The Fundamentals of Aerobic Exercise and Cardiorespiratory Conditioning: What Trainers Should Know

Aerobic Training

Aerobic exercise, an integral part of every personal trainer’s programming for apparently healthy individuals, can take on many forms. Knowing how much aerobic exercise to suggest (in addition to resistance training) depends in large part on each client’s current health status as well as their fitness goals.

Whether working with a fairly sedentary individual or a world-class athlete, trainers and clients together establish fitness goals upfront. Recommendations can vary from specific muscles to whole-body training; regardless, trainers strive to find movements that feature measurable aspects to ensure that the client progresses toward those goals. This article focuses on perhaps the most often-requested set of goals among training clients: general fitness.

Safety First, Safety Always

Conducting a screening for health concerns before embarking on any fitness activity remains fundamentally important. This definitely should come into play if your client demonstrates any cardiovascular risk factors, is over the age of 35, and/or currently leads a sedentary lifestyle.

According to the President’s Council on Physical Fitness, being physically fit means “having the energy and strength to perform daily activities vigorously and alertly, with energy left over to enjoy leisure activities or meet emergency demands.”  Heart, lungs, and muscles should remain healthy and strong, with weight and body fat falling within optimal ranges.

To determine a level of physical fitness, the NFPT suggests considering the following five measurable parts:

  1. Cardiorespiratory Conditioning
  2. Muscular Endurance
  3. Muscular Strength
  4. Flexibility
  5. Body Composition

Progression of Cardiorespiratory Conditioning

The NFPT promotes the following three stages of progression in the aerobic exercise or endurance phase of the exercise prescription:

  1. The Initial Conditioning Stage: During the first 4 to 6 weeks of engaging in an aerobic activity for the first time, we suggest 10-15 minute increments of low-intensity activity at 60-70% of maximum heart rate for the average healthy individual. This might include stretching and light calisthenics, such as abdominal work.
  2. The Improvement Conditioning Stage: Initially, this phase progresses to a slight increase in exercise intensity. Thereafter, the client can safely increase every 2 to 3 weeks. The ACSM warns that older individuals may take longer to adapt to increases in conditioning.
  3. The Maintenance Conditioning Stage: Typically, after 6 months of aerobic training, the average individual successfully achieves his goal of general fitness and wants to maintain this level. The ACSM states that just three 30-minute workouts a week, training at 60-70% of one’s maximum heart rate, proves sufficient.

Exercise Metabolism & Fuel Sources

Recall that glycogen gets converted to glucose, used for energy during average to high-intensity aerobic and resistance exercise, the bulk of which comes from the liver and bloodstream. With a proper pre-workout meal and sufficient glycogen stores in the liver, the body can provide anywhere from 30-400 calories’ worth of glycogen, for conversion to glucose and gradual release into the bloodstream during aerobic exercise.

Through our use of oxygen, liver glycogen/ fatty acids, circulating blood, and adipose tissue, the body provides the energy needed during aerobic activity. The contribution of the primary substrates (sources) of that energy however, may differ significantly depending upon the effort necessary to meet the activity’s imposed demands.

The higher the intensity (greater effort) of an activity, the more the body counts on stored carbohydrate sources for energy.

The lower the intensity (lesser effort) of an activity, the more the body depends on greater percentages of stored fat sources for energy.

It is important to understand that at any and every moment, energy is being provided from both primary energy sources and proteins. For our purposes, we will focus on the primary energy sources.

Primary Energy

When beginning any activity that elicits a rise above resting metabolism, the energy will first come from anaerobic (carbohydrate) metabolism, (as it takes several minutes to mobilize fat in sufficient quantities to sustain an activity, and therefore not eligible as an instant energy sources).

As the activity continues, the body “catches up” as oxygen uptake increases; this allows greater utilization of oxidized carbohydrates/ fats for energy. The transition from an anaerobically-dominant metabolism to an aerobically-dominated one is dependent on two main factors: an activity’s intensity and duration.

If an exercise calls for near-maximal effort, such as resistance training with weights, something that cannot be sustained for long periods, exercise metabolism will remain dependent primarily anaerobic. In doing so, it will rely upon carbohydrate sources without a need for a significantly increased oxygen uptake to perform the activity. We can define aerobic exercise as an activity of lower intensity, such as a brisk walk or a slow jog, sustained for a significant length of time. As the name implies, it allows for an increase in oxygen uptake and the release of fat stores necessary to make use of oxidized carbohydrates and fat as the primary energy sources.

Muscular Strength versus Muscular Endurance

Muscular endurance, the ability of muscles to perform contractions for extended periods, has evolved as a measure of general fitness. The number of curl-ups one can perform demonstrates abdominal endurance; likewise, advancing from a 5-second modified plank in good form to a 60-second plank reflects increased muscular endurance.

The muscular endurance phase of a general exercise program typically includes a circuit routine consisting of multiple compound exercises targeting different muscle groups. The program strives for the goal of affecting as much total muscle tissue as possible. To achieve this, experts suggest execution of 3 to 4 continuous circuits of one or more exercises in each category of the following movements:




Muscular strength focuses on how much weight one can hold in place (static strength) as well as how much weight one can move (dynamic strength).

The more balanced strength one develops moving heavier loads, the lower the chance of injury in everyday life.

Programming Aerobic Exercise for All Fitness Levels

General fitness clients may already actively engage in some form of fitness program. They may come to a personal trainer in search of guidance and direction in further improving their fitness levels. Others may have moved on from an introductory or fat reduction stage to a place where they may want to develop a higher level of fitness.

According to the American College of Sports Medicine, personal trainers can adhere to the following principles when developing an exercise prescription to enhance a client’s cardiorespiratory fitness:

  1. Type of Activity: The activity must use large muscle groups and last for some sustained period.
  2. Intensity: The average conditioning intensity for healthy adults approximates 60-70% of the functional capacity, referred to as maximum heart rate. Monitoring a target heart rate training zone during exercise can help maintain an appropriate intensity.
  3. Duration: The duration of the exercise will depend on the intensity of the exercise. Usually, activities of lower intensity, such as walking, can last longer than a high-intensity exercise like running. One can also accomplish aerobic fitness by alternating high and low-level activities, like walking between brief periods of jogging. The ACSM recommends 15-60 minutes of continuous or discontinuous aerobic activity.
  4. Conditioning Frequency: In order to make a difference in one’s health, the client needs to strive for an exercise frequency of 3 to 5 days a week.
  5. Rate of Progression: In the first 6-8 weeks of exercise, significant conditioning effects likely occur. The fitness professional may need to adjust the intensity and duration of the activity to keep seeing progress and avoid a training plateau.

Exercise intensities for a client interested in improving his or her fitness level tend to hover within the range of 70% to 85% of functional capacity. Intensity shifts according to a client’s current fitness level as well as his or her tolerance, based on the Rate of Perceived Exertion (RPE). For the general fitness client, this should range from 13, somewhat hard, to 17, very hard. Again, evaluations conducted in the beginning help to accommodate each client’s initial and individualized fitness level. Some trainers use sub-maximal pre-exercise testing to establish a baseline.

Progressing, or increasing, exercise intensities call for a corresponding decrease in the frequency of aerobic training. A general fitness client can engage in aerobic exercise 3-5 times per week. The client who seeks higher-intensity exercise sessions can taper the total sessions per week, to allow for adequate recovery between bouts. This will prevent burnout from overtraining and reduce the risk of injuries.

An inverse relationship exists between the duration of an exercise session and its intensity as well. Due to the stress applied to both anaerobic and aerobic systems during higher-intensity training sessions, maintaining the appropriate exercise intensity presents a challenge, due to the progressive depletion of carbohydrate sources. Depending on current fitness levels and applied exercise intensity, this level of exercise might seem too uncomfortable to continue for extended periods.

Typical aerobic training session prescriptions for general fitness clients last from 15 to 60 minutes. depending on current fitness levels, with the average duration falling between 20 and 30 minutes (not including warm-up). Please note that although a progressive increase in aerobic exercise intensity certainly comes with benefits, an eventual “burning sensation” in localized working muscles (sometimes attributed to localized lactic acid build-up) indicates an overemphasis on the body’s anaerobic systems are being overemphasized; too great of an exercise intensity leads to this sensation.


We define flexibility as the ability to move muscles and joints through their full range of motion.  How close one can come to touching his toes with legs straight remains the gold standard when assessing a client’s flexibility.

Having flexible muscles and joints will help prevent injury, and maintain mobility as general fitness clients age. Many older clients present with challenges of lacking flexibility in the posterior thigh and lower back; this increases the risk of chronic lower back pain.

Static and dynamic stretching can improve flexibility and mobility, the latter of which reflects the body’s ability to move the body through a range of motion activelyStatic stretching involves holding a sustained stretch for 30 seconds or more, without pain (taking a stretch too far can cause a rebound effect where the muscle contracts to protect itself). Dynamic stretching describes actively moving a joint through its full range of motion.

We can further break down stretching movements into active and dynamic stretches.

NFPT recommends an active and dynamic warm-up before embarking upon an extended aerobic activity. Once finished with the cardio, trainers typically suggest performing a dynamic warm-up routine that directly relates to the planned resistance exercises, such as unloaded squats before loaded squats.

By reserving static stretching for the final phase of the general fitness exercise program, clients can take advantage of warm/loosened muscles. Self-myofascial release techniques such as foam rolling can help prepare muscles for stretching by pumping blood into the tissue, moving metabolites out, and sending neuromuscular feedback to relax fascial restrictions. Trainers might remind clients to execute static stretching slowly. with gradual increases in the range of motion; overactive and shortened muscles in particular can benefit from such action.

Body Composition

Body composition describes the ratio of lean body mass to fat body mass. Experts divide composition measurements into two parts: fat and non-fat.

Generally speaking, when dieting without exercise and/or excessive caloric restriction, weight loss occurs as a result of losing lean body tissue, mostly water, and not loss of fat.  A balance of healthy eating and exercise is the most effective way to lose adipose tissue while developing/maintaining lean body mass.

Clients often confuse ‘thinness’ with ‘leanness’. Leanness reflects a healthy body fat range with adequate muscle to perform activity. Muscle is denser and heavier per equal measure than body fat (i.e. a spoonful of muscle weighs more than a spoonful of fat). Thus, the traditional scale seems misleading as it cannot differentiate between lean (muscle) and non-lean (fat) tissue. The number on the scale does not tell the whole story.

Assessing Body Composition

Various tools can calculate body fat based on total weight to determine lean body mass. Scales that use electrical impedance, skinfold calipers, or underwater weighing have varying accuracy. After the initial body composition measurements are conducted in the assessment phase of client programming, retesting every 1-2 months helps gauge progress. For women, fat should not exceed 25% of their body weight. For men, fat should not exceed 18% of their body weight.

In review, a general fitness client’s aerobic exercise prescription should include:

  • Moderate to high aerobic intensity (70 to 85%)
  • Moderate frequency of aerobic exercise (3 to 5 times per week)
  • Moderate duration of aerobic exercise (average 20-30 minutes per session)
  • Selection from among a variety of sports or exercise activities that are continuous.

The development of speed, agility, and advanced coordination will also enhance overall physical performance, contributing greatly to improved general fitness. Trainers should remind clients of the principle of specificity: do the exercise relative to the area you wish to improve. “If it doesn’t challenge you, it will not change you!”

Endurance Training course


1. American College of Sports Medicine. ACSM’s guidelines for exercise testing and prescription. Lippincott Williams & Wilkins, 2013.

2. Ekkekakis, Panteleimon, Eric E. Hall, and Steven J. Petruzzello. “Practical markers of the transition from aerobic to anaerobic metabolism during exercise: rationale and a case for affect-based exercise prescription.” Preventive medicine 38.2 (2004): 149-159.

3. The National Federation of Professional Trainers. Sports Nutrition Manual. 2nd Ed. Lafayette, IN: NFPT, 2006.


Cathleen Kronemer is an NFPT CEC writer and a member of the NFPT Certification Council Board. Cathleen is an AFAA-Certified Group Exercise Instructor, NSCA-Certified Personal Trainer, ACE-Certified Health Coach, former competitive bodybuilder and freelance writer. She is employed at the Jewish Community Center in St. Louis, MO. Cathleen has been involved in the fitness industry for over three decades. Feel free to contact her at [email protected]. She welcomes your feedback and your comments!