The relationship between the amounts of oxygen consumed and work performed is predictable, and since the body has limited capacity for storing oxygen, the rate of oxygen uptake at the lung represents rate of cellular oxygen consumption. Oxygen uptake (VO₂) is proportional to the external work of physical activities. Maximal oxygen uptake is defined as the maximal amount of oxygen distributed and used by the body while performing heavy exercise. A high VO₂ requires the respiratory system to exchange oxygen and carbon dioxide within blood; the cardiovascular system to pump and distribute oxygen-laden blood (hemoglobin) to active skeletal muscle; and the ability of the skeletal muscle to convert stored substrates to power muscular contraction. Maximal oxygen uptake is abbreviated VO₂ max; where V represents the volume of oxygen per minute, O₂ represents oxygen, and max represents maximal amount or conditions. Thus, VO₂ max is the maximal volume of oxygen used by the body per minute [1–8]. This value is usually expressed in absolute terms as liters per minute (L min⁻¹) or in relative terms as milliliters per kilogram of body weight per minute (mL/kg min⁻¹). VO₂ max is considered the gold standard measure of maximal aerobic capacity or aerobic power. Because VO₂ max is a measure of energy transfer, it is also considered a measure of aerobic power rather than just volume or capacity [1–8]. The by-product of aerobic power production is carbon dioxide (CO₂), and the volume of CO₂ eliminated per minute from the body is the VCO₂. This variable is important and provides information regarding cardiopulmonary efficiency especially at the onset of nonaerobic or “anaerobic” metabolism (anaerobic glycolysis or lactic acid system) [1–8]. Except for steady state exercise, there is an additional variable of total metabolism from anaerobic processes. Indeed, aerobic metabolism persists in longer duration physical events; however, it is important to recognize that aerobic metabolism is only one component of metabolic rate during maximal exercise. At maximal exercise, metabolism (rate of energy transfer) is the sum of maximal aerobic power (VO₂ max) and maximal anaerobic power. Some commonly used calculations to estimate VO₂ are listed in Table 12.3. However, one should be cautious when using normative calculations in the pediatric population; especially for maximal heart rate calculations [17, 18]. Children are born with a set number of muscle cells, but development of strength which is characterized by increased diameter of muscle fibers (cells) is dependent upon changes into puberty. The emergence into Tanner 4–5 stages can be variable and use of age related calculations can be problematic.

Historically, the measurement of VO₂ max can be a costly and time consuming operation confined to research laboratories. With the development of rapidly responding gas analyzers and volume measuring devices, along with advances in computer technology, this measurement is now accessible to scientists, clinicians, and fitness personnel. In fact there are now portable and even hand held VO₂ testing devices available on the market [19–22]. When work rate increases on an ergonometric device (e.g., speed and grade on a treadmill or cycle ergometer) or during any physical activity, so does VO₂ as long as the subject is not limited or deficient in any of the aforementioned physiological systems (i.e., cardiovascular, respiratory, hematologic, and musculoskeletal system). When approaching VO₂ max work rate will usually peak or plateau. After the plateau, work rate may continue to increase with no further increase in VO₂ (Fig. 12.2). Therefore, VO₂ max represents the physiological limit of aerobic power. Although work rate may continue to increase following a plateau of VO₂, this increase may be a product of other energy systems such as anaerobic power or metabolism [1–8]. Nonetheless, sustained work output will soon subside as muscular fatigue sets in. The magnitude increase in work rate beyond plateau does not affect the measurement of VO₂ max, but instead can be used as an index anaerobic power as well as psychological motivation and persistence (Fig. 12.3) [1–8]. Table 12.4 lists important variables obtained during a CPET.

In the classic definition, VO₂ max was presumed to occur when there was less than a 2.1 mL/kg min⁻¹ increase in VO₂ with an increase in treadmill grade of 2.5 % at speed of 7 mph. Thus, there would be an increase of less than 0.6 METs (metabolic equivalent of task) or about 25 % of the expected increase of 2.5 METS [1–5]. A metabolic equivalent is a physiological measure of the energy cost of physical activity defined as the ratio of metabolic rate (rate of energy consumption) during a specific physical activity to a reference metabolic rate, set by convention to 3.5 ml O₂/kg min⁻¹.

Other criteria for achievement of VO₂ max in well-motivated subjects include (1) a respiratory exchange ratio (RER) greater than 1.0 and (2) a blood lactate concentration greater than 8 mmol L⁻¹ (Table 12.5, [1–3]). In several studies involving both younger and older subjects, less than 50 % of these individuals could reach a plateau in VO₂ at maximal work rates. Åstrand was the first to document that many children and adolescents complete a progressive exercise test to exhaustion without a plateau in VO₂. Subsequent studies have confirmed only a minority of young people exhibit a classical VO₂ plateau [23].

The purpose of exercise testing is to assess cardiopulmonary, metabolic musculoskeletal power, and motivation/effort during physical stress. Physical stress magnifies the pathophysiology and physical limits, thus allowing the clinician to diagnose and develop interventions which are sport or activity specific. By considering time dimensions, the faster the heart and respiratory rate, the more samples available for analysis. The concept that a classical laboratory based CPET is needed is not necessarily true as exercise intolerance should also be addressed to include the biomechanics which trigger the event. The core of this evaluation is to determine functional capacity of the cardiopulmonary system and power of metabolic-muscular activity while reproducing symptoms or signs of the complaint. This process and the adaptations that occur with training or any exercise are listed in Table 12.6. The goal of laboratory based exercise testing is to address four key issues; (1) cardiovascular limitations, (2) ventilatory limitation, (3) metabolic or musculoskeletal limitation (biomechanics of activity) and (4) motivation or effort limitations or perceptions. The psychological aspects and expectations associated with exercise by the patient, and parents or coaches, can be a major contributor to the complaint [26].

Infants and pre-school children are active in various ways and the activity intensity is dependent upon their neurodevelopmental age. Simple evaluation of child’s neurodevelopment may be a valuable starting point for infants and toddlers and ultimately determines in many cases the activity capacity, power needs, and ability for future participation at any level. Physical evaluation can identify issues related to musculoskeletal abnormalities and dysplasia which in exercise-activity evaluation involves either (1) muscle strength (hypo or hypertonia), (2) skeletal development of the limbs especially including skeletal dysplasia of the chest wall, extremities and/or spine and (3) cardiopulmonary performance. These three areas of exercise physiology are determined by muscular strength, endurance, power and leverage, and ability to provide cardiopulmonary aerobic and anaerobic output to meet power requirements. In reality, the efficiency of performance is the result of metabolic-mitochondrial biochemistry where energy is produced to meet power needs. The first objective data to be gathered is the cardiopulmonary exam. When examining the infant or toddler it is best to think in terms of the following physiologic concepts; (1) neurodevelopmental level of ability and agility, (2) chronotropy, (3) inotropy, (4) strength, (5) anatomy/anomaly, (6) chest wall kinetics, and (7) breathe sounds-airflow (gas exchange). While this may seem to be obvious, the evaluation often times is under-appreciated and key predictors of present and possibly future activity tolerance are missed. When a parent complains that their child cannot keep-up, the provider can use these concepts to follow and discuss the issue in an organized fashion. This also provides a base for longitudinal evaluations regarding activity/exercise tolerance.

Chronotropy is not only about the heart rate appropriate for age and activity level, but is the heart rate variability that is expected to meet the changing metabolic demand at all levels of activity [27–34]. The autonomic nervous system function can be assessed in clinical settings by measuring resting heart rate (HR), heart rate variability (HRV), or heart rate recovery-reserve (HRR) following exercise. During the last decade, heart rate variability (HRV), that is, a marker of parasympathetic heart rate (HR) modulation has also been used in exercise physiology to evaluate the fitness level and the physiological responses to physical activity. Regulating cardiovascular function to satisfy the metabolic demand of working muscles, the autonomic nervous system has been investigated during recovery after physical exercises performed at different intensities. Low heart rate variability, especially during sleep suggests lack of autonomic input or responsiveness resulting in decreased cardiac output and places burden for activity on inotropic response [35]. Chronotropic and cardiopulmonary inefficiency in general may be elucidated in part by a sleeping overnight oximetry study. Some information can also be derived from holter monitors. Overnight oximetry can identify periods of poorly compensated hypoxemia and or excessively high or low heart rate variability. Overnight oximetry may also provide information about autonomic modulation as periods of sleep require activation of the sympathetic and parasympathetic systems. Inefficiencies in this output especially in the face of transient hypoxemia-desaturations can demonstrate chronotropic demand problems. When desaturations occur, the auto-resuscitation response should consist of increase heart rate and minute ventilation. In addition, there should be an arousal response to hypoxemia-desaturation. Absence of any these responses is a sentinel for possible abnormal or immature autonomic loop-gain processes and these mechanisms are important for development of activity tolerance. Increased oxygen consumption associated with activity requires modulation of heart rate and changes in vascular resistances. Failure of these mechanisms results in decreased perfusion to areas most involved in the activity and decreased functional capacity. While seemingly simple, overnight oximetry and especially polysomnography is a physiologic window into the capacity for activity-exercise as sleep is not a period of energy storage or biophysical restoration, but a period of reorganization and growth that requires energy consumption and strategic delivery by changing vascular flows to specific areas of the body [34]. These simple techniques are easily performed (especially overnight-sleeping oximetry) but are not well understood and interpreted by clinicians. The difficulty in interpretation is really made more challenging by the quality of the recording and this is very dependent on the type of pulse oximeter used. The signal-to-noise ratio and signal variance and toleration of movement are important to consider in regards to obtaining an optimal signal for study. This is especially true of probe and placement on the body. For example, placement of digits may not be the best choice and often times the headband probe delivers a more consistent signal. In summary, there are no gold standard assessments of activity intolerance in infants and toddlers with the exception of observation, examination, and vital signs. There are pulmonary functions that can provide some information that are discussed in other chapters of this text book. Measurement of HRV by overnight oximetry and polysomnography may be of some value but there are no studies to provide any evidence based recommendations. The most important part of this evaluation is to potentially identify those children with the potential for activity intolerance and at risk for long-term sedentary behavior. This allows the clinician to develop a long-term strategy for evaluation and intervention which ultimately can impact the future health of these patients.

School-age children become much more “in tune” with their exercise or activity based symptoms and can give some age level description of potential problems. One technique that can be used in 5–6 year olds is to have them draw or select an illustration or picture of what happens when they participate in exercise or activity [36]. We use an I-pad or other tablet with a standard stylus when possible, but paper and pen work as well (Fig. 12.6). Use of an I-pad or tablet device allows us to annotate on the picture and makes it easy to import pictures into a report. Child participation allows the clinician to begin probing into more specifics and may also trigger responses from parent(s) which can add to or create confusion regarding the true issue. Well meaning parents can present their biases into the symptoms and signs which can often lead the clinician to a conclusion that the parent is in part facilitating the complaint or may have push levels of activity and skill (training) upon the child which is not age appropriate. The opposite may also occur where parents limit the child’s activity for fear of inducing or triggering an event or the assumption that the child cannot be active. In a way, the parent may be indirectly participating in the limitation or they may just be uninformed or have been given incorrect information by others. Pushing children and adolescents to unachievable levels of physical activity occurs at all ages and is the most common cause of exercise intolerance and avoidance of exercise in pediatrics [37]. Sports specific specialization with development of skill really occurs during teenage years. The amount of practice in early age groups does not necessarily increase the probability a child will become a performance level athlete in a particular sport. Furthermore, teens that participate later in sport specialization can make up for less practice time and experience [38]. Unfortunately, the clinician cannot often openly express their concern that the patient-child is being asked to participate at a level far above (overtraining) or below their physical age. This conclusion requires clinicians to investigate all physical causes first which may require some objective testing (CPET or ACPET) which can show parents that biomechanics and physiology of exercise at their child’s age level are very intact and the child is capable of engaging in some form of physical activity. The opposite may occur where pathology is demonstrated as previously reviewed. This also allows clinicians to develop a prescription plan which involves parental participation. This objectified evaluation and prescription is the core principle which allows us to promote healthy development in our children.

The initial evaluation of a child or adolescent and/or caregiver complaining of physical activity intolerance is to define the characteristics of the observed or perceived events. This physical activity history assists in the study design and directs a purpose for the evaluation. The indications for CPET or other types of field exercise capacity testing (ACPET) are listed in Tables 12.7 and 12.8. Once again, the ultimate goal is to provide the patient with an individualized exercise evaluation, plan, medical intervention, psychological counseling, and educational training to counter-balance their limitations and/or pathology. Secondary goals are to assist in longitudinal planning for interventions by assessing functional capacity at intervals to provide information about exercise training or disease progression. Longitudinal exercise evaluations are often used to (1) assess the effectiveness of exercise training programs, (2) plan surgical interventions (transplantation-lung volume reduction-bariatric surgery), (3) assist in evidence based interventions for chronic diseases, (4) evaluation of occupational injury and (5) for research development of new therapies or training modalities. CPET can be used to reassure the patient and/or parent that the biomechanics and cardiopulmonary systems are functioning well or demonstrate pathophysiology. CPET is also used to assist the patient and family toward developing healthy attitudes about the quantity and quality of the patient’s physical activity/exercise program (i.e., frequency, intensity, time, and type (Table 12.9). Reassuring the parents and patients that the physiological systems are working efficiently will allows the clinician to investigate deeper into the physical activity/sport execution-performance, attitudes of physical activity, and sport-specific movements and expectations. Addressing these issues can be challenging and our laboratory asks the will often ask parents to not be present during the study, CPET or other challenges are performed. Video recording the study can be helpful allowing the laboratory technicians and clinicians to review the events (or the absence of events) with parents and concerned coaches and trainers. We also ask parents and observers to video events they observe in real time at sporting events or during physical activity events. The team trainer and or physician should also be asked to record their observations. Video is a valuable tool and in some circumstances, we provide relatively inexpensive video cameras for home if smart phones are not available. The videographer is asked to share the files by uploading the movie to a secure site or we download the device in the office or lab. In some difficult high profile cases or circumstances clinicians should try if possible to attend the sporting event to make observations and perhaps plan interventions. Coaches are usually very willing to grant access to sidelines and welcome input. This is especially true in performance athletes whom are candidates for scholarships, participate in professional or collegiate athletics or are Olympians. Most training sessions and events (competitions) at this level are recorded and with permission the videographers that film these events are happy to provide footage of the observations. It is important the exercise team obtains written consent from the participant, parent or guardian, coaches, and institution. Patient confidentiality and consent must be followed as the ramifications of disclosure of this information can have significant monetary and psychosocial consequences. One of the most important aspects when dealing with any competitive athlete is to be positive when addressing the issues. Instilling confidence that the exercise limitation is solvable or self-limiting is extremely important for the success of any intervention. Performance athletes are often suspicious and susceptible to negative thoughts or demeanor. Negativity will not be received well by the patient and definitely not by good coaches. Persistence of negativity by the coaching staff and/or parents may in itself be a cause of perceived exercise limitation from overtraining or sports related anxiety. Always ask the coaches, parents or guardians, and patient about recent head injury or symptoms of sports related concussion regardless of the sport. While these recommendations may seem excessive, the purpose for collecting data in this manner is to be thorough, to instill confidence in the patient and family and be positive that exercise is possible in anyone with the right modifications.