From Joe DeFranco’s latest Q&A
As far as your question is concerned, here’s a recap of my explanation of a “positive shin angle”…
A positive shin angle is the angle of the shin that you should try to achieve when accelerating into your sprint. When you accelerate, you should have an incredible body lean (approximately 45-degrees in relation to the ground). When your upper body is in front of your lower body, you need your knee to be in front of your foot during each foot strike; this prevents overstriding & the creation of a “breaking force” with each step. If your foot lands out in front of your knee during acceleration, you will actually decelerate with each stride and slow yourself down (breaking force). Check out the picture of Randy Moss below. It gives a great visual of a positive shin angle.
Notice the angle of his left shin; when his foot strikes the ground, it will push BACK into the turf, which will propel him FORWARD. This is a great acceleration position to be in. The key to acceleration is to take the biggest steps possible, without overstriding. A positive shin angle enables you to accomplish this.
The fewest steps wins the race!
I have bolded the bit I found most intriguing and would appreciate comment on what he says.
I would agree with this comment. It is generally good practice to use long strides out of the blocks because the accleration will be greater due to longer contact times. change in momentum = force x time. Of course we only want the foot on the ground when it is applying force backwards, to propel us forwards, so the foot needs to be applying a backwards force at the moment of ground contact. Positive shin angle enforces this because it keeps the contact point (foot on the track) closer to BDC from the centre of mass. Obviously any force applied behind the centre of mass will propel the athlete forwards, including pure vertical force, which the actual force is close to at the intial moment at which ground contact occurs. Conversly vertical force ahead of BDC will push the athlete backwards. So as force is a vector so long as the angle of force relevant to the centre of mass is backwards, rather than forwards, all the time the foot is on the floor the force is aiding in acceleration so longer the better. As the athlete becomes more upright the centre of mass moves backwards and so does BDC to the foot stike is closer to being under the body but the rule of longer contact being better still applies even though in reality the time get shorter as the athlete improves, this is due the the increase in force. The range of motion covered by the leg with the foot on the ground is the same, just occuring in a shorter space of time.
So, of all these, what would you say to your athlete?
Idealy nothing. I beleive that resistance running is the best way to enforce it because they can feel quite quickly the effects of increasing the ground contact (lets call it) range of motion. If you are purley addressing it from a technical point of view i think you can get away with using quite heavy sleds.
Heavier sled pulls will decrease stride length and fatigue the leg muscles prematurely, increasing stride rate. This will affect technique, how can this be considered a “TECHNICAL POINT”?
Yeah obviosuly the stride length will be shorter when towing but that is obvious, the velocity is slower, velocity x time in the air = stride length, but what is the problem with that? All i am concerned with is how long the foot is on the ground for, for each stride. Acceleration is so dependent on how long the force is applied that you need to get the most out of each stride in terms of how long you can apply useful force, the drill is aimed taking the athlete to the boundaries of effective range of motion during which the foot is applying force.
Does it really prematurly fatigue the muscles? Load is load, the time under load for each drill when towing will obviously be higher, so the volume is dropped, simple. But the athlete will adapt quicker towing than repetative analytical imporvements which may take weeks. Plus we could get into the difference between the concentric and eccentric elements between the two methods and we would probably find the fatigues is not so different as you would think.