mike - 15 January 2007 05:35 PM
Barry welcome to the board. I'm glad you joined on here.
Hi Mike!
The source of the force is irrelevant for the purpose of this discussion. I am speaking from a purely mechanical sense….the most basic and fundamental understanding of kinetics….and this is where you go woefully off course. To say that the direction of the force vector does not matter is absurd. If you are in fact arguing this issue (that direction of the force vector doesn't matter) please let me know and we'll discuss.
I don't think I'm off course at all, let alone "woefully". Where did I say that the direction of the force vector does not matter? I think you may have gone off on a tangent, sort of a change in the vector of the discussion, so to speak.
What in the world is 'chemical work' and how does this apply to the discussion of force vectors.
Chemical work is the volitional contraction of the muscle by the athlete. An eccentric contraction does not require the brain to send a chemical signal to the muscle to contract. BTW this thread topic was about whether or not technique training is necessary, not about force vectors.
(regarding length of ground contact time…br) No argument here. In fact, I argue that because ground contact time is so short one must PREPARE for it while still in flight in order to position the limbs in the most appropriate way to apply the largest possible forces in the appropriate directions.
In the shot put analogy above, the thrower can make the adjustments to their throwing vector because they have the time to do so. The surface used on that day does not change between throws. It's a bad analogy, period.
But, preparing during flight time in the sprints? How does one "prepare" the limbs to be in the best position while traveling at speeds up to 26 mph? How does one think fast enough to send a signal for the muscles to react in hundredths of a second in order to line up a landing spot for the ultimate angle, adjusting for wind conditions and surface changes? How does one do this for stride after stride in rapid succession? I don't think so.
No but they clearly can in 0.8-0.10s….the ground contact time for the best sprinters in the world.
They clearly don't. Force plates show force begins to register shortly after toe-down and peaks before the midpoint of the total stance time. That means you can cut almost 2/3 off the time you're using for an elite sprinter's ground contact time for force to begin registering, build to a peak and dissipate..This is where "technique" trainers are woefully off course because they neither understand the cause of this force nor its effects, relying solely on what they see. Force plates don't lie, they don't make up numbers, they don't sell books, they don't put on seminars with snappy quotes (yes, I do sell books and I do put on seminars but I'm not a force plate :bigsmile:) and they ignore vectors. They merely register force. Despite all that, do you know anyone that can lift more than 3 times their bodyweight on one leg in .10 seconds? If you could find someone (you won't), could they do it over and over again for the number of strides in a 200m race? 100m? 50m?
"When locomotion experts talk about the athlete's force application, the phrase generally used is " force to offset gravity". The runner applies isometric force to withstand hitting the ground without collapsing."
Do you disagree with this point? I'm not clear by what you've said.
First, it's not a matter of whether or not I agree with it, or whether or not you agree with it. It is what it is. It's a phrase used by locomotion experts to describe what happens at ground contact. How could I not agree with it?
I am in no way a 'sprint guru' and would argue that I have a fairly decent understanding of mechanics (USATF biomechanist for past 5 years; biomechanics PhD). Kinematics beget kinetics. This is a fundamental concept of biomechanics.
Kinematics is defined as the branch of mechanics concerned with motion without reference to force or mass.
Kinetics is defined as the branch of mechanics concerned with the forces that cause motions of bodies.
The preponderance of sprint training protocols are based upon kinematics with little or no kinetic basis. From your background, you should know that force at ground contact comes from the runner as a falling and accelerating body and not as volitional chemical muscle mechanical work.
Please address the following questions:
1. Does the direction of force applied to the ground matter at all?
This is another "vector" question and an over simplification of the issue. Does it matter for the runner in the sense of volitionally creating a running vector? No. The combination of the runners isometric strength to offset gravity and the braking effect at toe-down create the runners vector. The vertical aspect dominates the horizontal, but both are necessary for forward propulsion.
2. What is chemical work?
Volitional contraction by the runner, with an excessive metabolic cost. The spring-mass model describes the method that minimises metabolic cost.
3. Does the runner need to offset the effects of gravity?
Yes, it is the most critical aspect. Mass-specific force to offset the effects of gravity has a linear relationship to speed. It is so critical that changing MSF by 1/10 the runners bodyweight could increase speed by 1 meter per second. It is not a "technique". MSF is primarily isometric.
4. What is more important to sprinting- concentric strength or eccentric strength or some other form of strength?
Concentric strength is critical at the start to overcome inertia and it is volitional. After the first few strides, pure concentric strength gives way to the spring-mass model's description of what is essenentially a bouncing ball using ground force reaction and eccentric loading that begins at ground contact. I have been diligent in looking, but so far, no one has come forward to take credit for the bouncing ball's training regime. Maybe they are too busy working on a new technique?
5. How is "elastic force" produced?
The short version: the overwhelming force at ground contact CAUSES the runner to dorsiflex the grounded foot, creating the eccentric contraction in the posterior muscles, tendons and ligaments. Potential elastic energy is produced and stored by the limb, then released as part of the vertical impulse. Elastic energy not used dissipates as heat.
As for running mechanics, this is what the Weyand study states, "Although sprinting abilities differed greatly among subjects and the top speeds of the same runners differed considerably on the different inclines, the mechanical means by which runners increased speed from a jog to top speed varied little. Across each individual's speed range, speed increases were achieved primarily by increasing stride lengths at lower speeds and stride frequencies at higher ones. The more rapid increases in stride frequency as subjects approached their top speeds were achieved through reductions in both the contact and swing times that make up the total stride time…These aerial time reductions resulted from decreases in effective impulse, the product of contact time and effective force, which determines the time a runner spends in the air. Reductions in vertical impulse as top speed was approached were due to decreases in the time of foot-ground contact that were larger than the increases in the effective force applied to the ground."
In a nut shell: There was little mechanical difference between a 11.1 mps runner and a 6.2 mps runner (no mention of significantly better technique causing significantly better running mechanics by the faster runners). Longer stride lengths early in the running gave way to increased stride frequency at higher speeds. Stride frequency increased because of reduced ground contact time and swing time (reduction in ground contact time because of better mechanics? No. Perhaps Newton's law? After all, it is the collision of 2 bodies, isn't it? ). Aerial times were reduced because because effective impulse (impulse in excess of mass) decreased (no mention of changes in running mechanics? better form? better angle of arms during armswings?). Effective impulse decreased because one of its factors, ground contact time, decreased. Top speed is the point where ground contact time has shortend to the point that no additional effective force can be applied to the ground.
No doubt this will tick off a lot of readers of this post, but none will be any more ticked off then I was for buying into a lot of unfounded, non-scientific nonsense.
Barry Ross
