The following article by Ron Daniel first appeared in the Fall Bulletin of

the North American Racewalking Foundation. It was subsequently published in the

Ohio Racewalker. It is being submitted to the racewalk list for further

exposure. Elaine Ward

WHEN IS LIFTING AN ADVANTAGE? - Ron Daniel

I don't know if anyone has "tested" whether loss of contact (when the

knee is straight) increases the speed of a walker and gives him or her an

advantage. However, I've taken some time to think about the physics and will use

simple examples to show just when loss of contact can help with velocity and when

it doesn't. More difficult to analyze are the forces and energy issues

associated with race walking, and the potential changes when walking with loss of

contact.

VELOCITY: Consider a walker who is making double contact and whose stride

rate is 200 steps per minute with a stride length of 40 inches. At 200 steps

per minute, the walker's foot is in contact for 0.3 second each stride. This is

a velocity of 11.11 feet per second.

Now suppose this walker has a loss of contact with a float time of 0.02

second. If he maintains the same ground time, 0.3 second and he floats 0.02

second, the total stride time is now 0.32 second (which results in a stride rate

of 187.5 steps per minute). If the float is 2 inches (5% gain in stride

length), his velocity is only 10.94 feet per second. The walker is slowing down!

On the other hand, if the float is 4 inches (10% gain in stride length) his

velocity is 11.46 feet per second. The walker is going faster. The difference

in float distance is governed by the push off angle. Because the ground time

0.3 second and the float time 0.02 second have not changed, the steps per

minute remain 187.5.

Greater velocity is not guaranteed by loss of contact. If the walker has

a longer float of say 0.03 second, he has to have a float distance of 4

inches to exactly match his non loss of contact velocity of 11.11 feet per second.

So this increase in the amount of time in the air requires a greater float

distance to equal the walker who is maintaining contact. In the context of

human eye acuity, a 0.02 second float is basically invisible to the human eye, but

a 0.03 second float is quit visible.

ENERGY: Now, let's consider the ground forces and energy issues associated

with race walking, and how they may change when shifting from full contact

walking to loss of contact walking. Forward propulsion is provided by the push

(force) against the ground. If there is little or no extra effort (energy) to

achieve the float, then there is little to no energy penalty for the loss of

contact. If the float is sufficient to produce a greater velocity, then by the

end of a long race, there is a net energy savings because there are fewer

steps; i.e. fewer push offs.

Once the walker has pushed forward, there is now momentum associated with

the forward progress. Ideally, the walker wants to maintain the momentum;

however, when the forward swinging leg makes contact (usually forward of the

body's center of gravity) , there is momentary braking prior to the body rotating

over the foot. Consequently, some of the walker's energy goes into

overcoming the braking.

The amount of braking is related to the rise and fall of the body mass

and the momentum (mass times velocity) of the upper and lower leg.

Biomechanically, the braking is resolved into vertical and backward directions. The

walker can minimize the backward braking by making the ground contact closer to

under the center off gravity. However, if the walker's stride in front of the

center of gravity stays the same from "full contact" to loss of contact walking,

then the only consideration is if the per stride braking has increased. If

the per stride braking has remained the same, then there is a net savings in

the overcoming of braking for the whole race.

For example, a walker in a 20,000 meter race who has a stride length of

one meter is taking 20,000 strides. Let's say that he is able to float forward

1/10th of a meter. He has now saved himself 10% of the needed strides, or

2000 strides, which is a saving of 10% of the push-off energy and 10% of the

braking energy.

In shorter races, floating may have a simple velocity value, but at the

longer distances, it has a significant energy saving value as well. The

energy saving is there for all distances, but depending on the overall condition of

the athlete, his susceptibility to fatigue may not come until after 10km. Or

he may feel significantly tired after one mile.

LEGALITY: Now, let's put the above information in a way specific to the

original question. What is the value of a float for someone who walks with a

straight leg and has a loss of contact within the non detectable eye range? This

walker is not progressing forward in a way that is totally within the original

idea of the contact rule. True. But because the rules state that judging is

with the unaided eye, if the loss of contact is not detectable, it's NOT

illegal.

Again, traditional physics says as soon as the walker is not on the

ground pushing, he is starting to lose forward velocity. That is a reason why a

hurdler, when he goes over the hurdle, works very hard on getting the lead foot

down on the ground as soon as possible rather than continuing a nice long

float.

A racewalker who has his knee straight as required by the bent knee rule

is subject to the same physics as the hurdler when he pushes off the ground.

Whether he gains or loses by floating two to four inches depends on what is

going on. Does the float, in fact slow him down? Is he in the air for such a

short period of time that he may not be slowing down measurably? Or, if he

pushes off against the ground, does the amount of push and the amount of inches

gained floating overcome the slowing down? Equations would prove these

variables.

Without writing an equation, the way I look at it is this: We know as

soon as a walker puts his foot down somewhere in front of the body, a little

braking motion is created that slows him down. If he is keeping his leg straight,

but pushing off the ground and taking 5-10% less strides, he is saving

energy. If he meets the criteria in the first part of the discussion, then he is

gaining a velocity advantage and an energy advantage on the walkers maintaining

ground contact. This is as true of elite race walkers as it is for masters.