Physical Activity as a Metabolic Stressor

Overview

It is becoming increasingly clear that a person’s health and well-being are improved by physical activity as well as by a nutritious diet. Both physical activity and diet stimulate processes that, over time, alter the morphological composition and biochemical function of the body. Physical activity and diet are interrelated in that optimal adaptation to the stress of exercise-training usually requires a diet that is not lacking in various nutrients. Physical activity should therefore be viewed as providing stimuli that stress various systems of the body to differing degrees depending upon the type and intensity of exercise. Furthermore, the progressive adaptations to regular physical activity are very specific in response to the stress encountered during physical activity.

Definitions

Physical activity is defined as “any bodily movement produced by the contraction of skeletal muscle.” 1
Exercise, a type of physical activity, is defined as “a planned, structured, and repetitive bodily movement done to improve or maintain one or more components of physical fitness.”‘
Physical fitness is defined as “a set of attributes that people have or acquire that relates to their ability to perform physical activity.””
Dietary supplementsin the United States are usually defined as comprising plant extracts, enzymes, vitamins, minerals, and hormonal products that are available without prescription and are consumed in addition to the regular diet. In the present context, the addition of macronutrients and water to the diet can also be viewed as supplements.

Stress of Physical Activity
As shown in Figure 1, physical activity increases metabolism in the active muscles and other cells, and in the process of muscle contraction, produces mechanical loading. These processes of altered metabolism and mechanical loading are key physiological stimuli that serve to alter protein synthesis and degradation. This forms the basic scheme by which the body produces specific adaptations to exercise training and thus develops physical fitness. Substrates, obtained from the diet or endogenous production, supply the energy for metabolism and work and also provide the amino acids for protein synthesis. These processes are typically regulated by enzymatic activity requiring certain co-factors. Furthermore, all the steps shown are influenced by humoral factors as well as other effects.

FIGURE 1.

The stress of physical activity is typically categorized in two ways: (1) by the metabolic aerobic stress it places on the cardiovascular system when exercise is performed for several minutes or longs (Table 1) and (2) by the percentage of the individual’s one-repetition maximum (1RM) for physical activity involving lifting weights for short periods of time (Table 2). Weightlifting is largely power by anaerobic metabolism.

TABLE 1. Classification of Intensity of Aerobic Exercise¹

Relative Intensity			Absolute Intensity in METS^a
Classification	% Max Heart Rate	%Max VO₂ or % Heart Rate Reserve	RPE^b	Young 20-29 years	Old 60-75 years

Very Low	<30	<25	<10	<3.0	<2.0
Low	30-49	25-39	10-11	3.0- 4.7	2.0- 3.1
Moderate	50-69	40-59	12-13	4.8- 7.1	3.2- 4.7
High	70-89	60-85	14-15	7.20- 10.1	4.8- 6.7
Very High	> or = 90	> or = 85	> or = 16	> or = 10.2	> or = 6.8
Maximum^c	100	100	20	12.0	8.0

^aAbsolute intensity, measured in metabolic equivalent units (METS), is an appropriate mean value for men. Mean values for women are approxiamately 1-2 METS lower than those for men. ^bBorg Rating of Perceived Exertion-original 7-20 scale. ^cMaximum values are mean values during maximal exsercises for healthy adults.

TABLE 2. Classification of Intensity of Weightlifting¹

Intensity	Percentage of 1RM	Estimated Number of Repititions Possible^a

Very Heavy	95-100	1-2
Heavy	90-95	2-6
Moderately Heavy	85-90	3-8
Moderate	80-85	5-10
Moderately Light	75-80	7-12
Light	70-75	10-15
Very Light	65-70	15

^aThe estimated number of repititions possible will depend upon the person’s strength and endurance ratio.

Examples of the Stress of Various Types of Physical Activity and the Specific Adaptations

Low-intensity aerobic exercise such as walking (e.g., 25-40 percent VO₂max) increases metabolism several fold above basal levels, with relatively large increases in free fatty acid mobilization and oxidation. ^3. The humoral responses are small but physiologically significant, and carbohydrate metabolism is stimulated slightly. Although the cardiovascular stress is mild, it may be sufficient to stimulate acute and chronic adaptations.

Aerobic exercise performed at moderate to high intensity (50-90 percent VO₂max) for 30-60 minutes stimulates a large increase in carbohydrate metabolism and oxidation with a continued stimulation of fat metabolism.
³ The humoral responses are large and characterized by increases in catecholamines and a reduction in insulin. After I week or more of chronic exercise training, the exercising musculature experiences sufficient mitochondrial biogenesis to increase oxidation of fat and carbohydrate oxidation with a reduced disturbance of cellular homeostasis.^4,5 Cardiovascular adaptations include an increase in plasma volume and stroke volume during exercise.

Moderate to heavy weightlifting stimulates neuromuscular recruitment to very high levels. A sufficient number of repetitions produces brief alterations in metabolic homeostasis within muscle and a postcontraction hyperemia. Chronic activity promotes muscle fiber hypertrophy, possibly from the stretch-overload activation of a promoter of protein synthesis (-actin),⁶ which requires certain humoral agents.⁷

References

Haskell WL. Physical activity, lifestyle, and health in America [abstract]. NIH consensus development conference: physical activity and cardiovascular health. December 18-20, 1995.
Wathen D. Load assignment. In: Baechle T. ed. Essentials of strength training and conditioning. Human Kinetics, 1994.
Coyle EF. Substrate utilization during exercise in active people. Am J Clin Nutr 1995; 61(suppl):968S-79S.
Hollosy JO, Coyle EF. Adaptations of skeletal muscle to endurance exercise and their metabolic consequences. J Appl Physiol 1984;56:831-8.
Chesley A, Heigenhauser GJF, Spriet LL. Regulation of muscle glycogen phosphorylase activity following short-term endurance training. Am J Physiol 1996;270:E328-35.
Carson JA, Yan Z. Booth FW, Coleman ME, Schwartz RJ, Stump CS. Regulation of skeletal a-action promoter in young chickens during hypertrophy caused by stretch overload. Am J Physiol 1995;268:C918-24.
Fluckey JD, Vary TC, Jefferson LS, Farrell PA. Augmented insulin action on rates of protein synthesis after resistance exercise in rats. Am J Physiol 1996;270:E313-9.