Borer, K. T. (1994). Neurohumoral mediation of exercise-induced growth. Medicine and Science in Exercise and Sports, 26(6), 741-754.

In the course of a life, control of growth undergoes remarkable changes. Growth factors vary in their degree of tissue specificity, as some, like the bone morphogenetic proteins, selectively promote growth in specific cells while others like IGF-1 can affect growth in more than one type of cell (growth in the fetus is evidenced by this). Over a life, the scope of growth changes from the primary guidance of body growth to a more restricted role in the maintenance, repair, and hypertrophy of musculoskeletal tissues.

The second stage of neurohumoral control is confined to the period of incremental growth (prenatal to early postnatal stages) that is characterized by dependence on nutrient abundance. Incremental growth is vulnerable to energy deficits. Rate of growth is dependent upon nutrient availability. In young organisms forced to exercise and denied abundance of food at this nutritionally-dependent stage of growth, nutrient energy is diverted from growth and the final body stature is stunted, the more so the more rapid the rate of growth.

In the third stage (postnatal) of neurohumoral control, the brain assumes a central role in growth facilitation. It stimulates the release of HGH, thyroid hormones, and sex steroid hormones in a fashion that favors their anabolic actions by channelling of the nutrient energy toward the biosynthetic processes of growth.

In the fourth and final stage of neurohumoral control of growth, from puberty through late adulthood, the brain has assumed an inhibitory control over growth so that little if any incremental growth occurs and anabolic hormonal actions are limited to maintenance and repair of lean body tissues. Nutrient intake per unit weight declines since energy no longer is needed for incremental growth. Nutrient intake should be matched to energy expenditure since the neurohumors assume energy-regulating metabolic roles.

At the cessation of growth and onset of energy regulation, the levels of spontaneous physical activity attain their life-time peak and become subject to hormonal and bioenergetic controls. Exercise-induced energy expenditure affects total energy balance and exerts important influence over the maintenance, repair, and functional hypertrophy of lean body mass in adulthood. Lifestyles that include an over abundance of food and limited physical activity appear to block full expression of the energy regulatory mechanism whose function is maintenance of adult lean body mass and regulation of the size of storage depots.

Thus, at the cellular level, incremental, and to a lesser extent reparative and hypertrophic growth, and biosynthetic events cannot occur simultaneously with energy requiring events such as exercise. Stimulation of growth by exercise has to be separated in time from the act of exercise.

Exercise intensity and expression of growth. Mechanical force is an important stimulus for cellular and tissue growth, for maintenance and structural remodeling of the skeleton, and for maintenance and functional hypertrophy of the muscles. Evidence that muscles not subjected to sufficient mechanical forces atrophy and that bones subjected to inactivity or to absence of gravitational forces demineralize is convincing.

The effects of mechanical force on the growth process, however, do not appear to be linear. Increased growth is seen within an optimal range of intensities of stimulation while mechanical forces that are outside of this range fail to stimulate or even suppress cell proliferation.

Temporal determinants of growth. The final complicating variable has to do with the importance of the timing of stimuli that control growth. Growth is a discontinuous, saltatory event occurring during the episodes of high nutrient abundance and anabolic neurohumoral stimulation, and of low energy need, interrupted by episodes of energy expenditure. A regular alternation between the energy intake and biosynthetic activities on one hand and the activities and processes requiring energy expenditure, on the other, manifests itself in the form of ultradian, circadian, or seasonal rhythms. There is a circadian rhythm in which periods of neurohumoral secretion favoring high levels of energy expenditure, spontaneous physical activity, and body temperature alternate with periods of reduced physical activity, energy needs, and a neurohumoral secretion favoring low energy expenditure. There are circadian rhythms of insulin, glucocorticoid, IGF-1, and insulin sensitivity. There are circadian rhythms of mitoses and of activities of biosynthetic enzymes in the epiphyseal growth zones regulating linear skeletal growth in diaphyseal bone influencing bone remodeling.

Within this circadian organization, are nestled neurohumoral, physiological, and behavioral ultradian rhythms, any of which individually and collectively may influence growth.

Specificity of exercise-induced growth. Submaximal endurance exercise stimulates pulsatile HGH secretion and promotes reparative growth of the lean body mass. It relies on nutrient abundance and draws on body lipids for energy. High-resistance exercise is a sufficient stimulus for hypertrophic growth of the muscle and for the mineralization and remodeling of bone. Such growth entails the expression of the growth factor and structural protein genes within the stimulated cells and is relatively independent of anabolic hormones and nutrient abundance.

The amplitude of HGH surge is proportional to the magnitude of physical effort. Submaximal exercise induces chronic endocrine adaptations that serve to facilitate oxidative fuel use. Increased thyroid hormone secretion and activity increases the efficiency of oxidative phosphorylation in the mitchondrion. The adaptations result in increased use of lipids for energy needs of exercise and result in a reduction of body fat depot. Endurance exercise appears to promote reparative or incremental growth of the lean body mass and maintenance of body fat stores at a level that is lower than in sedentary organisms.

Life-style. Introduction to either high-resistance or endurance exercise to a living routine counteracts the changes in body composition associated with aging humans living in developed societies. Endurance activity weight loss results in maintenance or small gains in lean body mass and is entirely composed of body fat reduction. The presence of endurance activity is an essential part of an adequate living pattern.

Incidentals

Menarche is a late maturational event that occurs, on average, a year or so after peak height velocity, and the same hormones are involved in the regulation of the growth spurt and sexual maturation.

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