# 'Force', the 'BRIDGE' between Stride length and frequency.

‘Force’, the ‘BRIDGE’ between Stride length and frequency. Discus.

The more force you apply to the ground, the longer your stride is going to be because of the vertical forces being applied to the ground resulting in greater vertical propulsion, thus naturally lengthening stride length.

Stride frequency…not important until you have reached top speed (maxV) in which you are trying to maintain this by resuming the application of great ground forces while quickening the limb movements to avoid negative acceleration.

Question:- What happens to ground resistance as the athlete applies more force throughout the race? How does this relate to stride frequency?

If I understand ground resistance correctly, it will increase because as you apply more force, more force is applied to you. Newton’s law of equal and opposite reactions. The harder you hit something, the harder it is going to hit you back.

I think stride frequency is inversely proportionate to ground forces/ground resistance.

During acceleration ground forces are low becuase of the horizontal component of the phase of the race since you are coming up, but notice how the legs are moving very quickly to get the athlete running and helping to get to top-speed running form. But, when you are in top-speed running form, ground forces and resistance are at its highest because the force is being applied vertically allowing for more forces in both directions increasing stride length, but decreasing stride frequency because more energy is expended applying force than repositioning limbs.

I’m not too well rehearsed in biomechanics and biophysics, but I have taken physics and have some idea, so please do not badger me if I don’t explain things too well.

I am not trying to badger you. We are discussing force and the relationship it has between SF and SL. First, it is harder to turn over fast at the beginning of the race. Newton’s first law states that. Why? because resistance is at its highest. A body at rest want to stay at rest. If the athlete is running correctly, the resistance of the ground will slow the cycle of the leg down. As the athlete picks up speed, the resistance the ground offers deminishes. As the resistance deminishes, the leg is able to cycle through faster if the same amount of force is offered. Ground resistance is at its lowest at top speed. That is the reason that the athlete is at top speed because the ground can offer no more resistance. It is the dynamic between the ground (resistance) and the athlete (force) that results in speed. Coaches and athletes often forget that athlete needs the ground to accelerate. Speed is a dance between the ground and the athlete to metaphorically put it. It is about give and take. Force is applied to ground and the reaction is the ground applying an equal and opposite force to the body. Here is an example:- get on a bike and try to turnover as fast as you can in a medium to high gear. Resistance is at its highest at the start but as the bike picks up speed, peddle resistance deminishes and frequency is gained.

Thanks for re-explaining that for me. You just answered your own question and discredited my response (in a positive way). Now I have new light to investigate.

Too many coaches are coaching the speed right out of their athletes. Some believe that it is stride frequency and other believe that it is stride length that causes speed. The answer is neither. They are both measurements of speed but not the cause of speed. Speed is the result of net forces acting on the ground. My premiss is that the more force applied will result in longer stride length and greater frequency. What it comes down to, is the active forces of the body (legs) working against the resistive forces of the ground. The legs need something to push against in order for the body to pick up speed. It is how fast the body is moving over the ground that matters. Apply more force and you will have an increase in SL and SF. Keep in mind that the athlete accelerates at a decreasing rate and the biggest gains in acceleration are at the beginning but deminishes as higher velocity is attained (until top speed is reached). At top speed, frequency should be it highest because the resistive force are low (meaning that the only thing to slow the leg down is fatigue). It is the resistive forces that slow frequency down but they enhance stride length. As force is applied, there is a blending of stride length and frequency and the end result is speed.

I thought applying greater ground forces resulted in longer SL. How does the amount of ground forces/resistance effect SF. I’m confused on that much.

I’m certainly not a biomechanics expert, but i’ll take a shot at this one. My guess is that, if you apply greater ground force your CG will be higher off the ground. And if you’re CG is higher obviously contact time would be shortened and as Charlie has noted, pretty much anyone can lie on their back and cycle their legs at 5strides/sec. So therefore, b/c more time is spent in the air the faster you can cycle your legs. If i’m wrong please feel free to correct me.

In the beginning of the race stride length is increases due to the forces applied. Let’s start from the beginning. The body is at rest. You want to to give the body the greatest velocity from the block. Note, that I did not say turnover or stride length. As I said earlier, it is how fast the body is moving and not stride frequency or how far you can reach. The gun goes and you push off not by trying to free wheel the legs, but to project the body from the block at it highest possible velocity. There are several combination that can be used. (High force, short time; High force, long time; Low force long time etc). Let say that you were playing tennis or baseball. You wanted to hit an ace or a home run. The first thing that you would do is to swing the racket or bat fast through the full range of motion. The next thing you will have to do is to follow through with the stroke. The longer the ball is in contact with the racket or bat, the more likely that you will hit a home run or an ace. Now you are going to say that the frequency is down. What matters most is how fast the body as unit is getting off of the ground. Coaches are in error when they say to pull the foot off of the ground or reduce contact time. The lack of resistance is what decreases contact time. They should say get the body away from the ground. Contact time is determined by the quantity of force applied and the amount of resistance that the ground is offering. In the beginning of the race it take more time to get get the body moving so applying the biggest amount of force for the longest time possible is critical because resitance is at it highest as stated before. This time is short considering the time it takes to extend the leg and becomes shorter with each progressive step. Yes it is but the body has a higher initial velocity which is critical in getting the the body moving. You can apply a big force in a little time but I promise that you that the body will not have any speed and you will just be skimming over the ground only to exhaust yourself around 45-50 meters. Let say that each step you want to hit a home run or an ace. As the body picks up speed, it becomes harder to hit a home run because the ground is receding progressively faster behind you. Because the ground is receding faster with every push, the resistive forces of the ground is deminishing. Why? Time! It take less time to go through each cycle. If you try to apply the same amount of force to the smaller resistance then the leg will slip through the cycle faster. This is how frequency is gain. Based on this principle, Stride length is increased in the early stages of the race at a decreasing rate while each step gets quicker. Freqency is gain because of the decrease in resistive forces.of A good book to get is the Mechanics of Athletics by Geoffrey Dyson. It is out of print but I sure a book company can order it for you

A 100m sprinter reaches his highest stride frequency before his top speed, and reaches his highest stride length after top speed. So speed is about good balance between SF and SL.

One of the main effect of sprint training is to allow sprinter to achieve the maximum force on the ground in the shortest time possible. That’s why Ground Contact Time shall be reduce.
As far as top speed is concerned, the relation between SL and GCT is the main factor to improve: trying to improve SL with a reduced GCT. Here, force is the key.

Top speed should come where the athlete is making his fastest longest strides. The reach may lenghten after top speed by I can’t say that the stride is actually lengthening. If the reach is lengthening, it show that the athlete is rapidly decelerating. The key to finishing the race is having and maintaining a high frequency after top speed. The stride can not get any longer because there is no longer productive (net horizontal)force. As mentioned earlier, it ibecomes harder to project the body when there is nothing to push against. To get productive force, the athlete will have to slow down and then try to reaccelerate himself which does not make sense. The whole idea is to reach the highest angular velocity and hold it. Think in terms of a track racing bike with a lock gear (where the peddle is use for going and stopping). To get to top speed the cyclist would have to max his legs out. The faster he can peddle, the more speed the bike will pick up until the cyclist has maxes his legs out. After he has maxed out, it would be to his advantage to maintain the high frequency. If he fail to, he would decelerate accordingly to the cadence he has. He has two option to maintain that high speed:- keep peddling fast or take his feet off of the peddle. In running, we don’t have the second option.

I agree that maintaining a high frequency is important, how much is enough? What about stride length?

Analysing the fastest women ever and their race pattern, it’s impossible to say that those who are lengthening their stride until the finish line aren’t better finishers than those who maintain nearly perfectly the same stride length in the last half of the race.

We can make a list with those who keep nearly exactly the same stride length:
Florence Griffith-Joyner
Marion Jones
Evelyn Ashford
Inger Miller
Gwen Torrence
Kelli White
Chryste Gaines
Marlies Göhr
Marita Koch…
(basically, US and GDR sprinters)

those who lengthen their stride clearly and steadily:
Christine Arron
Merlene Ottey
Irina Privalova
Zhanna Block
Anelia Nuneva…

We have strong finishers in both case, and actually, this opposition and their own characteristics make their advantage.

Looking closely to what’s happen in the “maintening frequency group”, we se that ground contact time is far to remain the same in the last half of the race, the time increase steadily (e.g. Marion Jones), while the other group maintain a good ratio SL/GCT, showing that they still overcome the reduction of strength due to “fatigue”.

Not to be a smart arse but the track bike analogy removes all aspects of vertical force (and replaces them with tyres). Vertical force is the primary quality that allows an athlete to maintain turnover and stride length once acceleration is complete.

The legs are cycling in both instances. That was the analogy of both situations. The output is different of course. But we still agree that force is what cause the athlete maintain top speed.

The more force applied, makes the stride longer and the frequency faster. We always have to consider ground resistance when talking about applying force (equal and opposite reaction). What it all comes down to is how much force is applied against a resistance. Frequency in the early stages of the sprint means nothing without resistance. Good athletes know how to work the resistance that the ground offers. This leads to good acceleration.

I’ll agree with that. We’ve all seen the athlete that has extreme frequency early, with little force. They reach top end at 30m hold for a short time and decelerate for 60m, only to get passed by everyone.

A former athlete of mine had a great race, where she had longer, powerful strides out of the blocks. She reached top end later in the race than her competitors and thus had a shorter distance to decelerate. It looked like she accelerated through the tape, but we know that wasn’t true. All of the armchair coaches came up to me later letting me know that her “slow start” almost cost her and that as soon as we fixed it she’d be great!

She and I were very happy with her PR (12.04 f.a.t), set that day. Fixing her start would’ve killed her.

Are you sure? Isn’t it the time in which the force is produced as well as the amount of force, particurly at max v. Also, of special importance in max v is rate and level of antagonistic muscular relaxation.

Time is very important in the early stages of the race. Remember that force x time is impulse. Impulse is actually the change in momentum…meaning with every postive impulse, the body moves a bit faster down the track. The changes are greater at the beginning of the race as the body is accelerating. Time helps stride length (Body of speed) but reduces frequency. With each step, time gets smaller for completing each cycle which makes frequency faster.

She reached top end later in the race than her competitors and thus had a shorter distance to decelerate

Correct me if I’m wrong ( and I’m sure u will lol ) but -

I thought the idea was to get to ur max speed as soon as possible - then develop ur SE to match ur speed - if ur energy system isn’t strong enough to sustain this speed then its a weakness that should be addressed - holding back on the start might be a good tactic in the short term but holding back against someone who has the start and the finish will cost you every time .

Having said this my 200m pb this year came off the back of a diabolical start - but this is a slightly different matter to the 100m where every stride counts .

I don’t think cadence and stride length should be confused with efficiency - if a slower stride in the first 30m is going faster than using a faster cadence - then no problemo - but holding back sounds like a recipe for disaster in the 100m and avoiding an issue of a weakness that could be fixed .