Neural Adaptations to Resistance Training Implications for Movement Control

It has long been believed that resistance training is accompanied by changes within the nervous system that play an important role in the development of strength. Many elements of the nervous system exhibit the potential for adaptation in response to resistance training, including supraspinal centres, descending neural tracts, spinal circuitry and the motor end plate connections between motoneurons and muscle fibres. Yet the specific sites of adaptation along the neuraxis have seldom been identified experimentally, and much of the evidence for neural adaptations following resistance training remains indirect. As a consequence of this current lack of knowledge, there exists uncertainty regarding the manner in which resistance training impacts upon the control and execution of functional movements. We aim to demonstrate that resistance training is likely to cause adaptations to many neural elements that are involved in the control of movement, and is therefore likely to affect movement execution during a wide range of tasks. We review a small number of experiments that provide evidence that resistance training affects the way in which muscles that have been engaged during training are recruited during related movement tasks. The concepts addressed in this article represent an important new approach to research on the effects of resistance training. They are also of considerable practical importance, since most individuals perform resistance training in the expectation that it will enhance their performance in related functional tasks. CURRENT OPINION Sports Med 2001; 31 (12): 829-840 0112-1642/01/0012-0829/$22.00/0 © Adis International Limited. All rights reserved. Muscular strength is fundamental to the successful and efficient performance of many tasks that are encountered in daily living. Strength is defined as the capacity to exert force under a particular set of biomechanical conditions. The amount of force that an isolated muscle can exert is influenced by factors such as: the number and size of muscle fibres, the orientation of fibres with respect to the line of muscle action, and the proportion of myosin heavy and light chain isoforms that are expressed within the muscle fibres. [1] However, in natural tasks individual muscles are seldom required to generate force in isolation. Rather, most movements arise from the cooperation of a number of muscles acting together as functional synergists. The amount of force that can be generated in a particular movement context is therefore determined not only by intramuscular factors, but also by the effectiveness of muscular coordination. It is well documented in the literature that resistance training can lead to increases in muscular strength. There is a compelling body of evidence that many of the physiological adaptations that underlie increments in strength occur within the muscles themselves. [1-3] It has long been believed that resistance training is also accompanied by changes within the nervous system, that play an important role in the development of strength. [4,5-7] However, the specific sites of adaptation along the neuraxis have seldom been identified experimentally, and much of the evidence for neural adaptations following resistance training remains indirect. For example, in a recent review, Enoka[5] identified phenomena such as strength changes in the absence of substantial muscular adaptations, strength changes in the limb contralateral to the trained muscles (cross education), and specificity of strength adaptations to the training movements, as the strongest evidence that resistance training is accompanied by neural adaptations. While it is clear that intramuscular adaptations induced during a programme of resistance training lead to strength increases by increasing the forcegenerating capacity of individual muscles, it is likely that neural adaptations also contribute to strength increments by enhancing the effectiveness of muscular coordination. [8-10] That is, some of the adaptations associated with resistance training may be regarded as motor learning, in so much as individuals learn to produce the specific patterns of muscle recruitment that are associated with optimal performance of the training tasks. At present, it is difficult to predict the impact that the physiological adaptations that underlie motor learning during resistance training will have upon the control and execution of functional tasks. However, in the present article, we aim to demonstrate that resistance training has the potential to alter the manner in which muscles that have been recruited during training are controlled by the central nervous system (CNS). We review a small number of experiments that provide evidence that the changes that occur within the nervous system in response to resistance training may affect patterns of muscle recruitment within a variety of movement contexts. 1. Implications for Movement Control It is well established that resistance training results in greater increases in force-generating capacity in tasks that closely resemble the exercises performed during training than in novel tasks. That is, strength increases are somewhat specific to the tasks performed during training. [11-13] Many researchers have inferred from this concept that some of the adaptations that cause strength increases have an effect only on movements that are similar to the training tasks. However, while the principle of training specificity suggests that the adaptations within the nervous system that underlie motor learning do not greatly contribute to increases in strength when novel resistance training tasks are performed, it does not necessarily imply that the physiological changes associated with motor learning do not affect the manner in which novel movements are controlled and executed. When resistance training influences the execution of another movement task the effect can be regarded as a ‘transfer of learning’. Transfer occurs when training for one task affects the performance or learning of a subsequent task. Positive transfer takes place if training for the original task improves performance during a subsequent task, whereas transfer is negative if training causes a reduction in performance on the transfer task. The transfer of learning has been extensively studied over the past century. [14] A number of researchers have also attempted to understand the principles that govern transfer within the context of motor learning, particularly from a behavioural viewpoint. [14,15] Much of this ‘motor learning’ research was concerned with transfer between tasks requiring considerable cognitive, as well as motor, processing. In the present article, we attempt to understand the impact of resistance training on the performance of a wide range of transfer tasks in terms of the physiological adaptations that accompany training. We argue that resistance training has the capacity 830 Carroll et al.  Adis International Limited. All rights reserved. Sports Med 2001; 31 (12) to cause adaptations to many neuromuscular elements that are involved in the control and execution of movement, and is therefore likely to affect performance during a wide range of movement tasks. Our reasoning is based on the classic work of Thorndike and Woodworth[16] who stated that ‘spread of practice occurs only where identical elements are concerned in the influencing and influenced function’ (page 249). The degree and direction (i.e. whether transfer is negative or positive) of transfer from resistance training to other tasks is determined by the interaction of the various neuromuscular adaptations associated with training, and the characteristics of the transfer tasks. Figure 1 depicts a simplified model that illustrates how 2 independent neuromuscular adaptations could interact to affect the performance of a particular movement task. In reality, it is certain that transfer from resistance training to other movements will depend on the interaction of more than 2 forms of adaptation. The challenge for future researchers is to determine the precise nature of the neuromuscular adaptations that accompany resistance training, and to identify patterns of interaction among these adaptations for various classes of movement. 1.1 Individual Muscles In a recent experiment, we sought to establish whether resistance training has the capacity to influence the activation patterns of muscles that are Performance on transfer task High Low Neuromuscular variable 1 Neuromuscular variable 2