Post by John A. Casler on Apr 26, 2008 15:54:28 GMT -8
Force/Speed Relationship
No where will you find much more confusion than in the discussion and understanding of the Relationship of Muscular Force and the Speed of the Action.
Much of the confusion stems from a lack of clear focus on what one is observing, and how that observation might translate into training advantages.
There are so many elements that need to be clear here, I hardly know where to begin.
Firstly when discussing this Relationship, several issues must be perfectly clear:
1) You must be clear about whether you are discussing concentric or eccentric actions. They are slightly different in their force/speed relationships for different reasons
2) You must be clear as to the "type" of training force load (external force) that is being examined, since Isotonic and Isokinetic have different results due to their differing properties
3) You must be clear on the amount of effort that is being used during the observation. That is, is this a RM effort, or is it a Sub-Maximal Effort? The only effective method to offer meaning is a Maximal or RM effort for either Isokinetic or Isotonic.
Let us for the sake of this post focus on the Concentric Action, and a Single (1 rep) Rep Maximum.
Isokinetic is an exercise action whereby the Load varies and the Speed is Controlled. That means a MAX EFFORT would travel at the same speed as a sub maximal effort, and the Force would show us how Maximal the effort is.
Isotonic is an exercise action whereby the Speed varies and the Load is Controlled. That means that MAX EFFORT would travel at a speed limited by the ability of the Muscle to produce Speed to the Load. This means that in a REP MAX effort, the ability to supply a Maximum Force and maintain a Speed that provided Maximum Force would be the Key Elements.
To that end, it is easy to see that a Single Rep Maximum Effort would most always be Slow in comparison to a lesser effort, isotonically (meaning using a lighter weight that would allow for a faster speed, and the same speed in an Isokinetic Rep but the force output would be lower if one did not attempt to maintain Maximum Effort.
So now we turn to just how fast a muscle can shorten from sliding filament muscle action.
In reality it is pretty fast. Very cautiously flex your elbow as quickly as you can. What did it take, maybe a 10th of a second? This demonstrates that sliding filament action can act quite quickly.
But that is with only your forearm as load. You know that with a 30# dumbbell, that this speed would be reduced substantially. But knowing that the muscle CAN react with that speed, it is the "recruitment" of more Motor Units to act on the DB, that will create a force to move it at its fastest speed based on you ability to apply more force, and accelerate that dumbbell. The limitation is determined by elements of your strength. (I say elements, since speed of muscle actions is not simply determined by how strong a muscle is. The other Primary Ingredient is RFD or Rate of Force Development.)
If you could curl a 100# dumbbell, then a 30# db might move quite quickly, and unless you attempted to move it quickly, it would provide very little load for such strength. The key issue is that Moving it Slowly, WILL NOT make its force to the muscle increase. The only way to make it cause more load, is to attempt to accelerate it.
When launching a Rocket, the greatest amount of force and fuel are used during acceleration.
(to be continued)
In discussing force/speed, you also need to separate the discussion of concentric action and eccentric action.
In the concentric action the greatest forces and muscle tensions will occur when the trainee attempts to move or accelerate against the load as quickly and with the greatest effort possible.
In the eccentric action the "potential" greatest forces will be possible when trying to "slow" or negatively accelerate the load or action.
The difficulty of grasping the negative or eccentric loads and tensions relationship is that if the load is not sufficient, simply slowing a load that is too light WILL NOT produce the greatest forces, but obviously it will produce the slowest speeds and even cessation of movement without volitional lowering.
This difference between the eccentric and concentric, coupled with the "lack" of adequate loading if the load is not sufficient to create the higher muscle tensions causes much misunderstanding of just what the force/speed relationship means.
Even more incorrect, is when one interprets an intentional concentric slowing to having the product of "higher muscular tensions" when that IS NOT the case.
Any concentric speed reduction, should be the product of additional load to the action, and not the "reduction" off effort to produce a slower speed.
This is an incorrect interpretation of the "Force/Speed" relationship.
The eccentric action might be even more confusing, since we know the we are trying to "slow" or brake the load/force with muscular tension.
The confusion occurs again with submaximal loads when the load is small enough to allow it to be overcome. This means that it is "underloaded" and as such it will recruit fewer MU's (motor units) and subsequently produce lower tensions, and lower motor impulses.
The load can be increased during some portions of the ROM, by allowing controlled acceleration (if the load is weight based) and this acceleration would them require greater muscle tensions to be used against it.
This speed variation to produce this load is also confusing to most who reason that the "ideal" is to slow the eccentric action, not realizing that an underloaded action is far less effective a stimulus, than a partial ROM that IS more adequately loaded.