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.