[quote]AtleticoMadrid wrote:
CT,
During the concentric phase of lifts, generally speaking, motor units are recruited from largest to smallest as needed and when rate coding of various MU’s can no longer increase, more MU’s are recruited for the task. IS the recruitment pattern for the eccentric phase different, and if so, in what ways? [/quote]
First, that is not the recruitment pattern during concentric movements.
The smallest (weaker more enduring) motor units are recruited first … if the amount of force required is too high for these fibers (either because the load is heavy or that fatigue starts to set in) the body will recruit larger, stronger fibers.
During eccentric (and electrostimulation) the recruitment pattern is reversed, with the fast-twitch/stronger fibers being recruited first.
Here is a little something from my second book ‘‘Theory and Application of Modern Strength and Power Methods’’
Why is eccentric training effective?
Eccentric training allows one to stimulate greater strength and size gains than pure concentric training. Why is that? There are five major reasons why:
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There is a greater neural adaptation to eccentric training than to concentric training (Hortobagyi et al. 1996).
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There is a more important force output produced during a maximal eccentric action (greater overload) because you can use a higher external load (Colliander and Tesch 1990).
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There is a higher level of stress per motor unit during eccentric work. Less motor units are recruited during the eccentric portion of a movement, thus each of the recruited motor units receives much more stimulation (Grabiner and Owings 2002; Linnamo et al. 2002). Furthermore, since the nervous systems seems to recruit less motor units during a maximal eccentric action, the potential for improvement could be greater than with maximal concentric action.
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There is some evidence that maximal eccentric actions will preferably recruit fast-twitch muscle fibers, which are more responsive to muscle growth and strengthening (Nardone et al. 1989, Howell et al. 1995, Hortobagyi et al. 1996). In fact, eccentric training may stimulate an evolution towards a faster contractile profile (Martin et al. 1995).
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Most of the micro-trauma to the muscle cells incurred during training is a result of the eccentric action (Brown et al. 1997, Gibala et al. 2000). It has been established that this micro-trauma acts as the signal to start the muscle adaptation process (Clarke and Feedback, 1996).
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Greater cross-education (Hortobagyi and Lambert 1997). Cross-education refers to transfer of strength gains from one limb-side to the other. In practical terms it means that if you were to work only your right arm using eccentric actions, some of the strength gains would transfer to the left arm. This can be very beneficial to prevent excessive strength loss if one limb is immobilized.
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Eccentric training is also a superior method to treat tendinosis when compared with concentric exercise (Mafi et al. 2001). So it could be argued that this form of training is adequate for use by injured athletes and that it is relatively safer than concentric training even if the loads used are greater.
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A last point of interest is that strength gains from eccentric training are maintained longer during a period of detraining than the gains from concentric-only training (Collinder and Tesch 1992, Housh et al. 1996), which may be very important for athletes who cannot train as much during the season as they can in the off-season.
And from the same book, some info on electrostimulation and fiber recruitment:
Preferential recruitment of fast-twitch fibers
During voluntary contractions motor recruitment is done according to a rigid pattern known as the ?size principle? or the ?Henneman law.? According to this principle, the smallest motor units (slow twitch), which have the lowest recruitment threshold, are activated first. As the demand on the muscle increases, the bigger motor units (fast twitch), which have a higher recruitment threshold, are brought into play. This pattern doesn?t change except for a few noted exceptions (maximal eccentrics for example).
With EMS there is an inverted recruitment pattern. This means that the bigger motor units are actually recruited first. Why? There are three reasons:
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Because EMS works the following way: The electric current stimulates the nerve cells (and not the muscle fibers themselves as it is believed by some), which then innervate the motor units. Without going into too much physiology, motor units with bigger axons are more responsive to an external current; bigger axons are more excitable (Blair and Erlanger, 1933; Solomonow, 1984). Fast twitch motor units have been shown to have larger axons; the bigger the axon is, the more likely it is part of a fast-twitch motor unit. So understandably, EMS will preferentially recruit the bigger, fast-twitch motor units first (Solomonow, 1984; Enoka, 1988; Duchateau and Hainaut, 1988)
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EMS has also been shown to preferentially recruit superficial (closer to the skin surface) motor units over deeper motor units (Beulke, 1978). Snyder-Macier et al. (1993) have established that fast-twitch motor units have a tendency to be closer to the surface. So, since EMS works best on superficial muscle fibers, this also explains the preferential fast-twitch recruitment pattern.
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Stimulation of the cutaneous receptors (skin receptors) tends to increase the recruitment of fast-twitch fibers over slow-twitch fibers (Garnett and Stephens, 1981; Kanda and Desmedt, 1983). Since the electrode is placed on the skin and the electric current must go through the skin, this could also increase fast-twitch motor unit activation.
On top of the direct evidence supporting the preferential activation of fast-twitch fibers/motor units by EMS, we have some indirect evidence as well, provided by a recent study.
Maffiuletti et al. (2000) found that EMS training significantly increased eccentric strength and high-speed concentric strength, but not slow-speed concentric strength. We know that during maximal eccentric efforts the fast-twitch muscle fibers play a bigger role, and that high-speed concentric strength is highly dependent on fast-twitch fiber capacities. These results are thus highly indicative of a preferential fast-twitch recruitment pattern with EMS training.
Preferential recruitment of the fast-twitch fibers is very interesting for athletes. We know that under normal circumstances it is very hard to stimulate these fibers. The training means required to do so (maximal eccentrics, intense plyometrics) can often be extremely taxing on the CNS and joints. Because of this, EMS appears to be a good supplementary tool for the athlete. EMS enables the athlete to reduce his volume of maximal training (but not eliminate it) while still getting the same (if not superior) training effect.
