from Curtin University of Technology
Neural mechanisms are the most important determinants of strength adaptations.
Proposition for Debate - by Amanda Broughton
Statement of the Topic
Neural mechanisms are the most important determinants of strength adaptations.
This debate addresses factors influencing an increase in muscle strength. This debate can be simply affirmed by the fact that we have all witnessed improvement in performance of a repeated strength test without evidence of muscle hypertrophy. Two definitions to clarify any misunderstandings are:
"The greatest amount of force that muscles can produce in a single maximal effort" (Lamb, 1984).
"motor unit activation (recruitment, discharge rate), synchronization, and cross education" (Enoka and Fuglevand, 1993).
Literature suggests that physical training causes adaptations in the brain and spinal cord and that the ability of humans to recruit motor units increases with training (Lamb, 1984). Neural factors involved in muscle strength are: activation of motor units (frequency and quantity), involvement of afferent and efferent pathways, synchronization, and cross-education.
In addition to neural factors, we must consider other factors involved in muscle strength. Increased muscle cross sectional area (CSA) has a strong relationship with muscle strength (Lamb, 1984). Muscle length, rate of change of muscle length, and the alignment of the muscle with respect to the axis of joint rotation (Enoka and Fuglevand, 1993) are also involved in determining the strength of a muscle upon testing.
Considering all factors influencing muscle strength, it is important to ensure that a standard test procedure is used to evaluate muscle strength. As such, a maximal voluntary isometric contraction (MVIC) is the preferred option (Rutherford and Jones, 1986, cited in Enoka and Fuglevand, 1993). This minimizes the influence of neural components associated with muscle co-ordination, and removes influence from rate of change of a muscle. It also requires that muscle length and joint position are the same for each test. Mechanical and electromyographic (EMG) measurements are taken during the contraction to evaluate changes to the neuromuscular apparatus. EMG measurements are used as an indicator of motor unit activity, which gives an indication of the muscle force generated (Enoka and Fuglevand, 1993. Komi (1986) points out that the EMG recordings do not indicate whether the increased motor unit activity comes from the cortical or reflex sources, or from both.
Lawrence and DeLuca (1983, cited in Enoka and Fuglevand, 1993), suggest that EMG measurements during a MVIC are known to be somewhat unreliable. Howard and Enoka (1991, cited in Enoka and Fuglevand, 1993) found that on three repetitions of a knee extensor MVIC the average EMG varied substantially while the force remained constant. The authors therefore cautioned against using EMG as a direct representation of the activation of motor units of a muscle at high forces such as during an MVIC. The EMG recordings from surface electrodes are a result of summation of randomly occurring action potentials from numerous motor units. According to an unpublished dissertation by Fuglevand (1989, cited in Enoka and Fuglevand, 1993, p222), a motor unit action potential is influenced by:
- the number and size of fibers innervated by the motor unit,
- the spatial orientation of the fibers relative to the electrode,
- the electrode configuration and dimensions,
- the conduction velocity of the fiber action potential,
- the spatial relationship of the electrode to the innervation zone, and the length of the muscle fibers.
Figure 2 (Lamb, 1984)
(Plowman and Smith, 1997)
The motor unit consists of the motor nerve cell (neuron) that originates in the spinal cord (indicated by '3' in figure 1) and all the muscle fibers it supplies. All fibers in a motor neuron are of the same fiber type and are distributed throughout the muscle (Lamb, 1984). Slow twitch fibers are usually recruited first, and once a motor unit is activated, all muscle fibers in that unit are activated equally. To modulate muscle force, motor units change their firing frequency, and the number of active motor units changes. The motor units do not all fire in unison, except under conditions of maximal stimulation. "The CNS remains capable of fully activating all motor units to respond with maximum force under conditions of extreme contractile failure" (Thomas, Woods, and Bigland-Ritchie 1989, p. 1835, cited in Enoka and Fuglevand, 1993).
A motor unit is influenced by reflex pathways, muscle spindle input, input from higher and lower spinal cord levels, and from nerves on the opposite side of the cord as shown in figure 2 (Lamb, 1984). According to Enoka and Fuglevand (1993) many authors suggest that facilitation of the MVIC is due to the descending command being supplemented with afferent feedback. Komi (1986) suggests that training intensity must be periodically varied and/or progressively increased to maintain an increase in maximal neural activation. During detraining, or immobilisation, the neural input is decreased resulting in a decreased force production and muscle atrophy.
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. (See Figure 3).
"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.
Muscle hypertrophy seems mostly to result after training periods greater than six weeks, and is predominantly related to fast rather than slow twitch fibers (Bandy et al, 1990). Komi (1986) suggests that the increased alpha-motor neuron activation with motor neuron synchronization may stimulate hypertrophic factors that are expected to result after a period of progressively increasing strength training.
Figure 4 Figure 4
(Moritani and deVries, 1979)
Figure 3 Figure 3
(Plowman and Smith, 1997)
When considering a resistance training program, it is important to understand what you are improving at various stages of the program. Initially improvement will be due to neural adaptation. To maximise this potential, the program needs to be modified and/or progressed regularly so that neural adaptation does not plateau too soon. It is also necessary to consider the phenomenon of specificity. The muscle will improve in performing the task it is trained to do, there is minimal crossover to other tasks, and so a variety of contraction modes and joint positions will need to be employed for a more comprehensive program. Ensure that the task that is being trained will have functional relevance for day-to-day living. After a time hypertrophy will become evident. To maintain muscle strength and bulk, the training program needs to continue and be progressed and modified.
The phenomenon of bilateral deficit needs to be considered. A muscle can generate a greater force if worked in isolation. Unilateral training will therefore result in a more rapid strength increase than a bilateral task. Considering specificity, it may be necessary to train both ways.
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.
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