Generally as a muscle is required to generate greater force, there will be a concomitant increase in the number of high threshold motor units that are recruited.
So for a concentric action, that means the more quickly the load is accelerated (requiring greater force), the more fast twitch motor units will be recruited.
However, for an eccentric action the resultant force will be gravity/elastic recoil/whatever. This means that the muscle must generate contractile force in opposition to that, decelerating the load. Greater deceleration requires greater force, which will typically result in greater recruitment of fast twitch units.
The way the nervous system recruits motor units in the 2 muscle actions is distinct. The size principle only applies during concentric efforts. During eccentrics there is not an ordered recruitment of fibres from slow --> fast, but what appears to be a preferential recruitment of fast twitch fibres.
For that very reason I wouldn’t over think the use of eccentrics. So long as you make a concerted effort to control the load during the yielding phase you’ll probably get sufficient stimulus.
As far as speed strength/ strength speed- what you lift with is what you get better at. The loads you train with are the ones you get the greatest power adaptations in. So all you have to do is pick the load that applies most specifically to your goal and train accordingly. [/quote]
the size principle applies to eccentrics… pretty much every recent study has proven preferential recruitment to be bunk… with EMS it is possible apparently.
here’s a review study, there’s alot more recent studies on this topic:
Literature examining the recruitment order of motor units during lengthening (eccentric) contractions was reviewed to determine if fast-twitch motor units can be active while lower threshold slow-twitch motor units are not active. Studies utilizing surface electromyogram (EMG) amplitude, single motor unit activity, spike amplitude-frequency analyses, EMG power spectrum, mechanomyographic, and phosphocreatine-to-creatine ratio (PCr/Cr) techniques were reviewed. Only single motor unit and PCr/Cr data were found to be suitable to address the goals of this review. Nine of ten single motor unit studies, examining joint movement velocities up to 225Â°/s and forces up to 53% of a maximum voluntary contraction, found that the size principle of motor unit recruitment applied during lengthening contractions. Deviation from the size principle was demonstrated by one study examining movements within a small range of low velocities and modest forces, although other studies examining similar low forces and lengthening velocities reported size-ordered recruitment. The PCr/Cr data demonstrated the activation of all fibre types in lengthening maximal contractions. Most evidence indicates that for lengthening contractions of a wide range of efforts and speeds, fast-twitch muscle fibres cannot be selectively recruited without activity of the slow-twitch fibres of the same muscle.
"As far as speed strength/ strength speed- what you lift with is what you get better at. The loads you train with are the ones you get the greatest power adaptations in. So all you have to do is pick the load that applies most specifically to your goal and train accordingly. "
well, if you want to jump squat 135 and you squat 225, then it would be misleading to simply train only within that range (lighter weights)… if you end up squatting 335, then you’ll be able to throw 135 around alot easier than if you squatted 225… so here, the carry over in explosive strength from improving maximal strength would prove beneficial.
maybe i misunderstood your comment, or took it wrong.