CARDIOVASCULAR FITNESS

Cardiovascular fitness is the most important aspect of any fitness program. Cardiovascular fitness should be the mainstay of any fitness program. This section is designed to explain the definition of cardiovascular fitness, the benefits of cardiovascular fitness program, warm up and cool down aspects of cardiovascular fitness, the different modes of cardiovascular fitness, the criteria of cardiovascular fitness, explain how to monitor cardiovascular intensity levels, the factors of improving cardiovascular fitness, and the methods of cardiovascular training.

Cardiovascular Fitness: A Definition

Cardiovascular fitness also called cardiorespitory fitness is the ability of the lungs to provide oxygen to the blood and the heart to transport the oxygenated blood to the cells of the body. It is also the ability of the body to sustain an activity for an extended period of time.

Participating in a cardiovascular conditioning program can help the participant to:

- lower blood pressure
- increase HDL cholesterol
- decrease total cholesterol
- decrease body fat due to utilizing fat as energy
- increase heart function and its ability to pump more blood
- decrease stress reactions and anxiety
- reduce glucose-stimulated insulin
- increase oxygen output to body
- decrease resting heart rate
- increase cardiac output
- increase aerobic work capacity

Now that we know the benefits of a cardiovascular program lets examine the components of a proper program. Which include warm-up, cool down, and criteria for cardiovascular exercises.

The warm-up should increase the heart rate, blood pressure, oxygen consumption, dilate the blood vessels, increase elasticity of the active muscles, and increase the heat produced by the muscle groups to be used during exercise.

2) Stretching or Flexibility - This is to be performed on the muscle groups you will be exercising during your cardiovascular activity. An example would be a quadriceps stretch before bicycling.

The warm-up phase should last anywhere from 5 - 10 minutes. For most activities 5 minutes may be good enough.

Stretching should always follow the low intensity aerobic warm-up activity as it is easier to stretch the muscle groups when they are warm. An example of a good warm-up would be 5 minutes of walking followed by 3 minutes of light stretching. Try to hold your stretch for at least 20 seconds during the warm-up

The purpose of a cool down is to slowly decrease the cardiovascular work and overall metabolism that were elevated during the cardiovascular activity. A cool down consists of slowing down the intensity level of the cardiovascular activity you are performing slowly. Stopping exercise suddenly can be dangerous. A cool down keeps the body circulating blood and keeps blood from pooling in the veins. A cool down is extremely important for people who are in the beginning stages of an exercise program.

A proper cool down should last about 5 -10 minutes for each thirty minutes of exercise. If the cardiovascular activity is performed for longer periods or is of higher intensity the cool down phase should last longer.

The American College of Sports Medicine (ACSM) defines exercise mode as any exercise that utilizes the large muscle groups and can be maintained continuously, and is rhythmical and cardiovascular in nature. The ACSM classifies cardiorespitory fitness in three groups.

Group A - These are physical activities such as running, jogging, biking, in-line skating, swimming, or cross country skiing, or cardiovascular glide in which the intensity level is easily maintained with little variable in heart rate. Interval training is included in group A exercises. Beginners should start their cardiovascular training using group A exercises because this group gives a more precise measure of intensity levels.

Group B - This is the group that relates to those exercises in which a certain skill level is needed to complete the exercise efficiently. Choreographed dance routines, and figure skating are examples of group B Activities.

Group C - These are exercises which vary quite a bit in exercise intensity level. Exercise where heart rate is not maintained at a certain level for any length or time. These exercises include most team sports such as basketball, softball, racquetball, and soccer.

Frequency
Intensity
Time

Frequency - This refers to the number of times per week you perform cardiovascular exercise. To improve cardiovascular fitness levels a minimum of three days a week is recommended. The American Council of Exercise Guidelines recommend 3 -5 days per week. When starting a program for the first time a person should perform Group A exercises such as brisk walking at least every other day with two days in a row thrown in during the week. For some people it may be of benefit to exercise every day and for some people doing two cardiovascular exercises a day may be of benefit. Remember that a person should not try to overdo it to soon but progress slowly and efficiently.

Intensity - Intensity level is a very important part of any exercise program. Exercising at correct levels can make a big difference in the effectiveness of a program. Intensity can be defined as speed or workload of an activity. Many people, such as walkers, do not exercise at the correct intensity level for cardiovascular and fat burning enhancement. On the other hand some people can actually exercise at too high an intensity level. Exercising at a high intensity level can increase a persons chance of injury. There are also people who may not get maximum fat burning potential and cardiovascular improvement because they do not keep their intensity level sustained for a long enough period of time. Tennis is a good example. Knowing intensity level is very important in getting the desired results from a program. Intensity is important for a person who wants to build a base of fitness, a person who wants to burn fat, or a person who just want to increase performance levels for athletic competition.

The Guidelines for intensity levels from the American College of Sports Medicine are 60 -90 % of maximal heart rate. This range is approximately 50 to 85% of maximal oxygen consumption (functional capacity or VO2 max) and 50 - 85% of heart rate maximum reserve. We will discuss monitoring intensity in the next section. A person who is beginning a program should start at 50 - 60% of their maximum oxygen consumption although some beginners need to start at levels of 40 - 50%. For fitter people exercising at 75 - 85% is probably more appropriate. Overall for most healthier adults it is good to be at 60 - 70%. The key to all of this is knowing where he/she is at as far in their cardiovascular fitness level is concerned and starting slowly, progressing as cardiovascular fitness levels increase. Starting a program too fast is not good, but not doing enough is not good either. A comfortable balance which progresses to higher levels over time is recommended.

Time - This refers to the duration of an exercise session or the length of time to complete an exercise. For many the duration of an exercise depends upon the intensity of an exercise. In general the cardiovascular exercise should last a minimum of 20 minutes with the best results coming after a longer period of time. A person can get good cardiovascular fitness and body fat burning utilization at 20 - 30 minutes of cardiovascular exercise. People who are just beginning a program should start slowly and progress over time. A person who rarely has ever exercised should start at 10 -12 minutes including a 5 minute warm-up and increase this by about 2 minutes per week until they are above 20 minutes. The key is to spend some time doing some cardiovascular activity every day if possible.

There are many ways to monitor cardiovascular exercise intensity levels. Measuring exercise heart rate level can be tricky because everyone is different. Not everyone has the same resting heart rates and not everyone has the same maximum heart rate and therefore not the same exercising heart rate. To get a true reading on max heart rate and exercise heart rate an EKG stress test from a physician or sports medicine exercise laboratory can be performed. An EKG stress test can give a true indication of overall cardiovascular fitness levels and make the exercise intensity levels more accurate. Remember the following formulas may not be exactly accurate and you need to be aware of this and adjust accordingly. Remember it is better to be on the safe side to prevent injury.

The following formulas we will use are
1) Percentage of Maximal Heart Rate
2) Intensity by perceived exertion
3) Talk Test.

This method is the most popular used formula today. This formula can have a variability of + or - 10 to 12 beats per minutes (Dursitne, 1988). This method is measured as follows.

Training Heart Rate Range = Maximal predicted heart rate x % of intensity range x 1.15.

A 50 year old man for whom a 60% to 70% maximal heart rate is desired:

    220 - 50 = 170 x .60 = 102 x 1.15 = 117.3 (60% lower limit exercise rate)
    220 - 50 = 170 x .70 = 119 x 1.15 = 136.9 (70% higher limit exercise rate)

This person would exercise at between 117 - 136 bpm.

Remember that a person who is just starting a program should not push it to fast. A person starting a program would be advised to exercise at 50% - 60% of max heart rate to get acclimated to aerobic type exercise.

Monitoring your heart rate is important for assessing fitness goals, regulating exercise intensity, and documenting progress to exercise. The three methods to measuring cardiovascular exercise are heart rate, perceived exertion, and laboratory testing and monitoring.

Heart Rate - Knowing heart rate and exercise intensity is essential to progress in a persons program. The two ways to measure heart rate during exercise are obtained from measuring the pulse rate and using a heart rate monitor. Measuring or feeling a pulse is the easiest and less costly method although it does have its drawbacks. During exercise such as swimming or running it is often difficult to stop and take a pulse. To take a pulse place an index finger or middle finger over the radial artery which is located on the edge of the wrist just below the thumb.

Heart Rate monitors are a very good way to measure heart rate during exercise. Heart rate monitors are fairly accurate digital devices which use sensors strapped to the chest or wrist which can measure heart rate. The main advantage to heart rate monitors is the ability to accurately measure heart rate during exercise without stopping to take a pulse. The main drawback is cost. When picking a heart rate monitor look for quality as some heart rate monitors are sensitive to motion which can cause erroneous readings at times. Polar Heart Rate monitors are recommended for their accuracy.

Intensity by perceived exertion is also called measuring exertion using the Borg Scale developed by Dr. Gunner Borg. This scale uses a numbered scale from 6 -20 to measure feelings of exertion level. This is a good system in that it takes into account a persons fatigue level. Using this scale a reading of 12 on the scale would be approximately 50 -74% or maximum heart rate reserve (Karvonen formula). A reading of 16 would be 85% maximum heart rate reserve . Most people should try to stay between 12 and 16 on the borg scale.

RPE
6
7       Very, Very Light
8
9       Very Light
10
11       Fairly Light
12
13       Somewhat Hard
14
15       Hard
16
17       Very Hard
18
19       Very, Very Hard
20

Talk Test Method

This method is the simplest to use and understand. The talk test is exercising in an intensity level where a person is breathing comfortably and rhythmically. A person should be slightly winded but still be able to talk with someone somewhat easily when exercising. If a person is out of breath they should slow down. If they are not winded enough they should speed it up. The key again is not to overdo it too soon. Work up the intensity level over time. Using the talk test is great for beginners because it ensures a safe and comfortable level of exercise.

A person will begin to notice increases and improvement in cardiovascular fitness level almost immediately, although improving cardiovascular fitness level is a slow process. People who have good cardiovascular fitness capacities will notice less of an increase than a person who has a lower cardiovascular capacity. The criteria for aerobic fitness or FIT (Frequency, Intensity, and time of exercise) are the factors which indicate one’s level to improve cardiovascular fitness levels. Age can be a factor as most people show smaller improvements as they age due to the lower exercise intensities they must start at.

As a person stays with a cardiovascular program their cardiovascular capacity will increase. These gains will be at a higher level in the beginning and begin to level off as training continues, however improvements in cardiovascular levels will continue at a slower rate and improvements in cardiovascular capacity and endurance can be expected to continue. Setting goals and a progression plan will help to improve cardiovascular fitness levels over time.

Once a person has chosen the type of cardiovascular exercise they prefer and know the intensity, duration, and frequency to exercise at it is time to choose the training method.. There are five training methods we will use to help you increase cardiovascular conditioning: Continuous training, interval training, fartlek training, circuit training, and aerobic composite training

This is a conditioning stage involving cardiovascular exercises such as walking, jogging, cycling, swimming, step aerobics, aerobic dancing or in-line skating. During continuous training your intensity level should be between 50 - 85% of functional capacity (karvonen formula) depending on cardiovascular fitness level when starting the program. There are two types of continuous training to be concerned with. Intermediate slow distance and long slow distance.

Long slow distance - This kind of continuous training should be preceded by at least 6 months of intermediate slow distance training as prescribed above. This type of training is usually reserved for endurance sports such as long distance running (marathons), long distance cycling, and triathlons. This type of training should include adequate recovery time between sessions to help the body recuperate properly. This type of training may increase the risk of injury such as knee problems, achilles problems, and tendinitis.

Interval training can be a useful training method for all wishing to improve conditioning and performance levels. Beginners can benefit form this type of training when starting a cardiovascular program. People with higher fitness levels can use this method to improve endurance levels. Interval training consists of mixing more intense sessions with less intense sessions of the same or similar exercise. An example would be jogging or walking for a short period of time followed immediately by brisk walking and alternating these sessions for a complete cardiovascular workout. Interval sessions should use the same time principal as FIT or 15 - 60 minutes in length. There are two types of interval training. Aerobic Interval training and anaerobic interval training.

Aerobic Interval Training - This is most suited for people with a low or poor aerobic fitness capacity. This type of training uses bouts of exercise between 2 and 15 minutes in length. 2 -3 minutes is recommended for beginners. The intensity level should be between 50- 80 % of functional capacity with beginners staying within 50 -70 % range. The rest interval should be completed in the same time as the exercise interval. A person should complete between 5 - 10 exercise intervals and 5 - 10 rest intervals each. An example of aerobic intervals for a beginner would include a brisk walk at 60 - 70 % functional heart rate for 3 minutes followed by a slow walk at 50% functional heart rate for 3 minutes. A beginner could shoot for 5 exercise intervals and 5 slow walk resting intervals. A more advanced person could do 3 minutes of stationary cycling at a workload of 60 - 70 % with 3 minutes of stationary cycling at an easy pace with zero resistance for 3 minutes. Higher intensity and duration should be reserved for those in a higher cardiovascular functional capacity and for those seeking to increase endurance and speed.

Anaerobic Interval Training - This type of training is for people with a high level of fitness who want greater speed and aerobic power. This type of training results in more muscle discomfort from lactic acid build up in the muscles. This type of training is not for beginners. This type of training is usually 30 seconds to 4 minutes in length and is performed at 85% - 100% of functional capacity. Again the potential for injury is also higher with this type of training. An example of this type of training might be sprinting at a high rate of intensity for 30 seconds followed by walking for 1 1/2 minutes. The longer your exercise intervals toward 4 minutes the shorter the rest time. For example a high intensity interval of one minute would be followed by a two minute rest interval. This type of training increased the probability of injury due to the intensity levels used.

Fartlek is a Scandinavian term which roughly means speed play. This type of training is similar to interval training except the work rest intervals are not measured to any degree. This type of training can be performed by a person at any level of cardiovascular fitness ability. The intervals used generally depends on how a person feels during a session. A useful example would be a runner who warms up by running for 5 - 10 minutes then significantly varies the pace every 5 - 10 minutes of so. Fartlek training can mean playing with the intensity level time and duration of the intervals. Remember a person should not overdo it too soon.

This type of training takes a person through a series of exercise stations with little rest between the stations. Circuit training is a form of interval training. The number of stations is usually between 5 - 10. The stations most of the time involve weight or resistance equipment but calisthenics such as push ups and crunches can be classified as a station. This type of training usually involves all the major muscle groups and helps to develop cardiovascular and muscular endurance levels. The key to this type of training is to keep moving through the stations with little rest in between. A 15 second rest is appropriate although beginners may want to rest at 20 -25 seconds. A beginner should remember not to overdo it too quickly and use the talk test or measure heart rate. It is important as with all the training methods to include a thorough warm up of 5 - 10 minutes, warming up all the muscle groups. This type of training can significantly improve cardiovascular endurance.

Aerobic composite training is a cross training session all rolled up into one session and is for more advanced cardiovascular capabilities. Aerobic composite can help prevent boredom in training. A good example of Aerobic composite training would be a 45 minute workout of 15 minutes jogging to the pool followed by 15 minutes of swimming followed again by 15 minutes of jogging.

Strength Training is another important aspect of any exercise program. Strength training strengthens our muscles and connective tissues. Muscles are used every day for a variety of different activities. The human body have over 600 muscles. Muscles provide movement which is caused from stimulus and input from the nervous system. Every movement that we make from getting out of bed in the morning, to cooking dinner, to running a marathon, to the twitching of our eye lids involves some kind of muscle movement. Some of this movement is voluntary such as taking out the garbage or lifting a weight. Some of our muscle movements are involuntary such as twitching of the eyes. The muscle system has the marvelous ability to become stronger and bigger with the proper stimulus such as weight training. One the other hand our muscles can become smaller and weaker if not used. This chapter will cover the benefits of training for strength , factors which affect strength, the relationship between size and strength, strength training guidelines and strength program consideration.

Physical Appearance - Physical appearance can be greatly influenced by our muscle gain and loss of fat. More muscle gives a fitter, healthier appearance

Metabolic Function - Muscle tissue is very active and is responsible for much of the calories that are burned. Even when sleeping our bodies burn up to a quarter of all total daily calories. Muscle or lean tissue means a higher metabolism. This equates to more fat burning which leads to better health. In a non-trained person there is about .5% reduction in metabolic rate per year due to loss of lean tissue of muscle. This is one of the reasons why we may gain body fat weight as we grow older.

Increased Energy - Because of higher metabolic rates due to increased lean muscle tissue we can increase energy levels as more calories are burned for fuel.

Decreased Risk of Injury - Muscles also serve the purpose to balance and act as shock absorbers. Our bodies are able to act more effectively during activity thereby reducing the risk of energy. When a person strength trains the muscles, ligaments, and tendons are strengthened which decreases the risk of injury.

Age - Both men and women can benefit from a strength training program. The biggest benefits in size and strength come usually between the ages of 10 - 20 which are during the years of normal development and growth. Strength training in this age group should be measured in progressive and slow gains. Strength training gains come more slowly after maturity is reached.

Limb Length - The length of ones limbs (arms, legs) can make a difference in overall strength. Shorter limbs generally mean more leverage, which equates to higher strength output. Two people may have the same capacity for strength gains but a person with shorter limbs will show greater output.

Muscle Length - Muscles are attached to the bones by connective tissue called tendons. People have different muscle and tendon length. Some people may have shorter muscle with longer tendons. Some people may have longer muscles with shorter tendon attachments. The longer the muscle the greater the capacity for strength and size.

Muscle Fiber Type - There are two different types of muscle fiber. Fast twitch muscle fiber and slow twitch muscle fiber. A person with a majority slow twitch muscle fibers are best suited for endurance events such marathons. A person with a majority fast twitch muscle fibers would be best suited for sprinting type events. Most men and women have about the same proportion of fast twitch and slow twitch fibers.

Muscle strength is generally defined as a person’s ability to perform a single repetition of an exercise with maximum resistance. Muscle Endurance is generally defined as a person’s ability to perform many repetitions of an exercise with less than maximum resistance. This means that for endurance a person should exercise with less resistance and more repetitions and for maximum strength gains a person should exercise with heavier resistance and lower repetitions.

There are many ways to develop strength. The trick is to exercise the muscles effectively for strength, endurance, and/or size while avoiding injury. The following can be used as guidelines for achieving these goals.

Selection - It is important that a person has at least one strength exercise for each major muscle group to ensure balance in strength gains and decrease the chance of injury. The major muscle groups include the following:

Other groups of muscles which should be trained regularly are: Gluteals (butt), Obliques (side of stomach), Gastrocnemius (calves), Anterior Tibealis (front lower leg), Forearm flexors and extensors (forearm).

Sequence - When performing strength training exercise a person should always work from the larger muscles of the legs to the smaller muscles. This allows a person to perform the most demanding exercise of the large muscles when fatigue is at its lowest point.

Speed - Speed plays a major part of strength training. Performing each exercise movement in a slow and rhythmical movement is important. A person should not jerk or go at a fast pace when exercising. Concentrate on slowing down and completing each movement correctly. Slower movements put more stress on the muscle which gives more resistance and force throughout a full range of motion. Controlled movements equate to greater strength and size gains. Always emphasize the lowering movement called the negative or eccentric phase of the exercise. For example when lowering a barbell during a biceps curl go slower. A pace of about 3 seconds is recommended.

Sets - A set is defined as a number of successive repetitions performed without resting. The number of sets performed is a personal preference although beginners should try to stick to one to two sets of lighter resistance in the beginning.. A person should do a set until there is momentary muscle failure which is lifting to the point where a person cannot lift anymore without failing, jerking, or doing the exercise incorrectly. Those who do multiple set training should rest or recover at 1 - 3 minutes between sets.

Resistance and Repetitions - When exercising for strength the trick is to exercise to the point of momentary muscle failure. Most people can complete about 6 repetitions at 85% maximum resistance, 8 repetitions at about 80%, 10 repetitions at about 75%, and 12 repetitions at about 70% before reaching momentary muscle failure. This is a generally because everyone is different. Maximum resistance is measured in how much resistance a person can lift in one repetition. Beginners should not measure maximum one repetition resistance as this can be dangerous and lead to injury. A professional instructor help a person find their maximum resistance. Because training at 85% or above increases the risk of injury it is recommended that a person train at 8 - 12 repetitions with beginners working with higher repetitions at lower weight. This makes strength training safe and effective. Some people who are advanced may benefit using higher weight at lower than 8 reps but keep in mind this kind of training requires more recovery time before exercising the same muscle group again.

Range - The term full range of motion means completing the full movement of an exercise. Exercising at a full range of motion enhances both muscle and joint strength and flexibility. An example of using a full range of motion could be a biceps curl. During the down phase or negative phase bringing the bar all the way down until the arm is straight. On the up phase or positive phase bringing the bar all the way up to the shoulder all the while keeping the upper arm still and using the elbow as a hinge.

Progression - The key to gaining strength is to increase the resistance a person is working with progressively as the muscles adapt. A person who starts out at 8 repetitions should progress at that resistance weight until 12 repetitions can be performed before momentary muscle failure. After reaching 12 repetitions is accomplished the key is to add 5% of that weight and do 8 repetitions again working back up to 12 reps. Beginners can use this progression with 12 - 15 repetitions adding 5% as 15 repetitions are reached using lighter weights. The key is feeling the momentary muscle failure at the highest repetition being worked with. Progression strengthens the muscle and keeps the risk of injury lower. In some instances it may not be advisable to add resistance but to work at a certain resistance for a while. It is not advisable to add more than 10% resistance increases between successive exercise sessions. The key is progress slow and steady.

Frequency - Exercising the muscles causes micro-trauma to the muscle tissue which can cause a temporary reduction in strength output and cause muscle soreness. It is therefore very important to give the muscles plenty of recovery time between sessions to allow muscle tissue to rebuild itself. At least 48 hours of recovery time between sessions on the same muscle group is advisable. Recovery allows for the muscle to rebuild itself allowing for size and strength gains. Not allowing enough recovery can cause strength and size to increase much slower while increasing the risk of injury. Muscle tissue synthesizes protein and builds slightly higher gains during the recovery process This process happens over time and recovery is important in this process.

Improper form and technique - It is the tendency of most people to use too much resistance and cheat or use poor technique to lift the resistance. Improper technique not only decreases the effectiveness of training but can also cause injury. An example of poor technique would be bouncing the bar off the chest during a bench press or using the hip or back to start a barbell curl.

Warm up - Warm up is important to prepare the muscles for the work ahead. A proper warm up is the same warm up explained in the cardiovascular section. Which includes getting loose by walking, jogging or biking at a slow pace for approximately 5 minutes followed by stretching the muscles to be used during the training. A proper warm up will reduce the chance of muscle strains and pulls.

Cool Down - When ever an exercise session is suddenly stopped, blood tends to pool, or gather, in the muscles of the lower body which can cause light headiness. It is important to keep the muscles moving in a low intensity activity such as walking to keep pooling from occurring. Stretching is also recommended after a cool down. A proper cool down should last for at least 10 minutes after strength training. Cardiovascular complications can occur from sudden exercise stoppage.

Flexibility is an important part of any fitness program yet is often forgotten and ignored. Many people in a fitness program work their cardiovascular systems and muscles but forget about flexibility. This section will cover a definition of flexibility, The benefit of a flexibility program, principles of flexibility, flexibility guidelines, stretching properly, and the types of stretching.

“Flexibility is a joint’s ability to move freely in every direction, or more specifically, through a full and normal range of motion (ROM).”4

There are many benefits that a stretching program provide including the following:

Reduced Risk of Injury - Flexibility training reduces the resistance
about a joint as range of motion increases. This reduced resistance decreases the risk of injury

Gender - Evidence shows that women are more flexible than males. This is thought to be because women were created more flexible to accommodate child bearing especially in the pelvic region. The female hormone relaxin which relaxes ligaments and is present during pregnancy may also be a reason for more flexibility in women.

Age and Inactivity - As we age our flexibility decreases. Aging causes significant changes in the connective tissue. Flexibility is usually highest before the age of 12 and starts to decline as a person reaches their early 20’s. As we age the nutrients about a joint begin to decline. Activity can decrease this rate of deterioration as we age.

Strength Training - There is a common misconception that strength training causes a person to become less flexible as they gain muscle mass. If stretching is incorporated into a strength program flexibility can actually be increased. Flexibility is more dependant on the person and the activity they are involved in than muscle mass.

There are two main categories of stretching: Passive stretching and Active stretching. For the purposes of this manual we will only use the passive stretch

Passive Stretch - An example of a passive stretch is called the static stretch. The static stretch involves a slow, gradual, and controlled stretch which is held through a full range of motion. This involves holding the stretch for a certain length of time. The focus should be on stretching gradually until you feel a slight pull in the muscle. This should not involve any pain. This type of low intensity high duration stretch results in better flexibility in the muscle and connective tissue. A reason for static stretching is to prevent delayed muscle soreness which occurs at 24 - 48 hours after strenuous exercise and is thought to be caused by waste accumulation such as lactic acid and microtrauma to the muscle tissue.

4 Cyphers, Mari, “Flexibility”, Personal Trainer Manual, The Resource for Fitness Professionals. The American Council on Exercise, 1991.