Neural Contributions to Muscle Growth

The following is an exclusive excerpt from the book Strength Training, Second Edition, published by Human Kinetics. All text and images provided by Human Kinetics.

Neural adaptations help mediate growth-related processes. The early increase in strength is attributable mainly to improved neural function, primarily from the eccentric component of the resistance. (This again points to the importance of concentric−eccentric repetition with training.) Greater strength in turn increases the quality of the training stimulus, which also contributes to a greater anabolic stimulus for muscle hypertrophy.

A dramatic increase in strength occurs during the first few weeks of a strength training program. However, intact muscle and muscle fiber size change very little during this initial phase. The observed changes take place in the quality (e.g., increased packing density of the myofibrils) and type of protein (i.e., change in the myosin isoform X to A) in the growth of the muscle fibers. In general, resistance training stimulates a transition of Type II fibers: The fiber profile of the muscle shifts from Type IIX toward Type IIA. This is part of the adaptation of the muscle to a more metabolically active fiber (Staron et al. 1994). However, it is doubtful that muscle fibers transform from Type II to Type I, or vice versa, under normal training conditions. Thus, changes in the quality of muscle proteins occur with shifts in the myosin heavy chains toward the Type IIA fiber. Enzyme changes also occur that help mediate growth-related metabolism. Many of these changes start very quickly within the first week of training. Thus, along with neural changes, various aspects of the protein metabolism in the muscle are changing in the early phases of training.

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