see if this helps....
What Contemporary Research Tell us about Sprinting
Illinois Track and Cross Country Coaches Association Clinic
Ken Jakalski
January 12, 2002
Reconsidering the Conceptual Paradigm of Running Mechanics
The history of speed training makes it clear that our philosophical approach changes with the current thinking of the time. For example, several years ago, coaches believed that the only way to increase speed was to increase stride length. Indeed, stride length is a function of running speed, since stride lengths do increase as our speed increases. The natural way for a runner to increase stride length is for him or her to increase the force against the ground in each driving phase. This, of course, requires increased leg strength.
Then, we began almost two decades of sprint training that emphasized what is now referred to as neuro-biomechanics. This approach challenged the commonly held notion that stride frequency was too difficult to improve, and noted that, of the two factors, stride length/stride frequency, frequency was actually more important. The training approach today focuses on reducing the time it takes to get necessary force into the ground. The goal is to increase stride frequency and reduce the time it takes to recycle the leg.
To this end we designed drills to train athletes to place their limbs in more appropriate positions to improve the rate of force development. Since the ground phase was dictated by ground preparation, the key was to generate high speeds backward to minimize breaking forces and maximize propulsive forces. The secret seemed to be the ability to generate high negative thigh speed, or what came to be known in coaching circles as negative vertical velocity.
Unfortunately, the drills we've designed have been based more on coaching insight and observation than on hard science, and it's clear that the questionable carry-over of these drills to actual sprinting has left many coaches and runners frustrated. I have watched many colleagues teaching dorsi-flexion, pawing, clawing, fast foot, stepping over the opposite knee, appropriate arm carriage, etc. only to observe with dismay that these movements don't appear to be repeated when their athletes begin sprinting. What's the problem here? Insufficient time to fix these new patterns of movement? Poor coaching of these techniques? Inappropriate cues? Improper drills for appropriate mechanics?
My contention, based upon a wonderful opportunity I had to study over thirty years worth of locomotion research under the direction of renowned Harvard physiologist Peter Weyand, is that we may very well be attempting to make modifications to non trainable entities. I first began to consider this possibility when conventional speed training could not explain to me how it was possible for an athlete without feet to dorsi-flex, or arms to aid in propulsion, could run 22.94, which is exactly what World Paralympic Sprint Champion Tony Volpentest did in Lisle four years ago!
We believe that athlete's faster muscle fibers can improve stride frequency by reducing the time spent on the ground and in the air. In fact, reducing ground time and air time has been the basic approach to speed training since the early eighties. However, what if we discovered that the mechanical energy to reposition the free swinging limb is actually provided passively through elastic recoil and energy transfer between body segments instead of power generated within muscles?
If this were the case, if muscle speed has little effect on minimum swing time, then training to improve stride frequency, what we now refer to as maximum velocity mechanics, would be of little value.
If frequency is revealed not to be a contributor to faster top end speed, what is? Stride length must be critical. But how do athletes increase stride length? One way to achieve longer strides is to apply greater support forces to the ground. This makes sense, since we know that, at any speed, applying greater force to oppose gravity will increase a runner's vertical velocity at take-off. As a result, the forward distance traveled between steps will increase.
This was the Harvard researchers' hypothesis: that greater ground forces rather than minimum swing times enable sprinters to achieve faster top end speeds. In this process, the team re-considered the elementary mechanics of running. First, they explored the possibility that runners reach maximum velocity simply by taking more frequent steps. Next, they explored their original hypothesis, that speed could be achieved by the athlete increasing mass specific force to oppose gravity during the time the foot is in contact with the ground. Finally, they attempted to take into account the fact speed might be achieved by increasing the forward distance the body moves during this contact period, which is referred to as contact length.
The Harvard team expected to find that top speed was indeed more a product of forces applied to the running surface rather than increases in step frequency or contact length. Why did they feel this would be the case? For one, swing time comprises the majority of total stride time, and is the primary determinant of the frequency of a runner's steps. However, because the range of stride frequencies used by runners at different speeds tends to be narrow, the researchers expected little variation in step frequencies at top speed.
This similarity in step frequency is a difficult concept for most of us to grasp, since video analysis seems to reveal some fundamental yet critical movement “commonalities†consistent at high speed running, and that these commonalities indicate optimal positions of the leg during the recovery cycle. As a result of these observations, we concluded that the fastest sprinters in the world actually reposition their limbs appreciably faster than sub-elite sprinters, but this is not the case.
Second, contact lengths at high speeds do not vary significantly, yet faster runners still take considerably longer strides. Length of contact was clearly not a factor. Even though it would seem as if sprinters would benefit from additional time they have to apply force, but attempting to increase stride length through unnaturally longer steps is actually mechanically inefficient.
So what did the Harvard study reveal?
The more rapid increases in stride frequency as athletes approach top speed are achieved through reductions in both the contact and swing times that make up total stride time. The time when neither foot is on the ground—the aerial time—also decreases as top speed is approached.
Thus, the traditional concept of speed being the product of reduction of the time spent on the ground and in the air was correct, but the process by which this occurs never considered running mechanics as a function of speed—at least not the way the locomotion experts have been understanding mechanics. In other words, we believed that specific limb movements associated with high speed running were trainable entities that could impact upon performance. This is simply not the case.
Faster runners apply greater forces during briefer contact periods, but because of the narrow constraints in the minimum swing times and maximum contact lengths of sprinters, speed is conferred predominantly by an enhanced ability to generate and transmit muscular force to the ground.
The Conclusion
Of the mechanical means by which runners can reach faster top end speeds, the Harvard team found that runners rely on stride frequency to a limited extent, support forces predominantly, and contact lengths not at all.
The mission statement of previous speed programs was to reduce the time spent on the ground and in the air. The Faster Than Gravity mission statement, at least based on the new paradigm proposed in the Harvard study, is to alter support forces applied to the ground by one tenth of one body weight, which will be sufficient to alter top speed by one full meter per second!
The Limits to Top End Speed
Top speed is reached when increases in speed and decreases in foot ground contact times reduce effective aerial times to the minimum values, providing sufficient time to swing the leg into position for the next step. In other words, Leigh Kolka's theory that we can sprint as fast as we can put the free swinging limb in front of us was indeed accurate, but for reasons that, at the time, none of us truly understood.
The fastest runners do have faster muscle fibers and do have great muscular power available to reposition their limbs, but the reality is that they reposition their limbs little or no faster than average and slow runners do.
Activation of the flexor muscles and tendons that reposition the limb as a runner swings his leg is considerable at high speeds, but this activation likely occurs to increase the storage and release of mechanical energy in the oscillating limb rather than to generate mechanical power chemically within these muscles.
Once again, faster muscles fibers confer faster top end speeds not by decreasing minimum swing times, but by increasing the maximum rate at which force can be applied to the ground. An enhanced ability to quickly reposition their limbs in the air is not the reason why sprinters achieve faster top end speeds. They are simply applying greater support forces to the ground.
