Spinal Engine and Cross Extensor Reflex

I am not sure I completely agree with Carl or his therapist friend here. Stiffness increases as acceleration approaches its limit, but power is closely tied to impulse and both are very important with respect to acceleration and velocity.

Power is Work/time or Force * velocity
Impulse is Force * time or mass * velocity when we look at how impulse relates to momentum.

Not to meaning to get too sidetracked, but now we have new meaning to mass-specific force and its relationship to power and not strength. In sprinting mass-specific forces are power values, but they are also impulse values. However, we are talking about the integration of net horizontal impulses/forces of each step in the analysis and not vertical forces which are the only forces to affect stiffness. There is not a direct link to stiffness only that the legs must increase stiffness as acceleration diminishes which has to do with translation of horizontal forces to vertical forces to maintain at least the current speed. How stiffness affects acceleration doesn’t happen in the first 10m or 10 yards except for the slowest individuals known to mankind (ie… the human versions of the three toed sloth).

So how is force * velocity which is horizontal only linked to mass * velocity which is both vertical and horizontal in vector analysis because of gravity acting on a mass. The force required to overcome inertia in the horizontal direction on each consecutive step is dictated by drag and friction only. So lets put on our horizontal eyes only. If power is increased then both force and velocity in the horizontal direction are increased if there is link between power and impulse. You cannot increase force and run faster by having a reduced power output and on the same token you cannot run faster with increased force outputs by having a reduced impulse or momentum. Therefore running faster over a certain distance requires greater forces done in less time which really means greater power if we are creating greater power in less time that means we have less time in which to create a greater impulse. Impulse and power can only be generated when the foot is on the ground and the longest periods of time of ground contacts take place early in acceleration and those periods get progressively smaller with each step. This is the part which links stiffness to acceleration, but stiffness is a by product of translating horizontal to vertical and is by no means limited in the first 10yds or 10m. This leads to the problem of suppleness or flexibility/mobility/ROM which would mean to run faster you need larger ROM’s during ground contact progressing to smaller ones, but each step is getting progressively longer than the previous step. The swing phase ROM gets a progressively larger on step as well and so does the distance. This means if you are going to take more steps you have to take smaller ones, but ones where if you integrated the net horizontal impulse values of each step taken you would get a larger than if you took less steps. Which brings us to our final problem, which happens to be if you are going to take more steps and run faster you have to have a lower stiffness value to start with than those who are taking less steps at the same speed or slower speeds. All of which may be true and is unproven and I agree up to this point, but the link between power, stiffness, and suppleness depends on the ability to generate power, the range of stiffness (min-max), and having enough suppleness to allow the limbs to move freely. Then trying to link these to the spinal engine (CPGs) is another story. If one of those is limiting the expression of the others from the CPG will be dampened in the execution of the skill. This fits in line with equilibrium point hypothesis and its offshoots in motor learning which as Mike has correctly stated there is not any applied research as I know it to exist, most of the applied research is with single joint movements on human subjects and most of the research on multi-joint movements has been done on spinalized laboratory animals such as frogs. Most individuals who are athletic have the suppleness to go faster than they currently can, what they typically lack is the power or range of stiffness to maximize velocity or create the impulse necessary.

Is therapy needed to run faster? Occasionally. Will it make someone run faster over 10yds and then subsequently later over 40yds? Absolutely not. Shin angles still mean little because we then are confusing joint stiffness with vertical stiffness which are linked by the angle swept during ground contact. In fact it seems having the ability to produce locomotive power at a lower minimum stiffness to start locomotion at greater acceleration values and the ability to reach a higher top end velocity requires a greater maximum stiffness. You can provide artificial means to alter the range of stiffness you are accelerating through with hills, sleds, vests, etc, but you can alter the range of stiffness during acceleration by having larger power outputs. This requires an across the board change in both neural and structural changes in the muscle fibers and neural changes at the spinal level to say the least. I still don’t see getting to seven steps for most individuals and for those that do get to 7 steps and sub 4.3 well they could be world class sprinters, but I think most world class sprinters would still be around 6 steps at most over 10m and definitely over 10yds. Those who are shooting for 5 steps may be reaching as well in every sense of the word. My personal opinion is give the athlete the right environment and training the number of steps will optimize to the local environment and training otherwise known as a local optima. That doesn’t mean you cannot cue or give feedback in training, it just means give less of both as you cannot teach angles or stiffness from words, but through a culmination of training practices to include words, visualizations, specific and general power work.

Sorry about the slight disagreement, but you weren’t as pointed as usual this time around and somewhat vague which is what I guess people want.

“In research the spinal engine Carl refers to more typically goes by the name of central pattern generators or spinal generators. There won’t be much, if any, applied writings on the topic. Certainly nothing as it applies directly to sprinting beyond some passing reference.”

Mike

Doesnt bosh and klump refer to the cross extensor reflex and another reflex used during gait?

Yes but cross extensor reflex is not altogether the same as CPGs. CPG, if theory holds up, is responsible for some of our most highly coordinated movements (like walking, running, breathing, maybe even mastication) and not extension / flexion reflexes. CPG is really theoretical at this point. There’s a growing amount of research on the subject, especially on decerebrate cats, but it hasn’t been proven in the same way as cross extensor reflex has. And the two things are related in that they’re sub-cortical but are not the same thing.

ELITETRACK Founder