Methodological Issues of Measuring Physical Activity and Physical Fitness When Evaluating the Role of Dietary Supplements for Physically Active People

Dietary supplements may be used by physically active people to increase their physical performance (physical fitness), improve their health, or reduce the potential negative consequences of physical activity (injury, suppressed immune function, etc.). To appropriately assess these effects, it is essential that reliable and accurate measures of physical activity, physical fitness, and health-related outcomes be used. All of these outcomes are complex entities consisting of a number of different characteristics or components that need to be differentially considered depending on the specific scientific/clinical questions being addressed. This presentation focuses on some of the key issues that need to be considered in the measurement of physical activity and physical fitness when the effects of dietary supplements in physically active people are assessed.

Physical Activity
Physical activity is a very complex and not easily measured set of behaviors. Numerous different approaches have been used to assess physical activity or change in activity in studies where health and performance status are the primary outcomes. Most frequently used are self-reported surveys, but other measures have included job classification, behavioral observation, motion sensors, and physiologic markers. The strength of the relation between physical activity and health or performance is highly dependent on the effectiveness of the measurements used. Of particular concern is the accuracy and reliability of the measurements of physical activity and their appropriateness for documenting the primary outcome of the study. The methods used for measuring various aspects of physical activity have become highly developed, but still need additional standardization for use with specific populations, especially in older persons, women, and ethnic minorities.

Whether one assesses the effects of dietary supplements on exercise performance, the effects of dietary supplements on the positive or negative health consequences of exercise, or the interaction of dietary supplementation and exercise on health or performance parameters, it is critical to carefully define and measure the exercise characteristic(s) of interest. These characteristics include the total amount or volume of exercise performed and the intensity, frequency, and duration of each bout or combined bouts.

Amount

When dietary supplementation is considered, the total amount of activity performed, expressed as the total amount of energy expended (kilo joules or kilo calories) is likely one of the more important characteristics to assess accurately. The requirements for certain dietary constituents may be related to a person’s total energy expenditure and thus closely tied to body size and the total amount of activity performed. There are increasing data indicating that some of the health benefits of exercise are related to total energy expenditure during exercise performed at moderate intensity or higher. Total activity or energy expenditure has been measured by questionnaires such a diary or recall and by doubly labeled water. Questionnaires can be used for large groups of subjects over extended periods of time but lack precision in estimating the energy expenditure for an individual. Doubly labeled water appears quite accurate and reliable for estimating total energy expenditure over days or weeks, but is limited in use by high isotope costs and expensive analysis equipment.

Intensity

Intensity of an activity can be described in both absolute and relative terms. In absolute terms, intensity usually is expressed as either the increase in energy expenditure required to perform the activity or the force produced by the muscle contraction. The intensity of endurance activity usually is expressed in units of oxygen or converted to a measure of heat (calories) or a measure of energy expenditure (joules). The force of the muscle contraction is usually measured by how much weight is being moved or the force exerted against an immovable object. In relative terms, the intensity of the activity is expressed in relation to the capacity of the person performing the activity. For energy expenditure, the intensity usually is expressed as a percent of the person’s aerobic power (percent of maximal oxygen uptake). Intensity is an important characteristic of exercise because certain performance changes are highly related to intensity of activity, including increases in aerobic power in response to endurance training and muscle strength in response to resistance training. Utilization of carbohydrate, fat, and protein as energy substrate during exercise is substantially influenced by the intensity of the exercise relative to the person’s capacity. Also, intensity of activity is a major factor contributing to overuse injuries. High-intensity exercise can be measured accurately and reliably by a variety of questionnaires, heart rate monitors, and for certain types of exercise by motion sensors or accelerometers. Doubly labeled water does not provide accurate information on the “intensity profile.”

Physical Fitness

To define more accurately the outcomes of physical activity programs for improving health rather than maintaining or enhancing physical or athletic performance, the concept of performance-related fitness versus health-related fitness has evolved. Although it has been proposed that there is a clear separation between the health- and performance-related components of physical fitness, this is clearly not always the case. For example, cardiorespiratory endurance and muscle strength are highly important components of both. In Figure 1, the contribution of each of the components to health- and performance-related fitness is qualitatively rated.

This figure is designed to show that most components of physical fitness contribute to both performance and health status. The magnitude of the contribution of any one component will depend on the specific objective. When the effects of dietary supplementation on “physical fitness” are evaluated or when an interaction between supplementation and fitness is being investigated, the various components to be included as dependent variables need to be considered. As discussed by other conference speakers, there is some evidence, or at least claims by numerous athletes and the general public, that nutrition supplementation enhances various components of fitness, especially muscle strength and endurance, muscle power, and aerobic power.

Cardiorespiratory Endurance

The “gold standard” or criterion measure of cardiorespiratory fitness is maximal oxygen uptake or aerobic power (VO₂ max). Measured in healthy persons during large muscle, dynamic activity such as walking, running, or cycling, it is primarily limited by the oxygen transport capacity of the cardiovascular system. The most accurate assessment of VO₂ max is determined by measuring expired air composition and respiratory volume during maximal exertion. This procedure requires relatively expensive equipment, highly trained technicians, and time and cooperation from the subject, all of which make it difficult to use in large-scale studies.

Because the interindividual variation in mechanical and metabolic efficiency is quite low in adults during activities that do not require much skill. Such as walking or running on a motor-driven treadmill or cycling on a stationary ergometer, oxygen uptake can be quite accurately estimated from the rate of work (speed, grade, and resistance). Thus, VO₂ max can be estimated from the peak exercise intensity during a maximal exercise test without measuring respiratory gases. This procedure requires the use of an accurately calibrated exercise device, careful adherence to a specific protocol, and good cooperation by the subject.

Having a subject perform any maximal test to assess cardiorespiratory fitness carries a substantial burden for both the subject and examiner. The burden for the To reduce this burden, various submaximal exercise testing protocols have been developed and used in numerous observational and intervention studies for evaluating the relationship of physical activity, cardiovascular fitness, and cardiovascular health. In most protocols, the estimate of cardiovascular fitness is made from the heart rate response to a set workrate or workloads and data from the submaximal response are used to extrapolate to a predicted VO₂ max.

Another approach to assessing cardiorespiratory fitness has been the use of field testing, where subjects usually perform a walk, jog, or run of a specified time or distance and their performance is converted to an estimate of VO₂ max or aerobic power . In many cases, these tests require maximal or near-maximal effort by the subject and thus have not been used for older persons or those at increased risk for cardiovascular disease. The advantage is that large numbers of subjects can be tested rapidly at low cost, but to obtain an accurate evaluation, subjects must be willing to exert themselves and know how to set a proper pace.

Muscle Endurance

Muscle endurance is specific to each muscle group, whereas cardiorespiratory endurance is general (i.e., not specific to any muscle group). Few tests for use in the general population are pure measures of muscle endurance, as most tests of muscular endurance are also tests of muscle strength. Tests of muscular endurance and strength include situps, pushups, bent arm hang, and pullups. These tests need to be properly administered and may not discriminate well in some populations (e.g., pullups are not good for many populations because a percentage of those tested will have 0 scores). Few laboratory tests of muscle endurance have been developed. Such tests usually involve having the subject perform a series of contractions at a set percentage of maximal strength and at a constant rate until the person can no longer continue at that rate. The total work performed or the test duration is used as a measure of muscle endurance.

Muscle Strength

Muscle strength can be measured during performance of either static or dynamic muscle contraction. Like muscle endurance, strength is very specific to the muscle group, so the testing of one group (e.g., using hand grip) does not provide accurate information about the strength of other muscle groups. Thus, to be effective, strength testing must involve at least several major muscle groups, including the upper body, trunk, and lower body. Standard tests have included the bench press, leg extension, and biceps curl using free weights. The heaviest weight a person can lift only one time through the full range of motion is considered the person’s maximum strength.

Flexibility

Flexibility is a difficult component to measure accurately and reliably because it is specific to the joint being tested; no one measure provides a satisfactory index of an individual’s overall flexibility. Field testing of flexibility frequently has been limited to the sit-and-reach test, which is considered to be a measure of lower back and hamstring flexibility. Other tests have been used to determine the flexibility of the shoulder, hip, knee, and ankle. The criterion method for measuring flexibility in the laboratory is goniometry, which is used to measure the angle of the joint at both extremes in the range of motion.

Balance, Agility, and Coordination

There are no generally accepted standard techniques for measuring balance, agility, and coordination, especially in older persons. Field methods have included various “balance stands” (one foot stand with eyes open, and with eyes closed; standing on a narrow block, etc.) and “balance walks” on a narrow line or rail. In the laboratory, computer-based technology is now being used to evaluate balance measured on an electronic force platform or by analysis of a videotape of the subject walking. Agility or coordination is measured most frequently using a field test such as an agility walk or run, and while in the laboratory, coordination or reaction/movement time is determined using electronic signaling and timing devices. More test development is needed to establish norms using standardized tests for the measurement of balance, agility, and coordination of older persons.

Body Composition

In most population-based studies that have provided information on the relation between physical activity and morbidity or mortality, body composition has been estimated by measuring body height and weight and calculating body mass index (weight/height2). The preferred method for determining fat and lean body mass in exercise training studies has been hydrostatic or underwater weighing. This method has been considered the criterion for estimating fat and lean body mass in clinical studies, but it lacks accuracy in some populations, including older persons. Various anthropometric measurements (i.e., girths, diameters, and skinfolds) have been used to calculate percent body fat with varying degrees of accuracy and reliability. Usually the measurements and prediction equations are age and sex specific and the equations need to be quadratic rather than linear to reduce individual estimation errors. Data now exist demonstrating that the distribution of body fat, especially accumulation in the abdominal area, as well as total body fat is a significant risk factor for cardiovascular disease and diabetes. The magnitude of this abdominal or central obesity has been determined by the waist-to-hip circumference ratio or by new electronic methods that can image regional fat tissue. New technologies that have been used to determine body composition include total body electrical conductivity; bioelectrical impedance; magnetic resonance imaging, which has the potential for assessment of regional body composition; and duel photon absorptiometry. None of these new procedures has yet produced data that have influenced our understanding of the relation between physical activity and morbidity or mortality, but the procedures have substantial potential to provide new information on how changes in physical activity along with the use of dietary supplementation effects body composition.

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