Dear Group,
I have had some help from Roger M. Enoka and Per Aagaard, both of their writings are in quotes.
The main part of the debate we are having is on what two members of the forum said; They said in a slow rep set of roughly 4/4 {4 seconds positive, 4 seconds negative} speed for 6 reps, there will be more overall force/tension out/on the slow muscle fibers, than the following faster rep speed, which we think is not so. There would be far more tension on all muscle fibers in a fast rep set, and more overall force output by all muscle fiber, on a .5/.5 speed for 30 reps. Both using, 70% of the persons 1RM. Note the TUT in the faster set is shorter, as you always fail faster in a faster rep speed.
[i]Member wrote;
When you purposely slow down a rep speed you are recruiting less fibers, these are the lower threshold fibers. There is a greater tension on the lower threshold muscle fibers when you purposely rep slower due to their fiber type suiting slower velocities.
When you move a weight fast, you are recruited higher threshold motor units to do so, sharing the load amongst more motor units generates more force yes, but the actual tension placed on each of the muscle fibres is less due to more motor units being recruited.
Think 10 people pushing a car instead of 5
When you use a slow tempo you do 2 things.
1/ Recruit less motor units so less muscle fibres have to lift the same load, more tension is placed on these muscle fibres.
2/ Increase the TUT per recruited muscle fibre increasing fatigue.[/i]
I wrote;
1,
If as you say there is more tension on less muscle fibers when moving slow, why do you not fail faster in the slower rep ??? As if there was more tension placed on these muscle fibers the set should terminate faster, but it does not, as you always fail faster in a faster set using the same weight. That proves that there has been more overall force output by all muscle fibers, thus more tension on the muscle fibers as a whole, thus the fast set terminates faster.
2,
The power/strength overall output is far higher in the faster rep.
Lets calculate how much power/strength I would be used on both rep speeds. Distance weight moved 1.85 M. Since we are using the metric system, we will first need to convert the mass of the barbell into kilograms (200 lb ÷ 2.2 = 91 kg). Secondly, to determine the force we will need to figure out what the weight of the barbell is (W = mg = 91 kg x 9.81 m/s² = 892 kg.m/s² or 892 N). Now, if work is equal to Force x distance then, U = 892 N x 1.85 m = 1650 Nm.
We can calculate that lifting a 200 lb barbell overhead a distance of 1.85 m required 1650 J of work. You will notice that the time it took to lift the barbell was not taken into account.
Let us only add up the positive part of the lift.
The concept of power however, takes time into consideration. If for example, it took .5 seconds to complete the lift, then the power generated is 1650 J divided .5 s = 3300 J/s.
If it took 2 seconds to complete the lift, then the power generated is 1650 J divided 4 s = 412.5 J/s.
Slow set,
412.5 x 6 = 2475.
Fast set,
3300 x 99000
3,
There is a big difference in the poundage moved if the same weight is used, thus more aver all force out put by all fibers, and tension on them.
If your doing 6 reps at 4/4 with 200Ibs = 48 seconds, 200 x 6 = 1200Ibs
If your doing only half the time in the faster reps, 24 reps at .5/.5 with 200Ibs x 24 = 4800Ibs.
Let us say we are using 80% and rep it as fast as possible, this is what in my opinion happens to all muscle fibers ???
4,
My take of how muscle fibers fire.
At repping 70% at a .5/.5 speed, would they all come into play at once, and generate their higher and highest force, as the reps go on, and then their max force, and then some would lose some force, but are still contributing.
If you believe that all muscle fibers have at one point generated their max force. In your scenario above in a slow rep all muscle fibers have not generated max force, untill the very end of the set. In a fast rep all muscle fibers have generated max force at one point, and are still trying to generate as much force as possable.
With far more force, power, strength and overall more accumulated weight moved in the faster rep, I declare that all fibers have generated well over three times as more force and thus tension on all muscle fibers in the faster rep.
[b][quote]Per Aagaard Professor, PhD
Institute of Sports Science and Clinical Biomechanics
University of Southern Denmark
I think that you are right in questioning the above notions. When a given load is lifted very fast, the acceleration component means that the forces exerted on the load (and thereby by the muscles) by far exceeds the nominal weight of the load. For instance, a 120 kg squat can easily produce peak vertical ground reaction forces (beyond the body mass of the lifter) of 160-220 kg’s when executed in a very fast manner! Same goes for all other resisted movements with unrestricted acceleration (i.e. isokinetic dynamometers (and in part also hydralic loading devices) do not have this effect).
This means that higher forces may be exerted by more muscles fiber when a given load is moved at maximal high acceleration and speed - i.e. contractile stress (F/CSA) may well be the same or even greater for the activated muscle fibers than when the load is lifted slowly…
best wishes
Per [/b][/quote]
[i][b][quote]Roger M. Enoka, Ph.D. Wrote;
One important point to emphasize in these types of discussions is the concept of slow and fast twitch muscle fibers. Unfortunately, this terminology is misleading because there are not two (or three) types of muscle fibers; rather, there is a continuous distribution in every muscle from the fibers with slow contractile kinetics through to those with fast kinetics. Because there are not distinct types of muscle fibers, it is not possible to design an exercise program that stresses either “fiber type”.
A more appropriate functional distinction between muscle fibers is the force at which the motor units are activated during a muscle contraction, which is known as recruitment threshold. Motor units with low recruitment threshold can be either slow or fast twitch, whereas motor units with high recruitment thresholds are all fast twitch. But, recruitment thresholds decrease with contraction speed so that all motor units in a muscle are activated when rapid contractions are performed with loads 40% of maximum.
Wayne,
The force that a muscle must exert to move a load depends on two factors: the mass of the load and the amount of acceleration imparted to the load. The number of muscle fibers recruited during the lift increases with the speed the lift.
I hope this helps.
Wayne,
The rate at which any motor unit, low or high threshold, can discharge action potentials is not maximal during slow contractions. As contraction speed increases, so does discharge rate for all motor units.
Wayne,
The number of muscle fibers activated to lift a weight depends on two factors: (1) the amount of weight; and (2) the speed of the lift. Although more muscle fibers are activated during fast lifts, they are each generating MORE force. We know this because the rate at which the muscle fibers are activated by the nervous system increases with contraction speed.
Roger M. Enoka, Ph.D.
Professor and Chair
Department of Integrative Physiology
University of Colorado[/i][/b][/quote]
Comments on who is right and why please.
Wayne