling motion with the arms and hip and knee flexion and extension. These simultaneous motions make modifications for use with specific orthopedic problems quite difficult. Cross-country ski machines require bilateral, reciprocal leg and arm motions and are also difficult to modify for orthopedic patients with acute disorders. However, stair climbers, rowing machines, and cross-country ski machines can be effective tools in aerobic conditioning programs after the acute phase of recovery from injury or surgery.

MUSCULAR ENDURANCE TRAINING

Even though they share the term endurance, muscular endurance and cardiovascular endurance are not one and the same. One can demonstrate functional muscular endurance but not possess functional cardiovascular endurance. What is the difference? As mentioned earlier in the chapter, cardiovascular endurance involves one’s cardiovascular system to perform an aerobic activity. For an activity to be considered aerobic, the oxidative energy system is used. On the other hand, muscular endurance exercises typically only use the ATP-CP and glycolysis energy systems.

The key variables that define how a muscle(s) is(are) being trained are: the number of repetitions performed during a set, the amount of weight lifted during the set, and the period of rest between sets.

To increase muscular endurance, sets of high repetitions should be performed. At least 15 repetitions should be performed per set.2,10 Each repetition should be performed at or below 67% of one’s 1 RM (repetition maximum). A period of 1 to 2 minutes should be allowed for rest in between each set. Table 5-4 compares variables associated with the four main types of muscular training: power, strength, hypertrophy, and endurance.

Table 5-4

Training Variables Associated with Training Goals

Training Goal	Repetitions Goal	Load (%of 1 RM)	Rest Interval Between Sets (min)
Power	1-2	80-90	2-3
Strength	≤6	≥85	2-3
Hypertrophy	6-12	67-85	2-3
Endurance	15+	≤67	1-2

Muscular endurance training plays a key role when rehabilitating patients with an orthopedic injury.14,22,24,27,28 Using the muscular endurance training principles allows the physical therapy team to prescribe strengthening exercises during the subacute phase of healing (see Chapter 11) while reducing the risk of overstressing the healing tissue.