Neural adaptations to resistive exercise: mechanisms and recommendations for tra

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Sports Med. 2006;36(2):133-49.

Neural adaptations to resistive exercise: mechanisms and

recommendations for training practices.

DA, Kamen G, Frost G.

Department of Physical Education and Kinesiology, Brock University,

St Catharines, Ontario, Canada.

It is generally accepted that neural factors play an important role

in muscle strength gains. This article reviews the neural adaptations

in strength, with the goal of laying the foundations for practical

applications in sports medicine and rehabilitation.An increase in

muscular strength without noticeable hypertrophy is the first line of

evidence for neural involvement in acquisition of muscular strength.

The use of surface electromyographic (SEMG) techniques reveal that

strength gains in the early phase of a training regimen are

associated with an increase in the amplitude of SEMG activity. This

has been interpreted as an increase in neural drive, which denotes

the magnitude of efferent neural output from the CNS to active muscle

fibres. However, SEMG activity is a global measure of muscle

activity. Underlying alterations in SEMG activity are changes in

motor unit firing patterns as measured by indwelling (wire or needle)

electrodes. Some studies have reported a transient increase in motor

unit firing rate. Training-related increases in the rate of tension

development have also been linked with an increased probability of

doublet firing in individual motor units. A doublet is a very short

interspike interval in a motor unit train, and usually occurs at the

onset of a muscular contraction. Motor unit synchronisation is

another possible mechanism for increases in muscle strength, but has

yet to be definitely demonstrated.There are several lines of evidence

for central control of training-related adaptation to resistive

exercise. Mental practice using imagined contractions has been shown

to increase the excitability of the cortical areas involved in

movement and motion planning. However, training using imagined

contractions is unlikely to be as effective as physical training, and

it may be more applicable to rehabilitation.Retention of strength

gains after dissipation of physiological effects demonstrates a

strong practice effect. Bilateral contractions are associated with

lower SEMG and strength compared with unilateral contractions of the

same muscle group. SEMG magnitude is lower for eccentric contractions

than for concentric contractions. However, resistive training can

reverse these trends. The last line of evidence presented involves

the notion that unilateral resistive exercise of a specific limb will

also result in training effects in the unexercised contralateral limb

(cross-transfer or cross-education). Peripheral involvement in

training-related strength increases is much more uncertain. Changes

in the sensory receptors (i.e. Golgi tendon organs) may lead to

disinhibition and an increased expression of muscular force.Agonist

muscle activity results in limb movement in the desired direction,

while antagonist activity opposes that motion. Both decreases and

increases in co-activation of the antagonist have been demonstrated.

A reduction in antagonist co-activation would allow increased

expression of agonist muscle force, while an increase in antagonist

co-activation is important for maintaining the integrity of the

joint. Thus far, it is not clear what the CNS will optimise: force

production or joint integrity.The following recommendations are made

by the authors based on the existing literature. Motor learning

theory and imagined contractions should be incorporated into strength-

training practice. Static contractions at greater muscle lengths will

transfer across more joint angles. Submaximal eccentric contractions

should be used when there are issues of muscle pain, detraining or

limb immobilisation. The reversal of antagonists (antagonist-to-

agonist) proprioceptive neuromuscular facilitation contraction

pattern would be useful to increase the rate of tension development

in older adults, thus serving as an important prophylactic in

preventing falls. When evaluating the neural changes induced by

strength training using EMG recording, antagonist EMG activity should

always be measured and evaluated.