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Exercise Prescription
Maureen C. Ashe, BScPT, MSc, and Karim M. Khan, MD, PhD
Abstract
Although many studies suggest that exercise provides various health benefits, the
optimal dimensions of exercise (ie, frequency, intensity, duration, type) remain to
be carefully defined. Research encompasses many areas, from activities of daily living to elite athletic performance. To properly prescribe a beneficial exercise program,
a physician must have a basic understanding of the elements of exercise physiology
and energy metabolism, as well as of the cardiovascular, respiratory, and musculoskeletal responses to exercise and training. Also, integration of specific programs
for muscle strengthening with aerobic conditioning is important to achieve optimal
results in patients with musculoskeletal conditions such as osteoarthritis and osteoporosis.
J Am Acad Orthop Surg 2004;12:21-27
Determining appropriate exercise prescriptions for postsurgical patients or
individuals diagnosed with osteoporosis or osteoarthritis can be confusing;
although recent studies have suggested
that exercise is helpful, optimal parameters have not been carefully defined. Exercise physiology and prescription encompass a range of
intensity, from activities of daily living to elite athletic performance. Understanding the principles of exercise
physiology, such as how the body responds and adapts to the workload
imposed on it, is an important component of effective prescription. Without underestimating the importance
of cardiovascular fitness, it is critical
to emphasize specific elements of exercise that directly affect the musculoskeletal system.
Exercise Physiology
Energy Pathways
Performance during exercise depends on energy. Adenosine 5'triphosphate (ATP) is the substrate
that most cells use as their energy
source because it can both receive and
Vol 12, No 1, January/February 2004
provide energy; the supply is constantly replenished.1 The ATP molecule has three components. The center is ribose, with a nitrogenous base
(adenine) attached to one side and,
on the other side, a string of phosphate groups that provides the site of
energy transfer. With enzymatic
cleavage, the terminal phosphate is
released, energy is given off, and adenosine 5'-diphosphate is formed.1
Energy metabolism is a complex
series of steps that depends on the
available type of fuel (ie, carbohydrate, fat, protein). The fuel source
and the type of exercise determine the
physiologic pathway for energy production. The three pathways of carbohydrate metabolism are the ATPphosphocreatinine system (anaerobic),
glycolysis (aerobic and anaerobic), and
oxidative phosphorylation and the electron transport chain (aerobic). Which
pathway responds depends on the individual and the challenge that is presented. The three basic areas of exercise are power, speed, and endurance.
Depending on the sport and the type
of energy required, the athlete trying
to maximize performance needs to develop the anaerobic energy pathway
(eg, sprinter, basketball center), the aerobic pathway (eg, endurance runner),
or a combination of both (eg, soccer
player).
Cardiovascular System
The cardiovascular system is the
principal determinant of oxygen consumption. VO2 is the maximum volume of oxygen consumed per minute.
Vmax is the maximum volume of oxygen that can be consumed. Vmax can
be expressed as an absolute value (liters per minute) or a relative value
(with reference made to body weight
and so measured in milliliters per
minute per kilogram of body weight).2
During exercise, the cardiovascular system must meet the increased
demand for fuel and oxygen while removing metabolic by-products. Consequently, blood flow is redistributed to the exercising muscle while
cardiac and cerebral homeostasis is
Ms. Ashe is Doctoral Candidate in Medicine,
University of British Columbia, Vancouver, BC,
Canada. Dr. Khan is Assistant Professor, Department of Family Practice, University of British
Columbia.
None of the following authors or the departments
with which they are affiliated has received anything
of value from or owns stock in a commercial company or institution related directly or indirectly
to the subject of this article: Ms. Ashe and Dr.
Khan.
Reprint requests: Ms. Ashe, University of British Columbia, Suite 211, 2150 Western Parkway,
Vancouver, BC V6T 1V6, Canada.
Copyright 2004 by the American Academy of
Orthopaedic Surgeons.
21
Exercise Prescription
maintained. As a result, cardiac output, the total volume of blood
pumped by the ventricles per minute,
can increase in a highly trained athlete from an average resting value of
5 L/min to 30 L/min. The initial increase in cardiac output is the result
of increases in both stroke volume
(amount of blood ejected from the
ventricles during systole) and heart
rate. However, as exercise progresses above approximately 40% Vmax,
the stroke volume remains stable and
the accelerating heart rate increases
cardiac output.3
Blood pressure is controlled by cardiac output and total peripheral resistance. Baroreceptors in blood vessel walls respond to both average
pressure and the rate of change; these
receptors exert their influence through
the autonomic nervous system. The
autonomic nervous system, in turn,
influences heart rate, stroke volume,
and peripheral vascular resistance.
The lactate threshold is used to estimate fitness level. Lactic acid is produced when oxygen is absent during
glycolysis, and its accumulation in the
body can lead to early fatigue. Once
the lactic acid level begins to rise, it
continues to do so rapidly. Reaching
the lactate threshold means that the
body has enough lactic acid to affect
performance and can cause performance to cease. Training increases the
lactate threshold; a highly trained athlete will reach the threshold much later than an average person exercising
at the same level of intensity.4 Training also causes changes in the cardiovascular system, increasing resting
stroke volume, cardiac output, blood
volume, and hemoglobin, and decreasing resting heart rate and blood
pressure.2 Some elite athletes have
resting heart rates as low as 40 to 50
beats per minute.
Respiratory System
Alveolar ventilation, which allows
simple diffusion of oxygen and carbon dioxide by a pressure gradient
across the alveolar-capillary mem-
22
branes, is tightly controlled by peripheral sensors; the brainstem control center, which detects balance between
frequency and depth of breathing; and
the respiratory muscles.5 Although
the average intake of air is 6 L/min,
during exercise it can increase to 192
L/min and 48 breaths/min.5
The normal respiratory response to
steady-state exercise occurs in three
stages. Initially (within 30 seconds),
there is a rapid increase in ventilation
(breaths/min), which may be followed
by a plateau. As steady-state exercise
progresses, there is a gradual increase
in ventilation that continues until either maximum voluntary ventilation
or fatigue is reached.5,6 Training can
increase ventilatory efficiency by increasing diffusion capacity, thus improving the match of ventilatory efficiency with ventilation/perfusion.2
Neuromuscular System
Skeletal muscle fibers are either
slow twitch or fast twitch, with the
type of muscle fiber determined by
the nerve supply7 (Table 1). Muscle
fibers are recruited in a specific pattern, according to the size of the motor unit, to produce the necessary
force and speed. This is the so-called
size principle. For example, motor
units with smaller diameter innervations (slow-twitch muscles) are recruited before fast-twitch muscles.7
Muscular response to exercise depends on the length and type of ex-
ercise done. Endurance training can
result in hypertrophy of skeletal muscle with a concomitant increase in capillary density, the number and size of
mitochondria, and oxidative enzymes
to boost the overall oxidative capacity of the muscles. With strength training, there is an increase in muscle glycogen energy stores and muscle fiber
size, resulting in improved strength,
increased speed of contraction, and
increased skill and endurance.2
Two types of muscle soreness can
occur with exercise. Acute soreness is
caused by fatigue or lactic acid buildup
within the muscle. Exercise should not
continue if such discomfort persists.8
The other type of muscle soreness,
delayed-onset muscle soreness (DOMS),
can occur 12 hours or more after activity. DOMS is thought to be the result of microtears within the working
muscles, with subsequent swelling. Eccentric contractions, in which the muscle is slowly elongated during the contraction, are a major cause of DOMS.8,9
Adequate warm-up, cooldown, and
moderate progression of intensity can
help prevent DOMS.
Exercise Prescription
Applying the principles of exercise
physiology can help improve the
health of special populations. When
considering exercise, the following
parameters should be considered to
Table 1
Fiber Types
I
(slow twitch)
IIA
(fast twitch)
IIB
(fast twitch)
Properties
Red, high myoglobin content, tonic,
oxidative (aerobic),
small force developed slowly
White, phasic, oxidative-glycolytic (aerobic/anaerobic), medium force developed
with greater speed
Work
Endurance sports
Middle distance
running
White, phasic, glycolytic
(anaerobic),
high force
developed
rapidly
Sprinting
Fiber Type
Journal of the American Academy of Orthopaedic Surgeons
Maureen C. Ashe, BScPT, MSc, and Karim M. Khan, MD, PhD
achieve a training effect (eg, increased
cardiac output, increased lactate
threshold): frequency, intensity, duration, and type of exercise. With these
elements in mind, the physician can
determine how often the patient
should exercise, which exercise is
best, the optimal amount of exercise,
and how challenging the exercise
should be. A sound program includes
three key types of exercise: aerobic (ie,
cardiovascular), resistance training
(ie, strengthening), and stretching.
Done in moderation, exercise can help
provide a balanced approach to improving and maintaining health and
fitness10-12 (Table 2).
Aerobic Training
Walking, jogging, running, swimming, bicycling, cross-country skiing,
and aerobic dance are examples of
aerobic exercises that use large muscle groups for a long duration. Aerobic exercise relies on the oxidative
pathways; therefore, it is necessary to
Table 2
Exercise Prescription: Designing an Exercise Program
Match goals to exercise preferences
Set realistic and achievable goals
Provide regular follow-up to monitor progress; think of exercise as a
medication
Review goals regularly and adjust accordingly
Involve family members; encourage the individual to exercise with other
people
teach proper breathing technique (eg,
diaphragmatic breathing). Adequate
hydration and nutrition also are important.
Recently there has been a shift from
prescribing vigorous exercise several times a week to prescribing moderate daily activity.13 The American
College of Sports Medicine (ACSM)
recommends moderate-intensity exercise with longer duration range, that
is, “20 to 60 minutes of continuous or
intermittent (minimum 10-minute
bouts accumulated throughout the
day)” aerobic activity.14 The US Surgeon General’s Report15 recommends
a total of 30 minutes of moderate
activity (eg, walking, doing housework), preferably daily. For older
adults with limitations or who are
extremely sedentary, the first goal of
exercise prescription is to reduce time
spent sitting. Exercise can progress to
parking farther away from entrances at malls and shopping centers, using the stairs instead of the elevator,
or taking several brief walks a day
(Table 3).
Table 3
Exercise Prescription: Two Case Studies
Case 1*
Parameter
Exercise prescription
goals
Frequency
Intensity
Time
Type
Improve overall aerobic conditioning,
encourage weight loss, increase
flexibility and strength
3-5 times/wk
Target heart rate, 55%-64% of
maximum heart rate
Review perceived rate of exertion
20-60 min/session, eg, 10-min warmup, 20-min exercise session, 5-10–min
cooldown
Walking, with biking on alternate days
Case 2†
Improve overall aerobic conditioning, increase
flexibility and strength, improve balance
2-3 times/wk as tolerated initially
Target heart rate, 55%-64% of maximum heart
rate
Review perceived rate of exhaustion
Patient should be able to talk easily during
exercise session
Combined total of 20-30 min on exercise days
(minimum of 10-min sessions accumulated
throughout the day)
Increase walking tolerance up to 20 min at
one time
Alternative options include a water-based or
community program similar to those offered
by The Arthritis Society
*
A 40-year-old computer programmer presents with intermittent back pain. He is 30 pounds overweight and leads a sedentary lifestyle. After a thorough workup, the diagnosis is functional back pain. There are no other contraindications.
†
A 65-year-old has become increasingly sedentary over the last 5 years because of severe osteoarthritis of the hip. She had a hip replacement and is asymptomatic 6 months after surgery. There are no other contraindications.
Vol 12, No 1, January/February 2004
23
Exercise Prescription
A common challenge is prescribing aerobic exercise for those who cannot bear weight on the lower extremities. Low-impact activities such as
swimming and stationary cycling are
ideal for individuals with conditions
such as arthritis or poor balance. Hydrotherapy, pool running or swimming, use of a rowing machine, cycle arm ergometer, wheelchair training,
and games can each be effective in the
appropriate situation. Even when one
limb is immobilized, training the unaffected limb can maintain or improve
function in the affected limb.16
Aerobic exercise intensity is measured in several ways. For healthy
adults, the ACSM recommends a training rate between 55% and 90% of maximum heart rate, which can be measured in a maximal test or estimated
by subtracting the individual’s age
from 220.14 It is also recommended that
the lower intensity value (55% to 64%
of maximum heart rate) be used for
unfit or deconditioned individuals if
appropriate. Aerobic exercise intensity also can be measured by the rate
of perceived exertion (Table 4).
Resistance Training
Healthy persons of all ages, as well
as many with chronic diseases, should
engage in single-set resistance exercise programs of up to 15 repetitions
at least 2 days per week.18 Each workout session should consist of 8 to 10
different exercises that train the major muscle groups. A typical program
might include leg extension, knee flexion, hip abduction and adduction, and
upper body work. The program should
be designed to fit individual needs and
abilities. Resistance training is not restricted to free weights; weight machines and elastic bands or tubing are
other methods. Participants should be
able to complete two sets of 10 repetitions before resistance is progressed.
The goal of this type of program is to
gain a significant amount of muscle
mass, endurance, and strength.
Patients with chronic diseases (eg,
arthritis) may have to limit range of
24
motion for some exercises and therefore use lighter or easier weights with
additional repetitions. Proper breathing and equipment techniques are important to minimize injury. Educating patients about the normal response
to exercise and the possibility of
DOMS may assist with compliance.
Strengthening can be grouped into
three categories: isotonic, isometric,
and isokinetic. Isotonic exercise involves the development of muscular
force through range of motion or movement. Isokinetic exercise involves the
force being generated through a constant speed. Isometric exercise involves
the development of force without
movement, as in tensing and holding
a muscle at a certain part of the range.
Core stability (ie, abdominal)
strengthening exercises are still possible when some limb exercises are
contraindicated. For example, if the
lower leg cannot be exercised because
of recent injury or surgery, the abdominal muscles still can be worked.
Pilates exercises are ideal upper body
and abdominal workouts for those
whose lower leg resistance training
is limited. The Pilates method is a series of approximately 500 exercises
done on specialized equipment or
mats. The principles behind the
Pilates method include proper breathing technique, positioning of the
body, balance, coordination, strength,
and flexibility. Instructors must be
certified and can provide either oneon-one or group training.19
Patients often are interested in using their noninjured extremities for
functional activities that allow them
to remain in condition and retain
some skill practice. For instance,
Thomas Muster, the Austrian professional tennis player, underwent significant lower extremity surgery after a motor vehicle accident in the
spring of 1989, but within weeks he
returned to the tennis court, playing
in a wheelchair. By doing so, he was
able to maintain some aerobic and
anaerobic fitness, practice his motor
skills, and use his time productively.
Table 4
Borg Scale: Rate of Perceived
Exertion17
Scale Severity
0
0.3
0.5
0.7
1
1.5
2
2.5
3
4
5
6
7
8
9
10
11
Nothing at all
Extremely weak (just
noticeable)
Very weak
Weak (light)
Moderate
Strong (heavy)
Very strong
Extremely strong (“maximal”)
Absolute maximum (highest
possible)
On this scale, the patient rates how he or
she feels in terms of fatigue and physical
and mental well-being. It has been recommended that patients work between 4
(somewhat strong) to 5 or 6 (strong).
For correct usage of the scale the user must
go to the instruction and administration
given by Borg. See Borg G: Borg’s Perceived
Exertion and Pain Scales. Champaign, IL:
Human Kinetics, 1998, or write Borg
Products USA, Inc, 1579 F Monroe Drive,
#416, Atlanta, GA 30324-5022. Website:
www.borgproducts.com.
That training was combined with
clinic-based physical therapy, and
Muster returned to competition much
more rapidly than had originally been
anticipated.
Stretching
The scientific evidence supporting
stretching as part of an overall healthand-fitness program is somewhat
controversial. A recent meta-analysis
reviewed the evidence for stretching,
both immediately before activity and
Journal of the American Academy of Orthopaedic Surgeons
Maureen C. Ashe, BScPT, MSc, and Karim M. Khan, MD, PhD
independent of activity.20 There was
little evidence of favorable effects for
stretching before exercise, but stretching independently of activity periods
may have some benefits.21 Despite the
lack of solid evidence in favor of
stretching, most health professionals
regularly prescribe it as a preventive
measure.
The three main types of stretching
or flexibility exercise are static and
ballistic stretching and proprioceptive
neuromuscular facilitation. Static
stretching is the type most often recommended by health professionals.
It involves slowly stretching the muscle and holding the pain-free position
for 10 to 30 seconds without bouncing or other movement. Ballistic
stretching involves bouncing, and the
final position is not held. Proprioceptive neuromuscular facilitation has its
origins in physical therapy. Muscular contractions are elicited to produce relaxation within a muscle.
Properly done stretching appropriate
for a specific orthopaedic problem
also can incorporate breathing exercises to promote overall relaxation
and a sense of well-being. Yoga or tai
chi may be indicated, but patient limitations must be considered.
Precautions and
Contraindications
When prescribing exercise, the
physician should consider the patient’s age, medical history, and current level of health and fitness. In patients with a history of a cardiac event
and pulmonary and/or metabolic
disease, a medically supervised program is recommended. The American
College of Sports Medicine Position
Stand states:
The contraindications to exercise
testing and exercise training for
older men and women are the
same for young adults. The major
absolute contraindications precluding exercise testing are recent
ECG changes or myocardial infarction, unstable angina, uncon-
Vol 12, No 1, January/February 2004
trolled arrhythmias, third degree
heart block and acute congestive
failure. The major relative contraindications for exercise testing
include elevated blood pressure,
cardiomyopathies, valvular heart
disease, complex ventricular ectopy and uncontrolled metastatic
diseases. It is of paramount importance to remember that symptomatic and asymptomatic CV disease
and the absolute and relative contraindications precluding exercise
testing are more prevalent in older adults. In addition, there is an
increased prevalence of comorbidities in older adults that affect CV
function, including diabetes, hypertension, obesity and left ventricular dysfunction. This adherence to the general ACSM testing
guidelines with respect to the necessity for exercise testing and for
medical supervision of such testing is imperative.13
Although isometric exercise is prescribed to maintain muscle mass and
strength when range of motion is restricted, it may be contraindicated in
a patient with a history of cardiac
problems. Resistance training can
cause a dramatic increase in blood
pressure, especially if the patient
holds her or his breath and evokes a
Valsalva maneuver. Therefore, a team
approach to the treatment of the patient is ideal. The orthopaedic surgeon, family physician, cardiac specialist, and treating therapist can
work together to prescribe the right
exercise and then monitor and assist
the patient to maintain benefits.
Osteoarthritis and
Osteoporosis
Osteoarthritis
For patients with osteoarthritis of
the knee or hip, exercise can provide
modest improvements in measures
of disability and physical performance.22,23 Although both resistance
and aerobic exercise programs are
helpful, as is a mixture of several
types of therapy, data are insufficient
to suggest that one protocol is better
than another.
One successful exercise protocol is
a 3-month facility-based program of
walking 3 times a week with an instructor, followed by a 15-month,
home-based, 3-times-a-week walking
program closely monitored by an instructor.24 Each aerobic session consists of 10 minutes of warm-up and
cooldown bracketing a 40-minute
walk at between 50% and 70% of heart
rate reserve, as determined by a
screening exercise test. Time commitments for resistance training are similar to those for the aerobic exercise
program. The 40-minute program depends on individual findings. It includes exercises for strength, mobility, and coordination. Weight is begun
at the lowest possible level (1 kg) and
increased in a stepwise fashion as
long as the participant can complete
two sets of 10 repetitions of each exercise. Once a plateau is reached,
weight is increased when the patient
successfully completes two sets of 12
repetitions for 3 consecutive days. Although more research is needed to test
the various modalities of exercise for
osteoarthritis of the knee or hip,24 this
protocol provides a basis for prescribing exercise.
Osteoporosis
Part of managing osteoporosis itself, not just its complications, is an
exercise program.25 Most types of activityarepreferabletoasedentarylifestyle and should be encouraged. The
patient may choose to exercise alone,
or, if one is available, join a group
program for individuals with osteoporosis.
Resistance training should be considered for persons with osteoporosis because such training can cause
mild improvement in, or at least help
maintain, bone mineral density.26,27 In
a 1-year prospective randomized controlled trial, Kerr et al28 studied the
25
Exercise Prescription
impact of resistance training (8 to 12
repetitions) on bone mass in the forearm and hip in postmenopausal women. There was a statistically significant (P < 0.05) improvement in bone
mass, which was site-specific and dependent on the load applied.28 In their
meta-analysis of resistance training
and bone mineral density in premenopausal and postmenopausal women,
Kelley et al29 concluded that resistance
training had a positive impact on the
lumbar spine in all women, but only
postmenopausal women gained bone
mineral at the femur and radius. Resistance training also improves lower limb muscle strength, which is a
key factor in preventing falls and fractures.30
Exercise sessions should begin
with an 8- to 15-minute warm-up of
gentle stretching and joint motion,
followed by cardiovascular exercises
for 5 to 10 minutes. Targeted heart
rate should be 60% of 220 less the pa-
tient’s age for a beginner or a deconditioned person, and 70% to 75% of
220 less the patient’s age for those in
better shape. Range-of-motion exercises for the shoulder and hip joints
should follow. Gradual weight training with light free weights and rubber tubing then can be incorporated.
Effective upper arm exercises include
pushing against a wall or pulling and
twisting against a partner.31
Quadriceps muscle strength can be
improved with a wall slide exercise
or by practicing standing from a seated position. Back posture correction
can be done standing, sitting in a chair,
or lying prone. Abdominal strengthening and leg strength exercises also
are important. The exercise program
should conclude with a 7- to 8-minute
cooldown period; postexercise stretches can be done while sitting on the
floor. Although no guidelines have
been scientifically validated, these
steps represent current best practice
guidelines that are safe and appear
to be appropriate from clinical experience. The physician should discover activities the patient values; many
popular activities require some degree
of strength, flexibility, endurance, balance, and coordination.
Summary
Exercise provides major health benefits, but specific prescription is necessary. The basis for prescription is
the exercise physiology of the energy metabolism pathways and the
response of various systems to different types of training. Despite inconsistencies in exercise prescription, a
commonsense approach based on the
principles of exercise physiology provides a starting point. Specifically,
patients with osteoarthritis or osteoporosis may benefit from participation in exercise programs.
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