Muscle Physiology in Athletes

Abstract

Skeletal muscle exhibits plasticity in response to physical training. The functional consequences of these adaptations are determined by training volume, intensity and frequency. One could oppose the practice of endurance sports to sports involving strength and speed. Endurance exercise leads to physiological and biochemical adaptations in skeletal muscle which sustain aerobic metabolism capacity such as mitochondrial biogenesis, angiogenesis, and fiber type transformation. Strength training stimulates synthesis of contractile proteins that are responsible for muscle hypertrophy and increased maximal contractile force output. The increase in muscle mass observed in response to strength training is related to hypertrophy of cellular components, with an increase in their number referred to as hyperplasia. These adaptive changes are responsible for the improvement of physical performance. This review focuses on the mechanisms involved in these adaptations. Modifications of muscle typology under the effect of training result from three main factors: nerve stimulation, mechanical stress resulting from the type of physical activity, and the metabolic response to effort. Beside these main factors of muscle adaptation, hormonal response and nutrition can modulate their expression. Recent findings have revealed some of the mechanisms of various signal transduction pathways and gene expression programs in exercise-induced skeletal muscle adaptations. It is now possible to study the effects of various training interventions on a variety of signaling proteins and early-response genes in skeletal muscle. A practical question is whether it is possible to relate muscle structural and functional capacities to performance. Physiological and possibly pathological structural modifications are appreciated through the various imaging techniques, such as dual energy X-ray absorptiometry (DEXA), radiography and computed tomography, magnetic resonance imaging and ultrasound, all of which have been applied to the study of how changes in muscle mass are effected by training. Measuring cross-sectional surface area by means of ultrasound helps evaluate with precision the increase in segmental cross-sectional surface area. A non-invasive measurement of the effects of training on muscle typology can be realized using magnetic resonance spectrometry, 31P-MRS.