The nervous system's role in strength gains
02-24-2010 11:23 PM
The nervous system's role in strength gains
Significant gains in muscle strength have been shown following short periods of resistance training, which are generally regarded as being too short to elicit morphological changes in the muscle (Moritani and deVries, 1979). It would therefore seem that this strength increase is due to an ability to better activate the muscle. Over time the muscle activation plateaus and CSA increases, suggesting that after a time, hypertrophy is the more significant factor in increased strength. Various suggestions regarding these two factors are explored below.
"Neural adaptation after resistance training has been inferred on the basis of several studies reporting increases in muscle strength with little or no change in cross sectional area of the muscle." (Bandy et al, 1990, p.252). Most research into neural adaptations after resistance training looks mainly at motor unit activation by using EMG. It is widely accepted that increases in EMG is a result of increased firing frequency of motor units in combination with an increased recruitment of motor units.
Cross education is evidenced by an increased strength in the contralateral limb and is likely due to cross talk between nerves in the spinal cord from one side to the other. Moritani and deVries (1979) reported an increase in MVIC force of 36% in isometrically trained elbow flexors versus a 25% increase in the contralateral untrained limb. The changes in the untrained limb occurred without changes in CSA or enzyme activities. Butler and Darling (1990, cited in Enoka and Fuglevand, 1993) found an increase in EMG in the contralateral untrained limb. Subjects have exhibited a lower single limb MVIC when both limbs are active simultaneously than when tested in isolation (Howard and Enoka, 1991, cited in Enoka and Fuglevand, 1993). It could be postulated that this is due to cross talk from the contralateral side during a single limb effort that is not present to the same extent during a bilateral task.
Research Update - New Findings
Central Nervous System
Increases in strength have been shown when a subject shouts during exertion, or if a pistol is fired near the subject shortly before the test procedure (Ikai and Steinhaus, 1961, cited in Lamb, 1984). Similar strength changes have also been noted when the subject is given hypnotic suggestions of strength (Morgan, 1972, cited in Lamb, 1984). Yue and Cole (1992, cited in Enoka and Fuglevand, 1993) observed an increase in MVIC and EMG following imagery.
It has been shown that a voluntary contraction is not a strong as a contraction stimulated electrically (Ikai and Yabe, 1969, cited in Lamb, 1984, and Stephens and Taylor, cited in Lamb, 1984).
Electrical stimulation - training
It has been shown that strength development can be achieved through electrical stimulation of a muscle, however the strength gains from this method of training are less than those noted in a voluntary training program (Massey, 1964, cited in Moritani and deVries, 1979, and Nowakowska, 1962, cited in Moritani and deVries, 1979). This is likely due to the lack of involvement of the motor pathways in electrically stimulated training. Lyle and Rutherford (1998) however, found no significant difference between strength gains in adductor pollicis of voluntary versus stimulated contractions. The large gains shown in stimulated training argues against central adaptations as a major contributor to the strength increases following training.
In most studies, the EMG/force slope initially remained the same as in the pre-trial testing with an increase in muscle activation (EMG values). After a few weeks resistance training the EMG slope started to decrease, indicating muscle hypertrophy gradually becoming integrated in the strength increase and the rapidly increasing muscle activation slowed to a lesser rate. (See figure 3)
Disproportionate CSA increase
After a number of weeks of resistance training, an increase in CSA can be measured. This increase is proportionally smaller than the increase in MVIC (Narici et al, 1989, cited in Enoka and Fuglevand, 1993). Nonetheless, CSA is the single best predictor of muscle strength. Larger muscles have a greater amount of actin and myosin, therefore a greater number of cross bridges, which results in a greater potential for force production during contraction.
Motor Unit Synchronisation
Strength training can increase motor unit synchronization. Friedeboldet et al (1957, cited in Komi, 1986) was among the first to suggest that, in particular, the early part of strength training is associated with an increase in motor unit synchronization. Komi goes on to suggest two possible explanations for this increased synchronization.
The dendrites of alpha-motor neurons receive increased input from sensory fibers, and the higher motor centers increase their descending activity.
Rasch and Morehouse (1957, cited in Moritani and deVries, 1979) demonstrated strength gains from a six-week training program in tests where muscles were used in a familiar way, but not when unfamiliar test procedures were involved. This suggests that larger test results were mainly due to skill acquisition.
Initial changes to muscle strength are due to neural factors (motor unit activation, firing frequency, input from the opposite side of the spinal cord, input from muscle spindles and reflexes, input from lower and higher spinal cord levels). Over time, the increased rate of neural activation decreases to a slower rate and muscle hypertrophy commences (this is postulated to be stimulated by the neural system). The muscle CSA increases with continued training. This also results in increased strength. The CSA does not increase to the same extent as the muscle strength. The total strength increase is a combination of increased neural activation and muscle hypertrophy.
Research data compiled by Curtin University of Technology
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